The Hertzsprung–Russell diagram (HR diagram) shows the relationship between the temperature and luminosity (absolute magnitude) of stars. It is a “map” of stellar evolution.
Basic Knowledge
The horizontal axis of the HR diagram represents surface temperature (lower temperatures toward the right), and the vertical axis represents luminosity (absolute magnitude) (brighter toward the top). Spectral types (O, B, A, F, G, K, and M, from high to low temperature) may also be used on the horizontal axis.
Most stars are located on the main sequence, which extends from the upper left (high temperature and high luminosity) to the lower right (low temperature and low luminosity). The Sun is near the center of the main sequence (G type, surface temperature 5800K).
Stars in special positions outside the main sequence:
・Upper right (low temperature, high luminosity): red giants and supergiants (enormous radii)
・Lower left (high temperature, low luminosity): white dwarfs (high density and small radii)
Luminosity classes are classified as I (supergiants), II (bright giants), III (giants), IV (subgiants), V (main-sequence stars), VI (subdwarfs), and VII (white dwarfs).
HR diagram (a diagram showing the distribution of stars, with surface temperature on the horizontal axis (lower temperatures to the right) and luminosity on the vertical axis (greater brightness upward))
Main sequence (the diagonal band of the HR diagram; a stable stage in which hydrogen fusion occurs)
Spectral type (O-B-A-F-G-K-M; from high to low temperature. The Sun is G type)
Red giant (an evolved star that has left the main sequence; low temperature, high luminosity, and a radius expanded to several hundred times its original size)
White dwarf (the burnt-out core of a star; a hot, faint, Earth-sized, high-density object)
Hertzsprung gap (the empty region between giants and the main sequence in the HR diagram; evolution is rapid and few stars are found there)
Further Study (Background and Significance)
The time a star remains on the main sequence (main-sequence lifetime) strongly depends on its mass. The Sun (1M☉) remains there for about 10 billion years. The more massive a star is, the faster its nuclear fusion proceeds and the shorter its lifetime: a 10M☉ star remains on the main sequence for several tens of millions of years, whereas a 0.1M☉ star may remain there for several trillion years. The dependence is approximately lifetime ∝ M⁻²·⁵.
In an HR diagram of a star cluster (a group of stars formed at the same time), the cluster’s age can be estimated from the position of the upper end of the main sequence (the main-sequence turnoff point). The farther the turnoff lies toward the low-luminosity side of the main sequence, the older the cluster is.
According to the Stefan–Boltzmann law L ∝ R²T⁴, a star with a larger radius has greater luminosity even at the same temperature. The vertical axis of the HR diagram is absolute luminosity and includes information about radius.
- HR diagram horizontal axis: left = high temperature (O type), right = low temperature (M type)
- Main sequence: a band from the upper left (high temperature and luminosity) to the lower right (low temperature and luminosity)
- Sun: G type, near the center of the main sequence
- Red giants: upper right (low temperature and high luminosity)
- White dwarfs: lower left (high temperature and low luminosity)
- Main-sequence lifetime: shorter for more massive stars
- Cluster turnoff point → used to estimate age
Points of Caution (Easily Confused)
① On the HR diagram, the left side is hotter and the right side is cooler (the reverse of an ordinary temperature axis). ② Do not confuse luminosity class with magnitude (brightness). ③ Red giants (cool and large) and white dwarfs (hot and small) occupy diagonally opposite positions on the HR diagram. ④ While a star is on the main sequence, hydrogen fusion occurs; after it leaves the main sequence, other types of nuclear fusion begin.
Practice
- What kinds of stars are located on the left side of the HR diagram’s horizontal axis?
- Where are white dwarfs located on the HR diagram in terms of temperature and luminosity?
- State the spectral type of the Sun.