Last updated: 3 September 2000
Stars range tremendously in size, temperature, color, and brightness. These characteristics are used to classify stars.
Types Of Stars And Related Objects
The Seven Main Spectral Types
||Very hot and blue in color.|
||Hot and blue in color.
Although spectral type B main-sequence giant and supergiant stars are relatively rare, they are so luminous they can be seen across great distances. Consequently, they make a great contribution to the night sky; five of the twenty brightest stars in the night sky are spectral type B.
||Hot and white in color.
The most luminous stars within thirty light-years of the Sun are spectral type A. Five of the twenty brightest (apparent magnitude) stars in the night sky are spectral type A.
||Warm to hot, and yellow-white in color.|
||Warm and yellow.
The Sun is spectral type G2.
||Slightly cooler than the Sun,
and orange in color.
K-type stars come in two main varieties:
-- modest stars on the main sequence; these are the more numerous variety.
-- bright giant stars; these are more noticeable because of their much greater luminosity.
||Cool and red in color.
M-type stars come in two main varieties:
-- dim main sequence stars, termed red dwarfs, which emit far less light than the Sun.
White Dwarf: A star on the main sequence that is a little larger than Earth in size, containing around 60% of the mass of the Sun. A white dwarf is the end for a star that is born with less than eight times the Sun's mass. After leaving the main sequence, such a star evolves into a red giant, then sheds its outer layers (sometimes called its atmosphere) thereby exposing its hot core. This residual hot core of degenerate matter is a "white dwarf". It is estimated that ~5% of all stars are white dwarfs. The shed outer layers expand in a spherical shell, termed a "planetary nebula", surrounding the star. This expanding cloud of gas glows from the energy received from the newly exposed core.
Orange Dwarf: A main-sequence star that is orange in color and of spectral type K. Orange dwarfs are common; five lie within 12 light years of the Sun.
Red Dwarf: A main-sequence
star of spectral type M. Such stars are cool, small, and faint. Although
red dwarfs make up ~80% of all stars in the Milky Way, none is visible
to the unaided eye from Earth.
Red Giant: Giant stars of spectral type M; occasionally used to describe some giant stars of spectral type K (orange) or G (yellow). Red giants evolve from main-sequence stars that were born with less than eight times the mass of the Sun. Some time in the distant future, the Sun will become a red giant.
Giant Star: A large star, not in the main sequence, that shines ~100 times more brightly than the Sun. Giant stars no longer fuse hydrogen into helium in their cores. Rather, they may fuse hydrogen into helium outside their cores, or engage in other fusion reactions, or both. Giant stars evolve from stars with less than eight times the mass of the Sun. Most giants are yellow (G), orange (K), or red (M).
Supergiant: A large, very
luminous star. While supergiants come in all colors, they are most commonly
Neutron star: A collapsed star that forms when a massive star explodes. A typical neutron star has a mass about 1.4 times the mass of the Sun but measures on ~16 km (10 mi) across. The matter in a neutron star is degenerate.
Black Hole: An object with a gravitational field of such great strength that light can not escape. Black holes form when stars born with more than about 40 times the mass of the Sun run out of fuel and collapse.