Astronomers have released a new, extremely detailed three-dimensional census of our Milky Way galaxy.

Gaia map of the Milky Way
Data from more than 1.8 billion stars have been used to create this map of the entire sky. 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 colour 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.
ESA / Gaia / DPAC / CC BY-SA 3.0 IGO; acknowledgement: A. Moitinho

On December 3rd, the team behind the European Space Agency's Gaia mission published the third and most accurate edition of its star catalog.

The catalog, titled Early Data Release 3, contains ultra-precise coordinates for 1,811,709,771 stars between 3rd and 21st magnitude, with a precision of a few tens of micro-arcseconds; most of these also have brightness measurements. In addition, the catalog provides parallaxes (aka, distances) and proper motions on the sky for a subset of 1,467,744,818 stars.

“This is certainly one of the largest catalogs ever compiled in the history of science,” says consortium chair Anthony Brown (Leiden Observatory, The Netherlands).

Big Data from Gaia

The European Space Agency’s Gaia spacecraft has been scanning the skies from its vantage point 1.5 million kilometers away from Earth in the opposite direction from the Sun, at the L2 Lagrangian point, since July 2014, following its launch in late 2013. The mission team has been releasing gradually improved catalogs, with the first coming out in September 2016 and then in April 2018. The next release is expected in early 2022. To date, over 3,600 refereed papers have already been published on the basis of earlier Gaia results.

This most recent release includes almost three full years of data. Due to the longer time baseline, the parallax estimates, which astronomers use to derive stellar distances, are 30% more precise than the last data release. Proper motions across the sky are also twice as precise. The data also contains information on 1.6 million quasars, gas-guzzling black holes in distant galaxies. Eventually, with 10 years of data, all measurements will become twice as accurate, while proper motion precision will increase sevenfold. Gaia is expected to operate until 2025.

Gaia infographic
Gaia's Early Data Release 3, by the numbers.

First Look: Satellite Galaxies and Star Clusters

In a series of papers published in Astronomy & Astrophysics, the Gaia Collaboration provides a taste of what can be done with this new data.

For instance, color information on individual stars in the Large and Small Magellanic Clouds reveals that both older (redder) and younger (bluer) populations of stars are present in the gaseous bridge that connects our two largest satellites galaxies. Proper motion data also show that while the LMC is rotating in an orderly way, many of the stars in the SMC appear to be flowing towards and into the Magellanic Bridge. These observations will influence ideas about how these satellites are interacting with each other and with the Milky Way.

This image shows the stellar density of the Large and Small Magellanic Clouds, as seen by Gaia. The image is composed of red, green and blue layers, which trace mostly the older, intermediate age, and younger stars respectively.
ESA / Gaia / DPAC / CC BY-SA 3.0 IGO; Acknowledgement: L. Chemin; X. Luri et al (2020)

Using colors and proper motions of stars opposite the galactic center, Gaia scientists have distinguished stars that once belonged to Gaia-Enceladus – a companion galaxy that collided and merged with our Milky Way some 8–11 billion years ago.

“We can see its debris clearly out to 65,000 light-years from the center of the galaxy,” says Amina Helmi (University of Groningen, The Netherlands). Her team discovered the galactic intruder in 2018 from a previous Gaia data release. A preliminary analysis of the new data indicates that the disk of the Milky Way was almost 20% smaller at the time of the encounter. This agrees with computer simulations that show evidence of disk growth after galaxy mergers.

One other impressive result is the precise determination of our solar system’s acceleration. While the Sun has a more or less constant orbital velocity of about 230 kilometers per second, it is continuously “falling” in the direction of the galactic center, causing a minute shift in the perceived positions of distant quasars. Based on the new data, the Gaia team arrives at a centripetal acceleration of 7 millimeters per second per year – in good agreement with current models of the mass distribution in the Milky Way. “This is extreme astrometry,” says Floor van Leeuwen (University of Cambridge, UK).

Where the stars are going
The trails on this image show how 40,000 stars, all located within 326 light-years of the solar system, will move across the sky in the next 400,000 years.
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

Finally, the Gaia collaboration has produced a subcatalog — and an accompanying 3D movie (below) — of 331,312 stars within 326 light-years of the Sun. This local census is estimated to be 92% complete (some extremely dim stars may still be missing). Our “solar backyard” contains two open star clusters: the Hyades and the Coma Berenices star cluster, the members of which can easily be recognized by their common proper motion. The 1,000 or so stars belonging to the Hyades cluster spread over a distance of almost 100 light-years, an elongated structure that spans many tens of degrees on the sky.

“We already knew that the Hyades cluster is dispersing due to the Milky Way’s tidal forces,” says Brown, “but the new data provide a much more precise picture.”

The worldwide astronomical community received access to Gaia’s newest data release at 6:00 a.m. EST (11:00 a.m. UT), and as you read this, many groups all around the globe will be carrying out their own research. According to Gerry Gilmore (University of Cambridge, UK), “A vast wealth of science is coming out of Gaia.” The first new results will probably hit the arXiv preprint server within days, and many more are expected to follow over the coming weeks and months.

“Almost every field is benefitting from this mission,” says Helmi. “Gaia is transformational. It is revolutionizing astronomy.”

Explore Gaia data using the Gaia Sky software.




Image of Andrew James

Andrew James

December 3, 2020 at 5:34 pm

Gosh! It is hard to keep up!
Soon astronomers will become so specialised, they will have trouble communicating their work to each other.

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