Key Ideas

The Main-Sequence Lifetime: The duration of a star’s main-sequence lifetime depends on the amount of hydrogen available to be consumed in the star’s core and the rate at which this hydrogen is consumed.

• The more massive a star, the shorter is its main-sequence lifetime. The Sun has been a main-sequence star for about 4.56 billion years and should remain one for about another 7 billion years.
• During a star’s main-sequence lifetime, the star expands somewhat and undergoes a modest increase in luminosity.
• If a star’s mass is greater than about 0.4 M_⊙, only the hydrogen present in the core can undergo thermonuclear fusion during the star’s main-sequence lifetime. If the star is a red dwarf with a mass less than about 0.4 M_⊙, over time convection brings all of the star’s hydrogen to the core where it can undergo fusion.

Becoming a Red Giant: Core hydrogen fusion ceases when the hydrogen has been exhausted in the core of a main-sequence star with mass greater than about 0.4 M_⊙. This leaves a core of nearly pure helium surrounded by a shell through which hydrogen fusion works its way outward in the star. The core shrinks and becomes hotter, while the star’s outer layers expand and cool. The result is a red giant star.

• As a star becomes a red giant, its evolutionary track moves rapidly from the main sequence to the red-giant region of the H-R diagram. The more massive the star, the more rapidly this evolution takes place.

Helium Fusion: When the central temperature of a red giant reaches about 100 million K, helium fusion begins in the core. This process, also called the triple alpha process, converts helium to carbon and oxygen.

• In a more massive red giant, helium fusion begins gradually; in a less massive red giant, it begins suddenly, in a process called the helium flash.
• After the helium flash, a low-mass star moves quickly from the red-giant region of the H-R diagram to the horizontal branch.

Star Clusters and Stellar Populations: The age of a star cluster can be estimated by plotting its stars on an H-R diagram.

• The cluster’s age is equal to the age of the main-sequence stars at the turnoff point (the upper end of the remaining main sequence).

• As a cluster ages, the main sequence is “eaten away” from the upper left as stars of progressively smaller mass evolve into red giants.
• Relatively young Population I stars are metal rich; ancient Population II stars are metal poor. The metals (heavy elements) in Population I stars were manufactured by thermonuclear reactions in an earlier generation of Population II stars, then ejected into space and incorporated into a later stellar generation.

Pulsating Variable Stars: When a star’s evolutionary track carries it through a region in the H-R diagram called the instability strip, the star becomes unstable and begins to pulsate.

• Cepheid variables are high-mass pulsating variables. There is a direct relationship between their periods of pulsation and their luminosities.
• RR Lyrae variables are low-mass, metal-poor pulsating variables with short periods.
• Long-period variable stars also pulsate but in a fashion that is less well understood.

Close Binary Systems: Mass transfer in a close binary system occurs when one star in a close binary overflows its Roche lobe. Gas flowing from one star to the other passes across the inner Lagrangian point. This mass transfer can affect the evolutionary history of the stars that make up the binary system.