STELLAR EVOLUTION

We have learned that lifetime of a star depends upon its mass. Well, the way a star dies depends upon its mass too.

Low mass stars die peacefully as a "planetary nebula" and then become a white dwarf, which is the left over core of the star

High mass stars die in huge explosions called "supernova", some form into neutron stars and others become black holes

EvolutionaryTracks

During each stage of the evolution, the star is adjusting its structure as it battles its own crushing weight under gravity. As the star adjusts, its surface temperature and luminosity change. Thus, where it falls on the HR diagram changes with time. The changes of its locations on the HR diagram are called evolutionary tracks.

LOW MASS STAR EVOLUTION

There are seven (7) stages in the evolution of a low mass star.

• Main Sequence:

  The star is happy fusing hydrogen into helium in its core.

• Red Giant:

  The star has built up enough helium in its core that hydrogen fusion is becoming less efficient. The star adjusts by heating up in its core. This causes the star to expand; its luminosity goes up and its surface temperature goes down (the star moves toward the upper right in the HR diagram). As time continues, the core becomes inert helium and so the hydrogen fusion must now occur in a shell surrounding the inert helium core.

• Helium Flash:

  The inert helium core grows large enough that helium can now begin to fuse into carbon and oxygen. The onset of core helium fusion is very quick (thus, the term "flash"). This is because the helium core was degenerate matter.

• Horizonatal Giant:

  Following the Helium Flash, the star settles down once again and is happy fusing helium into carbon and oxygen in its core. The star contracts and this causes its luminosity to go down and its surface temperature to rise (the star moves slightly downward and to the left on the HR diagram, but it still has a higher luminosity than when it was on the Main Sequence). At this time, it is ALSO fusing hydrogen into helium in a shell surrounding the core.

• Super Red Giant:

  As with the Red Giant stage, the build up of carbon and oxygen in the core makes helium fusion very innefficient. The core gets hotter, and this makes the star expand a lot. This, again, causes the star's luminosity to increase and its surface temperature to decrease. As time continues, the core becomes inert carbon and oxygen. The helium fusion migrates out to a shell surrounding the inert core. There is also an outer shell where hydrogen continues to fuse into helium.

• Planetary Nebula:

  The star never can get its core hot enough to fuse carbon and oxygen into heavier elements. The helium shell fusion is very temperature sensitive, and thus the star becomes unstable and goes through periods of rapid expansion and contraction (it becomes a variable star). With each expansion, outer layers of the star are ejected into space and this causes a nebula around the star. Each time more material is ejected, the remaining star is getting smaller, so we are seeing the inner layers of the star now. These inner layers are hotter, and so the star moves toward the left in the HR diagram.

• White Dwarf:

  Eventually, the outler layers are gone and all that is left is the inner core of the star. This inner core is the inert carbon and oxygen left over, called a stellar remnant. It is very hot and small, and thus it resides in the lower left corner of the HR diagram. The largest mass a white dwarf can have is 1.4 times the sun. Above this mass the matter becomes too compacted and the star will collapse into a neutron star or a black hole.