200 billion stars in the Milky Way, stretched across space in a disk shaped like a ninja’s throwing star. It’s so big that, traveling at the speed of light, it’d still take you 100,000 years to traverse it. But if you could find the ideal point in space to stare at these stars around the clock for, say, eight years, tracking their movements and studying their brightness with highly accurate astronomy tools, you’d have created a pretty good moving, living map of the galaxy.
Every six hours, with its back pointing toward our sun, Gaia scans a great circle of the sky, spinning at a steady, slow rate and taking in tiny pinpricks of light from distant stars. That light is captured by its two telescopes, CCDs, photometers, and a spectrometer to measure each star’s position, motion, distance, radial velocity, brightness, and color—details that can reveal everything from a star’s mass to its makeup.
Gaia’s second release in 2018 jumped to 1.6 billion objects, with 1.3 billion measurements of parallax distance and proper motion. It also collected the accurate brightnesses and colors of these stars. This allowed scientists to better understand each star’s temperature, luminosity, and more. The mission also collected the radial velocity of stars—which combined with “proper motion” data shows where each one is going and how fast—for 7 million objects.