Best map of Milky Way reveals a billion stars in motion

The best available map of the Milky Way just got better. The latest update from the Gaia space observatory which is tracking more than 1 billion stars in the Galaxy provides not just a static image, but a moving picture of how stars will shift over time. The data will underpin studies that range from the origins and evolution of the Galaxy to locating its dark matter.

"I am yet to see another project in astronomy or any science that has had such an impact on such a short timescale," says Amina Helmi, an astronomer at the University of Groningen in the Netherlands. "My group is ready to go and very excited to find out what is there to discover and learn about the Milky Way." Using data that Gaia released in 2018, Helmi and her collaborators have studied the motions of large numbers of stars to reveal evidence of galactic mergers that happened billions of years in the past.

Gaia lifted off in late 2013, and began observing stars in July 2014 from a perch 1.5 million kilometres from Earth. The European Space Agency (ESA) probe continuously scans the sky as it slowly spins on itself, and it has now measured the positions of the same stars multiple times. This enables scientists to track stars' nearly imperceptible motions across the Galaxy year after year. As Gaia orbits the Sun, its changing perspective also makes the stars' apparent position change by tiny amounts typically 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 the bread and butter of the field. Without a reliable distance measurement, in particular, it can be difficult to guess a star's size, age and brightness, and therefore to model its structure and evolution.

Researchers have pored over the mission's two previous data sets, released in 2016 and 2018. Those are now being cited in the literature at a rate of 3,000 times per year, according to Floor van Leeuwen, an astronomer at the University of Cambridge, UK. One website has catalogued 4,324 refereed papers based on Gaia data so far. "You can see the influence of Gaia data spreading through all of astronomy," he says.

After the latest update was released on 3 December, astronomers began tweeting about checks they had done on their favourite stars. "It's like an early Christmas for Galactic Astronomers," tweeted Michelle Collins of the University of Surrey, UK. Joao Alves of the University of Vienna posted plots of the same group of stars to compare the latest Gaia data set with the previous one, thanking ESA "and the 400 scientists in Europe that make this mission a dream come true".

Data dump

Gaia's latest update consists of 1.3 terabytes versus the 551 gigabytes of the previous one and is based on around three years of data. The mission has expanded its catalogue of stars by 15%, to 1.8 billion, and its measurements have become more precise. Compared with 2018, Gaia's distance measurements are 50% better, and those of stellar velocities are 100% better, van Leeuwen says.

To achieve this improvement, the mission team had to overcome an unexpected issue with the probe. As the spacecraft spins, sunlight falls on it at varying angles, which slightly deforms its shape. This has affected its measurements of stellar positions more than expected. But the team has now learnt how to correct for this effect, at least in part, says van Leeuwen. This means that for stars up to about 5,000 parsecs (16,000 light years) of the Solar System, it can measure distances with an accuracy of up to 10%. By the time the mission concludes, the team expects to achieve this level of accuracy up to distances of 10,000 parsecs, which was its original plan (see 'Gaia's gold').

The data release includes a complete census of the Sun's neighbourhood: all but the faintest stars within 100 parsecs (326 light years), totalling more than 300,000 objects. Gaia's detailed measurement of stellar motions has also enabled the researchers to predict what Earth's night sky will look like for 1.6 million years to come: as stars move around, all the constellations we currently see will eventually disappear.

In addition to stars, Gaia also maps quasars, the fiery hearts of other galaxies much farther away. Quasars are much too distant to show any parallax, and they appear essentially immobile which makes them ideal points of reference for tracking the motions of other things, including the tectonic plates on Earth. But because of an optical effect of relativity, the sky appears slightly deformed in the direction of the Solar System's motion in the Milky Way. Now Gaia has measured how that direction changes slightly, as a result of the Galaxy's gravitational attraction: over a year, the Solar System accelerates by 7 millimetres per second.

Denis Erkal, an astronomer at the University of Surrey, UK, quickly used the data about the Solar System acceleration to rule out the presence of massive clouds of dark matter in nearby space. The plot he tweeted provides only a rough calculation, but it already hints at studies that might become feasible as the mission gathers more data.

A more-complete data set is due to be published in 2022, and will include updated stellar spectra. It should also show thousands of stars wobbling under the gravitational pull of another object, providing a new tool for discovering thousands of massive exoplanets. After that, Gaia's team expects to produce at least one more vastly improved map of the Galaxy. The probe has enough fuel to keep operating until 2025.

Source: Nature