Helium and hydrogen found in the sun make up most of the ISM. Hydrogen clouds in the ISM can cool, collapse under the force of gravitation and eventually form new stars. If this gas did not exist, no new stars could form. The M33 galaxy is a classic example of a massive stellar nursery. About 3 million years ago, a giant gas cloud in this galaxy began collapsing and forming stars that the Hubble telescope can see.