The Hubble Space Telescope found a galaxy that was completely formed within 480 million years of the Big Bang. How could gas and dust clouds condense into stars and organize themselves via gravity into an entire galaxy in less time than it takes the Milky Way to make two rotations?

Galaxies like the Milky Way indeed could not have formed as a unit at such an early era after the Big Bang. The first galactic objects were smaller and denser than present-day galaxies, and thus could collapse and pull in gas on timescales of tens, rather than hundreds, of millions of years. Once gas compressed inside such a protogalaxy, stars could begin to form ò?? an especially tricky step that is not fully understood ò?? and then galactic evolution could proceed quickly.

Astronomy magazine subscribers can read the full answer for free. Just make sure you're registered with the website.