The color of a star (measured from its spectrum or "fingerprint") tells us how
hot it is. If we also know how far away it is we know what its true
brightness by how bright it appears in the sky. All stars with the
same spectrum and the same true brightness are the same type of star.
If we measure these two properties for all nearby stars (where we know
the distance fairly well) we can produce a picture of how the stars
are spread in brightness and temperature and we can see if there is a
pattern.
By using the temperature (or spectrum) of a star combined with its brightness
(or luminosity) we can set up a scheme to
compare it with all other stars. This was done most elegantly by two astronomer in 1908: Ejnar Hertzsprung in Denmark and Henry Russell in the United States. These astronomers devised a figure which
beautifully illustrates how different stars compare with each other. This is
now known as the Hertzsprung-Russell (or HR)
diagram.
The key feature in the HR-diagram is the band of stars running from
top left to bottom right (high luminosity-high temperature to low
luminosity-low temperature) known as the main sequence.
About 90% of all nearby stars lie on the main sequence. The Sun is a
typical main sequence star.
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