III IV V VI

– ordinary giants – subgiants – main sequence stars (aka dwarfs) – subdwarfs

This system of classification, in which the Sun is a G2V star, has been so successful that it has remained largely unchanged for nearly 75 years. It is embodied in the Hertzsprung-Russell diagram, shown left, which is a two-dimensional plot of stars according to their temperature and luminosity. A young star joins the main sequence as a dwarf. As its hydrogen is exhausted, the star leaves the main sequence and becomes a giant. A Sun-like star will eventually throw off its outer layers as a planetary nebula, while the nuclear reactions subside and all that remains is an inert, cooling, white dwarf. Stars larger than eight solar masses will evolve more rapidly, executing complicated loops on the Hertzsprung-Russell diagram, before exploding as supernovae.

As our knowledge increased, more classifications have been added. The cool red and brown dwarfs are classified as L, T and Y, so the full spectral sequence runs OBAFGKMLTY. There are also some stars that don’t fit and run parallel to the sequence. These include the Wolf-Rayet stars (W) at the hot end, and the Carbon (C) and very rare S stars at the cool end.

Examples of each of the main categories in the winter sky are (main sequence and giant/supergiant respectively):

– Sigma Orionis, O9.5V; Alnitak (Zeta Orionis), O9.5Ib – Gomeisa (Beta Monocerotis), B8V; Rigel (Beta Orionis), B8Ia – Castor (Alpha Geminorum), A2V; Deneb (Alpha Cygni), A2Ia – Procyon (Alpha Monocerotis), F5IV-V; Polaris (Alpha Ursae Minoris), F7Ib – Kappa Ceti, G5V; Mebsuta (Epsilon Geminorum), G8Ib – 61 Cygni A, K5V; Pollux (Beta Geminorum), K0III – No easily visible main sequence red dwarfs; Betelgeuse (Alpha Orionis), M2Ib