You'll still run across these letters from time to time, but beginning in 1941 they were replaced by a more detailed scheme first published by William W. Morgan and Philip C. Keenan. Thus a designation such as G 2V, the Sun's spectral type, tells temperature and luminosity. This has been a fundamental astrophysical tool ever since it was invented around 1911. Most stars gather in certain narrow regions of the H-R diagram according to their masses and ages. Stars arrive on the main sequence soon after they are born, and this is where they spend most of their lives.
Massive stars blaze brightly on the hot, blue end of the main sequence.
They burn up their nuclear fuel in only millions or tens of millions of years. Stars with lower masses comprise the yellow, orange, and red dwarfs on the lower-right part of the main sequence, where they remain for billions of years. As a star begins to exhaust the hydrogen fuel in its core, it evolves away from the main sequence toward the upper right and becomes a red giant or supergiant. Stars that began with more than six times the Sun's mass then evolve left and right through complicated loops on the H-R diagram as if in a frenzy to keep up their energy production, then finally explode as supernovae. Odds and Ends Spectra can reveal many other things about stars.