click for full image and caption

At the beginning of the universe, when the galaxies first appeared, there were no stars at all. The galaxies began as condensations of hot gas, and only after they had time to cool did the first stars begin to form. The densest galaxies worked quickly, turning all of their gas into stars when the universe was still quite young. Others, such as our own Milky Way, puttered along at a relatively modest rate of star formation. Indeed, our Galaxy has a lot of gas left, and is still forming stars today at the rate of about 10 new stars every year. These stars are formed in what might be called a stellar nursery: a vast cloud of gas and dust where hundreds or thousands of stars may be born within a few million years of each other. The authors describe how stars form in such large groups, and why some groups stay together as they age, whereas others disperse.