Evolutionary Track
When a main-sequence star has consumed the hydrogen at its core, the loss of energy generation causes its gravitational collapse to resume and the star evolves off the main sequence. The path which the star follows across the HR diagram is called an evolutionary track.
White Dwarf
If the star has a mass less than 0.23Mʘ, it is predicted to directly become a white dwarf when the energy generation by nuclear fusion of hydrogen at its core comes to a halt.
Massive Stars
In stars more massive than 0.23Mʘ, the hydrogen surrounding the helium core reaches sufficient temperature and pressure to undergo fusion, forming a hydrogen-burning shell and causing the outer layers of the star to expand and cool.
The stage as these stars move away from the main sequence is known as the subgiant branch which is relatively brief and appears as a gap in the evolutionary track since few stars are observed at that point.
Red Giants
When the helium core of low-mass stars becomes degenerate, or the outer layers of intermediate-mass stars cool sufficiently to become opaque, their hydrogen shells increase in temperature and the stars start to become more luminous.
This is known as the red giant branch, which is a relatively long-lived stage, and it appears prominently in H-R diagrams. These stars will eventually end their lives as white dwarfs.
You might want to see Solar system: Some Quick facts you need to know.
Supergiants
The most massive stars do not become red giants, instead, their cores quickly become hot enough to fuse helium and eventually heavier elements and they are known as supergiants. They follow approximately horizontal evolutionary tracks from the main sequence across the top of the H-R diagram. Supergiants are relatively rare and do not show prominently on most H-R diagrams.
Their cores will eventually collapse, usually leading to a supernova and leaving behind either a neutron star or black hole.
You might be interested in Voyager Probes: Top 10 Amazing Facts To Make You Awestruck.
Gravitational Collapse
Gravitational collapse occurs when an object’s internal pressure is insufficient to resist the object’s own gravity. For stars, this usually occurs either because a star has too little “fuel” left to maintain its temperature through stellar nucleosynthesis, or because a star that would have been stable receives extra matter in a way that does not raise its core temperature.
In either case, the star’s temperature is no longer high enough to prevent it from collapsing under its own weight.
The collapse may be stopped by the degeneracy pressure of the star’s constituents, allowing the condensation of matter into an exotic denser state. The result is one of the various types of compact stars. Which type forms depends on the mass of the remnant of the original star left after the outer layers have been blown away. Such explosions and pulsations lead to planetary nebula.
This mass can be substantially less than the original star. Remnants exceeding 5Mʘ are produced by stars that were over 20Mʘ before the collapse.
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Tolman-Oppenheimer-Volkoff limit
If the mass of the remnant exceeds about 3-4Mʘ (Tolman-Oppenheimer-Volkoff limit), either because the original star was very heavy or because the remnant collected additional mass through accretion of matter, even the degeneracy pressure of neutrons is insufficient to stop the collapse.
No known mechanism is powerful enough to stop the implosion and the object will inevitably collapse to form a black hole.
Also see Betelgeuse is Surprisingly Smaller, Closer to Us.
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