The Milky Way probably first formed in much the same way as our solar system. An enormous cloud of gas—mostly hydrogen—began to collapse under its own gravity 10 to 15 billion years ago. In the more densely packed central region of this cloud, the first stars turned on. Today, these are the old stars in globular clusters found in the galactic bulge and the halo.

The gravity of stars and gas and dust attracted more stars and gas and dust, a rotational direction emerged from the twist of the cloud, and the central region began to flatten out into the galactic disk.

That galactic disk became the new region of star formation, drawing raw material in and leaving a vacuum in the halo so that few new stars could form there. Today, we see only older stars and globular clusters in the halo.

By the time the Milky Way started to coalesce as a galaxy, about twelve billion years ago, many stars had already grown old enough to exhaust their supplies of hydrogen fuel, and the very largest began to die the spectacular deaths we call supernovas.

But in every death there is birth, and the supernovas explosively seeded their surroundings with new elements they'd cooked up deep within their fusion furnaces. Each new generation of stars came together out of a richer interstellar medium. And as new stars spawned planets, these worlds could be more and more complex.

One by-product of this process is us.