The HR diagram is the "Rosetta Stone" of stellar astronomy. Simply put,
it plots a star's luminosity against its surface temperature. As simple as that
sounds, it is the key to understanding stellar evolution.
The luminosity of a star is its rate of energy output, i.e. wattage. The Sun's
output is about 4 x 1026 watts.
The surface temperature of a star and its color are directly tied to each other. Hotter stars
are toward the blue end of the spectrum. Cooler stars are red.
Other colors are inbetween. Stars are grouped into
spectral types which can be viewed as either temperature or color groupings. From hottest to
coolest the group designations are: O B A F G K M. The order is remembered by astronomy students
with the mnemonic "Oh Boy Another Fine Girl (or Guy) Kissed Me."
The majority of stars lie roughly on a line from hot and bright, to cool and dim. This line is
known as the "main sequence." Mathematically speaking, stars live most of their lives on the
Most stars begin their lives hot and bright on the map, then progress to a semi-permanent address
farther down on the main sequence. What happens later depends on the star's mass. For example, the Sun
will eventually move upwards and to the right, into the realm of the red giants. Then it
will move up and to the left, and then drop straight down into the graveyard of white dwarfs.