Minimize GAIA (Global Astrometric Interferometer for Astrophysics)

Gaia Astrometry Mission

Spacecraft   Launch   Mission Status   Payload Module   Ground Segment   Big Data Archive   References

 

Gaia (mother Earth in Greek mythology) is an ESA cornerstone space astrometric mission, part of the Horizon 2000 Plus long-term scientific program, with the goal to compile a 3D space catalog of > 1000 million stars, or roughly 1% of the stars in our home galaxy, the Milky Way. Gaia will monitor each of its target stars about 70 times to a magnitude of G=20 over a period of 5 years. It will precisely chart their positions, distances, movements, and changes in brightness. It is expected to discover hundreds of thousands of new celestial objects, such as extra-solar planets and brown dwarfs, and observe hundreds of thousands of asteroids within our own Solar System. The mission will also study about 500,000 distant quasars and will provide stringent new tests of Albert Einstein’s General Theory of Relativity. 1) 2) 3) 4) 5)

Cataloguing the night sky is an essential part of astronomy. Before astronomers can investigate a celestial object, they must know where to find it. Without this knowledge, astronomers would wander helplessly in what Galileo once termed a ‘dark labyrinth’.

During the satellite’s expected lifetime of five years, Gaia will observe each star about 70 times, each time recording its brightness, color and, most importantly, its position. The precise measurement of a celestial object’s position is known as astrometry, and since humans first started studying the sky, astronomers have devoted much of their time to this art. However, Gaia will do so with extraordinary precision, far beyond the dreams of those ancient astronomers.

By comparing Gaia’s series of precise observations, today’s astronomers will soon be able to make precise measurements of the apparent movement of a star across the heavens, enabling them to determine its distance and motion through space. The resulting database will allow astronomers to trace the history of the Milky Way.

In the course of charting the sky, Gaia’s highly superior instruments are expected to uncover vast numbers of previously unknown celestial objects, as well as studying normal stars. Its expected haul includes asteroids in our Solar System, icy bodies in the outer Solar System, failed stars, infant stars, planets around other stars, far-distant stellar explosions, black holes in the process of feeding and giant black holes at the centers of other galaxies.

The primary mission objectives are:

• Measure the positions and velocity of approximately one billion stars in our Galaxy

• Determine their brightness, temperature, composition and motion through space

• Create a three-dimensional map of the Galaxy.

Additional discoveries expected:

- hundreds of thousands of asteroids and comets within our Solar System

- seven thousand planets beyond our Solar System

- tens of thousands of ‘failed’ stars, called brown dwarfs

- twenty thousand exploding stars, called supernovae

- hundreds of thousands of distant active galaxies, called quasars.

The Gaia objective is to provide a very accurate dynamical 3D map of our Galaxy by using global astrometry from space, complemented with multi-color multi-epoch photometric measurements. The aim is to produce a catalog complete for star magnitudes up to 20, which corresponds to more than one billion stars or about 1% of the stars of our Galaxy. The instrument sensitivity is such that distances beyond 20-100 kiloparsec (kpc) will be covered, therefore including the Galaxy bulge (8.5 kpc) and spiral arms. The measurements will not be limited to the Milky Way stars. These include the structure, dynamics and stellar population of the Magellanic Clouds, the space motions of Local Group Galaxies and studies of supernovae, galactic nuclei and quasars, the latter being used for materializing the inertial frame for Gaia measurements.

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Figure 1: Gaia measurements objectives (image credit: ESA, Airbus Defence and Space) 6)


Background: Gaia is ESA's second space mission dedicated to astrometry. It builds on the legacy of the successful Hipparcos mission (1989-1993). 7) Like Hipparcos, Gaia's observation strategy is based on detecting stellar positions in two fields of view separated by a 'basic angle', which for Gaia is 106.5º. This strategy allows astronomers to establish a coherent reference frame over the entire sky, yielding highly accurate measurements of stellar positions.

After a detailed concept and technology study during 1998–2000, Gaia was selected as a confirmed mission within ESA’s scientific program in October 2000. It was confirmed by ESA’s Science Program Committee following a re-evaluation of the science program in June 2002, and reconfirmed following another re-evaluation of the program in November 2003. The project entered Phase-B2/C/D in February 2006. As of the summer 2012, Gaia is in Phase-D (Qualification and Production) and will be launched in the second half of 2013. 8) 9) 10)

• In June 2013, ESA's billion-star surveyor, Gaia, has completed final preparations in Europe and is ready to depart for its launch site in French Guiana. The Gaia spacecraft arrived in Cayenne, French Guiana, on August 23, 2013 on board the Antonov 124 aircraft.

• On Oct. 23, 2013, ESA postponed the launch of the Gaia mission. The decision was taken due to a technical issue that was identified in another satellite already in orbit. The issue concerns components used in two transponders on Gaia that generate ‘timing signals’ for downlinking the science telemetry. To avoid potential problems, they will be replaced.

The transponders were removed from Gaia at Kourou and returned to Europe, where the potentially faulty components were replaced and verified. After the replacements have been made, the transponders will be refitted to Gaia and a final verification test made. As a consequence of these precautionary measures, it will not be possible to launch Gaia within the window that includes the previously targeted launch date of 20 November. The next available launch window is 17 December to 5 January 2014. 11)

• Update Oct. 20, 2013: The upcoming launch manifest of Arianespace has now been established. Gaia is scheduled for launch on 20 December.

• Update Nov. 22, 2013: The checks on the Gaia satellite are proceeding well, enabling the launch to take place on December 19, 2013 (Ref. 11).


Some astrometry basics:

The precise measurement of a celestial object’s position is known as astrometry, and since humans first started studying the sky, astronomers have devoted much of their time to this art. However, Gaia will do so with extraordinary precision, far beyond the dreams of those ancient astronomers (Ref. 21). 12)

By comparing Gaia’s series of precise observations, today’s astronomers will soon be able to make precise measurements of the apparent movement of a star across the heavens, enabling them to determine its distance and motion through space. The resulting database will allow astronomers to trace the history of the Milky Way.

In the course of charting the sky, Gaia’s highly superior instruments are expected to uncover vast numbers of previously unknown celestial objects, as well as studying normal stars. Its expected haul includes asteroids in our Solar System, icy bodies in the outer Solar System, failed stars, infant stars, planets around other stars, far-distant stellar explosions, black holes in the process of feeding and giant black holes at the centers of other galaxies. Gaia will be a discovery machine.

Stars as individuals and collectives:

To understand fully the physics of a star, its distance from Earth must be known. This is more difficult than it sounds because stars are so remote. Even the closest one is 40 trillion km away, and we cannot send spacecraft out to them to measure as they go. Nor can we bounce radar signals off them, which is the method used to measure distances within the Solar System. Instead, astronomers have developed other techniques for measuring and estimating distances.

The most reliable and only direct way to measure the distance of a star is by determining its 'parallax'. By obtaining extremely precise measurements of the positions of stars, Gaia will yield the parallax for one billion stars; more than 99% of these have never had their distances measured accurately. Gaia will also deliver accurate measurements of other important stellar parameters, including the brightness, temperature, composition and mass. The observations will cover many different types of stars and many different stages of stellar evolution.

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Figure 2: Distance to a star can be calculated with simple trigonometry from the measured parallax angle (1 a.u. is 1 Astronomical Unit, or 149.6 million km), image credit: ESA/Medialab

The principles of Gaia:

At its heart, Gaia is a space telescope – or rather, two space telescopes that work as one. These two telescopes use ten mirrors of various sizes and surface shapes to collect, focus and direct light to Gaia’s instruments for detection. The main instrument, an astrometer, precisely determines the positions of stars in the sky, while the photometer and spectrometer spread their light out into spectra for analysis.

Gaia’s telescopes point at two different portions of the sky, separated by a constant 106.5º. Each has a large primary mirror with a collecting area of about 0.7 m2. On Earth we are used to round telescope mirrors, but Gaia’s will be rectangular to make the most efficient use of the limited space within the spacecraft. These are not large mirrors by modern astronomical standards, but Gaia’s great advantage is that it will be observing from space, where there is no atmospheric disturbance to blur the images. A smaller telescope in space can yield more accurate results than a large telescope on Earth.

Gaia is just 3.5 m across, so three curved mirrors and three flat ones are used to focus and repeatedly fold the light beam over a total path of 35 m before the light hits the sensitive, custom-made detectors. Together, Gaia’s telescopes and detectors will be powerful enough to detect stars up to 400,000 times fainter than those visible to the naked eye.

Gaia uses the global astrometry concept demonstrated by Hipparcos. The principle is to link stars with large angular distances in a network where each star is connected to a large number of other stars in every direction. The condition of closure of the network ensures the reduction of the position errors of all stars. This is achieved by the simultaneous observation of two fields of views separated by a very stable basic angle. The spacecraft is slowly rotating at a constant angular rate of 1º/min around a spin axis perpendicular to both fields of view, which describe a great circle on the sky in 6 hours. The spacecraft rotation axis makes an angle of 45º with the Sun direction (Figure 3). A slow precession around the Sun-to-Earth direction, with a 63.12 days period, enables to repeat the observation of sky objects with 86 transits on average over the 5 years of mission.

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Figure 3: Illustration of the sky scanning principle (image credit: ESA)

The resulting performance will enable a breakthrough in the astrometry field, as well regarding star position and velocity performance as for the number of objects observed.

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Figure 4: Gaia will improve the accuracy of astrometry measurements by several orders of magnitude compared with previous systems and observations (image credit: ESA)



Spacecraft:

Gaia is an exceptionally complex space observatory. ESA awarded Airbus Defence and Space (former Astrium SAS,Toulouse, France) the prime contract in May 2006 to develop and build the spacecraft. Together with the German and British branches of Astrium, more than 50 industrial subcontractor companies from across Europe are involved in building this discovery machine. The Gaia DPAC (Data Processing and Analysis Consortium) will process the raw data to be published in the largest stellar catalog ever made. 13) 14) 15) 16) 17) 18) 19) 20)

The Gaia spacecraft is composed of two sections: the Payload Module and the Service Module. The Payload Module is housed inside a protective dome and contains the two telescopes and the three science instruments. They are all mounted on a torus made of a ceramic material (silicon carbide). The extraordinary measurement accuracy required from Gaia calls for an extremely stable Payload Module that will barely move or deform once in space; this is achieved thanks to the extensive use of this material. 21)

Underneath the Payload Module, the Service Module contains electronic units to run the instruments, as well as the propulsion system, communications units and other essential components. These components are mounted on CFRP (CarbonFiber Reinforced Plastic) panels in a conical framework.

Finally, beneath the Service Module, a large sunshield keeps the spacecraft in shadow, maintaining the Payload Module at an almost constant temperature of around -110ºC, to allow the instruments to take their precise and sensitive readings. The sunshield measures about 10 m across, too large for the launch vehicle fairing, so it comprises a dozen folding panels that will be deployed after launch. Some of the solar array panels that are needed to generate power are fixed on the sunshield, with the rest on the bottom of the spacecraft.

The Gaia spacecraft configuration is driven by the required very high thermo-mechanical stability of the entire spacecraft. A low disturbance cold gas micro-propulsion is used for fine attitude control. The astrometric instrument is used for precise rate sensing in fine pointing operating mode.

Spacecraft launch mass

~2034 kg (dry mass = 1392 kg)

Spacecraft power

1.91 kW (EOL)

Spacecraft dimensions

Height = 3.5 m, deployed diameter = 10 m

Mission duration

5 years

Table 1: Parameters of the Gaia spacecraft

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Figure 5: Artist's rendition of the deployed Gaia spacecraft (image credit: ESA)


SVM (Service Module):

The SVM is generally referred to as 'the platform'. The SVM in turn is comprised of MSM (Mechanical Service Module) and an ESM (Electrical Service Module). 22) 23) 24)

MSM (Mechanical Service Module): The spacecraft main structure is of hexagonal conical shape. It is a sandwich panel structure with CFRP (Carbon Fiber Reinforced Plastic) face sheets, and a central cone supporting the propellant tanks. The MSM houses instruments needed for the basic control and operation of the satellite; this includes all mechanical, structural and thermal elements that support the instrument payload and spacecraft electronics. It also includes the chemical & micro propulsion systems, the deployable sunshield with solar arrays, the payload thermal tent and harness. The module consists of a central tube that is about 1.17 m long and hosts six radial panels to create a hexagonal spacecraft shape.

The service module also houses the communication subsystem, central computer and data handling subsystem, the high rate data telemetry, attitude control and star trackers. For telemetry and telecommand, low gain antenna uplink and downlink with a few kbit/s capacity are employed. The high gain antenna used for the science telemetry downlink will be used during each ground station visibility period of an average of about 8 hours per day.

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Figure 6: Photo of the SVM integration (image credit: EADS Astrium)

ESM (Electrical Service Module): The ESM design is driven by the science performance (attitude control laws with the hybridization of star tracker and payload measurements, high rate data telemetry, and regulated power bus for thermal stability). It houses the AOCS units, the communication subsystem, central computer and data handling subsystem, and the power subsystem.

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Figure 7: Diagram of the ESM (image credit: EADS Astrium)

AOCS (Attitude and Orbit Control Subsystem). The AOCS subsystem is characterized by:

- High precision 3-axis control

- The ASTRO (Astrometric) instrument is used for precise rate sensing during the fine pointing operational mode

- A high precision gyroscope is used for quick and efficient transitions during the fine pointing operational mode. Three FOGs (Fiber Optics Gyroscopes) use the interference of light to detect mechanical rotation. Each unit contains four closed-loop gyroscope channels to provide built-in redundancy.

- Rugged flight-proven initial acquisition and safe modes

- Three sun acquisition sensors plus one gyroscope provide spin-axis stabilization during the L2 transfer phase of the mission

- One large field of view star sensor plus use of the main instrument SM (Sky Mapper) for the 3-axis controlled operational phase.

Gaia AOCS architecture is based on a fully redundant set of equipment. Moving parts on board are strictly minimized (e.g. no reaction wheels, no mechanically steerable antenna). The data is downlinked through a novel electromagnetically steerable phased array antenna and attitude control is provided by a micro propulsion system that has its first flight use with Gaia. An atomic clock is used for precise time-stamping.

Two Autonomous Star Trackers are used in cold redundancy three FSS (Fine Sun Sensors) are used in hot redundancy through triple majority voting. Three Gyro packages provide coarse rate measurements where each gyro package comprises two fully independent co-aligned channels (i.e. a fiber optic gyroscope sensor plus associated electronics per channel), with the channels being used in cold redundancy. As mentioned above, the payload module provides very precise rate measurements when the spacecraft is operated in fine pointing science modes. 25)

For actuators, a bi-propellant Chemical Propulsion System (2 x 8 10 N thrusters used in cold redundancy) is used for orbit maintenance and attitude control in coarse AOCS modes (circa 350 kg MON + MMH).

The Micro-propulsion System (2 x 6 proportional micro-thrusters) can provide a range of 0 - 1000 µN at a resolution of 0.1 µN. The individual thrusters are driven by the micro-propulsion electronic, which is internally redundant and used in cold redundancy (circa 57 kg of GN2). The nominal science AOCS mode uses the cold gas micro-propulsion system.

C&DMS (Command & Data Management Subsystem). The C&DMS is characterized by:

- An ERC-32 based central computer and distinct input/output units for efficient software development

- Two segregated MIL-STD-1553 B data buses: one for the payload module and one for the service module

- SpaceWire data links for high-speed payload data

- FDIR architecture aiming at preserving payload integrity, with built-in autonomy for increased availability.

The PDHU (Payload Data Handling Unit) is, among other things, the 'hard-disk' of Gaia, responsible for temporary storage of science data received from the telescope before transmission back to Earth. It will receive thousands of compressed images per second from the observing system; this data will be sorted and stored. The individual star data objects will be prioritized based on the magnitude of the star. A complex file management system allows deletion of low-priority data in the event of data rates or volumes that exceed the capacity of the storage or transmission systems.

The solid-state storage subsystem of the PDHU has a capacity of 960 GB which, while not impressive by terrestrial standards, is extremely large for a space system. It uses a total of 240 SDRAM modules, each with a capacity of 4 GB, which populate six memory boards. The PDHU controller board is responsible for communication with the other spacecraft subsystems, file system management and the management of telemetry and telecommands. 26) 27)

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Figure 8: The PDHU (Payload Data Handling Unit), image credit: ESA

The PDHU communicates with the gigapixel focal plane over seven redundant 40 Mbit/s SpaceWire channels to acquire the scientific data coming from the seven VPUs (Video Processing Units) of the camera. The unit's controller sorts the incoming data according to star magnitude and manages deletion of low priority data should this become necessary. It sends data for transmission to Earth under the control of the CDMU (Command and Data Management Unit). The PDHU communicates with the CDMU via a MIL-STD-1553 data bus and delivers the science data over two 10 Mbit/s PacketWire channels.- The PDHU consumes only 26 W, has a mass of 14 kg, and occupies a volume of 2.3 liter.

EPS (Electrical Power Subsystem): The spacecraft is equipped with a 12.8 m2 high-efficiency triple-junction GaAs (Gallium-Arsenide) cell solar array, of which 7.3 m2 is in the form of a fixed solar array and 5.5 m2 is covered by 6 panels mechanically linked to deployable sunshield assembly.

For the launch, the deployable sunshield is folded against the payload module. After separation from the launch vehicle, it is deployed around the fixed solar array, in the same plane. During LEOP (Launch and Early Operations Phase), power is supplied by a 60 Ah mass-efficient Lithium-ion battery.

Optimum power supply during all phases of the mission is ensured by a PCDU (Power Control and Distribution Unit) with maximum power point tracking. The PCDU performs power management by generating a 28 V primary power bus that supplies power to all spacecraft subsystems. It also controls the battery state of charge and generates pyrotechnic commands as well as heater actuation as commanded by the C&DMS (Command & Data Management Subsystem).

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Figure 9: Photo of the battery (image credit: ABSL)

Propulsion: After injection into the L2 transfer orbit by the Soyuz-Fregat launcher, a chemical bi-propellant propulsion system (8 x 10 N) is used for the transfer phase. It will cover attitude acquisition, spin control, mid-course corrections, L2 orbit injection, and safe mode.

After arriving at L2, one redundant set of micro-propulsion thrusters will control the spin and precession motion of the spacecraft. Regular orbit maintenance will be performed by using the chemical propulsion thrusters. - The spacecraft uses a cold gas micropropulsion system for fine attitude control.

CPS (Chemical Propulsion Subsystem): CPS is a bi-propellant system using two tanks of Herschel/Planck heritage filled with with a total of ~400 kg of propellant featuring a blowdown ratio of 4:3. Use of monomethylhydrazine as fuel and nitrogen tetroxide as oxidizer. The 10 N thrusters are manufactured by Astrium consisting of a platinum alloy combustion chamber and nozzle that tolerates the operational temperature of 1,500°C. The thruster can be operated in a thrust range of 6 to 12.5 N with a nominal thrust of 10 N which generates a specific impulse of 291 seconds.

MPS (Micro Propulsion Subsystem): The MPS is being used for fine attitude pointing and spin rate management. A total of 12 cold gas thrusters are installed on the spacecraft being grouped in three clusters each featuring four cold gas thrusters. The thruster system uses high-pressure nitrogen propellant to provide very small impulses with a thrust range of 1 - 500 µN. The system uses two nitrogen tanks, each containing 28.5 kg of N2, stored at a pressure of 310 bar (Ref. 19). The CG-MPS (Cold Gas-Micro Propulsion Subsystem) was developed by TAS-I and Selex ES S.p.A., Italy. 28)

RF communications: All communication with the Gaia spacecraft is done using the X-band. For TT&C (Tracking Telemetry and Command), a low gain antenna uplink and downlink with a few kbit/s capacity and an omnidirectional coverage are employed. The science telemetry X-band downlink is based on a set of electronically-scanned phased array antennae accommodated on the service module bottom panel. This high gain antenna is used during each ground station visibility period of about 8 hours per day.

The X-band payload downlink rate is 10 Mbit/s from L2. To achieve this, Gaia uses a specially designed on-board phased array antenna to beam the payload data to Earth (a conventional steerable antenna would disturbed the very precise measurements).

Gaia is equipped with a total of three communication antennas – two LGAs (Low Gain Antennas) and a single X-band Medium Gain Phased Array Antenna. One LGA is located pointing in the +X direction while the other points to –X being located on the Thermal Tent and the base of the spacecraft, respectively. The two LGAs build an omni-direction communications system for housekeeping telemetry downlink and command uplink with data rates of a few kbit/s.

The MGA (Medium Gain Antenna) is located on the base of the Payload Module, protruding the DSA(Deployable Sunshield Assembly) . This directional antenna can achieve data rates of up to 10 Mbit/s for science data and telemetry downlink and telecommand reception.

Demodulation of the uplink signal is completed by the transponder units before the data flow is passed on to the CDMU (Command & Data Management Unit). The downlink data is encoded by the CDMU and modulated in X-band within the transponders before being amplified by the SSPA (Solid State Power Amplifier). The signal is combined in the phased array of the active antenna in order to orient the beam towards the Earth.

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Figure 10: Photo of the phased array antenna (image credit: EADS Astrium)

CCSDS Image Data Compression ASIC: In order to transmit all the data generated on board, a particularly challenging compression factor averaging 2.8 was necessary. Unfortunately the standard suite of algorithms was not able to reach this target, because of the peculiarities of Gaia imagery, which include ‘outliers’, such as bright stars and planets, and which are marred by the momentary ‘hot pixels’ due to cosmic rays in deep space. Instead, with the support of ESA compression experts, industry developed an ad hoc solution, enabling all Gaia mission data to reach their home planet. 29)

CWICOM (CCSDS Wavelet Image COMpression ASIC) is a very high-performance image compression ASIC that implements the CCSDS 122.0 wavelet-based image compression standard, to output compressed data according to the CCSDS output source packet protocol standard. This integrated circuit was developed by Airbus DS through an ESA contract.

CWICOM offers dynamic, large compressed-rate range and high-speed image compression potentially relevant for compression of any 2D image with bi-dimensional data correlation (such as a hyperspectral data cube). Its highly optimized internal architecture allows lossless and lossy image compression at very high data rates (up to 60 Mpixels/s) without any external memory by taking advantage of its on-chip memory – almost 5 Mbit of embedded internal memory).

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Figure 11: The CWICOM ASIC is a customized microchip for imaging data compression (image credit: Airbus DS, ESA)

CWICOM is implemented using the largest matrix of the Atmel ATC18RHA ASIC family, and is provided within a standard surface mount package (CQFP 256). CWICOM offers a low-power, cost-effective and highly integrated solution for any image compression application, performing CCSDS image compression treatments without requiring any external memory. The simplicity of such a standalone implementation is achieved thanks to a very efficient internal embedded memory organization that removes any need for extra memory chip procurement and the potential obsolescence threatened by being bound to a specific external memory interface.

Change in data compression implementation:

In order to transmit all the data generated on board, a particularly challenging compression factor averaging 2.8 was necessary. Unfortunately the standard suite of algorithms [which includes CCSDS standards, including the 122.0 implemented by CWICOM] was not able to reach this target, because of the peculiarities of Gaia imagery, which include ‘outliers’, such as bright stars and planets, and which are marred by the momentary ‘hot pixels’ due to cosmic rays in deep space. Instead, with the support of ESA compression experts, industry developed an ad hoc solution,enabling all Gaia mission data to reach their home planet.

Instead of CWICOM, Gaia applies a tailored data compression algorithm (using a heavily tailored pre-processing stage followed by a variant of the Rice coder), using a software implementation running on VPUs (Video Processing Units). The GOCA (Gaia Optimum Compression Algorithm) project was entrusted by ESA to GTD System & Software Engineering (project prime) and IEEC (Institut d'Estudis Espacials de Catalunya), scientific partner), aimed in providing a deep understanding of the Gaia compression problem and offering a complete data compression system, both at an algorithm and implementation level. The main objectives of GOCA not only encompassed the review and evaluation of the already proposed compression scheme but the design of new algorithms for the mission. 30) 31) 32)

The CCDs in the focal plane are commanded by video-processing units (VPUs). Gaia has seven identical VPUs, each one dealing with a dedicated row of CCDs. Each CCD row, contains in order, two SM CCDs (one for each telescope), 9 AF CCDs, 1 BP CCD, 1 RP CCD, and 3 RVS CCDs (the latter only for four of the seven CCD rows). The VPUs run seven identical instances of the video-processing algorithms (VPAs), not necessarily with exactly the same parameter settings though. This (mix of some hardware and mostly) software is responsible for object detection (after local background subtraction), object windowing (see below), window conflict resolution, data binning, data prioritization, science-packet generation, data compression, etc.) 33)

TCS (Thermal Control Subsystem): A deployable sunshield with optimal thermoelastic behavior, made of multi-layer insulation sheets, is attached to the service module and folded against the payload module for the launch. After separation of the Gaia spacecraft from the launch vehicle, the Sun shield is deployed around the fixed solar array, in the same plane. - A thermal tent covers the payload, offering extra protection against micrometeoroids and radiation.

The very high stability thermal control is mostly passive and is achieved through optical surface reflector material, multilayer insulation sheets on the outer faces of the service module, and a black painted cavity, supplemented by heaters where required. Thermal stability is guaranteed by a constant solar aspect angle and the avoidance (as far as possible) of any equipment switch-ON/OFF cycles during nominal operation.

DSA (Deployable Sunshield Assembly): The bottom floor of the SVM is a dodecagonal-shaped panel to comply with the 12 frames of the DSA The main structure consists of carbon-fiber reinforced plastic face sheets.

DSA is folded up during launch and is deployed early in the flight. It is required to shade the payload unit and protect it from direct sunlight that could compromise instrument accuracy. Keeping the instrument at a constant temperature prevents expansion and contraction during temperature variations which would alter the instrument geometry ever so slightly with a large effect on data quality. The DSA is 10 m in diameter.

The DSA is an umbrella-type structure that consists of MLI (Multilayer Insulation) as the primary shield material and six rigid deployment booms as well as six secondary stiffeners. These booms have a single articulation on the base of the Service Module for easy deployment in the radial direction by a spring system. Spacing cables link the booms to the others to ensure a synchronized deployment sequence. The booms and strings are located on the cold side of the cover to limit thermoelastic flexing.

Attached to the DSA are six rectangular solar panels (with triple-junction solar GaAs cells) that are constantly facing the sun once the shield is deployed. They provide 1910 W of EOL (End of Life) power.

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Figure 12: Photo of Gaia's DSA deployment (image credit: Astrium SAS)

Legend to Figure 12: The DSA during deployment testing at Astrium Toulouse. Since the DSA will operate in microgravity, it is not designed to support its own weight in the one-g environment at Earth's surface. During deployment testing, the DSA panels are attached to a system of support cables and counterweights that bears their weight, preventing damage and providing a realistic test environment. The flight model thermal tent is visible inside the deploying sunshield and the mechanically representative dummy payload can be seen through the aperture in the tent.

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Figure 13: Photo of the Gaia SVM in the EMC chamber at Intespace, Toulouse, during launcher EMC compatibility testing (image credit: Astrium SAS)

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Figure 14: Exploded view of the Gaia spacecraft (image credit: EADS Astrium)

Propulsion systems

- 8 x 10 N bipropellant thrusters for coarse attitude control, orbit injection and trajectory corrections
- Low disturbance cold gas micro-propulsion for fine attitude control

Command and data management system

- ERC 32 based central computer with embedded solid state mass memory, complemented by a distinct input / output management unit
- Two segregated MIL-STD-1553B data bus, one for Payload Module and one for Service Module
- SpaceWire data links for high speed payload data

3-axes AOCS (Attitude and Orbit Control Subsystem)

- Payload instrument used for precise rate sensing in science mode
- High precision gyroscope to support transition to fine science pointing with instrument in the loop
- 3-axes autonomous star sensor for inertial attitude measurement
- Fine sun sensor for independent attitude monitoring

TT&C (Telemetry, Tracking and Command Subsystem)

- All X-band system
- Low Gain Antenna for spacecraft commanding, used also for spacecraft telemetry during critical phases
- Electrically steerable high gain antenna with built in solid state amplifiers for science downlink up to 8.7 Mbit/s from L2 orbit without mechanical disturbances

EPS (Electrical Power Subsystem)

- 60 Ah Li-ion battery
- 13 m2 triple junction cell solar array
- Power control and distribution with maximum power point tracking system

Payload data handling

- 7 Video Processing Units in parallel, each provided with 1 GIPS (Giga Instruction/s) processing capability
- 1 Terabit mass memory
- Atomic rubidium clock for accurate sequencing of the focal plane and star measurement dating

TCS (Thermal Control Subsystem)

- Fully passive thermal control
- Thermal tent for payload instrument, providing extra protection against space environment
- 100 m2 sunshield featuring a 0.1º flatness

Structure

- All Silicium Carbide instrument: structure & mirrors
- CFRP (Carbon Fiber Reinforced Plastic) SVM structure

Table 2: Summary of spacecraft subsystems (Ref. 6)

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Figure 15: Alternate exploded view of the Gaia spacecraft elements (image credit: EADS Astrium)

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Figure 16: The Gaia flight model spacecraft undergoing final electrical tests at Astrium Toulouse in June 2013 (image credit: EADS Astrium)

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Figure 17: Photo of the Gaia spacecraft in Nov. 2013 with an Astrium AIT engineer installing the transponders at the launch site (image credit: ESA)

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Figure 18: Photo of the Gaia spacecraft, tucked up inside the Soyuz fairing, ready to be mated with the Soyuz lower stages (image credit: ESA, M. Pedoussaut) 34)


Launch: The Gaia spacecraft was launched on December 19, 2013 (09:12:19 UTC) from Kourou by Arianespace, Europe’s Spaceport in French Guiana. The launch vehicle was a Soyuz-STB with a Fregat-MT upper stage The launch is designated as Soyuz flight VS06. 35) 36) 37)

- About ten minutes later, after separation of the first three stages, the Fregat upper stage ignited, delivering Gaia into a temporary parking orbit at an altitude of 175 km.

- A second firing of the Fregat 11 minutes later took Gaia into its transfer orbit, followed by separation from the upper stage 42 minutes after liftoff. Ground telemetry and attitude control were established by controllers at ESOC (European Space Operation Centre) in Darmstadt, Germany, and the spacecraft began activating its systems.

- The sunshield, which keeps Gaia at its working temperature and carries solar cells to power the satellite, was deployed in a 10 minute automatic sequence, completed around 88 minutes after launch. Gaia is now en route towards L2 (Ref. 35).

Orbit: Large Lissajous orbits around L2 (Lagrangian Point 2), about 1.5 million km from Earth. L2 offers a stable thermal environment because the sunshield will protect Gaia from the Sun, Earth and Moon simultaneously, allowing the satellite to keep cool and enjoy a clear view of the Universe from the other side. In addition, L2 provides a moderate radiation environment, which benefits the longevity of the instrument detectors.

• The critical LEOP (Launch and Early Orbit Phase) will last approximately four days. In this phase, Gaia will perform the first activations – transmitter switch ON, priming of the chemical thrusters, first attitude control and finding of the sun position – followed by the sun shield deployment. Engineers on ground will perform orbit determination, then prepare and execute the critical 'Day 2' maneuver to inject Gaia into its final transfer trajectory toward the L2 Lagrange point (Ref. 92).

• LEOP will be followed by the transfer cruise phase, lasting up to 30 days, an L2 orbit injection maneuver, then the in-orbit commissioning phase, during which all operations to prepare for the routine operational phase are performed. In particular, the scientific FPA (Focal Plane Assembly) and related avionics will be thoroughly tested and calibrated. The commissioning phase is expected to last four months.

• The insertion into the final 300,000 x 200,000 km Lissajous orbit around L2 was performed one month after launch.

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Figure 19: Gaia mission scenario, from launch to in-orbit operations (image credit: ESA)


Note: As of 3 March 2020, the previously single large Gaia file has been split into two files, to make the file handling manageable for all parties concerned, in particular for the user community.

This article covers the Gaia mission and its imagery in the period 2020, in addition to some of the mission milestones.

Gaia status and imagery in the period 2019-2013



Mission status

• May 13, 2022: ESA’s Gaia space telescope revolutionises our understanding of the Milky Way. It scans the sky to measure the position, movement, distance, and characteristics of billions of stars. 38)

Figure 20: Gaia is creating the most precise map of our home galaxy yet, providing clues to its origin and evolution. Gaia not only studies the stars, but also what is in between them, as well as asteroids and planetary moons in our Solar System, binary stars and exoplanets, and quasars and galaxies outside of the Milky Way. Gaia provides us with a wealth of data, giving us a new sense of our place in the Universe (video credit: ESA)

- Gaia’s data release 3 will be made public on 13 June 2022: https://www.cosmos.esa.int/web/gaia/data-release-3

• March 23, 2022: Using data from ESA’s Gaia mission, astronomers have shown that a part of the Milky Way known as the ‘thick disc’ began forming 13 billion years ago, around 2 billion years earlier than expected, and just 0.8 billion years after the Big Bang. 39)

- This surprising result comes from an analysis performed by Maosheng Xiang and Hans-Walter Rix, from the Max-Planck Institute for Astronomy, Heidelberg, Germany. They took brightness and positional data from Gaia’s Early Data Release 3 (EDR3) dataset and combined it with measurements of the stars’ chemical compositions, as given by data from China’s Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) for roughly 250,000 stars to derive their ages.

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Figure 21: Basic structure of our home galaxy, edge-on view. The new results from ESA's Gaia mission provide for a reconstruction of the history of the Milky Way, in particular of the evolution of the so-called thick disc (image credit: Stefan Payne-Wardenaar / MPIA)

- They chose to look at sub giant stars. In these stars, energy has stopped being generated in the star’s core and has moved into a shell around the core. The star itself is transforming into a red giant star. Because the sub giant phase is a relatively brief evolutionary phase in a star’s life, it permits its age to be determined with great accuracy, but it’s still a tricky calculation.

How old are the stars?

- The age of a star is one of the most difficult parameters to determine. It cannot be measured directly but must be inferred by comparing a star’s characteristics with computer models of stellar evolution. The compositional data helps with this. The Universe was born with almost exclusively hydrogen and helium. The other chemical elements, known collectively as metals to astronomers, are made inside stars, and exploded back into space at the end of a star’s life, where they can be incorporated into the next generation of stars. So, older stars have fewer metals and are said to have lower metallicity.

- The LAMOST data gives the metallicity. Together, the brightness and metallicity allow astronomers to extract the star’s age from the computer models. Before Gaia, astronomers were routinely working with uncertainties of 20-40 percent, which could result in the determined ages being imprecise by a billion years or more.

- The LAMOST data gives the metallicity. Together, the brightness and metallicity allow astronomers to extract the star’s age from the computer models. Before Gaia, astronomers were routinely working with uncertainties of 20-40 percent, which could result in the determined ages being imprecise by a billion years or more.

Milky Way anatomy

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Figure 22: An artist’s impression of our Milky Way galaxy, a roughly 13 billion-year-old ‘barred spiral galaxy’ that is home to a few hundred billion stars. - On the left, a face-on view shows the spiral structure of the Galactic Disc, where the majority of stars are located, interspersed with a diffuse mixture of gas and cosmic dust. The disc measures about 100 000 light-years across, and the Sun sits about half way between its centre and periphery. - On the right, an edge-on view reveals the flattened shape of the disc. Observations point to a substructure: a thin disc some 700 light-years high embedded in a thick disc, about 3000 light-years high and populated with older stars. - The edge on view also shows the Galactic Bulge, located in the central portion of the Milky Way and hosting about 10 billion stars, which are mainly old and red. The bulge, also visible in the face-on view on the left, has an overall elongated shape that resembles that of a peanut-shaped bar, with a half-length of about 10,000 light-years, making the Milky Way a barred spiral galaxy. - Beyond the disc and bulge is the stellar halo, a roughly spherical structure with a radius of about 100 000 light-years, containing isolated stars as well as many globular clusters – large, compact conglomerations of some of the most ancient stars in the Galaxy. On a grander scale, the Milky Way is embedded in an even larger halo of invisible dark matter (image credit: Left: NASA/JPL-Caltech; right: ESA; layout: ESA/ATG medialab)

- Our galaxy is made of different components. Broadly, these can be split into the halo and the disc. The halo is the spherical region surrounding the disc, and has traditionally been thought to be the oldest component of the galaxy. The disc is composed of two parts: the thin disc and the thick disc. The thin disc contains most of the stars that we see as the misty band of light in the night sky that we call the Milky Way. The thick disc is more than double the height of the thin disc but smaller in radius, containing only a few percent of the Milky Way’s stars in the solar neighbourhood.

- By identifying sub giant stars in these different regions, the researchers were able to build a timeline of the Milky Way’s formation – and that’s when they got a surprise.

Two phases in Milky Way history

- The stellar ages clearly revealed that the formation of the Milky Way fell into two distinct phases. In the first phase, starting just 0.8 billion years after the Big Bang, the thick disc began forming stars. The inner parts of the halo may also have begun to come together at this stage, but the process rapidly accelerated to completion about two billion years later when a dwarf galaxy known as Gaia-Sausage-Enceladus merged with the Milky Way. It filled the halo with stars and, as clearly revealed by the new work, triggered the nascent thick disc to form the majority of its stars. The thin disc of stars which holds the Sun, was formed during the subsequent, second phase of the galaxy’s formation.

- The analysis also shows that after the star-forming burst triggered by the merger with Gaia-Sausage-Enceladus, the thick disc continued to form stars until the gas was used up at around 6 billion years after the Big Bang. During this time, the metallicity of the thick disk grew by more than a factor of 10. But remarkably, the researchers see a very tight stellar age—metallicity relation, which indicates that throughout that period, the gas forming the stars was well-mixed across the whole disk. This implies that the early Milky Way’s disk regions must have been formed from highly turbulent gas that effectively spread the metals far and wide.

A timeline thanks to Gaia

- The earlier formation age of the thick disc points to a different picture of our galaxy’s early history. “Since the discovery of the ancient merger with Gaia-Sausage-Enceladus, in 2018, astronomers have suspected that the Milky Way was already there before the halo formed, but we didn’t have a clear picture of what that Milky Way looked like. Our results provide exquisite details about that part of the Milky Way, such as its birthday, its star-formation rate and metal enrichment history. Putting together these discoveries using Gaia data is revolutionising our picture of when and how our galaxy was formed.” says Maosheng.

- And we may not yet be looking far enough into the Universe to see similar galactic discs forming. An age of 13 billion years corresponds to a redshift of 7, where redshift is a measure of how far away a celestial object is, and so how long its light has taken to cross space and reach us.

- New observations could come in the near future as the James Webb Space Telescope has been optimised to see the earliest Milky Way-like galaxies in the Universe. And on 13 June this year, Gaia will release its full third data release (Gaia DR3). This catalogue will include spectra and derived information like ages and metallicity, making studies like Maosheng’s even easier to conduct.

- “With each new analysis and data release, Gaia allows us to piece together the history of our galaxy in even more unprecedented detail. With the release of Gaia DR3 in June, astronomers will be able to enrich the story with even more details,” says Timo Prusti, Gaia Project Scientist for ESA.

- A time-resolved picture of our Milky Way’s early formation history” by Maosheng Xiang and Hans-Walter Rix is published in Nature. 40) doi 10.1038/s41586-022-04496-5

Figure 23: Gaia's Milky Way discoveries. ESA's Gaia mission is surveying more than a billion stars in our cosmic neighbourhood to chart the history and evolution of our home galaxy, the Milky Way. This video highlights some of the mission's discoveries based on the first 22 months of scanning the sky (video credit: ESA)

• March 16, 2022: On 18 February, the NASA/ESA/CSA James Webb Space Telescope was photographed by ESA’s Gaia observatory. 41)

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Figure 24: Gaia snaps photo of Webb. Gaia’s sky mapper image showing the James Webb Space Telescope. The reddish colour is artificial, chosen just for illustrative reasons. The frame shows a few relatively bright stars, several faint stars, a few disturbances – and a spacecraft. It is marked by the green arrow (image credit: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO)

- Both spacecraft are located in orbits around the Lagrange point 2 (L2), 1.5 million km from Earth in the direction away from the Sun. Gaia arrived there in 2014, and Webb in January 2022.

- On 18 February 2022, the two spacecraft were 1 million km apart, with an edge-on view of Gaia towards Webb’s huge sunshield. Very little reflected sunlight came Gaia’s way, and Webb therefore appears as a tiny, faint spec of light in Gaia’s two telescopes without any details visible.

Sky mapper

- A few weeks before Webb’s arrival at L2, Gaia experts Uli Bastian of Heidelberg University (Germany) and Francois Mignard of Nice Observatory (France) realised that during Gaia’s continuous scanning of the entire sky, its new neighbour at L2 should occasionally cross Gaia’s fields of view.

- Gaia is not designed to take real pictures of celestial objects. Instead, it collects very precise measurements of their positions, motions, distances, and colours. However, one part of the instruments on board takes a sort of sky images. It is the ‘finder scope’ of Gaia, also called the sky mapper.

- Every six hours, Gaia’s sky mapper scans a narrow 360-degree strip around the entire celestial sphere. The successive strips are slightly tilted with respect to each other, so that every few months the entire sky is covered – touching everything that’s there and that’s bright enough to be seen by Gaia. Within seconds, these slices are automatically scrutinized for star images, the positions of which are then used to predict when and where those stars could be recorded in Gaia’s main scientific instruments. Then they are routinely deleted.

- But the computer can be manually requested to exceptionally keep a stretch of the image data. The sky mapper was originally planned for technical servicing purposes, but during the mission it has also found some scientific uses. Why not use it for a snapshot of Webb?

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Figure 25: Gaia orbits the second Lagrange Point (L2) in a Lissajous orbit. The James Webb Space Telescope orbits L2 in a halo orbit. The telescopes are between 400,000 and 1,100,000 km apart, depending on where they are in their respective orbits. This image shows the relative sizes and locations of the Gaia orbit (yellow) and the Webb orbit (white). In this view Earth is located to the left, not far outside of the frame. Gaia’s Lissajous loops have L2 right in their centre, while Webb’s halo orbit loops are closer to Earth by about 100,000 km on average (image credit: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO)

Got it!

- After Webb had reached its destination at L2, the Gaia scientists calculated when the first opportunity would arise for Gaia to spot Webb, which turned out to be 18 February 2022.

- After Gaia’s two telescopes had scanned the part of the sky where Webb would be visible, the raw data was downloaded to Earth. In the morning after, Francois sent an email to all people involved. The enthusiastic subject line of the email was "JWST: Got it!!"

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Figure 26: Images of the James Webb Space Telescope taken by ESA’s Gaia observatory on 18 February 2022. — Background frame: Cutout of the specially recorded image from Gaia’s sky mapper instrument at the first of the two observations from Gaia’s two telescopes. The reddish colour is artificial, chosen just for illustrative reasons. The frame shows a few relatively bright stars, several faint stars, a few disturbances – and a spacecraft. It is marked by the green circle. — Left grey inset: Zoom into the frame showing the Webb image at full resolution. It is the slightly extended speck of light in the centre. The other three bright dots are traces of energetic cosmic-ray particles which hit the CCD chip during the 2.5 seconds of exposure. The on-board software is capable of autonomously and reliably distinguishing these from star images. — Right grey inset: The second “photo” of Webb, taken in the second field of view of Gaia’s telescopes about 106.5 minutes after the first one. Each of the two images were created by just under 1000 sunlight photons arriving from the Webb spacecraft (image credit: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO)

- The astronomers had to wait a few more days for Juanma Martin-Fleitas, ESA’s Gaia calibration engineer, to identify Webb in the sky mapper images. "I've identified our target" was the message sent by him, with the images attached and the two tiny specks labelled as ‘Webb candidates’.

- After scrutinising these carefully, Uli replied: “Your ‘candidates’ can be safely renamed ‘Webb’”.

- Gaia now has a spacecraft friend at L2, and together they will uncover our home galaxy, and the Universe beyond.

Figure 27: Gaia snaps photo of Webb (animation), video credit: ESA/Gaia/DPAC

• February 17, 2022: Our galaxy, the Milky Way, began forming around 12 billion years ago. Since then, it has been growing in both mass and size through a sequence of mergers with other galaxies. 42)

- Perhaps most exciting is that this process has not quite finished, and by using data from ESA’s Gaia spacecraft, astronomers can see it taking place. This in turn allows to reconstruct the history of our galaxy, revealing the ‘family tree’ of smaller galaxies that has helped make the Milky Way what it is today.

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Figure 28: This image shows the Milky Way as seen by Gaia. The squares represent the location of globular clusters, the triangles the location of satellite galaxies, and the small dots are stellar streams. The dots and squares in purple are objects brought into the Milky Way by the Pontus merging galaxy (image credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO)

- The latest work on this subject comes from Khyati Malhan, a Humboldt Fellow at the Max-Planck-Institut für Astronomie, Heidelberg, Germany, and colleagues. Together, they have analysed data based on Gaia’s early third data release (EDR3) looking for the remains of smaller galaxies merging with our own. These can be found in the so-called halo of the Milky Way, which surrounds the disc of younger stars and central bulge of older stars that comprise the more luminous parts of the Milky Way.

- When a foreign galaxy falls into our own, great gravitational forces known as tidal forces pull it apart. If this process goes slowly, the stars from the merging galaxy will form a vast stellar stream that can be easily distinguished in the halo. If the process goes quickly, the merging galaxy’s stars will be more scattered throughout the halo and no clear signature will be visible.

- In total they studied 170 globular clusters, 41 stellar streams and 46 satellites of the Milky Way. Plotting them according to their energy and momentum revealed that 25 percent of these objects fall into six distinct groups. Each group is a merger taking place with the Milky Way. There was also a possible seventh merger in the data.

- Five had been previously identified on surveys of stars. They are known as Sagittarius, Cetus, Gaia-Sausage/Enceladus, LMS-1/Wukong, and Arjuna/Sequoia/I’itoi. But the sixth was a newly identified merger event. The team called it Pontus, meaning the sea. In Greek mythology, Pontus is the name of one of the first children of Gaia, the Greek goddess of the Earth.

- Based upon the way Pontus has been pulled apart by the Milky Way, Khyati and colleagues estimate that it probably fell into the Milky Way some eight to ten billion years ago. Four of the other five merger events likely also took place around this time as well. But the sixth event, Sagittarius, is more recent. It might have fallen into the Milky Way sometime in the last five to six billion years. As a result, the Milky Way has not yet been able to completely disrupt it.

- Piece by piece, astronomers are fitting together the merger history of the Galaxy, and Gaia data is proving invaluable.

- On 13 June 2022, the Gaia mission will issue its data release 3, which will provide even more detailed information about the Milky Way’s past, present, and future. 43)

• January 3, 2022: If it’s gigantic enough, a cold cloud of molecules can collapse and fragment under its own gravity to give birth to a litter of thousands of new stars: an open cluster. The stars’ mutual gravity is strong enough to hold the cluster together as it orbits its galactic host, but the attraction is too weak to keep cluster members from eventually straying, either on their own or as result of a dynamically disruptive event. 44)

- Galileo Galilei and other early telescope-wielding astronomers identified open clusters as improbable congregations of similar stars. Now clusters can be automatically cataloged by algorithms that trawl through astrometric data. In April 2021 Wilton Dias of the Federal University of Itajubá in Brazil and his collaborators published an updated catalog of 1743 open clusters based on an analysis of data gathered by the European Space Agency’s Gaia spacecraft. Andrés Piatti of the Interdisciplinary Institute of Basic Science in Mendoza, Argentina, and Khyati Malhan of Stockholm University in Sweden have now used that catalog set to look for pairs of clusters that are close together.

- Close clusters are rare. And when they do occur, they tend to be of similar ages, which suggests that they formed from the same giant molecular cloud. But the members of one pair that Piatti and Malhan found, IC 4665 (shown here) and Collinder 350, have ages that differ by more than 500 million years. What’s more, the clusters’ stellar populations overlap: IC 4665 and Collinder 350 appear to be merging.

- The disparate ages suggest that IC 4665 and Collinder 350 could have formed in different parts of the Milky Way. To see whether that was the case, Piatti and Malhan tracked the members of the two clusters back in time. They first noted the stars’ current positions in the six-dimensional phase space made up of the stars’ 3D positions and 3D velocities. For the source of gravity acting on the stars, they used two different continuous models of the distribution of gravitational mass in the galaxy. Both models include an extended disk, a central bulge, and a dark-matter halo. They differ in total mass.

- Piatti and Malhan integrated the stars’ equations of motion backward for 80 Myr. Although the two mass models gave slightly different answers, the result was the same: 60 Myr ago, IC 4665 and Collinder 350 were around 500 parsecs (1500 light-years) apart. That distance is more than twice as large as the diameters of the biggest molecular clouds. The two clusters had different parents.

- Whether IC 4665 and Collinder 350 will coalesce or separate after passing through each other remains to be determined. The question pertains to how an observed spread in the ages and chemical composition of a cluster are accounted for. If coalescence is a possibility, distinct episodes of star formation need not be invoked. 45)

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Figure 29: Two loosely bound groups of stars in the Milky Way are passing through each other. A simulation of IC 4665 (image credit: Roberto Mura, CC BY-SA 3.0)

• December 1, 2021: Astronomers at The University of Texas at Austin’s McDonald Observatory have discovered an unusually massive black hole at the heart of one of the Milky Way’s dwarf satellite galaxies, called Leo I. Almost as massive as the black hole in our own galaxy, the finding could redefine our understanding of how all galaxies — the building blocks of the universe — evolve. The work is published in a recent issue of The Astrophysical Journal. 46)

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Figure 30: McDonald Observatory astronomers have found that Leo I (inset), a tiny satellite galaxy of the Milky Way (main image), has a black hole nearly as massive as the Milky Way's. Leo I is 30 times smaller than the Milky Way. The result could signal changes in astronomers' understanding of galaxy evolution. Credit: ESA/Gaia/DPAC; SDSS (inset) McDonald Observatory astronomers have found that Leo I (inset), a tiny satellite galaxy of the Milky Way (main image), has a black hole nearly as massive as the Milky Way's. Leo I is 30 times smaller than the Milky Way. The result could signal changes in astronomers' understanding of galaxy evolution [image credit: ESA/Gaia/DPAC; SDSS (inset)]

- The team decided to study Leo I because of its peculiarity. Unlike most dwarf galaxies orbiting the Milky Way, Leo I does not contain much dark matter. Researchers measured Leo I’s dark matter profile — that is, how the density of dark matter changes from the outer edges of the galaxy all the way into its center. They did this by measuring its gravitational pull on the stars: The faster the stars are moving, the more matter there is enclosed in their orbits. In particular, the team wanted to know whether dark matter density increases toward the galaxy’s center. They also wanted to know whether their profile measurement would match previous ones made using older telescope data combined with computer models.

- Led by recent UT Austin doctoral graduate María José Bustamante, the team includes UT astronomers Eva Noyola, Karl Gebhardt and Greg Zeimann, as well as colleagues from Germany’s Max Planck Institute for Extraterrestrial Physics (MPE). 47)

- For their observations, they used a unique instrument called VIRUS-W on McDonald Observatory’s 2.7-meter Harlan J. Smith Telescope.

- When the team fed their improved data and sophisticated models into a supercomputer at UT Austin’s Texas Advanced Computing Center, they got a startling result.

- “The models are screaming that you need a black hole at the center; you don’t really need a lot of dark matter,” Gebhardt said. “You have a very small galaxy that is falling into the Milky Way, and its black hole is about as massive as the Milky Way’s. The mass ratio is absolutely huge. The Milky Way is dominant; the Leo I black hole is almost comparable.” The result is unprecedented.

- The researchers said the result was different from the past studies of Leo I due to a combination of better data and the supercomputer simulations. The central, dense region of the galaxy was mostly unexplored in previous studies, which concentrated on the velocities of individual stars. The current study showed that for those few velocities that were taken in the past, there was a bias toward low velocities. This, in turn, decreased the inferred amount of matter enclosed within their orbits.

- The new data is concentrated in the central region and is unaffected by this bias. The amount of inferred matter enclosed within the stars’ orbits skyrocketed.

- The finding could shake up astronomers’ understanding of galaxy evolution, as “there is no explanation for this kind of black hole in dwarf spheroidal galaxies,” Bustamante said.

- The result is all the more important as astronomers have used galaxies such as Leo I, called “dwarf spheroidal galaxies,” for 20 years to understand how dark matter is distributed within galaxies, Gebhardt added. This new type of black hole merger also gives gravitational wave observatories a new signal to search for.

- “If the mass of Leo I’s black hole is high, that may explain how black holes grow in massive galaxies,” Gebhardt said. That’s because over time, as small galaxies like Leo I fall into larger galaxies, the smaller galaxy’s black hole merges with that of the larger galaxy, increasing its mass.

- Built by a team at MPE in Germany, VIRUS-W is the only instrument in the world now that can do this type of dark matter profile study. Noyola pointed out that many southern hemisphere dwarf galaxies are good targets for it, but no southern hemisphere telescope is equipped for it. However, the Giant Magellan Telescope (GMT) now under construction in Chile was, in part, designed for this type of work. UT Austin is a founding partner of the GMT.

• November 24, 2021: Data from ESA’s Gaia mission is re-writing the history of our galaxy, the Milky Way. What had traditionally been thought of as satellite galaxies to the Milky Way are now revealed to be mostly newcomers to our galactic environment. 48)

- A dwarf galaxy is a collection of between thousand and several billion stars. For decades it has been widely believed that the dwarf galaxies that surround the Milky Way are satellites, meaning that they are caught in orbit around our galaxy, and have been our constant companions for many billions of years. Now the motions of these dwarf galaxies have been computed with unprecedented precision thanks to data from Gaia’s early third data release and the results are surprising.

- François Hammer, Observatoire de Paris - Université Paris Sciences et Lettres, France, and colleagues from across Europe and China, used the Gaia data to calculate the movements of 40 dwarf galaxies around the Milky Way. They did this by computing a set of quantities known as the three-dimensional velocities for each galaxy, and then using those to calculate the galaxy’s orbital energy and the angular (rotational) momentum.

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Figure 31: Our galaxy, the Milky Way, is surrounded by about fifty dwarf galaxies. Most of these galaxies are only identifiable through telescopes and have been named after the constellation in which they appear on the sky (for example, Draco, Sculptor or Leo). However, the two most obvious dwarf galaxies are called the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), and these are easily visible to the unaided eye. Traditionally these dwarf galaxies have been thought of as satellites in orbit around the Milky Way for many billions of years. Now, however, new data from ESA’s Gaia spacecraft have shown that the majority of the dwarf galaxies are passing the Milky Way for the first time. This forces astronomers to reconsider the history of the Milky Way and how it formed, along with the nature and composition of the dwarf galaxies themselves (image credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO)

- They found that these galaxies are moving much faster than the giant stars and star clusters that are known to be orbiting the Milky Way. So fast, that they couldn’t be in orbit yet around the Milky Way, where interactions with our galaxy and its contents would have sapped their orbital energy and angular momentum.

- Our galaxy has cannibalized a number of dwarf galaxies in its past. For example, 8-10 billion years ago, a dwarf galaxy called Gaia-Enceladus was absorbed by the Milky Way. Its stars can be identified in Gaia data because of the eccentric orbits and range of energies they possess.

- More recently, 4-5 billion years ago, the Sagittarius dwarf galaxy was captured by the Milky Way and is currently in the process of being pulled to pieces and assimilated. The energy of its stars is higher than those of Gaia-Enceladus, indicating the shorter time that they have been subject to the Milky Way’s influence.

- In the case of the dwarf galaxies in the new study, which represents the majority of the dwarf galaxies around the Milky Way, their energies are higher still. This strongly suggests that they have only arrived in our vicinity in the last few billion years.

- The discovery mirrors one made about the Large Magellanic Cloud (LMC), a larger dwarf galaxy so close to the Milky Way that it is visible as a smudge of light in the night sky from the southern hemisphere. The LMC was also thought to be a satellite galaxy of the Milky Way until the 2000s, when astronomers measured its velocity and found that it was travelling too fast to be gravitationally bound. Instead of a companion, LMC is visiting for the first time. Now we know that the same is true for most of the dwarf galaxies too.

- So will these newcomers settle into orbit or simply pass us by? “Some of them will be captured by the Milky Way and will become satellites,” says François.

- But saying exactly which ones is difficult because it depends on the exact mass of the Milky Way, and that is a quantity that is difficult for astronomers to calculate with any real accuracy. Estimates vary by a factor of two.

- The discovery of the dwarf galaxy energies is significant because it forces us to re-evaluate the nature of the dwarf galaxies themselves.

- As a dwarf galaxy orbits, the Milky Way’s gravitational pull will try to wrench it apart. In physics this is known as a tidal force. “The Milky Way is a big galaxy, so its tidal force is simply gigantic and it's very easy to destroy a dwarf galaxy after maybe one or two passages,” says François.

- In other words, becoming a companion to the Milky Way is a death sentence for dwarf galaxies. The only thing that could resist our galaxy’s destructive grip is if the dwarf had a significant quantity of dark matter. Dark matter is the mysterious substance that astronomers think exists in the universe to provide the extra gravity to hold individual galaxies together.

- And so, in the traditional view that the Milky Way’s dwarfs were satellite galaxies that had been in orbit for many billions of years, it was assumed that they must be dominated by dark matter to balance the Milky Way’s tidal force and keep them intact. The fact that Gaia has revealed that most of the dwarf galaxies are circling the Milky Way for the first time means that they do not necessarily need to include any dark matter at all, and we must re-assess whether these systems are in balance or rather in the process of destruction.

- “Thanks in large part to Gaia, it is now obvious that the history of the Milky Way is far more storied than astronomers had previously understood. By investigating these tantalizing clues, we hope to further tease out the fascinating chapters in our galaxy’s past,” says Timo Prusti, Gaia Project Scientist, ESA (publication in The Astrophysical Journal). 49)

• June 7, 2021: It's hard to see more than a handful of stars from Princeton University, because the lights from New York City, Princeton and Philadelphia prevent our sky from ever getting pitch black, but stargazers who get into more rural areas can see hundreds of naked-eye stars—and a few smudgy objects, too. 50)

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Figure 32: A team of astrophysicists led by Princeton University's Luke Bouma has confirmed that open cluster NGC 2516, also known as the Southern Beehive, extends at least 1,600 light-years — 500 parsecs — from tip to tip. To an Earth-based stargazer, that would look as big as 40 full moons, side by side, stretching across the sky (image credit: Princeton University)

- The biggest smudge is the Milky Way itself, the billions of stars that make up our spiral galaxy, which we see edge-on. The smaller smudges don't mean that you need glasses, but that you're seeing tightly packed groups of stars. One of the best-known of these "clouds" or "clusters"—groups of stars that travel together—is the Pleiades, also known as the Seven Sisters. Clusters are stellar nurseries where thousands of stars are born from clouds of gas and dust and then disperse across the Milky Way.

- For centuries, scientists have speculated about whether these clusters always form tight clumps like the Pleiades, spread over only a few dozen lightyears.

- "We call them 'open clusters'—the 'open' part refers to the expectation that these things formed in a much denser group that then dispersed," said Luke Bouma, a graduate student in astrophysical sciences at Princeton and the lead author on an upcoming paper published by the American Astronomical Society. "But we never thought we'd be able to find the stars that were lost."

- Then, two years ago, a machine-learning algorithm using data from the Gaia satellite identified that many far-flung stars are moving at the same speed and direction and could therefore be part of the same open cluster—but as more of a stream or a string than a clump.

- Now, a team of astrophysicists led by Bouma can confirm that one of these streams of stars, NGC 2516, also known as the Southern Beehive, extends at least 1,600 light-years—500 parsecs—from tip to tip. To an Earth-based stargazer, that would look as big as 40 full moons, side by side, stretching across the sky.

- "Gaia data let us trace the process of star cluster formation and dissolution in unprecedented detail—but to complete the picture, we need independently estimated ages," said Lynne Hillenbrand, a 1989 Princeton alumna and a professor of astronomy at Caltech, who was not involved in this research. "Bouma's paper brings together several different methods to consistently age-date stars at both the core and the outer reaches of this cluster."

- "In retrospect, the existence of this large stellar stream is not too surprising," said Bouma, who recently won the prestigious 51 Pegasus b Fellowship. One interpretation could be that a cluster starts as a tight clump that expands through time to form "tidal tails" stretching in front of it and behind it, as it moves through the Milky Way.

- "The broader implication is that there are bound to be other enormous open clusters like this," he said. "The visible part of the cluster, where we can easily see the stars close together, may be only a small part of a much, much larger stream."

- "I have seen the Southern Beehive many times through a pair of binoculars under the dark skies of Chile," said Gaspar Bakos, a professor of astrophysical sciences and the director of Princeton's program in planets and life, who was a co-author on the paper. "The cluster nicely fits the view of the binoculars, because its apparent size in the sky is something like the tip of my thumb at arm's length. It is curious to know, thanks to Luke's research, that the cluster actually spans an area as big as my entire palm held toward the sky."

- Bouma and his colleagues used data from the Transiting Exoplanet Survey Satellite (TESS) to precisely measure the rotation rates of stars that the Gaia study had assigned to NGC 2516. The researchers demonstrated that many stars with similar masses are all spinning at (or very near) the same rate, confirming that they were born in the same stellar nursery.

- Bouma has spent years developing the tools to measure a star's rotation so that he can calculate its age, a technique called gyrochronology (from the Greek words for "spin" and "time"). Our sun, which at the age of 4.6 billion years old is in its sedate middle age, rotates once every 27 days. The stars Bouma measured in NGC 2516 are rotating 10 times faster than our sun, because they are so much younger. Those stars are barely out of their infancy, only about 150 million years old.

- "In addition to expanding our knowledge of this and other star clusters, Luke has given us an expanded list of young stars that we can search for planets," said Joshua Winn, Bouma's adviser and co-author and a professor of astrophysical sciences. "Finding planets around young stars will help us understand how planetary systems form and change with time."

- "What's so surprising about this work - what's so exciting - is that we confirmed that Gaia, because it really precisely measures the positions and the motions of stars, can find these 'needles in the haystack' of the Milky Way," Bouma said. "Gaia can identify all the stars that are moving in the same direction, at the same rate. And we don't have to just trust the machine learning algorithm saying that they're related - we can verify it with TESS data, using our gyrochronological technique."

- This open cluster also has an intriguing connection with Greek mythology, Bouma said. "In the southern night sky, NGC 2516 is near a constellation called the Argo Navis, which was the boat sailed by Jason and the Argonauts to obtain the golden fleece." He added with a smile: "Jason and the Argonauts are sailing on a stream of stars made by the open cluster NGC 2516." 51)

• April 21, 2021: Astronomers using data from NASA and ESA (European Space Agency) telescopes have released a new all-sky map of the outermost region of our galaxy. Known as the galactic halo, this area lies outside the swirling spiral arms that form the Milky Way’s recognizable central disk and is sparsely populated with stars. Though the halo may appear mostly empty, it is also predicted to contain a massive reservoir of dark matter, a mysterious and invisible substance thought to make up the bulk of all the mass in the universe. 52)

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Figure 33: Images of the Milky Way and the Large Magellanic Cloud (LMC) are overlaid on a map of the surrounding galactic halo. The smaller structure is a wake created by the LMC’s motion through this region. The larger light-blue feature corresponds to a high density of stars observed in the northern hemisphere of our galaxy. The highlight of the new chart is a wake of stars, stirred up by a small galaxy set to collide with the Milky Way. The map could also offer a new test of dark matter theories (image credit: NASA/ESA/JPL-Caltech, Conroy et. al. 2021)

- Astronomers using data from NASA and ESA (European Space Agency) telescopes have released a new all-sky map of the outermost region of our galaxy. Known as the galactic halo, this area lies outside the swirling spiral arms that form the Milky Way’s recognizable central disk and is sparsely populated with stars. Though the halo may appear mostly empty, it is also predicted to contain a massive reservoir of dark matter, a mysterious and invisible substance thought to make up the bulk of all the mass in the universe.

- The data for the new map comes from ESA’s Gaia mission and NASA’s NEOWISE (Near Earth Object Wide Field Infrared Survey Explorer) mission, which operated from 2009 to 2013 under the moniker WISE. The study makes use of data collected by the spacecraft between 2009 and 2018.

- The new map reveals how a small galaxy called the Large Magellanic Cloud (LMC) – so named because it is the larger of two dwarf galaxies orbiting the Milky Way – has sailed through the Milky Way’s galactic halo like a ship through water, its gravity creating a wake in the stars behind it. The LMC is located about 160,000 light-years from Earth and is less than one-quarter the mass of the Milky Way.

Figure 34: A simulation of dark matter surrounding the Milky Way galaxy (small ring at center) and the Large Magellanic Cloud (LMC) reveals two areas of high density: the smaller of the two light blue areas is a wake created by the LMC’s motion through this region. The larger corresponds to an excess of stars in the Milky Way’s northern hemisphere (video credits: NASA/JPL-Caltech/NSF, R. Hurt, N. Garavito-Camargo & G. Besla)

- Though the inner portions of the halo have been mapped with a high level of accuracy, this is the first map to provide a similar picture of the halo’s outer regions, where the wake is found – about 200,000 light-years to 325,000 light-years from the galactic center. Previous studies have hinted at the wake’s existence, but the all-sky map confirms its presence and offers a detailed view of its shape, size, and location.

- This disturbance in the halo also provides astronomers with an opportunity to study something they can’t observe directly: dark matter. While it doesn’t emit, reflect, or absorb light, the gravitational influence of dark matter has been observed across the universe. It is thought to create a scaffolding on which galaxies are built, such that without it, galaxies would fly apart as they spin. Dark matter is estimated to be five times more common in the universe than all the matter that emits and/or interacts with light, from stars to planets to gas clouds.

- Although there are multiple theories about the nature of dark matter, all of them indicate that it should be present in the Milky Way’s halo. If that’s the case, then as the LMC sails through this region, it should leave a wake in the dark matter as well. The wake observed in the new star map is thought to be the outline of this dark matter wake; the stars are like leaves on the surface of this invisible ocean, their position shifting with the dark matter.

- The interaction between the dark matter and the Large Magellanic Cloud has big implications for our galaxy. As the LMC orbits the Milky Way, the dark matter’s gravity drags on the LMC and slows it down. This will cause the dwarf galaxy’s orbit to get smaller and smaller, until the galaxy finally collides with the Milky Way in about 2 billion years. These types of mergers might be a key driver in the growth of massive galaxies across the universe. In fact, astronomers think the Milky Way merged with another small galaxy about 10 billion years ago.

- “This robbing of a smaller galaxy’s energy is not only why the LMC is merging with the Milky Way, but also why all galaxy mergers happen,” said Rohan Naidu, a doctoral student in astronomy at Harvard University and a co-author of the new paper. “The wake in our map is a really neat confirmation that our basic picture for how galaxies merge is on point!”53)

A Rare Opportunity

- The authors of the paper also think the new map – along with additional data and theoretical analyses – may provide a test for different theories about the nature of dark matter, such as whether it consists of particles, like regular matter, and what the properties of those particles are.

- “You can imagine that the wake behind a boat will be different if the boat is sailing through water or through honey,” said Charlie Conroy, a professor at Harvard University and an astronomer at the Center for Astrophysics | Harvard & Smithsonian, who coauthored the study. “In this case, the properties of the wake are determined by which dark matter theory we apply.”

- Conroy led the team that mapped the positions of over 1,300 stars in the halo. The challenge arose in trying to measure the exact distance from Earth to a large portion of those stars: It’s often impossible to figure out whether a star is faint and close by or bright and far away. The team used data from ESA’s Gaia mission, which provides the location of many stars in the sky but cannot measure distances to the stars in the Milky Way’s outer regions.

- After identifying stars most likely located in the halo (because they were not obviously inside our galaxy or the LMC), the team looked for stars belonging to a class of giant stars with a specific light “signature” detectable by NEOWISE. Knowing the basic properties of the selected stars enabled the team to figure out their distance from Earth and create the new map. It charts a region starting about 200,000 light-years from the Milky Way’s center, or about where the LMC’s wake was predicted to begin, and extends about 125,000 light-years beyond that.

- Conroy and his colleagues were inspired to hunt for LMC’s wake after learning about a team of astrophysicists at the University of Arizona in Tucson that makes computer models predicting what dark matter in the galactic halo should look like. The two groups worked together on the new study.

- One model by the Arizona team, included in the new study, predicted the general structure and specific location of the star wake revealed in the new map. Once the data had confirmed that the model was correct, the team could confirm what other investigations have also hinted at: that the LMC is likely on its first orbit around the Milky Way. If the smaller galaxy had already made multiple orbits, the shape and location of the wake would be significantly different from what has been observed. Astronomers think the LMC formed in the same environment as the Milky Way and another nearby galaxy, M31, and that it is close to completing a long first orbit around our galaxy (about 13 billion years). Its next orbit will be much shorter due to its interaction with the Milky Way.

- “Confirming our theoretical prediction with observational data tells us that our understanding of the interaction between these two galaxies, including the dark matter, is on the right track,” said University of Arizona doctoral student in astronomy Nicolás Garavito-Camargo, who led work on the model used in the paper.

- The new map also provides astronomers with a rare opportunity to test the properties of the dark matter (the notional water or honey) in our own galaxy. In the new study, Garavito-Camargo and colleagues used a popular dark matter theory called cold dark matter that fits the observed star map relatively well. Now the University of Arizona team is running simulations that use different dark matter theories to see which one best matches the wake observed in the stars.

- “It’s a really special set of circumstances that came together to create this scenario that lets us test our dark matter theories,” said Gurtina Besla, a co-author of the study and an associate professor at the University of Arizona. “But we can only realize that test with the combination of this new map and the dark matter simulations that we built.”

- Launched in 2009, the WISE spacecraft was placed into hibernation in 2011 after completing its primary mission. In September 2013, NASA reactivated the spacecraft with the primary goal of scanning for near-Earth objects, or NEOs, and the mission and spacecraft were renamed NEOWISE. NASA’s Jet Propulsion Laboratory in Southern California managed and operated WISE for NASA’s Science Mission Directorate. The mission was selected competitively under NASA’s Explorers Program managed by the agency’s Goddard Space Flight Center in Greenbelt, Maryland. NEOWISE is a project of JPL, a division of Caltech, and the University of Arizona, supported by NASA’s Planetary Defense Coordination Office.

• March 24, 2021: Data from ESA’s Gaia star mapping satellite have revealed tantalizing evidence that the nearest star cluster to the Sun is being disrupted by the gravitational influence of a massive but unseen structure in our galaxy. 54)

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Figure 35: The Hyades star cluster is gradually merging with the background of stars in the Milky Way. The cluster is located 153 light years away and is visible to the unaided eye because the brightest members form a ‘V’-shape of stars in the constellation of Taurus, the Bull. This image shows members of the Hyades as identified in the Gaia data. Those stars are marked in pink, and the shapes of the various constellations are traced in green. Stars from the Hyades can be seen stretching out from the central cluster to form two ‘tails’. These tails are known as tidal tails and it is through these that stars leave the cluster. The image was created using Gaia Sky (image credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO; acknowledgement: S. Jordan/T. Sagrista)

- If true, this might provide evidence for a suspected population of ‘dark matter sub-halos’. These invisible clouds of particles are thought to be relics from the formation of the Milky Way, and are now spread across the galaxy, making up an invisible substructure that exerts a noticeable gravitational influence on anything that drifts too close.

- ESA Research Fellow Tereza Jerabkova and colleagues from ESA and the European Southern Observatory made the discovery while studying the way a nearby star cluster is merging into the general background of stars in our galaxy. This discovery was based on Gaia’s Early third Data Release (EDR3) and data from the second release.

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Figure 36: The Hyades and their tidal tails. The true extent of the Hyades tidal tails have been revealed for the first time by data from the ESA’s Gaia mission. The Gaia data has allowed the former members of the star cluster (shown in pink) to be traced across the whole sky. Those stars are marked in pink, and the shapes of the various constellations are traced in green. The image was created using Gaia Sky (image credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO; acknowledgement: S. Jordan/T. Sagrista)

- The team chose the Hyades as their target because it is the nearest star cluster to the Sun. It is located just over 153 light years away, and is easily visible to skywatchers in both northern and southern hemispheres as a conspicuous ‘V’ shape of bright stars that marks the head of the bull in the constellation of Taurus. Beyond the easily visible bright stars, telescopes reveal a hundred or so fainter ones contained in a spherical region of space, roughly 60 light years across.

- A star cluster will naturally lose stars because as those stars move within the cluster they tug at each other gravitationally. This constant tugging slightly changes the stars’ velocities, moving some to the edges of the cluster. From there, the stars can be swept out by the gravitational pull of the galaxy, forming two long tails.

- One tail trails the star cluster, the other pulls out ahead of it. They are known as tidal tails, and have been widely studied in colliding galaxies but no one had ever seen them from a nearby open star cluster, until very recently.

Figure 37: Locating the Hyades tidal tails. The Hyades is an easily recognizable star cluster in the night sky. The brightest handful of stars define the face of Taurus, the Bull. Telescopes show that the central cluster itself contains many hundreds of fainter stars in a spherical region roughly 60 light years across. Previous studies have shown that stars were ‘leaking’ out of the cluster to form two tails that stretch into space. Gaia has now allowed astronomers to discover the true extent of those tails by tracing former members of the Hyades across the whole sky. The animation was created using Gaia Sky (video credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO; acknowledgement: S. Jordan/T. Sagrista)

- The key to detecting tidal tails is spotting which stars in the sky are moving in a similar way to the star cluster. Gaia makes this easy because it is precisely measuring the distance and movement of more than a billion stars in our galaxy. “These are the two most important quantities that we need to search for tidal tails from star clusters in the Milky Way,” says Tereza.

- Previous attempts by other teams had met with only limited success because the researchers had only looked for stars that closely matched the movement of the star cluster. This excluded members that left earlier in its 600–700 million year history and so are now travelling on different orbits.

- To understand the range of orbits to look for, Tereza constructed a computer model that would simulate the various perturbations that escaping stars in the cluster might feel during their hundreds of millions of years in space. It was after running this code, and then comparing the simulations to the real data that the true extend of the Hyades tidal tails were revealed. Tereza and colleagues found thousands of former members in the Gaia data. These stars now stretch for thousands of light years across the galaxy in two enormous tidal tails.

- But the real surprise was that the trailing tidal tail seemed to be missing stars. This indicates that something much more brutal is taking place than the star cluster gently ‘dissolving’.

Figure 38: Evolution of Hyades star cluster from ~ 650 million years ago until now. Located 153 light years away, the Hyades is between 600 and 700 million years old. During that time, stars have been ‘leaking’ out of the central cluster to form two ‘tidal tails’ that stretch across space. Gaia data has now allowed these tails to be traced across the whole sky and a mystery has been uncovered. The tails should contain roughly the same number of stars as each other but there are many more stars in the leading tail than in the trailing one. This simulation shows why that might be true. The left panel shows a schematic of the Milky Way galaxy. The Hyades star cluster is shown in yellow. The right-hand panel shows a close-up of the cluster. The grey spots show clumps of matter in the Milky Way. These could be molecular clouds, other star clusters, or clumps of dark matter termed sub-halos. As time passes, the Hyades and the other clumps orbit the centre of the galaxy. Close to the end of the simulation, one of the clumps passes through one of the Hyades tidal tails, scattering stars out of the tail (video credit: Jerabkova et al., A&A, 2021)

- Running the simulations again, Tereza showed that the data could be reproduced if that tail had collided with a cloud of matter containing about 10 million solar masses. “There must have been a close interaction with this really massive clump, and the Hyades just got smashed,” she says.

- But what could that clump be? There are no observations of a gas cloud or star cluster that massive nearby. If no visible structure is detected even in future targeted searches, Tereza suggests that object could be a dark matter sub-halo. These are naturally occurring clumps of dark matter that are thought to help shape the galaxy during its formation. This new work shows how Gaia is helping astronomers map out this invisible dark matter framework of the galaxy.

- “With Gaia, the way we see the Milky Way has completely changed. And with these discoveries, we will be able to map the Milky Way’s sub-structures much better than ever before,” says Tereza. And having proved the technique with the Hyades, Tereza and colleagues are now extending the work by looking for tidal tails from other, more distant star clusters. 55)

December 03, 2020: The motion of stars in the outskirts of our galaxy hints at significant changes in the history of the Milky Way. This and other equally fascinating results come from a set of papers that demonstrate the quality of ESA’s Gaia Early third Data Release (EDR3), which is made public today. 56)

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Figure 39: Gaia’s Early Data Release 3 was made public on 3 December 2020. It contains detailed information on more than 1.8 billion sources, as measured by the Gaia spacecraft. This represents an increase of more than 100 million sources over the previous data release (Gaia DR2), which was made public in April 2018. Gaia EDR3 also contains color information for around 1.5 billion sources, an increase of about 200 million sources over Gaia DR2. As well as including more sources, the general accuracy and precision of the measurements has also improved (image credit: ESA; CC BY-SA 3.0 IGO)

- Gaia EDR3 includes:

a) 1,811,709,771 sources with positions to provide the best ever sky map

b) 1,467,744,818 sources with parallax and proper motion to reveal their distances and motions

c) 1,806,254,432 sources with the measurement of their brightness in white light

d) 1,542,033,472 sources with the brightness of the objects in blue light

e) 1,554,997,939 sources with the brightness of the objects in red light (a comparison of the blue and the red light provides information of the temperature of the object)

f) 1,614,173 extragalactic sources to provide a reference frame for measuring ‘absolute’ positions and motions.

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Figure 40: Gaia’s stellar motion for the next 400 thousand years. The stars are in constant motion. To the human eye this movement – known as proper motion – is imperceptible, but Gaia is measuring it with more and more precision. The trails on this image show how 40,000 stars, all located within 100 parsecs (326 light years) of the Solar System, will move across the sky in the next 400 thousand years. These proper motions are released as part of the Gaia Early Data Release 3 (Gaia EDR3). They are twice as precise as the proper motions released in the previous Gaia DR2. The increase in precision is because Gaia has now measured the stars more times and over a longer interval of time. This represents a major improvement in Gaia EDR3 with respect to Gaia DR2 (image credit: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO. Acknowledgement: A. Brown, S. Jordan, T. Roegiers, X. Luri, E. Masana, T. Prusti and A. Moitinho)

What’s new in EDR3?

- Gaia EDR3 contains detailed information on more than 1.8 billion sources, detected by the Gaia spacecraft. This represents an increase of more than 100 million sources over the previous data release (Gaia DR2), which was made public in April 2018. Gaia EDR3 also contains color information for around 1.5 billion sources, an increase of about 200 million sources over Gaia DR2. As well as including more sources, the general accuracy and precision of the measurements has also improved.

- “The new Gaia data promise to be a treasure trove for astronomers,” says Jos de Bruijne, ESA’s Gaia Deputy Project Scientist.

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Figure 41: The density of stars from Gaia’s EDR3. Data from more than 1.8 billion stars have been used to create this map of the entire sky. It shows the total density of stars observed by ESA’s Gaia satellite and released as part of Gaia’s Early Data Release 3 (Gaia EDR3). Brighter regions indicate denser concentrations of stars, while darker regions correspond to patches of the sky where fewer stars are observed. In contrast to the brightness map in color which is enhanced by the brightest and most massive stars, this view shows the distribution of all stars, including faint and distant ones. - The bright horizontal structure that dominates the image is the plane of the galaxy. It is a flattened disc that hosts most of our galaxy's stars. The bulge in the centre of the image is surrounding the centre of the galaxy. - The faint, elongated feature just visible below the Galactic centre and pointing in the downwards direction is the Sagittarius dwarf galaxy. This is a small satellite of the Milky Way that is leaving a stream of stars behind as an effect of our galaxy's gravitational pull. This faint feature is only visible in this view, and not in the all-sky map based on the luminosity of stars, which is dominated by brighter sources. - Darker regions across the Galactic plane correspond to foreground clouds of interstellar gas and dust, which absorb the light of more distant stars. Many of these clouds conceal stellar nurseries where new generations of stars are currently being born. - Dotted across the image are also many globular and open clusters, as well as entire galaxies beyond our own. The two bright objects in the lower right of the image are the Large and Small Magellanic Clouds, two dwarf galaxies orbiting the Milky Way. Other nearby galaxies are also visible, most notably the Milky Way's largest galactic neighbor the Andromeda galaxy (also known as M31), seen in the lower left of the image along with its satellite, the Triangulum galaxy (M33). - A number of artefacts are also visible on the image. These take the form of curved features. These features are not of astronomical origin but rather reflect Gaia's scanning procedure. They are much less pronounced than they were in previous data releases, and will fade even more as more data are gathered. - Gaia EDR3 was made public on 3 December 2020 and includes the position and brightness of more than 1.8 billion stars, the parallax and proper motion of almost 1.5 billion stars, and the color of more than 1.5 billion stars. It also includes more than 1.6 million extragalactic sources (image credit: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO. Acknowledgement: A. Moitinho and M. Barros)

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Figure 42: Gaia’s view of the Milky Way’s neighboring galaxies. The Large and Small Magellanic Clouds (LMC and SMC, respectively) are two dwarf galaxies that orbit the Milky Way. This image shows the stellar density of the satellite galaxies as seen by Gaia in its Early Data Release 3, which was made public on 3 December 2020. It is composed of red, green and blue layers, which trace mostly the older, intermediate age, and younger stars respectively. Astronomers place stars into categories that are often named for their color and appearance. - In this image, the red layer contains evolved stars that compose the Red Giant Branch and Red Clump stars. The green layer contains Main Sequence stars of mixed ages of up to two billion years. The blue layer contains stars younger than 400 million years, Asymptotic Giant Branch stars, and RR-Lyrae and classical Cepheid variable stars. - The brightnesses used in this image are based on a logarithmic scale to enhance low surface density regions in the galaxies, for example the outer spiral arm in the LMC visible in the upper left.- The density of younger stars has been artificially enhanced with respect to the other evolutionary phases to make them more clearly visible. This shows that younger stars mostly trace the inner spiral structure of the LMC, and the ‘bridge’ of stars between the two galaxies. Finally, intermediate age and older stars trace the LMC bar, spiral arms, and outer halo, as well as the SMC outer halo [image credit: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO. Acknowledgement: L. Chemin; X. Luri et al (2020)]

Figure 43: Astronomers have measured the acceleration of the Solar System around the center of the galaxy using data from Gaia’s Early Data Release 3 (Gaia EDR3), which was made public on 3 December 2020. - The velocity of the Solar System has been measured to change by 0.23 nm/s2. Due to this tiny acceleration, the trajectory of the Solar System is deflected by the diameter of an atom every second. In a year this adds up to around 115 km. The acceleration measured by Gaia shows a good agreement with the theoretical expectations and provides the first measurement of the curvature of the Solar System’s orbit around the galaxy in the history of optical astronomy. - The measurement was made by looking for minuscule changes in the positions of distant galaxies, called quasi stellar objects (QSOs), caused by the movement of our Solar System. The apparent changes in position is called aberration. Discovered by James Bradley, England’s Astronomer Royal, in 1727, aberration causes the positions of celestial objects to appear to move in the direction of the observer. In this case, the aberration caused by Gaia's orbit around the Sun was removed from the data. This left the aberration caused by the motion of the Solar System around the center of the galaxy. - The animation shows the systematic movement of 3000 simulated QSOs that would be induced by the Solar System’s measured acceleration. The video starts by showing the positions of the simulated QSOs. Then, it superimposes the ‘proper motion vectors’, which shows the direction and magnitude of the apparent QSO movements induced by the actual motion of the Solar System. Finally, the QSOs appear to move in the direction of the acceleration, which is close to the Galactic centre, but with highly exaggerated motion to make the effect visible [video credit: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO. Acknowledgement: S. Jordan, T. Sagristà and Klioner, et al, (2020)]

Figure 44: Orbits of the nearby stars around the galaxy. The movement of almost 75 thousand stars from the Gaia Catalogue of Nearby Stars in their orbits around the center of the galaxy are shown in this video. Their motion for the next 500 million years is shown from three different perspectives (face-on, side view, and perspective). Each second on the video corresponds to six million years, and the field of view for each image of the galaxy is 100,000 light years wide. - Stars in the Gaia Catalogue of Nearby Stars (GCNS) come from Gaia’s Early Data Release 3 (Gaia EDR3), which was made public on 3 December 2020. The GCNS contains 74,281 stars within 100 pc (326 light years) of the Sun that have measured radial velocities in EDR3. The orbits have been computed to take into account the gravitational field of the Milky Way, but not the gravitational interaction between the stars. - Most of the stars in the GCNS have a disc-like orbit, similar to the Sun, with small deviations away from circularity. They stay close to the Galactic plane but form long ribbons that eventually wind themselves around the galaxy. - The solar neighborhood is also visited by stars from the outer reaches of the galaxy. This part is known as the halo, and stars from here are shown in orange. Their orbits have larger deviations from circles, and can be seen heading into the outer parts of the galaxy, away from the Galactic plane. Stars coming from or going to the inner parts of the galaxy are also shown (yellow dots). - The Hyades and Coma Berenice star clusters are also clearly visible as small clumps of stars (blue dots) [video credit: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO. Acknowledgement: S. Payne-Wardenaar, S. Jordan, C. Reylé and Smart et al. (2020)]

To the galactic anticenter

- The new Gaia data have allowed astronomers to trace the various populations of older and younger stars out towards the very edge of our galaxy – the galactic anticenter. Computer models predicted that the disc of the Milky Way will grow larger with time as new stars are born. The new data allow us to see the relics of the 10 billion-year-old ancient disc and so determine its smaller extent compared to the Milky Way’s current disc size.

- The new data from these outer regions also strengthen the evidence for another major event in the more recent past of the galaxy.

- The data show that in the outer regions of the disc there is a component of slow-moving stars above the plane of our galaxy that are heading downwards towards the plane, and a component of fast-moving stars below the plane that are moving upwards. This extraordinary pattern had not been anticipated before. It could be the result of the near-collision between the Milky Way and the Sagittarius dwarf galaxy that took place in our galaxy’s more recent past.

- The Sagittarius dwarf galaxy contains a few tens of millions of stars and is currently in the process of being cannibalized by the Milky Way. Its last close pass to our galaxy was not a direct hit, but this would have been enough so that its gravity perturbed some stars in our galaxy like a stone dropping into water.

- Using Gaia DR2, members of DPAC had already found a subtle ripple in the movement of millions of stars that suggested the effects of the encounter with Sagittarius sometime between 300 and 900 million years ago. Now, using Gaia EDR3, they have uncovered more evidence that points to its strong effects on our galaxy’s disc of stars.

- “The patterns of movement in the disc stars are different to what we used to believe,” says Teresa Antoja, University of Barcelona, Spain, who worked on this analysis with DPAC colleagues. Although the role of the Sagittarius dwarf galaxy is still debated in some quarters, Teresa says, “It could be a good candidate for all these disturbances, as some simulations from other authors show.”

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Figure 45: The color of the sky from Gaia’s Early Data Release 3. Data from more than 1.8 billion stars have been used to create this map of the entire sky. It shows the total brightness and color of stars observed by ESA’s Gaia satellite and released as part of Gaia’s EDR3. - Brighter regions represent denser concentrations of bright stars, while darker regions correspond to patches of the sky where fewer and fainter stars are observed. The color of the image is obtained by combining the total amount of light with the amount of blue and red light recorded by Gaia in each patch of the sky. - The bright horizontal structure that dominates the image is the plane of our Milky Way galaxy. It is actually a flattened disc seen edge-on that contains most of the galaxy’s stars. In the middle of the image, the Galactic center appears bright, and thronged with stars. - Darker regions across the Galactic plane correspond to foreground clouds of interstellar gas and dust, which absorb the light of more distant stars. Many of these clouds conceal stellar nurseries where new generations of stars are currently being born. - Dotted across the image are also many globular and open clusters, as well as entire galaxies beyond our own. The two bright objects in the lower right of the image are the Large and Small Magellanic Clouds, two dwarf galaxies orbiting the Milky Way. - Gaia EDR3 was made public on 3 December 2020 and includes the position and brightness of more than 1.8 billion stars, the parallax and proper motion of almost 1.5 billion stars, and the color of more than 1.5 billion stars. It also includes more than 1.6 million extragalactic sources (image credit: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO. Acknowledgement: A. Moitinho)

Measuring the Solar System’s orbit

- The history of the galaxy is not the only result from the Gaia EDR3 demonstration papers. DPAC members across Europe have performed other work to demonstrate the extreme fidelity of the data and the unique potential for unlimited scientific discovery.

- In one paper, Gaia has allowed scientists to measure the acceleration of the Solar System with respect to the rest frame of the Universe. Using the observed motions of extremely distant galaxies, the velocity of the Solar System has been measured to change by 0.23 nm/s2 . Because of this tiny acceleration, the trajectory of the Solar System is deflected by the diameter of an atom every second, and in a year this adds up to around 115 km. The acceleration measured by Gaia shows a good agreement with the theoretical expectations and provides the first measurement of the curvature of the Solar System’s orbit around the galaxy in the history of optical astronomy.

Figure 46: Exploring Gaia's 2020 data release. New details of our Milky Way galaxy are being revealed in Gaia’s Early Data Release 3. This video summarizes the main highlights, which include a new census of stars in our cosmic neighborhood, a study of the motions of stars in the outskirts of our galaxy, details of the shape of the Solar System’s orbit around the centre of the galaxy, and an investigation of the Milky Way's nearby satellite galaxies (video credit: ESA)

A new stellar census

- Gaia EDR3 has also allowed a new census of stars in the solar neighborhood to be obtained. The Gaia Catalogue of Nearby Stars contains 331 312 objects, which is estimated to be 92 percent of the stars within 100 parsecs (326 light years) of the Sun. The previous census of the solar neighborhood, called the Gliese Catalogue of Nearby stars, was carried out in 1957. It possessed just 915 objects initially, but was updated in 1991 to 3803 celestial objects. It was also limited to a distance of 82 light years: Gaia’s census reaches four times farther and contains 100 times more stars. It also provides location, motion, and brightness measurements that are orders of magnitude more precise than the old data.

Beyond the Milky Way

- A fourth demonstration paper analyzed the Magellanic Clouds: two galaxies that orbit the Milky Way. Having measured the movement of the Large Magellanic Cloud’s stars to greater precision than before, Gaia EDR3 clearly shows that the galaxy has a spiral structure. The data also resolve a stream of stars that is being pulled out of the Small Magellanic Cloud, and hints at previously unseen structures in the outskirts of both galaxies.

- At 12:00 CET on 3 December, the data produced by the many scientists and engineers of the Gaia DPAC Consortium become public for anyone to look at and learn from. This is the first of a two-part release; the full Data Release 3 is planned for 2022.

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Figure 47: Bridge of stars. Data from Gaia’s Early Data Release 3 shows how stars are being pulled from the Small Magellanic Cloud, and heading towards the adjacent Large Magellanic Cloud, forming a stellar bridge through space [image credit: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO. Acknowledgements: S. Jordan, T. Sagristà, X. Luri et al (2020)]

- “Gaia EDR3 is the result of a huge effort from everyone involved in the Gaia mission. It’s an extraordinarily rich data set, and I look forward to the many discoveries that astronomers from around the world will make with this resource,” says Timo Prusti, ESA’s Gaia Project Scientist. “And we’re not done yet; more great data will follow as Gaia continues to make measurements from orbit.”

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Figure 48: Expert scientists and software developers from across Europe are teamed up in the Gaia Data Processing and Analysis Consortium (DPAC). DPAC is responsible for processing and analysing Gaia's data, and producing the Gaia Catalogues. With members from more than 20 countries, the consortium brings together skills and expertise from across the continent, reflecting the international nature and cooperative spirit of ESA itself. - DPAC data processing centers are located in six European countries. DPAC has been in place since 2006 developing the data processing algorithms, the corresponding software, and the IT infrastructure for Gaia. It also executes the algorithms that turn Gaia's raw telemetry into the final scientific data products that are then used by the wider scientific community. 57)

• October 15, 2020: Star clusters have been part of the Imaginarium of human civilization for millennia, as shown through their countless representations in arts and sciences across cultures and continents. The closest and brightest star clusters to Earth, like the Pleiades, are readily visible to the naked eye and are prominent members of our night sky, where they appear as tight concentrations of stars. A research team around astronomer Stefan Meingast at the University of Vienna has now revealed the existence of massive stellar halos, termed coronae, surrounding local star clusters. The paper will be published in "Astronomy & Astrophysics". 58) 59)

- "Clusters form big families of stars that can stay together for large parts of their lifetime. Today, we know of roughly a few thousand star clusters in the Milky Way, but we only recognize them because of their prominent appearance as rich and tight groups of stars. Given enough time, stars tend to leave their cradle and find themselves surrounded by countless strangers, thereby becoming indistinguishable from their neighbors and hard to identify" says Stefan Meingast, lead author of the paper published in "Astronomy & Astrophysics". "Our Sun is thought to have formed in a star cluster but has left its siblings behind a long time ago" he adds.

- Thanks to the ESA Gaia spacecraft’s precise measurements, astronomers at the University of Vienna have now discovered that what we call a star cluster is only the tip of the iceberg of a much larger and often distinctly elongated distribution of stars.

- "Our measurements reveal the vast numbers of sibling stars surrounding the well-known cores of the star clusters for the first time. It appears that star clusters are enclosed in rich halos, or coronae, more than 10 times as large as the original cluster, reaching far beyond our previous guesses. The tight groups of stars we see in the night sky are just a part of a much larger entity" says Alena Rottensteiner, co-author and master student at the University of Vienna. "There is plenty of work ahead revising what we thought were basic properties of star clusters, and trying to understand the origin of the newfound coronae."

- To find the lost star siblings, the research team developed a new method that uses machine learning to trace groups of stars which were born together and move jointly across the sky. The team analyzed 10 star clusters and identified thousands of siblings far away from the center of the compact clusters, yet clearly belonging to the same family. An explanation for the origin of these coronae remains uncertain, yet the team is confident that their findings will redefine star clusters and aid our understanding of their history and evolution across cosmic time.

- "The star clusters we investigated were thought to be well-known prototypes, studied for more than a century, yet it seems we have to start thinking bigger. Our discovery will have important implications for our understanding of how the Milky Way was built, cluster by cluster, but also implications for the survival rate of proto-planets far from the sterilizing radiation of massive stars in the centers of clusters", says João Alves, Professor of Stellar Astrophysics at the University of Vienna and a co-author of the paper. "Dense star clusters with their massive but less dense coronae might not be a bad place to raise infant planets after all."

Figure 49: The discovery of corona star clusters: the Alpha Persei case. The research team is focussing their efforts to unravel more mysteries surrounding the newly found cluster coronae (video credit: Alves Lab)

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Figure 50: A panoramic view of the nearby Alpha Persei star cluster and its corona. The member stars in the corona are invisible. These are only revealed thanks to the combination of precise measurements with the ESA Gaia satellite and innovative machine learning tools (image credit: Stefan Meingast, made with Gaia Sky)

• July 14, 2020: On this page a description is given of the expected contents of Gaia's Early Data Release 3. The Gaia EDR3 catalogue will be based on 34 months of data collection, and is expected to contain about 1.8 billion stars. Gaia EDR3 is on track for a release late 2020. A more exact date will be announced later. 60)

• July 1, 2020: ESA’s Gaia space observatory is an ambitious mission to construct a three-dimensional map of our galaxy by making high-precision measurements of over one billion stars. However, on its journey to map distant suns, Gaia is revolutionizing a field much closer to home. By accurately mapping the stars, it is helping researchers track down lost asteroids. 61)

Using stars to spot asteroids

- Gaia charts the galaxy by repeatedly scanning the entire sky. Over the course of its planned mission, it observed each of its more than one billion target stars around 70 times to study how their position and brightness change over time.

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Figure 51: These six images show the asteroid Gaia-606 (indicated by an arrow) on 26 October 2016. The images, spanning a period of a little more than 18 minutes, were taken at the Observatoire de Haute Provence in southern France by William Thuillot, Vincent Robert and Nicolas Thouvenin (Observatoire de Paris/IMCCE). Gaia-606 was discovered in October 2016 when Gaia data hinted at the presence of a faint, moving source in this region of the sky. Astronomers immediately got to work and predicted the asteroid's position as seen from the ground over a period of a few days. The follow-up observations by Thuillot and his colleagues showed this was an asteroid that did not match the orbit of any previously catalogued Solar System object. Further investigation revealed some sparse observations of this object already existed; Gaia-606 has now been renamed 2016 UV56. The star closest to the asteroid is USNO-A2-1125-19276564. North is up, east to the left ( image credit: Observatoire de Haute-Provence & IMCCE)

- The stars are so far from Earth that their movements between images are very small, hence why Gaia has to measure their positions so accurately to even notice a difference. However, sometimes Gaia spots faint light sources that move considerably from one image of a certain region of the sky to the next, or are even only spotted in a single image before disappearing.

- To move across Gaia’s field of view so quickly, these objects must be located much closer to Earth.

- By checking the positions of these objects against the catalogues of known Solar System bodies, many of these objects turn out to be known asteroids. Some, however, are identified as potentially new detections and are then followed up by the astronomy community through the Gaia Follow-Up Network for Solar System Objects. Through this process, Gaia has successfully discovered new asteroids.

Lost and found

- These direct asteroid observations are important for solar system scientists. However, Gaia’s highly accurate measurements of the positions of stars provide an even more impactful, but indirect, benefit for asteroid tracking.

- “When we observe an asteroid, we look at its motion relative to the background stars to determine its trajectory and predict where it will be in the future,” says Marco Micheli from ESA’s Near-Earth Object Coordination Centre. “This means that the more accurately we know the positions of the stars, the more reliably we can determine the orbit of an asteroid passing in front of them.”

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Figure 52: Lutetia at Closest approach (image credit: ESA 2010 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA)

- In collaboration with the European Southern Observatory (ESO), Marco’s team took part in an observation campaign targeting 2012 TC4, a small asteroid that was due to pass by the Earth. Unfortunately, since the asteroid was first spotted in 2012, it had become fainter and fainter as it receded form Earth, eventually becoming unobservable. Where it would appear in the sky at the time of the upcoming campaign was not well known.

- “The possible region of the sky where the asteroid might appear was larger than the area that the telescope could observe at one time,” says Marco. “So we had to find a way to improve our prediction of where the asteroid would be.”

- “I looked back at the initial observations from 2012. Gaia had since made more accurate measurements of the positions of some of the stars in the background of the images, and I used these to update our understanding of the asteroid’s trajectory and predict where it would appear.”

- “We pointed the telescope towards the predicted area of the sky using the data from Gaia and we found the asteroid on our first attempt.”

- “Our next goal was to accurately measure the asteroid’s position, but we had very few stars in our new image to use as a reference. There were 17 stars listed in an older catalogue and only four stars measured by Gaia. I made calculations using both sets of data.”

- “Later in the year, when the asteroid had been observed multiple times by other teams and its trajectory was better known, it became clear that the measurements I made using just four Gaia stars had been much more accurate than the ones using the 17 stars. This was really amazing.”

Keeping Earth safe

Figure 53: Animated view of 14,099 asteroids in our Solar System, as viewed by ESA’s Gaia satellite using information from the mission’s second data release. The orbits of the 200 brightest asteroids are also shown, as determined using Gaia data. In future data releases, Gaia will also provide asteroid spectra and enable a complete characterization of the asteroid belt. The combination of dynamical and physical information that is being collected by Gaia provides an unprecedented opportunity to improve our understanding of the origin and the evolution of the Solar System (video credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO. Gaia Data Processing and Analysis Consortium (DPAC); Orbits: Gaia Coordinating Unit 4; P. Tanga, Observatoire de la Côte d'Azur, France; F. Spoto, IMCCE, Observatoire de Paris, France; Animation: Gaia Sky; S. Jordan / T. Sagristà, Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Germany)

- This same technique is being applied to asteroids that were never lost, allowing researchers to use data from Gaia to determine their trajectories and physical properties more accurately than ever before.

- This is helping them update asteroid population models and deepen our understanding of how asteroid orbits develop, for example, by measuring subtle dynamical effects that play a key role in pushing small asteroids into orbits that could see them collide with Earth.

Dancing with daylight

- In order to make such accurate measurements of the positions of other stars, Gaia has a complicated relationship with our own.

- Gaia orbits around the second Lagrange point, L2, of the Sun-Earth system. This location keeps the Sun, Earth and Moon all behind Gaia, allowing it to observe a large portion of the sky without their interference. It is also in an even thermal radiation environment and experiences a stable temperature.

- However, Gaia must not fall entirely into Earth’s shadow, as the spacecraft still depends on solar power. As the orbit around the L2 point is unstable, small disturbances can build up and see the spacecraft heading for an eclipse.

Figure 54: Positioned at the second Lagrange point, Gaia is able to avoid falling into Earth's shadow (image credit: ESA)

- Gaia’s flight control team at ESA’s ESOC mission control center in Darmstadt are responsible for making corrections to the spacecraft’s trajectory to keep it in the correct orbit and out of Earth’s shadow. They ensure that Gaia remains one of the most stable and accurate spacecraft ever. On 16 July 2019, the team successfully performed a crucial eclipse avoidance maneuver, moving Gaia into the extended phase of its mission and allowing it to keep scanning the sky for several more years.

• June 5, 2020: Chance of finding young Earth-like planets higher than previously thought, say Sheffield scientists. The team studied groups of young stars in the Milky Way to see if these groups were typical compared to theories and previous observations in other star-forming regions in space, and to study if the populations of stars in these groups affected the likelihood of finding forming Earth-like planets. 62)

a) New research from the University of Sheffield has found that the chance of finding earth-like planets in their early formation is much higher than previously thought

b) The team of researchers and undergraduate students studied these young Earth-like planets called magma ocean planets

c) The research will be vital to understanding how habitable planets like Earth form.

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Figure 55: Artist's impression of magma ocean planet (image credit: Mark Garlick)

- The research, published in The Astrophysical Journal, found that there are more stars like the Sun than expected in these groups, which would increase the chances of finding Earth-like planets in their early stages of formation. 63)

- In their early stages of formation these Earth-like planets, called magma ocean planets, are still being made from collisions with rocks and smaller planets, which causes them to heat up so much that their surfaces become molten rock.

- The team, led by Dr Richard Parker, included undergraduate students from the University of Sheffield giving them the opportunity to apply the skills learnt on their course to leading published research in their field.

- Dr Richard Parker, from the University of Sheffield’s Department of Physics and Astronomy, said: “These magma ocean planets are easier to detect near stars like the Sun, which are twice as heavy as the average mass star. These planets emit so much heat that we will be able to observe the glow from them using the next generation of infrared telescopes.

- “The locations where we would find these planets are so-called ‘young moving groups' which are groups of young stars that are less than 100 million years old - which is young for a star. However, they typically only contain a few tens of stars each and previously it was difficult to determine whether we had found all of the stars in each group because they blend into the background of the Milky Way galaxy.

- “Observations from the Gaia telescope have helped us to find many more stars in these groups, which enabled us to carry out this study.”

- The findings from the research will help further understanding of whether star formation is universal and will be an important resource for studying how rocky, habitable planets like Earth form. The team now hopes to use computer simulations to explain the origin of these young moving groups of stars.

- The research team included undergraduate students Amy Bottrill, Molly Haigh, Madeleine Hole and Sarah Theakston from the University of Sheffield’s Department of Physics and Astronomy.

- The team said: “Being involved in this project was one of the highlights of our university experience and it was a great opportunity to work on an area of astronomy outside the typical course structure.

- “It was rewarding to see a physical application of the computer coding we learnt in our degree by sampling the initial mass distribution of stars and how this can relate to the future of exoplanet detection.”

- The Department of Physics and Astronomy at the University of Sheffield explores the fundamental laws of the universe and develops pioneering technologies with real-world applications. Researchers are looking beyond our planet to map out distant galaxies, tackling global challenges including energy security, and exploring the opportunities presented by quantum computing and 2D materials.

• June 4, 2020: An artificial intelligence system analyzing data from the Gaia space telescope has identified more than 2,000 large protostars - and they could hold clues to the origins of the stars in the Milky Way. 64)

- Protostars are young stars that are still forming. Scientists had previously catalogued only 100 of this type of forming star.

- The project was led by Miguel Vioque, a PhD researcher at the University of Leeds, and the findings have been published in the journal Astronomy and Astrophysics. 65)

- He believes investigation of these newly identified stars has the potential to change scientists’ understanding of massive star formation and their approach to studying the galaxy.

- Mr Vioque and his colleagues were interested in what are known as Herbig Ae/Be stars, stars that have a mass that is at least twice that of the Sun. They are also involved in the birth of other stars.

- The researchers took the vast quantity of data being collected by the Gaia spaceborne telescope as it maps the galaxy. Launched in 2013, data collected by the telescope has enabled distances to be determined for about one billion stars, about one per cent of the total that are thought to exist in the galaxy.

- The researchers cleaned that data and reduced it to a subset of 4.1 million stars which were likely to contain the target protostars.

- The artificial intelligence (AI) system sifted the data and generated a list of 2,226 stars with around an 85 percent chance of being a Herbig Ae/Be protostar.

- Mr Vioque, from the School of Physics and Astronomy, said: “There is a huge amount of data being produced by Gaia – and AI tools are needed to help scientists make sense of it.

- “We are combining new technologies in the way researchers survey and map the galaxy with ways of interrogating the mountain of data produced by the telescope - and it is revolutionizing our understanding of the galaxy.

- “This approach is opening an exciting, new chapter in astronomy.”

- Mr Vioque and his colleagues then validated the findings of the AI tool by investigating 145 of the stars identified by the AI system at ground observatories in Spain and Chile where they were able to measure the light, recorded as spectra, coming from the stars.

- He said: “The results from the ground-based observatories show that the AI tool made very accurate predictions about stars that were likely to fall into the Herbig Ae/Be classification.”

- One of the target stars is known as Gaia DR2 428909457258627200.

- It is 8,500 light years away and has a mass 2.3 times that of the sun. Its surface temperature is 9,400 degrees Celsius – the sun is about 5,500 degrees Celsius – and it has a radius that is twice that of the sun. It has existed for around six million years, which in astronomical terms makes it a young star that is still forming.

- Professor René Oudmaijer, from the School of Physics and Astronomy at Leeds, supervised the research. He said: "This research is an excellent example of how the analysis of the Big Data collected by modern scientific instruments, such as the Gaia telescope, will shape the future of astrophysics.

- “AI systems are able to identify patterns in vast quantities of data – and it is likely that in those patterns, scientists will find clues that will lead to new discoveries and fresh understanding.”

- The research was funded by the European Union’s Horizon 2020 research and innovation program, under the STARRY project.

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Figure 56: This image is an artist's impression of a protostar (image credit: European Southern Observatory/L. Calçada)

• May 25, 2020: The formation of the Sun, the Solar System and the subsequent emergence of life on Earth may be a consequence of a collision between our galaxy, the Milky Way, and a smaller galaxy called Sagittarius, discovered in the 1990s to be orbiting our galactic home. 66)

- Astronomers have known that Sagittarius repeatedly smashes through the Milky Way’s disc, as its orbit around the galaxy’s core tightens as a result of gravitational forces. Previous studies suggested that Sagittarius, a so called dwarf galaxy, had had a profound effect on how stars move in the Milky Way. Some even claim that the 10,000 times more massive Milky Way’s trademark spiral structure might be a result of the at least three known crashes with Sagittarius over the past six billion years. 67)

- A new study, based on data gathered by ESA’s galaxy mapping powerhouse Gaia, revealed for the first time that the influence of Sagittarius on the Milky Way may be even more substantial. The ripples caused by the collisions seem to have triggered major star formation episodes, one of which roughly coincided with the time of the formation of the Sun some 4.7 billion years ago.

- “It is known from existing models that Sagittarius fell into the Milky Way three times – first about five or six billion years ago, then about two billion years ago, and finally one billion years ago,” says Tomás Ruiz-Lara, a researcher in Astrophysics at the Instituto de Astrofísica de Canarias (IAC) in Tenerife, Spain, and lead author of the new study published in Nature Astronomy.

- “When we looked into the Gaia data about the Milky Way, we found three periods of increased star formation that peaked 5.7 billion years ago, 1.9 billion years ago and 1 billion years ago, corresponding with the time when Sagittarius is believed to have passed through the disc of the Milky Way.”

Figure 57: The Sagittarius dwarf galaxy has been orbiting the Milky Way for billions for years. As its orbit around the 10,000 more massive Milky Way gradually tightened, it started colliding with our galaxy's disc. The three known collisions between Sagittarius and the Milky Way have, according to a new study, triggered major star formation episodes, one of which may have given rise to the Solar System (image credit: ESA)