18.4 NOTES
How are the stars classified?
Objective: Explain how the Hertsprung-Russell diagram is used to classify stars.
Astronomers Ejnar Hertzsprung and Henry Russell each made a separate but
similar discovery about stars in the early 1900’s. They found that there is a
relationship between a star’s absolute magnitude and its surface temperature
and color. They developed a chart called the Hertzsprung-Russell, or H-R,
diagram.
A star’s position on the H-R diagram depends on its absolute magnitude and its
surface temperature, or color. Temperature is measured in degrees Kelvin (K).
For example, a blue star with a low absolute magnitude would be found in the
upper left corner of the diagram. A red star with a high absolute magnitude
would be found in the lower right corner. Most stars are found in a narrow
diagonal band that runs from the upper left to the lower right corner of the
diagram. These stars are called main sequence stars.
Some stars do not fall within the main sequence. They may be bright but not very
hot. Many of these stars are red, orange, or yellow. Even though they are not
very hot, they are usually bright because they are large in size. They have large
absolute magnitudes. These stars are called red giants. They are found in the
upper right corner of the H-R diagram. Stars that are larger and brighter than red
giants are called supergiants. Other stars outside the main sequence are hot but
very small. They are blue or white in color, and are called white dwarfs. They are
found in the lower part of an H-R diagram, below the main sequence stars.