This is Not a Photo of the Milky Way. It’s the Map of 1.8 Billion Stars From Gaia’s Major New Data Release

In 2013, the European Space Agency (ESA) deployed the Gaia mission to space, a next-generation observatory that will spend the next five years gathering data on the positions, distances, and proper motions of stars. The resulting data will be used to construct the largest 3D space catalog ever, totaling 1 billion stars, planets, comets, asteroids, quasars, and other celestial objects.

Since the mission began, the ESA has issued three early releases of Gaia data, each of which has led to new research findings and more detailed maps of our galaxy. Based on the third release of mission data, known as Early Data Release 3 (Gaia EDR3), astronomers have created a map of the entire sky that includes updated data on celestial objects and manages to capture the total brightness and color of stars in our galaxy.

The EDR3 was made public on December 3rd, 2020, and includes data on the position and brightness of more than 1.8 billion stars, the parallax and proper motion of close to 1.5 billion stars, and the color of more than 1.5 billion stars. It also includes data on more than 1.6 million extra-galactic sources of light, including stars, globular clusters, and more distant galaxies.
Density map of the galaxy based on EDR3 without added color. Credit: ESA/Gaia/DPAC/A. Moitinho and M. Barros

New Maps

Compared to the previous release (Gaia DR2) that was released in April of 2018, this represents an increase of more than 100 million sources. In addition, the latest release included improvements in the general accuracy and precision measurements. With this updated data, astronomers were able to create a map that shows not only the brightness, but also the density of our galaxy.

Whereas the brighter regions correspond to denser concentrations of bright stars, the darker regions are parts of the sky where fewer and fainter stars are located. Across the galactic plane, there are dark patches created by foreground clouds of interstellar gas and dust that absorbs light from more distant stars. The bright horizontal structure corresponds with the flattened disk of the Milky Way (aka. the plane of the galaxy) viewed edge-on.

Many of these are clouds that conceal stellar nurseries, diffuse nebula in the interstellar medium where new stars are being born. Across the galactic plane, there are regions of dark patches, which correspond to foreground clouds of interstellar gas and dust absorbing light from more distant stars. And of course, there’s the bright “bulge” at the middle is the dense concentration of stars and gas that is the Galactic center.

Then there are the many globular and open clusters that appear as bright spots dotted across the image, some of which are galaxies beyond our own. The two bright objects in the lower right of the image are the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), the two dwarf galaxies that orbit the Milky Way (and are expected to merge with it in a few billion years).

The color of the stars was reproduced by combining the total amount of light gathered by Gaia with all the blue and red light recorded from each patch of the sky. This is another improvement that the EDR3 offers, which is the presence of color information for around 1.5 billion sources (200 million more than DR2).

Galactic Collisions

Another breakthrough to come from this latest data release was the way it allowed astronomers to trace the populations of older and younger stars all the way out to the galactic anticenter (the very edge of the galaxy). This allowed astronomers to determine how past mergers affected the structure of the Milky Way disc, and to create computer models that predicted how it will grow larger with time.

The data showed that in the outer regions of the disc there is a slow-moving component of stars above the plane heading downwards and a fast-moving component below the plane moving upwards. This pattern came as a complete surprise to astronomers and has strengthened the case for a near-collision between the Milky Way and the Sagittarius Dwarf Galaxy in the recent past.

This dwarf galaxy, which contains a few tens of millions of stars, is located about 70,000 light years from Earth and orbits the Milky Way around its poles. This satellite galaxy is currently being cannibalized by the Milky Way, a process that has brought it close to our galaxy a few times in the past. With every pass, the gravitational influence of this galaxy has been enough to perturb some stars in our galaxy’s disc.
Gaia’s stellar motion for the next 400 thousand years. Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO./A. Brown, S. Jordan, T. Roegiers, X. Luria, E. Masana, T. Prusti and A. Moitinho

The ESA’s Data Processing and Analysis Consortium (DPAC), a pan-European team of expert scientists and software developers, were already able to detect a subtle ripple in the Milky Way. They attributed this to past collisions between Sagittarius and the Milky Way between 300 and 900 million years ago. This latest data bolsters the case for this based on the movement of stars in the galaxy’s disc.

Teresa Antoja, a Marie Curie Sk?odowska Fellow with the Institute of Cosmos Sciences at the University of Barcelona, worked on this analysis with DPAC colleagues. “The patterns of movement in the disc stars are different to what we used to believe,” she said in a recent ESA press release. “It could be a good candidate for all these disturbances, as some simulations from other authors show.”

The Motion of Stars

Additionally, researchers from the University of Helsinki created an animation using the proper motions of 40,000 randomly-selected stars over the next 1.6 million years. As it progresses, the stars appear to be moving from the left side of the galaxy and collecting at the right, which is result of the motion of the Solar System. Similarly, the apparent motions of quasars helped constrain the absolute motion of the Solar System.

“The knowledge accrued by Gaia affects the precision of satellite navigation in the future,” Said Professor Markku Poutanen of the Finnish Geospatial Research Institute (FGI). “The satellite positions and Earth orientation in space are determined in a reference frame tied to the directions of quasars. The precision and state of the art of the reference frame are critical for the precision in navigation.”

As Prof. Karri Muinonen of the University of Helsinki and a Research Professor with the FGI explained:

“In the animation, short and long trails describe changes in stellar positions with 80 000 years. The former are mostly related to distant stars, whereas the latter are solely due to the nearby stars. Every now and then, short trails expand into long ones, and long trails shrink into short ones. This is also related to the changing distances of the stars.”

“This shows the average motion of the Solar System with respect to the surrounding stars. From the Finnish point of view, it is intriguing that the motion documented by Gaia agrees with the pioneering research about the Solar System’s motion by Friedrich Wilhelm August Argelander (1799-1875) in the 19th century at the Helsinki Observatory.”

Argelander was a member of the University of Helsinki’s Observatory, which was known as the Imperial Alexander University at the time. While was the first astronomer to calculate the motion and direction of Solar System around the center of the Milky Way. These observations were made while Argelander was at the Turku Observatory from 1827 to 1831, along with the precise positions of 560 stars.

Further research performed by DPAC measured how the motion of the Solar System is accelerating over time. Using the observed motion of extremely distant galaxies, they estimated that the Solar System is accelerating at a rate of 0.23 nm/s2 (relative to the rest frame of the Universe), which adds up to an increase of around 115 km a year.

Star Census

The EDR3 data also allowed a new census of stars to be conducted, known as the Gaia Catalogue of Nearby Stars, which contains 331 312 objects (an estimated 92% of the stars) within 326 light-years of the Solar System. This is the first census to be obtained since the Gliese Catalogue of Nearby Stars was compiled in 1957. Initially, it contained just 915 objects, updated to 3,803 objects in 1991, and was limited to a distance of 82 light years.

In other words, the new Gaia’s census contains 100 times as many objects at four times the distance. It also provides location, motion, and brightness measurements that are orders of magnitude more precise than the Gliese catalog. As Muinonen described the data-mining process:

“We are responsible for the daily computation of orbits for asteroids discovered by Gaia. Based on these computations, ground-based follow-up observations are organized. Before data releases, we take part in the validation of Gaia observations of asteroid positions, brightnesses, and spectra.

Our research with Gaia data focuses on asteroid orbits, rotation periods and pole orientations, masses, shapes, and surface structural and compositional properties. In the computation of collision probabilities for near-Earth asteroids, the precision of reference frames is completely central.”

As Timo Prusti, the Gaia Project Scientist, added:

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. And we’re not done yet; more great data will follow as Gaia continues to make measurements from orbit.

These and other new insights are just the latest breakthroughs to come from the data Gaia has accumulated over the past seven years. EDR3 is the first of a two-part release, which will be followed by the issuing of the full Data Release 3 (DR3). Due to the pandemic, the date has been pushed back but is currently expected to take place by the first half of 2022.

On October 1st, at a meeting of the ESA’s Science Programme Committee (SPC), the Gaia mission was extended yet again – this time to Dec. 31st, 2022. Pending a mid-term review and confirmation by the SPC, the mission may even be extended to 2025. With all the data it’s providing, it is clear that the ESA wants to keep this mission going!

Further Reading: ESA, ESA (2), University of Helsinki