There are probably 200 billion stars in the Milky Way, stretched through space in a disk shaped like a ninja throwing star. It is so big that it would take you another 100,000 years to traverse it at the speed of light. But if you could find the ideal point in space to stare at these stars around the clock, say eight years, track their movements and study their luminosity with very precise astronomical tools, you’d have a pretty good moving, living map of the galaxy.
Since 2013, the European Space Agency Gaia probe has done just that. The latest result of the mission, Data Release 3, which came out two weeks ago, maps 1.8 billion stars in and around our galaxy — covering about 1 or 2 percent of all stellar objects in the Milky Way. It’s the most comprehensive star map ever created by mankind, and scientists are already using it to uncover new secrets about our galactic neighborhood.
“Like a study of stars in our galaxy, it blows all other studies out of the water,” says Conny Aertsa stellar astrophysicist at the Catholic University of Leuven and member of the Gaia consortium.
The Gaia mission was launched in 2013, but its history goes much deeper. Its predecessor, the Hipparcos mission, was launched in 1989 to measure the positions, distances and motions of stars with unprecedented precision — a field called “astrometry,” which pioneered the mission in space. Precision astrometry of the entire sky is difficult on Earth; before Hipparcos was launched, there were fewer than 9,000 accurate “parallax” measurements of stars. (Parallax means that as the Earth moves, nearby stars in the sky appear to shift, just as a lamppost appears to shift relative to the hills in the background when you cross the street. The degree of shift indicates how far away objects are. .) Hipparcos increased the number of those measurements to 120,000 by the end of the mission in 1993.
“But we knew we could do better even while Hipparcos was working,” says Anthony Brown, an astronomer at the University of Leiden and the leader of Gaia’s data processing team. Gaia, a nearly $1 billion mission, was approved in 2000 as an upgrade, with two much larger 1.5-meter telescopes and 106 charge-coupled devices, or CCDs, sensitive photon detectors. (In that respect, this instrumentation is relatively similar to that of the Hubble Space Telescope.) But unlike Hubble, which has a series of heavy instruments designed to focus its gaze on small regions of space, Gaia’s mission is extensive: to research. all over the sky and collect huge amounts of data.
“Our problem with understanding the Milky Way is that we’re in it,” says Timo Prustic, a star astronomer for the ESA and project scientist on the Gaia mission. “Suppose you want to know what shape a forest has. When you are dropped in that forest, you see a lot of trees, but no shape, because you are in the forest itself.”
In 2014, Gaia arrived at the second Lagrange point, an ideal, quiet place to gaze at the Milky Way. Then the craft, somewhat shaped like a top hat with a shiny brim, began to look.
