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Off the main sequence line are stars that are dying or dead. Dying stars are above the main sequence line, and dead (but persistent) stars are below. Above the line we see giant and supergiant stars: these are stars that are much brighter than they should be for their spectral class, because they have swelled in size in their dying phase. Betegeuse is a good example: even though it is in the spectral class "K" or "M", it is as bright as the main sequence "B" star Beta Centauri. This is because betegeuse is 1000 times greater in diameter than the Sun, while Beta Centauri is "only" 10 times greater in diameter than the Sun.
Stars below the main sequence line are dead stars called white dwarfs. When a star has a mass just before fusion ceases (a corpse mass) of less than 1.4 solar masses (the Chandrasekhar limit), it does not explode, but instead first becomes a giant, and then shrinks into a white hot cinder which we call a white dwarf. Stars that do explode when they cease fusion are not shown on the H-R diagram, because they do not give off significant (or any) light: they have essentially zero luminosity.
It is hypothesized that white dwarfs will eventually cool to the point of having no luminosity. At that time, they become black dwarfs. Since the time for a white dwarf to become a black dwarf is longer than the age of the universe, no black dwarf has ever been seen.
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Notice also the other diagonal lines ascending from right to left that show a relationship between the diameter of a star and its luminosity, for a particular spectral class.
Also on the main sequence line, in purple, we see the typical mass of a star for its spectral class: "O" stars typically have 30 solar masses, "G" stars, like our Sun, one solar mass, and "M" stars about 0.2 solar masses.
On the main sequence line, in green, we see the typical main sequence lifetimes of stars for a particular spectral classification. Orange or "K" stars can last 100 billion years, whereas blue or "O" stars last as little as 10 million years.
Stars on the diagonal line ascending from right to left are the main sequence stars. These are stars undergoing normal core fusion.
Notice on the horizontal axis that both surface temperature and spectral classification are shown. One can clearly see that a 'B" star has a surface temperature of about 15,000 to 18,000 K, and that our Sun, a "G" star, runs a surface temperature of just below 6000 K.
The horizontal axis shows the increase in surface temperature of stars.
The vertical axis shows the increase in total power output from stars. This is called luminosity.
Most all of the important defining characteristics of stars are incorporated in the Hertzsprung-Russell or H-R diagram shown to the left. Click on it to enlarge it further.
The H-R diagram is not a standard graph: for historical reasons, the horizontal axis' values increase from right to left, instead of the more common left to right.
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