We live in a great galaxy, one that’s far larger and brighter than most others in the cosmos. Abounding with countless stars and surrounded by dozens of satellite galaxies, the Milky Way is a giant barred spiral with a dark matter halo spanning some 2 million light-years. Our galaxy’s disk is spinning so fast that we race along at more than 800,000 kilometres per hour as the Solar System speeds around the galactic centre.
But how did this vast entity arise?
Remarkable new discoveries, enabled by the Gaia spacecraft’s precise measurements of star positions and velocities, are providing incredible insights into the Milky Way’s earliest epochs. These observations reach all the way back to our galaxy’s initial chaotic state, when the Milky Way was smaller and just starting to assemble itself. Astronomers are now discovering when the galaxy began to spin, when its disk started to spawn stars, and when its stellar halo formed — all thanks to these new observations of its oldest stars.
Meet the protogalaxy
Early on, more than 13 billion years ago, our galaxy was hardly the well-ordered system it is today. “It’s the time when the Milky Way was not the Milky Way — it was not a disk galaxy yet,” says Vasily Belokurov (University of Cambridge). “It was probably extremely lumpy, extremely incoherent, and kind of amorphous.” The early galaxy also only had a fraction of its current mass.
Many of those early Milky Way stars still shine now. As reported in the July 2022 , Belokurov and Andrey Kravtsov (University of Chicago) identified the stars by their chemical compositions. The astronomers knew that the first stars born in the galaxy should have little iron, because the Big Bang that created the universe 13.8 billion years ago made none of the metal; those early stars would have formed from near-pristine
