Milky Way’s best map reveals a billion moving stars

The best available map of the Milky Way just got even better. The latest update to Gaia’s Space Observatory – which tracks over a billion stars across the galaxy – provides not only a static picture, but a picture of how the stars will move over time. The data will support studies ranging from the origins and evolution of the galaxy to the location of its dark matter.

“I have yet to see another astronomical project – or any science – that has had such an impact in such a short period of time,” says Amina Helmi, an astronomer at the University of Groningen in the Netherlands. “My group is ready to go and very happy to discover what there is to discover and learn about the Milky Way.” Using data released by Gaia in 2018, Helmi and colleagues studied the motions of a large number of stars to reveal evidence of galactic mergers that occurred billions of years in the past.

Gaia took off in late 2013 and started stargazing in July 2014 from a perch 1.5 million kilometers from Earth. The European Space Agency (ESA) probe continuously scans the sky as it slowly spins on itself, and it has now measured the position of the same stars multiple times. This allows scientists to follow the almost imperceptible movements of stars across the galaxy year after year. As Gaia rotates around the Sun, her shifting perspective also causes the apparent position of the stars to change by tiny amounts – usually by an angle of millionths of a degree. These offsets can be used to calculate their distance from our solar system using a technique called parallax.

The type of information Gaia provides is bread and butter from the field. Without a reliable distance measurement, in particular, it can be difficult to guess the size, age and luminosity of a star, and therefore to model its structure and evolution.

The researchers looked at the mission’s two previous datasets, published in 2016 and 2018. These are now cited in the literature at a rate of 3,000 times a year, according to Floor van Leeuwen, an astronomer at the University. of Cambridge, UK. A website has cataloged 4,324 articles rated on Gaia’s data to date. “You can see the influence of Gaia’s data spreading throughout astronomy,” he says.

After the last update was released on December 3, astronomers began tweeting about the checks they’ve done on their favorite stars. “It’s like an early Christmas for galactic astronomers,” tweeted Michelle Collins from the University of Surrey, UK. João Alves from the University of Vienna published plots from the same group of stars to compare the latest Gaia dataset with the previous one, thanking ESA “and the 400 scientists in Europe who make this mission a dream come true”.

Data dump

Gaia’s latest update consists of 1.3 terabytes – up from 551 gigabytes of the previous one – and is based on roughly three years of data. The mission expanded its star catalog by 15%, to 1.8 billion, and its measurements became more accurate. Compared to 2018, Gaia’s distance measurements are 50% better and stellar velocities are 100% better, says van Leeuwen.

To achieve this improvement, the mission team had to overcome an unexpected problem with the probe. As the spaceship spins, sunlight falls on it at varying angles, slightly distorting its shape. This affected his measurements of stellar positions more than expected. But the team has now learned how to correct this effect at least in part, says van Leeuwen. This means that for stars up to nearly 5,000 parsecs (16,000 light years) from the solar system, it can measure distances with an accuracy of up to 10%. By the end of the mission, the team expects to achieve this level of accuracy up to distances of 10,000 parsecs, which was their original plan (see “Gaia’s Gold”).

The data release includes a comprehensive census of the Sun’s neighborhood: all but the weakest stars within 100 parsecs (326 light years), totaling more than 300,000 objects. Gaia’s detailed measurement of stellar movements also allowed researchers to predict what Earth’s night sky will look like for 1.6 million years to come: As the stars move, all of the constellations we currently see will eventually disappear.

In addition to stars, Gaia also maps quasars, the fiery hearts of other galaxies much further away. Quasars are far too far away to show parallax, and they appear to be essentially still – making them ideal reference points for tracking the movements of other things, including tectonic plates on Earth. But due to an optical effect of relativity, the sky appears slightly distorted in the direction of the solar system’s movement in the Milky Way. Now Gaia has measured how this direction changes slightly, due to the gravitational pull of the galaxy: over a year, the solar system accelerates by 7 millimeters per second.

Denis Erkal, an astronomer at the University of Surrey, UK, quickly used data on the acceleration of the solar system to rule out the presence of massive clouds of dark matter in near space. The plot he tweeted provides only a rough calculation, but it already hints at studies that may become feasible as the mission collects more data.

A more comprehensive data set is expected to be released in 2022 and will include updated stellar spectra. It is also expected to show thousands of stars flickering under the gravitational pull of another object, providing a new tool for discovering thousands of massive exoplanets. After that, the Gaia team plans to produce at least one even more vastly improved galaxy map. The probe has enough fuel to continue operating until 2025.