With its second data release, the European Space Agency's Gaia satellite has mapped 1.7 billion stars and redefined the way we look at our galaxy.
How many stars are out there – and how far are they from us? We'll probably never know the whole picture, but we now have a much clearer idea for our cosmic neighborhood. On April 25th, European astronomers published the most extensive and precise star catalog ever made.
Containing data from 22 months of observation by their Gaia satellite, the second data release (dubbed DR2) consists of precise parallaxes (and thus distances) for more than 1.3 billion stars in our local part of the Milky Way, as well as positions and brightnesses of almost 1.7 billion stars total. That's a huge leap compared to the mission’s first data release in 2016, which contained 2 million stellar distances. For the first time, distances rely solely on Gaia's own measurements – in DR1 they had to be augmented by data from the 1990s-era Hipparcos satellite. However, even this large number — 1.7 billion stars — makes up only slightly more than 1% of all the stars in our galaxy.
The Purpose of Parallax
Stars move throughout the year due to the parallax caused by Earth's movement around the Sun. During the year we see stars along slightly different viewing angles, causing them to perform a minute elliptical movement around their mean position. Parallax provides a way to measure stars’ distances through pure geometry, as long as astronomers know the distance between the Sun and Earth. But parallaxes are typically much smaller than an arcsecond — the farther the star, the smaller its parallax. Because atmospheric turbulence also causes stars to “wobble,” parallaxes of stars more than a few dozens of light-years away are best measured from space.
Even from space, though, astronomers must still watch out for stars’ real motions. While to the unaided eye, the stars may appear fixed in the sky, they actually move over time. (Very few stars reveal their proper motion to amateurs; Barnard's star is one famous exception.) A star’s motion across the sky — termed proper motion — along with its movement toward or away from Earth — called radial velocity — must be taken into account when measuring parallaxes and can be useful in their own right for determining stellar orbits.
Gaia: The Ins & Outs
Gaia is the finest instrument ever built for the purpose of measuring stellar parallax. After its launch on a Soyuz rocket from French Guiana in December 2013, it headed off to Lagrangian point L2, 1.5 million kilometers farther from the Sun than Earth. The satellite spins continuously around its axis, while its two telescopes scan great circles on the sky, observing about 100,000 stars every minute.
The optics feed three instruments: one for astrometry (to determine stars’ positions and motions), one for photometry (to measure the stars’ colors), and one for spectroscopy (to measure stars’ radial velocity, and to find out their composition). The heart of the instrument is a CCD with 848 million pixels, the largest digital camera ever used in space.
The Second Data Release
During its orbit around the Sun, Gaia covers the entire sky, measuring each star over and over again. In the DR2 positions and parallaxes are measured with unprecedented 0.00002-arcsecond accuracy for brighter stars (0.00007-arcsecond accuracy for proper motions), and 0.002-arcsecond accuracy for the faintest stars in the surveyed population (about 21 magnitudes in Gaia’s G band, between 330 and 1050 nm). Thanks to spectrometry Gaia's DR2 also contains estimates of the effective temperature, radius, and luminosity of 76 million stars, as well as time-dependent measurements about half a million variable stars – one of the largest available catalogs of variables.
Closer to Earth, Gaia observed about 14,000 known solar system objects, most of them asteroids, providing measurements that enable a more precise calculation of their orbits. On the other end of the distance scale, Gaia’s database also contains positions and brightness for more than 500,000 quasars. These extragalactic objects are so far away that their parallaxes are practically zero, making them useful references for the celestial coordinate system that’s used to define the positions of all other objects in the Gaia catalog.
Although most scientific discoveries are expected once astronomers start exploring Gaia’s newest release, the consortium has already used it to validate the catalog’s quality and found some promising surprises along the way.
For example, Gaia scientists built the most detailed Hertzsprung-Russell diagram of stars ever made and derived the orbits of 75 globular clusters and 12 dwarf galaxies revolving around the Milky Way. Gaia's improved stellar positions also aid the navigation of NASA’s New Horizons spacecraft, which, after its famous flyby of Pluto in 2015, is now heading for its next flyby in the Kuiper Belt on New Year’s Day.
The DR2 isn’t the end of the road for Gaia. There will be more data releases in the next couple of years, and the final Gaia catalog is scheduled to be published in late 2022. This will then be the definitive stellar catalog for the foreseeable future, playing a pivotal role in many fields of astronomy. It will take generations to fully exploit its treasures.
View an animated 3D view of the sky created using Gaia's DR2. The animation starts with the Orion constellation at the center; we then move towards the neighboring Taurus constellation and to the Hyades star cluster. The animation first shows the 3D structure of the cluster, then an animated view of the future motions of stars is shown – both in Hyades and beyond — based on the stars' current proper motions.