OUTSIDE THE MILKY WAY

When you're surveying between 1 and 2 billion stars, you can't help bagging numerous galaxies and quasars, too. Quasars are the luminous cores of remote galaxies, powered by supermassive black holes. They are found all over the sky, and since they don't exhibit parallax or proper motion, Gaia uses a selection of 1.6 million quasars as a reference frame — a coordinate system against which it measures stellar positions. But of course, quasars are interesting by themselves just like the many galaxies studied by Gaia, DR3 includes two separate catalogues of extragalactic sources, containing 6.6 million quasar candidates and 4.8 million galaxy candidates, respectively. For millions of those objects, astronomers have also determined redshifts, providing the distances. “The quality and homogeneity of the data are exceptional,” says astronomer, Christine Ducourant. One particularly…