Back to Stars and Nebulae Starlight - Main Sequence - O - B - A - F - G - K - M - Luminosity Classes - Spectral Class Codes
Eighty-five percent of stars are main sequence, luminosity class V stars. Most of the others fit into the luminosity classes below. Point to the items in the list to see the temperature-luminosity plot for each of these sequences. Ia-0 The most luminous stars known Ia Luminous Supergiants Ib Luminous Supergiants II Bright Giants III Giants IV Subgiants V Main Sequence Stars VI Subdwarfs D or VII White Dwarfs
Main Sequence stars maintain a fixed luminosity and temperature and stay at the same position on the Main Sequence for as long as they have hydrogen in their cores. Once core hydrogen is exhausted, other fusion reactions start. Luminosities and temperatures change. Stars migrate to positions in other luminosity sequences. The other sequences reflect the variety of energy production processes that occur as stars near their end points. Red Dwarfs, Red Giants and Red Supergiants for example, radiate energy in the same part of the spectrum, but generate vastly different amounts of energy by different processes. They belong to different luminosity classes. Consider these stars: Star Code Classification Details Barnard's Star M5V Main Sequence Red Dwarf A small star, fusing hydrogen in its core and radiating with less than one percent of the Sun's luminosity. Gamma Crucis M3.5III Red Giant A star whose atmosphere has swelled enormously because it is fusing helium. It is radiating with the luminosity of 1,300 suns. Betelgeuse M2Iab Red Supergiant A massive supergiant that has exhausted its hydrogen and is now fusing helium. It has swelled beyond the size of the orbit of Mars. It is radiating with the luminosity of 59,000 suns. Betelgeuse is thought to be in the closing stages of its short, 10 million year life. It will explode as a Type II supernova any time in the next million years.