Visual Timeline of the Universe

The largest inhabitants of galaxies are giant clouds of molecules that contain the raw material for stars and planets. A cloud with a diameter of 300 light years (1 light year is equal to about 10 trillion kilometers) contains enough mass to manufacture 10,000 to a million stars, each with the mass of our Sun. However, only about 10 percent of the cloud will be in clumps dense enough for stars to form -- enough to produce a few hundred to a few thousand new stars. Giant molecular clouds last for 10 to 100 million years before they dissipate.