One way is connected with the death of the stars themselves. Some of the stars in a given hydrogen cloud are more massive than the rest. These giant stars consume their hydrogen fuel voraciously, spouting enormous quantities of energy, living large, and dying quickly. At the end, they explode with such force that they rip themselves apart and briefly outshine the rest of the 100 billion stars in the galaxy where they reside. We need another cause for stellar formation, however.
Galaxies seem to go through a massive burst of star creation early in their lives.
The relatively infrequent supernovas in galaxies cannot possibly account for the billions of stars created in each galaxy. The answer comes when we consider the early life of the universe. Everything began in a cataclysmic explosion called the Big Bang. Early in the explosion, hydrogen gas swirled together to form hundreds of billions of spinning gaseous disks, called proto-galaxies, which must have had a few lumps in them. These lumps formed a few stars.