SUMMARY OF KEY IDEAS

Protostars and Pre–Main-Sequence Stars

Main-Sequence and Giant Stars

Clusters of Stars

Variable Stars

WHAT DID YOU THINK?

  • How do stars form? Each star forms from the collective gravitational attraction of a clump of gas and dust usually inside a giant molecular cloud.

  • Are stars still forming today? If so, where? Yes. Astronomers have seen stars that have just arrived on the main sequence, as well as infrared images of gas and dust clouds in the process of forming stars. Most stars in the Milky Way form in giant molecular clouds in the disk of the Galaxy.

  • Do more massive stars shine longer than less massive ones? Explain your reasoning. No. Lower-mass stars last longer because the lower gravitational force inside them causes fusion to take place at much slower rates compared to the fusion inside higher-mass stars. These latter stars therefore use up their fuel more rapidly than do lower-mass stars.

  • When stars like the Sun stop fusing hydrogen into helium in their cores, do the stars get smaller or larger? They get larger. Such stars start fusing hydrogen into helium outside their cores. This new fusion, closer to the star’s surface, is able to push the star’s outer layers out farther than they had been before.