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SDSS (Sloan Digital Sky Survey)

The Sloan Digital Sky Survey    Events and science results     References

SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory (APO) in south-east New Mexico, United States (Latitude 32° 46' 49.30" N, Longitude 105° 49' 13.50" W, Elevation 2788m). The project was named after the Alfred P. Sloan Foundation (New York City), which contributed significant funding.

The telescope is a modified two-corrector Ritchey-Chrétien design with a 2.5 m, f/2.25 primary, a 1.08 m secondary, a Gascoigne astigmatism corrector, and one of a pair of interchangeable highly aspheric correctors near the focal plane, one for imaging and the other for spectroscopy. The final focal ratio is f/5. The telescope is instrumented by a wide-area, multiband CCD camera and a pair of fiber-fed double spectrographs. 1)

Novel features of the telescope include the following:

1) A 3° diameter (0.65 m) focal plane that has excellent image quality and small geometric distortions over a wide wavelength range (3000-10,600 Å) in the imaging mode, and good image quality combined with very small lateral and longitudinal color errors in the spectroscopic mode. The unusual requirement of very low distortion is set by the demands of time-delay-and-integrate (TDI) imaging.

2) Very high precision motion to support open-loop TDI observations.

3) A unique wind baffle/enclosure construction to maximize image quality and minimize construction costs. The telescope had first light in 1998 May and began regular survey operations in 2000.

The Irénée du Pont Telescope at Las Campanas Observatory: The new fourth phase of the SDSS includes observations from the Southern Hemisphere for the first time. The southern observations are taken from the Irénée du Pont Telescope at Las Campanas Observatory in northern Chile (Latitude 29° 0' 52.56" S, Longitude 70° 41' 33.36" W, Elevation 2380 m). The du Pont telescope is a Ritchey-Chrétien 2.5-m f/7.5 telescope with a Gascoigne corrector lens. This telescope has been in operation at Las Campanas Observatory since 1977.


About the Sloan Digital Sky Survey

Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions. SDSS acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS web site is https://www.sdss.org

SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration including the Brazilian Participation Group, the Carnegie Institution for Science, Carnegie Mellon University, Center for Astrophysics | Harvard & Smithsonian, the Chilean Participation Group, the French Participation Group, Instituto de Astrofísica de Canarias, The Johns Hopkins University, Kavli Institute for the Physics and Mathematics of the Universe (IPMU) / University of Tokyo, the Korean Participation Group, Lawrence Berkeley National Laboratory, Leibniz Institut für Astrophysik Potsdam (AIP), Max-Planck-Institut für Astronomie (MPIA Heidelberg), Max-Planck-Institut für Astrophysik (MPA Garching), Max-Planck-Institut für Extraterrestrische Physik (MPE), National Astronomical Observatories of China, New Mexico State University, New York University, University of Notre Dame, Observatório Nacional / MCTI, The Ohio State University, Pennsylvania State University, Shanghai Astronomical Observatory, United Kingdom Participation Group, Universidad Nacional Autónoma de México, University of Arizona, University of Colorado Boulder, University of Oxford, University of Portsmouth, University of Utah, University of Virginia, University of Washington, University of Wisconsin, Vanderbilt University, and Yale University.

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Figure 1: SDSS 2.5 m at sunset, ready for a night’s observing. The picture is taken facing west, with the rolled-off telescope enclosure behind the viewer. The telescope is enclosed in its rectangular wind baffle (image credit: SDSS)

Figure 2: Scientists from the Sloan Digital Sky Survey (SDSS) have released a comprehensive analysis of the largest three-dimensional map of the universe ever created. The new results come from the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), an SDSS collaboration of more than 100 astrophysicists worldwide. SDSS-IV Director Michael Blanton (New York University) and eBOSS Survey Scientist Will Percival (Perimeter Institute and University of Waterloo) discuss the legacy of 20 years of SDSS galaxy surveys (video credit: Perimeter Institute for Theoretical Physics)

SDSS provides descriptions of the telescopes and instruments under: 2)




The Sloan Digital Sky Survey

Over the last 15 years, the Sloan Digital Sky Survey (SDSS) has revolutionized many fields of astronomy. It has produced some of the most detailed three-dimensional maps of the Universe ever made, with deep multi-color images of 1/3 of the sky and spectra for more than three million astronomical objects. 3)

All phases of the SDSS survey have been carried out by an ever growing consortium of universities and institutions. As the first European partner institution, MPIA (Max Planck Institute for Astronomy) has been part of the SDSS surveys since their inception in 1998.

In its next phase, SDSS-IV, the survey will begin including observations from the southern hemisphere for the first time. These will be taken from the Irénée du Pont Telescope at Las Campanas Observatory in northern Chile. The du Pont Telescope is a Ritchey-Chrétien 2.5-m ƒ/7.5 telescope with a Gascoigne corrector lens.

SDSS-IV

SDSS began regular survey operations in 2000 and it has progressed through several phases: SDSS-I (2000-2005), SDSS-II (2005-2008), SDSS-III (2008-2014), and SDSS-IV (2014-2020). Each of these has involved multiple surveys with interlocking science goals. Over the years, astronomers have used SDSS data to make numerous discoveries related to the nature of the Universe at all scales. SDSS-IV will extend precision cosmological measurements to a critical early time in cosmic history, expand its revolutionary infrared spectroscopic survey of the Milky Way in the northern and southern hemispheres, and for the first time use the Sloan spectrographs to make spatially resolved maps of individual galaxies.

The three surveys that comprise SDSS-IV are:

APOGEE-2 (APO Galaxy Evolution Experiment North and South)

A stellar survey of the Milky Way with northern and southern hemisphere components. MPIA researchers hope to combine the SDSS-IV information with data from the Gaia mission, in order to study how our Milky Way was built up over time. APOGEE-2 will explore the formation history of the Milky Way using the “archeological” record provided by hundreds of thousands of its individual stars. The details concerning how the Milky Way grew from small progenitor galaxies are preserved in the patterns seen today in the motions of stars and the abundances of chemical elements they contain. APOGEE-2 will map these patterns using observations of the entire Galaxy from both hemispheres using the Sloan Telescope at Apache Point site in New Mexico and the du Pont Telescope at Las Campanas Observatory in Chile to obtain a complete view of its evolutionary history. 4)

Key Science Questions

• What is the history of star formation and chemical enrichment of the Milky Way?

• What are the dynamics of the disk, bulge, and halo of the Milky Way?

• What is the age distribution of stars in the Milky Way?

• Do planet-hosting stars have different properties than stars that have no planets?

To answer these science questions, APOGEE-2 relies upon the spectroscopy of stars using near-infrared light, which can penetrate regions obscured by interstellar dust. The APOGEE-2 spectral data provide a comprehensive view of (1) element abundance distributions in Galactic stars and (2) the dynamical motions of stars at various locations throughout the Milky Way. A primer on spectroscopy, especially in the near-infrared, as well some background on the Milky Way Galaxy can be found here.

APOGEE-1 predominantly observed red giant stars distributed across several kiloparsecs of the Milky Way disk. APOGEE-2 continues to observe these evolved stars and with the Southern hemisphere component, extends into previously unreachable parts of the disk. APOGEE also acquires spectra of young stars and star-forming regions, variable stars, stars in clusters and satellite galaxies, and stars with asteroseismic measurements. See APOGEE Targeting Information for more details.

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Figure 3: APOGEE-2 extends the sky coverage of the SDSS by using telescopes at both Apache Point Observatory (APOGEE-2N) and Las Campanas Observatory in Chile (APOGEE-2S). Using telescopes in both hemispheres means that APOGEE-2 is able to view the entire Milky Way (image credit: Dana Berry / SkyWorks Digital Inc. and the SDSS collaboration)

APOGEE-2 Technical Details

• Bright-time observations at APO and LCO

• Duration: Fall 2014 - Fall 2020

• Fiber Complement: 300 fibers per 7 deg2 plate (APO 2.5-m) or 3.5 deg2 plate (LCO 2.5-m), or 10 fiber (APO 1-m)

• Wavelength Range: 1.51-1.70 µm

• Spectral Resolution: R~22,500

• DR16 Sample Size: ~430,000 stars

• Signal-to-Noise Goal: S/N > 100 per pixel

• Radial Velocity Precision: ~200 m/s

• Element Abundance Precision: ~0.1 dex for 20 calibrated species.

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Figure 4: The planned APOGEE-2 survey area overlain on an image of the Milky Way. Each dot shows a position where APOGEE-2 will obtain stellar spectra (image credit: Sloan Digital Sky Survey)

eBOSS (extended Baryon Oscillation Spectroscopic Survey)

A cosmological survey of quasars and galaxies, also encompassing subprograms to survey variable objects (TDSS) and X-Ray sources (SPIDERS). MPIA researchers will use the data to study the history of the intergalactic medium and large-scale structure in the distant Universe. eBOSS will precisely measure the expansion history of the Universe throughout eighty percent of cosmic history—back to when the Universe was less than three billion years old—and improve constraints on the nature of dark energy. “Dark energy” refers the observed phenomenon that the expansion of the Universe is currently accelerating, which is the most mysterious experimental result in modern physics.

eBOSS will concentrate its efforts on observing faraway galaxies, quasars in particular, in a range of distances (redshifts, denoted by z) currently unexplored by other 3-D maps of large-scale structure in the Universe. In filling this gap, eBOSS will create the largest volume survey of the Universe to date.

eBOSS will also include two subprograms to follow up on other types of objects: the Time-Domain Spectroscopic Survey (TDSS) for variable objects, and the SPectroscopic IDentification of ERosita Sources (SPIDERS) for X-ray sources.

The combination of eBOSS with the SPIDERS X-ray selected sample of quasars, and the TDSS variability selected AGN sample, will create a unique window into the full population of quasars at all epochs to redshift z = 3.

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Figure 5: Planned eBOSS coverage of the Universe. The region to be newly explored by eBOSS corresponds to the epoch when the Universe was transitioning from deceleration due to the effects of gravity, to the current epoch of acceleration due to dark energy (image credit: Sloan Digital Sky Survey)

MaNGA (Mapping Nearby Galaxies at APO)

MaNGA is a galaxy survey that will explore the detailed internal structure of nearly 10,000 nearby galaxies using spatially resolved spectroscopy. Building on its central role in the CALIFA galaxy survey, MPIA researchers hope to produce dynamical models for the vast set of MaNGA galaxies to get a comprehensive picture of these objects' internal structure.

Unlike previous SDSS surveys which measured spectra only at the centers of target galaxies, MaNGA's tightly-packed optical fiber arrays will enable spectral measurements across the face of each of ~10,000 nearby galaxies. MaNGA’s goal is to understand the “life cycle” of present day galaxies from imprinted clues of their birth and assembly, through their ongoing growth via star formation and merging, to their death from quenching at late times.

To answer these questions, MaNGA will provide 2D maps of stellar velocity and velocity dispersion, mean stellar age and star formation history, stellar metallicity, element abundance ratio, stellar mass surface density, ionized gas velocity, ionized gas metallicity, star formation rate, and dust extinction for a statistically powerful sample. The galaxies are selected to span a stellar mass interval of nearly 3 orders of magnitude. No cuts are made on color, morphology or environment, so the sample is fully representative of the local galaxy population. Just as tree-ring dating yields information about climate on Earth hundreds of years into the past, MaNGA’s observations of the dynamical structures and composition of galaxies will help unravel their evolutionary histories over several billions of years.

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Figure 6: An image of a spiral galaxy with the face of a MaNGA 127 fiber integral field unit (IFU) superimposed. Galaxies are being selected from the SDSS Main Galaxy Legacy Area, with selection cuts applied to only redshift and a color-based stellar mass estimate (image credit: Sloan Digital Sky Survey)




SDSS events and some science results

Cosmology: Measurements of large-scale structure in SDSS maps of galaxies, quasars, and intergalactic gas have become a central pillar for tests of the standard cosmological model that describes our understanding of the history and future of the Universe. SDSS data have helped to demonstrate that the Universe is dominated by unseen dark matter and pervasive dark energy, and seeded with structure by quantum fluctuations in the infant cosmos. Those fluctuations have grown into the large-scale structure we see today. 5)

- The SDSS's high-precision maps of cosmic expansion history using baryon acoustic oscillations (BAO) have been especially influential in quantifying these results, yielding exquisite constraints on the geometry and energy content of the universe. The BAO feature was first detected in galaxy clustering in SDSS-I and in the contemporaneous 2dF Galaxy Redshift Survey. Since then, SDSS researchers have measured the BAO feature to an unprecedented one-percent precision, using measurements of more than one million galaxies — and for the first time, in the distribution of quasars. SDSS was also the first survey to detect BAO in intergalactic hydrogen gas using Lyman-alpha forest techniques, and has continued to improve these measurements as well. These BAO measurements are beautifully complemented by the results of the SDSS-II Supernova Survey, which has provided the most precise measurements yet of cosmic expansion rates over the last four billion years.

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Figure 7: The SDSS map of the Universe. Each dot is a galaxy; the color bar shows the local density. (image credit: SDSS Collaboration)

- The extended Baryon Oscillation Spectroscopic Survey completed observations in March 2019, marking the conclusion of large-scale structure observations from APO and complete spectral coverage over 11 billion years of cosmic history. The full sample of SDSS BAO and RSD measurements cover a wider redshift range than any other probe of expansion history or structure growth, enabling unique cosmology constraints on the nature of dark energy, the curvature of the universe, the local expansion rate, neutrino masses, and the laws of gravity.


• August 4, 2020: The journal Nature Communications today is publishing the discovery of a new type of stars, very rich in phosphorus, which could help to explain the origin of this chemical element in our Galaxy. This achievement has been made by astronomers of the Instituto de Astrofísica de Canarias (IAC) and researchers in computer science from the Centre for Research in Information and Communication Technology (CITIC) at the University of La Coruña (Galicia), Spain. 6) 7)

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Figure 8: Scheme which represents the origin of phosphorus on Earth, with respect to possible stellar sources of phosphorus in our Galaxy [image credit: Gabriel Pérez Díaz, SMM (IAC)]

- All the chemical elements in the universe, except for hydrogen and most of the helium, were produced inside stars. But among them there are a few (carbon, nitrogen, oxygen, sulphur and phosphorus) which are particularly interesting because they are basic to life as we know it on Earth. Phosphorus is of special interest because it forms part of the DNA and RNA molecules and is a necessary element in the energetic interchange within cells, and for the development of their membranes.

- The study published in Nature, based on an analysis of a large number of infrared spectra (in the H band, with APOGEE) from the public data base of the SDSS (Sloan Digital Sky Survey), could offer a clear set of promising stellar candidates to clarify the origin and the quantity observed of phosphorus in the Galaxy, and specifically, in our Solar System, which until now none of the current models of Galactic chemical evolution have been able to explain.

- However, the peculiar chemistry which these stars show is still disconcerting. In fact, not only are they rich in phosphorus, but also in certain other elements, such as magnesium, silicon, oxygen, aluminum and even of heavier elements such as cerium. Surprisingly, after an extensive analysis of all the possible stellar sources and processes known to form chemical elements in the interiors of stars, this chemical pattern is not predicted by the current theories of stellar evolution and nucleosynthesis.

- "These results show that not only are we dealing with a new type of objects, but that their discovery opens the way for the exploration of new physical mechanisms and nuclear reactions which occur in stellar interiors" explains IAC researcher Thomas Masseron, the leader of the project and the first author of the article.

- "It could be an important clue about the origin of the phosphorus, which is a fundamental component of life", says Aníbal García-Hernández, another IAC researcher, who is the second author of the article.

- In addition, thanks to Spanish service time, they could obtain the optical spectrum of the most brilliant of the phosphorus stars with the Echelle spectrograph (FIES) on the Nordic Optical Telescope (NOT) at the Roque de los Muchachos Observatory, (Garafía, La Palma).

- "This spectrum allowed us to obtain the chemical abundances of further elements in these stars which are peculiar and rich in phosphorus, and to rule out definitively any known stellar candidate which could explain the stars which are rich in this elements", indicates Olga Zamora, a co-author of the article, and an IAC support astronomer.

- "A discovery which is so unexpected and extraordinary could not have been made without a close interdisciplinary collaboration between astronomers and experts in computation", points out Arturo Manchado, an IAC research and a co-author of the article.


• July 19, 2020: SDSS released today a comprehensive analysis of the largest three-dimensional map of the Universe ever created, filling in the most significant gaps in our possible exploration of its history. 8) 9)

“We know both the ancient history of the Universe and its recent expansion history fairly well, but there’s a troublesome gap in the middle 11 billion years,” says cosmologist Kyle Dawson of the University of Utah, who leads the team announcing today’s results. “For five years, we have worked to fill in that gap, and we are using that information to provide some of the most substantial advances in cosmology in the last decade.”

The new results come from the extended Baryon Oscillation Spectroscopic Survey (eBOSS), an international collaboration of more than 100 astrophysicists that is one of the SDSS’s component surveys. At the heart of the new results are detailed measurements of more than two million galaxies and quasars covering 11 billion years of cosmic time.

We know what the Universe looked like in its infancy, thanks to the thousands of scientists from around the world who have measured the relative amounts of elements created soon after the Big Bang, and who have studied the CMB (Cosmic Microwave Background). We also know its expansion history over the last few billion years from galaxy maps and distance measurements, including those from previous phases of the SDSS.

“Taken together, detailed analyzes of the eBOSS map and the earlier SDSS experiments have now provided the most accurate expansion history measurements over the widest-ever range of cosmic time,” says Will Percival of the University of Waterloo, eBOSS’s Survey Scientist. “These studies allow us to connect all these measurements into a complete story of the expansion of the Universe.”

The final map is shown in the image of Figure 9. A close look at the map reveals the filaments and voids that define the structure in the Universe, starting from the time when the Universe was only about 300,000 years old. From this map, researchers measure patterns in the distribution of galaxies, which give several key parameters of our Universe to better than one percent accuracy. The signals of these patterns are shown in the insets in the image.

This map represents the combined effort of more than 20 years of mapping the Universe using the Sloan Foundation telescope. The cosmic history that has been revealed in this map shows that about six billion years ago, the expansion of the Universe began to accelerate, and has continued to get faster and faster ever since. This accelerated expansion seems to be due to a mysterious invisible component of the Universe called “dark energy,” consistent with Einstein’s General Theory of Relativity but extremely difficult to reconcile with our current understanding of particle physics.

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Figure 9: The SDSS map is shown as a rainbow of colors, located within the observable Universe (the outer sphere, showing fluctuations in the Cosmic Microwave Background). We are located at the center of this map. The inset for each color-coded section of the map includes an image of a typical galaxy or quasar from that section, and also the signal of the pattern that the eBOSS team measures there. As we look out in distance, we look back in time. So, the location of these signals reveals the expansion rate of the Universe at different times in cosmic history (image credit: Anand Raichoor (EPFL), Ashley Ross (Ohio State University) and the SDSS Collaboration)

Combining observations from eBOSS with studies of the Universe in its infancy reveals cracks in this picture of the Universe. In particular, the eBOSS team’s measurement of the current rate of expansion of the Universe (the “Hubble Constant”) is about 10 percent lower than the value found from distances to nearby galaxies. The high precision of the eBOSS data means that it is highly unlikely that this mismatch is due to chance, and the rich variety of eBOSS data gives us multiple independent ways to draw the same conclusion.

“Only with maps like ours can you actually say for sure that there is a mismatch in the Hubble Constant,” says Eva-Maria Mueller of the University of Oxford, who led the analysis to interpret the results from the full SDSS sample. “These newest maps from eBOSS show it more clearly than ever before.”

There is no broadly accepted explanation for this discrepancy in measured expansion rates, but one exciting possibility is that a previously-unknown form of matter or energy from the early Universe might have left a trace on our history.

In total, the eBOSS team made the results from more than 20 scientific papers public today. Those papers describe, in more than 500 pages, the team’s analyzes of the latest eBOSS data, marking the completion of the key goals of the survey.

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Figure 10: This image illustrates the impact that the eBOSS and SDSS maps have had on our understanding of the current expansion rate and curvature of the Universe from the last 20 years of work. The gray region shows our knowledge as of 10 years ago. The blue region shows the best current measurement, which combines SDSS, eBOSS and other programs [image credit: Eva-Maria Mueller (Oxford University) and the SDSS Collaboration]

Within the eBOSS team, individual groups at Universities around the world focused on different aspects of the analysis. To create the part of the map dating back six billion years, the team used large, red galaxies. Farther out, they used younger, blue galaxies. Finally, to map the Universe eleven billion years in the past and more, they used quasars, which are bright galaxies lit up by material falling onto a central supermassive black hole. Each of these samples required careful analysis in order to remove contaminants, and reveal the patterns of the Universe.

“By combining SDSS data with additional data from the Cosmic Microwave Background, supernovae, and other programs, we can simultaneously measure many fundamental properties of the Universe,” says Mueller. “The SDSS data cover such a large swath of cosmic time that they provide the biggest advances of any probe to measure the geometrical curvature of the Universe, finding it to be flat. They also allow measurements of the local expansion rate to better than one percent.”

eBOSS, and SDSS more generally, leaves the puzzle of dark energy, and the mismatch of local and early Universe expansion rate, as a legacy to future projects. In the next decade, future surveys may resolve the conundrum, or perhaps, will reveal more surprises.

Meanwhile, with continued support from the Alfred P. Sloan Foundation and institutional members, the SDSS is nowhere near done with its mission to map the Universe. Karen Masters of Haverford College, Spokesperson for the current phase of SDSS, described her excitement about the next phase. “The Sloan Foundation Telescope and its near-twin at Las Campanas Observatory will continue to make astronomical discoveries mapping millions of stars and black holes as they change and evolve over cosmic time.” The SDSS team is busy building the hardware to start this new phase and is looking forward to the new discoveries of the next 20 years.

The final map released by SDSS reveals the filaments and voids that define the structure in the Universe. A characteristic of this distribution, the so-called "baryonic acoustic oscillations", is a very subtle signal from the early epochs of the universe, when sound waves got “frozen” after traveling about 500 million light years and imprinted a signal in the distribution of matter. This footprint is visible today in the distribution of galaxies, where it is a little more likely to find pairs of galaxies separated by this scale than at smaller or larger distances. Measurements of the apparent size of this scale allow the scientists to calculate cosmic distances with high precision (Ref. 9).

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Figure 11: Two-dimensional correlation function for the eBOSS quasar sample. The signal measured, the so-called Baryonic Acoustic Oscillation, is revealed as the ring feature at a characteristic scale of ~100 Mpc/h. In addition, also the so-called redshift space distortion can be identified as the anisotropic clustering towards the center (image credit: MPE)

In addition to that, galaxies also have peculiar motions that make their distribution appear anisotropic with respect to the line-of-sight direction, an effect known as “redshift-space distortions”. This characteristic anisotropic pattern allowed the eBOSS team to measure the rate at which cosmic structures grow due to gravitational interactions. “Using quasars, we can constrain the distance measurement to around 3 percent and the measurement of the growth rate of the Universe within 10 percent,” says Hou.

“In our group, we have had a continuous contribution to the SDSS cosmology programs since almost a decade ago”, says cosmologist Ariel Sánchez, a senior researcher at MPE. “Taken together, these studies allow us to reconstruct the expansion and growth of structure histories of our Universe over a range of cosmic time covering roughly 90% of its age.” Combined with additional information from the Cosmic Microwave Background and supernovae, this allows the scientists to determine several key parameters of our Universe to better than one percent accuracy. The cosmic history that has been revealed by these data shows that about six billion years ago, the expansion of the Universe began to accelerate, and has continued to get faster and faster ever since. This accelerated expansion seems to be due to a mysterious invisible component of the Universe called “dark energy”, consistent with Einstein’s General Theory of Relativity but extremely difficult to reconcile with our current understanding of particle physics.


• June 19, 2020: The eROSITA telescope has provided a new, sharp view of hot and energetic processes across the Universe – a view that the Sloan Digital Sky Survey (SDSS) and similar projects will use to enhance our knowledge of the Universe. 10)

“With eROSITA finding more than one million new X-ray sources, the time is ripe for SDSS to follow up the hot and energetic sky, hunting for distant black holes and the mysteries of Dark Energy,” says Andrea Merloni of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, eROSITA’s Project Scientist and a longtime member of the SDSS collaboration.

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Figure 12: The energetic universe as seen with the eROSITA X-ray telescope [image credit: Jeremy Sanders, Hermann Brunner and the eSASS team (MPE); Eugene Churazov, Marat Gilfanov (on behalf of IKI)]

After six months of uninterrupted scanning of the whole sky, eROSITA, the new powerful X-ray telescope onboard the Russian-German Spektrum-Roentgen-Gamma (SRG) space mission, has delivered a new map of the hot and energetic sky.

The map reveals spectacular details on the structure of the hot gas in and around the Milky Way, while detecting more than one million (mostly new) X-ray sources, about as many have been detected over the 60 years history of X-ray astronomy by all its predecessors.

“This all-sky image completely changes the way we look at the energetic universe,” says Peter Predehl, the Principal Investigator of eROSITA at the Max Planck Institute for Extraterrestrial Physics (MPE). “We see such a wealth of detail – the beauty of the images is really stunning.”

Most of these objects seen in the new image are Active Galactic Nuclei (AGN), growing supermassive black holes at cosmological distances, interspersed with clusters of galaxies, which appear in the map as extended X-ray nebulae shining thanks to the hot gas confined by their large potential wells.

Within our own Galaxy, eROSITA reveals a plethora of coronally-active stars, X-ray binary stars containing neutron stars, black holes or white dwarfs, and spectacular supernova remnants.

“The clusters of galaxies that mark out the large-scale structure of the Universe are filled with gas at temperatures of a million degrees or more,” Merloni says. “To see that gas directly, you have to use an X-ray telescope. With eROSITA covering the whole sky, we can see enough clusters to reconstruct their growth history extremely accurately, which in turns tells us the amount, and perhaps the nature, of dark matter and dark energy.”

One key step in the process that will lead astronomers to extract scientific information from these spectacular maps is the measure of the distance (or redshift) and physical properties of the hundreds of thousands of stars, AGN and clusters that eROSITA will discover. The SDSS, with its capability of taking large number of spectra over very wide areas of the sky in a short time is the perfect complement of eROSITA, and the two teams have partnered to explore the science reward of such X-ray/optical synergies for many years now.

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Figure 13: The Vela Supernova Remnant, about 800 light-years from Earth, as seen by eROSITA [image credit: Jeremy Sanders, Hermann Brunner and the eSASS team (MPE); Eugene Churazov, Marat Gilfanov (on behalf of IKI)]

The eROSITA telescope consists of seven identical telescopes, focusing X-rays in the energy range 0.1-10 keV onto seven pnCCD cameras, which not only image the sky, but are capable of measuring the energy of every incoming photon with per-cent accuracy. Launched in July 2019 into a large halo orbit around the second Lagrange equilibrium point of the sun-earth system (L2), SRG/eROSITA has scanned the whole celestial sphere in six months, from December 2019 to June 2020, and is planning to continue doing so for 3.5 more years. By the end of 2023, eROSITA will have reached a sensitivity more than 30 times higher than the only all-sky X-ray survey to date, completed by ROSAT 30 years ago. By that time, the fifth phase of the SDSS (SDSS-V) will be in full swing, providing a rich dataset for comparison with upcoming eROSITA results.

“The MPE has been a member of SDSS since 2012, and we have continued to be part of the program ever since” continues Andrea Merloni. “The long term objective has always been to deploy the multi—object spectroscopic capabilities of SDSS to exploit the full potential of eROSITA as a tracer of large scale structure. We have finally reach the point where this plan can be made concrete. The all-sky nature of SDSS-V is a match made in heaven for eROSITA.”


• January 8, 2019: Today, at the 233rd AAS meeting in Seattle, astronomers from the Sloan Digital Sky Survey (SDSS) announce that they have developed a new tool to find otherwise-hidden galaxy mergers in data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey of SDSS. These results show that by going beyond simple searches for merging galaxies based just on how they look, astronomers will now be able find more galaxy mergers than ever before. 11)

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Figure 14: Three galaxies observed by the SDSS MaNGA survey. The top row shows the galaxies’ images, while the bottom row shows the velocity of the stars within the galaxies; red means the stars are moving away from us and blue means towards us. The panel on the left shows an isolated spiral galaxy, not undergoing a merger. The middle panels show a spectacular pair of merging galaxies, obvious in both the image and the velocity map. The right panels show what appears in the image to be a single galaxy – but the velocity map reveals that it is actually a galaxy that has just merged. This is evident in the disturbed (counter-rotating) features in the velocity map. This example demonstrates the power of the team’s new method, which will identify merging galaxies using both imaging and kinematics (image credit: Rebecca Nevin (University of Colorado Boulder) and the SDSS collaboration)

“Merging galaxies are key to understanding galaxy evolution, but finding them can be tricky,” says Rebecca Nevin of the University of Colorado, the lead author of the study. Nevin is presenting this work this week as a Dissertation talk, as it formed the basis of her PhD thesis at Colorado with Professor Julie Comerford.

A pair of merging galaxies is one of the most beautiful sights in astronomy, with giant tidal streams of stars and unusual shapes sometimes resembling animals (e.g. the Antennae, Mice, Tadpole, or Penguin galaxies). However, these beautiful visible features are visible are only found in a small fraction of merging galaxies – and even then only for a small part of the billions of years it takes for two galaxies to fully merge into one. Some galaxies that otherwise look “normal” may still be in the process of merging.

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Figure 15: This montage of six images from the Hubble Space Telescope shows six real galaxies in different stages of the merger process. For more information about these galaxies, see the image description at the Hubble Space Telescope website [image credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University), K. Noll (STScI), and J. Westphal (Caltech)]

Astronomers have developed a way to find these hidden mergers. They created a method that uses simulations of merging galaxies to predict both how the mergers would look and how the stars in the galaxies would move. By comparing their results with observations of galaxies from the SDSS’s Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, astronomers will be able to do much better at identifying merging galaxies in the wild.

“These simulations allow us to predict the more subtle signs of merging galaxies, so we can find mergers in SDSS data that were previously hidden,” explains Laura Blecha (University of Florida), another key member of the team.

What the team is presenting today is the part of their method that analyzes galaxy images. They have essentially made a galactic photo album, including pictures of galaxies in all stages of merging. In the past, astronomers’ “photo albums” of galaxy mergers were sparse, including only galaxies in the stage of merging where they looked like spectacular mergers.

“Nowadays, it would be totally unthinkable to take only one or two selfies every year,” said Nevin. “We have modernized the galaxy merger photo album – now it’s like taking one galaxy merger selfie a day for years.”

The astronomers plan to make these extensive photo albums publicly available to everyone. Astronomers will use them to study how galaxies change as they undergo mergers.

The team’s work so far is already a giant step forward in merger identification, but they are already taking the next step. They have already begun to incorporate data on how the stars move in the galaxies from the SDSS MaNGA survey. This will allow the team to identify even more mergers – those where the galaxy looks completely “normal.”

The key to this new analysis is to incorporate data from MaNGA on how stars within galaxies are moving. “By going beyond images alone and incorporating stellar kinematics, we will find many more merging galaxies,” says Karen Masters of Haverford College, the Spokesperson for SDSS. “We’ll be able to learn how the merger process impacts how galaxies in our Universe evolve.”

These stellar kinematics are revealed in the maps created by the SDSS’s MaNGA survey. Because the spectra that MaNGA observes come from the light of all the stars in a particular part of a galaxy, stars, the spectra are slightly shifted by the Doppler Effect – blueshifted for the parts of a galaxy that are moving toward Earth and redshifted for the parts moving away from Earth. These subtle shifts reveal how the stars are moving around the galaxy.

When galaxies merge, the stars in them almost never collide, but they are thrown all around, creating dramatic distortions in the pattern of how stars move around the galaxy – patterns that astronomers refer to as “stellar kinematics.” In a typical, non-merging spiral galaxy, the stars rotate in a simple, predictable pattern. But if such a galaxy is undergoing a merger, that simple pattern becomes chaotic, creating wild (but predictable) arrangements of stellar motion. When a galaxy’s patterns of stellar motion have become distorted by a merger, the stellar kinematics data from MaNGA provides direct evidence of the merger. Nevin’s team, which includes astronomers from the University of Colorado Boulder, the University of Florida, and Princeton University, is beginning to add stellar kinematics data into their work.

Figure 16: This animation shows one of the galaxy merger simulations the team created. The first 38 seconds shows the simulation running, covering 2.5 billion years of history. From each step of the simulation, the team figures out what the galaxy would look like when viewed from Earth by the Sloan Digital Sky Survey (shown from 0:39 to 1:06). The last part of the video (1:06-1:30) shows a collection of simulated images and how they are used to create a classification method that can then be applied to real SDSS images (image credit: Rebecca Nevin (University of Colorado), Laura Blecha (University of Florida), and the SDSS collaboration)

“As we improve our machine learning algorithms to incorporate the stellar kinematics of merging galaxies, we are able to identify different stages of the merger. The disturbances in the stars can last longer than imaging signatures of a merger like faint tidal tails, which fade much quicker. This means we can identify later stages in the merger, when in the imaging the galaxies look just like normal galaxies. This is a powerful new technique in the study of merging galaxies.”

Understanding mergers is not only important to astronomers like Nevin’s team; this understanding can help us predict the future of our own Galaxy. The Milky Way will merge with the Large and Small Magellanic Clouds in about 2.5 billion years – and is then predicted to merge with the much more massive Andromeda galaxy in five billion years, combining to form a single super-galaxy, which some dub “Milkdromeda.” This event might throw the Sun out of the galaxy, but it won’t matter to future inhabitants of Earth, which will have been swallowed by the Sun as it turns into a red giant star at around the same time. But maybe our descendants will see this for themselves as they travel among the stars.


• May 9, 2018: This week marks the twentieth anniversary of “first light” for the telescope behind the Sloan Digital Sky Survey (SDSS), which has gone on to create by far the largest three-dimensional map of the Universe ever made. Early in the morning of May 10th, 1998, the observers and engineers pointed the Sloan Foundation Telescope to the celestial equator and light went through to the survey’s exquisitely sensitive camera. When dawn broke after a long night’s work, SDSS observer Dan Long emailed his usual observer’s log summarizing what happened. After describing the technical details of the observations, and before noting a series of newly identified problems to fix, he wrote: “Wow; What a night!” 12)

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Figure 17: A map showing some of what the Sloan Digital Sky Survey has discovered over the last twenty years. The dots show the distance to various sky objects that the SDSS has discovered. The horizontal axis of the map is labeled in light-years, stretching from our own solar system to the most distant reaches of the universe. Sample images at the top show some of the things the SDSS has seen (image credit: V. Belokurov, M. R. Blanton, A. Bonaca, X. Fan, M. C. Geha, R. H. Lupton, the SDSS Collaboration)

“That was the beginning of our survey, and we’re still going strong twenty years later,” says Michael Blanton, a professor at New York University and the Director of SDSS-IV. “We are now planning our fifth generation, called SDSS-V, and we’re still using the same Sloan Foundation Telescope.”

The telescope’s mirror, 2.5 meters in diameter, is small by astronomy research telescope standards, but powerful because it can see a large area of the sky simultaneously. Astronomers have used it to make an enormous, highly-detailed map – a map which covers one-third of the night sky, with measurements of hundreds of thousands of Milky Way stars and distances to more than four million galaxies. All the data collected by the telescope is available free to anyone online, and all images are open access under the SDSS’s Image Use Policy.

This map has played a key role in astronomical history, helping astronomers learn about our Milky Way, other galaxies, distant black holes, the nature of dark matter and dark energy and myriad types of stars in ways never imagined when the project started.

The SDSS has measured the universe’s expansion more precisely than ever before and has mapped how galaxies and larger structures grew over cosmic time, helping to establish our current standard model of cosmology. It discovered some of the nearest stars and the smallest companion galaxies to our own Milky Way galaxy, revealing how our galaxy grew by cannibalizing smaller galaxies. It has studied the Milky Way’s disk of stars more completely than ever before, using infrared light to peer through the obscuring dust. It has studied the dark matter content of the Milky Way and distant galaxies, and found some of the most distant quasars known.

And perhaps most importantly, the SDSS has achieved its remarkable scientific discoveries while establishing a new way of doing science. Since 2001, the project has made all its data freely available to the public in a series of roughly-annual releases of data – most recently with Data Release 14 in July 2017. The team who built and operated the survey recognized that the data were far too rich for them to reap all the science from it alone, and that maximizing the science return would require the entire worldwide community of astronomers. As a result, a generation of astronomers have grown up learning their craft with the help of SDSS data. Close to ten thousand astronomers have worked with SDSS data, publishing over eight thousand scientific papers – thus making it the most broadly used data set in astronomy.

Along the way, SDSS gained the support of numerous institutions. One-quarter of the funding for SDSS comes from the Alfred P. Sloan Foundation, and about five percent comes from the U.S. Department of Energy. The rest comes from more than fifty universities and research institutions on four continents, all of whom consider it so vital to their scientists that they contribute their time, effort, and resources to become members of the project. The next step, SDSS-V, will commence in 2020 and will continue to uncover the unexpected by observing millions of stars, galaxies and black holes across the entire sky.

At the Apache Point Observatory, home of the Sloan Foundation Telescope, astronomers will gather this Wednesday, 9th May 2018, to mark this anniversary, remember the early days of the project, and discuss the impact it has had on the field of astronomy. The telescope no longer hosts a camera, but it still measures spectra of hundreds of celestial objects every night – and is still operated by a dedicated on-site team of observers and engineers. True to the online nature of the Sloan Digital Sky Surveys over the last 20 years, this celebration will be broadcast live on the internet, and involve collaboration members past and present across all parts of the globe.

The core of the project – the people who get the data upon which so many of the world’s astronomers depend – sits the team of observers and engineers at Apache Point Observatory. A handful of the original team that led the efforts in 1998 are still with the project. Dan Long was promoted along the way to Chief Telescope Engineer, a position from which he recently retired. Many other members of the team have joined more recently. But they all share the excitement that Dan Long expressed the night of first light, and each night brings the prospect of new discoveries that will change the way we see our universe and make them and us say “Wow.”


• January 9, 2018: How far away is that galaxy? — Our entire understanding of the Universe is based on knowing the distances to other galaxies, yet this seemingly-simple question turns out to be fiendishly difficult to answer. The best answer came more than 100 years ago from an astronomer who was mostly unrecognized in her time — and today, another astronomer has used Sloan Digital Sky Survey (SDSS) data to make those distance measurements more precise than ever. 13)

“It’s been fascinating to work with such historically significant stars,” says Kate Hartman, an undergraduate from Pomona College who announced the results at today’s American Astronomical Society (AAS) meeting in National Harbor, Maryland. Hartman studied “Cepheid variables,” a type of star that periodically pulses in and out, varying in brightness over the course of a few days or weeks.

The pattern was first noticed in 1784 in the constellation Cepheus in the northern sky, so these stars became known as “Cepheid variables.” Cepheid variables went from interesting to completely indispensable in the early 1900s thanks to the work by astronomer Henrietta Leavitt. Leavitt’s contributions were largely ignored for one simple reason — she was a woman at time when women were not taken seriously as astronomers.

In fact, when Leavitt was first hired by Harvard College Observatory in 1895, she was hired as a “computer” — a term which meant something completely different from what it means today. In the days before modern computers or even pocket calculators, a “computer” was a person hired to perform complex calculations in their mind, assisted only by pencil and paper. Although the work was demanding, it was not taken seriously by the male professional scientists of the time — it was seen as rote work not requiring intelligence or insight that could be done by anyone, even a woman.

So in 1908 when Leavitt discovered a relationship between the brightness (or “luminosity”) of a Cepheid variable star and the time it took to go through a full cycle of change (its “period”), her work was not immediately recognized for its significance. It took years for the mostly-male astronomy community to realize that this relationship (today known as “the Leavitt Law”) means that measuring the period of a Cepheid variable immediately gives its true brightness — and furthermore, that comparing this to its apparent brightness immediately gives its distance.

Sadly, it was only after Leavitt’s death from cancer at age 53 that astronomers realized that she had found the key to unlocking distances to such stars everywhere — whether in our Milky Way or in a galaxy in the distant Universe.

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Figure 18: Henrietta Leavitt (left) and Kate Hartman (right) — two astronomers a century apart studying Cepheid variable stars [image credit: Cynthia Hunt (Carnegie Institution for Science)]


• 2018: SDSS-V: Pioneering Panoptic Spectroscopy. 14)

Overview: SDSS-V is an all-sky, multi-epoch spectroscopic survey that will yield optical and IR spectra of over 6 million objects during its five year lifetime (2020-2025). Using SDSS’s existing and anticipated new facilities at Apache Point and Las Campanas Observatories, SDSS-V will survey the entire sky — mapping the Milky Way using rapid, repeated observations, mapping Local Volume galaxies using wide-angle integral field spectroscopy, and mapping black holes using time domain spectroscopy of quasars and bright X-ray sources. Instead of SDSS’s classic plugplate system, SDSS-V will use a new, custom-built robotic positioning system to allow for rapid configuration of fibers. These fibers will lead to the existing APOGEE and BOSS spectrographs, enabling simultaneous IR and optical observations. In addition, new wide-field integral-field units are being deployed to observe the stars and interstellar medium in the Milky Way, Magellanic Clouds, and local galaxies at unprecedented spatial resolution. More information about SDSS-V and its place in the astronomical landscape during the 2020s can be found in our white paper SDSS-V: Pioneering Panoptic Spectroscopy.

Without federal or other large-scale funding, SDSS-V’s commitment to transformative science and public data release is not possible without membership contributions from institutional partners. SDSS-V is currently under development and actively seeking funding from partners. Institutional partners benefit from early access to data, participation in the development process, and access to our global science collaborative network.

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Figure 19: Although SDSS-IV will keep collecting and publishing data through 2020, we are excitedly planning for SDSS-V, a panoptic spectroscopic survey that will continue the SDSS tradition of innovative data and collaboration infrastructure. SDSS-V will be the first facility providing multi-epoch, all-sky, optical & IR spectroscopy, as well as offering contiguous integral-field spectroscopic coverage of the Milky Way and Local Volume galaxies. Our white paper SDSS-V: Pioneering Panoptic Spectroscopy contains a fuller description of the survey’s ambitious goals and scope (image credit: Juna A. Kollmeier and Hans-Walter Rix)

Mappers: SDSS-V comprises three key programs, called Mappers:

The Milky Way Mapper (MWM) will target 4-5 million stars across the Milky Way, collecting infrared spectra with an APOGEE spectrograph and/or optical spectra with a BOSS spectrograph. The MWM seeks to understand the evolution of the Milky Way, the physics of its stars and interstellar medium, and the architecture of multiple-star and planetary systems.

The Black Hole Mapper (BHM) will target over 400,000 sources, primarily black holes, with a BOSS optical spectrograph. Many of these will be observed numerous times, with the goal of measuring black hole masses, probing black hole growth across cosmic time, and characterizing the X-ray sky.

The Local Volume Mapper (LVM) will observe the interstellar medium and stellar populations in the Milky Way and several local galaxies, collecting more than 25 million contiguous spectra over 2,500 square degrees on the sky. The LVM will use new integral-field spectrographs to explore the physics of star formation and the interactions between stars and the interstellar medium.

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Figure 20: SDSS-V will be carried out in both hemispheres, at Apache Point Observatory (APO) in the USA and Las Campanas Observatory (LCO) in Chile. Multi-object fiber spectroscopy will be obtained with two 2.5 m telescopes, each feeding a near-infrared APOGEE spectrograph and an optical BOSS spectrograph, for the Milky Way Mapper and Black Hole Mapper programs. The Local Volume Mapper will make use of smaller telescopes to perform its optical integral-field spectroscopy (image credit: M. Seibert (OCIS) & SDSS-V team)



1) James E. Gunn, Walter A. Siegmund, Edward J. Mannery, Russell E. Owen, Charles L. Hull, R. French Leger, Larry N. Carey, Gillian R. Knapp, Donald G. York, William N. Boroski, Stephen M. Kent, Robert H. Lupton, Constance M. Rockosi, Michael L. Evans, Patrick Waddell, John E. Anderson, James Annis, John C. Barentine, Larry M. Bartoszek, Steven Bastian, Stephen B. Bracker, Howard J. Brewington, Charles I. Briegel, Jon Brinkmann, Yorke J. Brown, Michael A. Carr, Paul C. Czarapata, Craig C. Drennan, Thomas Dombeck, Glenn R. Federwitz, Bruce A. Gillespie, Carlos Gonzales, Sten U. Hansen, Michael Harvanek, Jeffrey Hayes, Wendell Jordan, Ellyne Kinney, Mark Klaene, S. J. Kleinman, Richard G. Kron, Jurek Kresinski, Glenn Lee, Siriluk Limmongkol, Carl W. Lindenmeyer, Daniel C. Long, Craig L. Loomis, Peregrine M. McGehee, Paul M. Mantsch, Eric H. Neilsen, Jr., Richard M. Neswold, Peter R. Newman, Atsuko Nitta, John Peoples, Jr., Jeffrey R. Pier, Peter S. Prieto, Angela Prosapio, Claudio Rivetta, Donald P. Schneider, Stephanie Snedden, and Shu-i Wang, ”The 2.5 m Telescope of the Sloan Digital Sky Survey,” The Astronomical Journal, Volume 131, Number 4, April 2006, https://doi.org/10.1086/500975, URL: https://iopscience.iop.org/article/10.1086/500975/pdf

2) ”Telescopes and instruments,” SDSS, 2018, URL: https://www.sdss.org/dr14/instruments/

3) ”The Sloan Digital Sky Survey,” MPIA, 2020, URL: https://www.mpia.de/en/research/collaborations/sdssiv

4) ”APOGEE-2 APO Galactic Evolution Experiment 2,” 2018, SDSS, URL: https://www.sdss.org/surveys/apogee-2/

5) ”Science Results,” SDSS, 2018, URL: https://www.sdss.org/science/

6) ”Stars rich in phosphorus: seeds of life in the universe,” IAC Press Release, 4 August 2020, URL: https://www.iac.es/en/outreach/news/stars-rich-phosphorus-seeds-life-universe

7) Thomas Masseron, D. A. García-Hernández, Raúl Santoveña, Arturo Manchado, Olga Zamora, Minia Manteiga & Carlos Dafonte, ”Phosphorus-rich stars with unusual abundances are challenging theoretical predictions,” Nature Communications, Volume 11, Article number: 3759 (2020), Published: 4 August 2020, https://doi.org/10.1038/s41467-020-17649-9, URL: https://www.nature.com/articles/s41467-020-17649-9.pdf

8) Jordan Raddick, ”No need to Mind the Gap: Astrophysicists fill in 11 billion years of our universe’s expansion history,” SDSS Press Release, 19 July 2020, URL: https://www.sdss.org
/press-releases/no-need-to-mind-the-gap/

9) Ariel Sánchez, Jiamin Hou, ”SDSS reveals 11 billion years of the history of our expanding Universe,” MPE, July 20, 2020, URL: http://www.mpe.mpg.de/7474540/news20200720?c=260780

10) ”Our deepest view of the X-ray sky,” SDSS Press Release, 19 June 2020, URL: https://www.sdss.org/press-releases/our-deepest-view-of-the-x-ray-sky/

11) ”Don’t judge a galaxy by its cover: Astronomers develop new tool to find merging galaxies,” SDSS Press Release, 8 January 2019, URL: https://www.sdss.org/press-releases
/dont-judge-a-galaxy-by-its-cover-astronomers-develop-new-tool-to-find-merging-galaxies/

12) “Wow, what a night” 20 years later: The Sloan Digital Sky Survey celebrates a milestone,” SDSS Press Release, 9 May 2018, URL: https://www.sdss.org/press-releases/sdss20/

13) ”A century of cepheids: Two astronomers, a hundred years apart, use stars to measure the Universe,” SDSS Press Release, 9 January 2020, URL: https://www.sdss.org/press-releases/cepheids/

14) ”The future SDSS-V,” SDSS, 2018, URL: https://www.sdss.org/future/


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).

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