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Summary of Key Ideas

Protostars and Pre–Main-Sequence Stars

Enormous, cold clouds of gas and dust, called giant molecular clouds, are scattered about the disk of the Galaxy.
Star formation begins when gravitational attraction causes clumps of gas and dust, called protostars, to coalesce in Bok globules within a giant molecular cloud. As a protostar contracts, its matter begins to heat and glow. When the contraction slows down, the protostar becomes a pre–main-sequence star. When the pre–main-sequence star’s core temperature becomes high enough to begin hydrogen fusion and stop contracting, it becomes a main-sequence star.
The most massive pre–main-sequence stars take the shortest time to become main-sequence stars (O and B stars), while the least massive pre-main sequence stars take the longest time.
In the final stages of pre–main-sequence contraction, when hydrogen fusion is about to begin in the core, the pre–main-sequence star may undergo vigorous chromospheric activity that ejects large amounts of matter into space. G, K, and M stars at this stage are called T Tauri stars.
A collection of a few hundred or a few thousand newborn stars formed in the plane of the Galaxy is called an open cluster. Stars escape from open clusters, most of which eventually dissipate.

Main-Sequence and Giant Stars

The Sun has been a main-sequence star for 4.6 billion years and will remain so for about another 5 billion years. Less massive stars than the Sun evolve more slowly and have longer main-sequence lifetimes. More massive stars than the Sun evolve more rapidly and have shorter main-sequence lifetimes.
Main-sequence stars with mass between 0.08 M_⊙ and 0.4 M_⊙ convert all of their mass into helium and then stop fusing. Their lifetimes last hundreds of billions of years, so none of these stars has yet left the main sequence.
Core hydrogen fusion ceases when hydrogen in the core of a main-sequence star with M > 0.4 M_⊙ is gone, leaving a core of nearly pure helium surrounded by a shell where hydrogen fusion continues. Hydrogen shell fusion adds more helium to the star’s core, which contracts and becomes hotter. The outer atmosphere expands considerably, and the star becomes a giant.
When the central temperature of a giant reaches about 100 million K, the thermonuclear process of helium fusion begins. This process converts helium to carbon, then to oxygen. In a massive giant, helium fusion begins gradually. In a less massive giant, it begins suddenly in a process called the helium flash.
Giants undergo extensive mass loss, sometimes producing shells of ejected material that surround the entire star.
Relatively young stars are metal-rich (Population I); ancient stars are metal-poor (Population II).

Clusters of Stars

Groups of between a few hundred and a few thousand stars, formed together from a single interstellar cloud in the disk of our Galaxy, are called open clusters.
Stars in open clusters go their separate ways.
Groups of hundreds of thousands to millions of stars formed together from a common interstellar cloud are called globular clusters.
Stars in globular clusters remain bound together.
The age of an open or globular star cluster can be estimated by plotting its stars on an H-R diagram. The upper portion of the main sequence disappears first, because more massive main-sequence stars become giants before low-mass stars do. Therefore, the age of the stars now leaving the main sequence is the cluster’s age.

Variable Stars

When a star’s evolutionary track carries it through a region called the instability strip in the H-R diagram, the star becomes unstable and begins to pulsate.
RR Lyrae variables are low-mass, pulsating variables with short periods. Cepheid variables are higher-mass, pulsating variables exhibiting a regular relationship between the period of pulsation and luminosity.
Mass can be transferred from one star to another in close binary systems. When this occurs, the evolutionary paths of the two stars change.

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? What is 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.