Classifying Stars

Last updated: 3 September 2000


Stars range tremendously in size, temperature, color, and brightness. These characteristics are used to classify stars.

Apparent Magnitude
Absolute Magnitude
Spectral Type
Types Of Stars And Related Objects

Apparent Magnitude measures how bright a star appears when viewed from Earth. Apparent magnitude depends on both

Absolute Magnitude provides a measure of a star's true, or intrinsic, brightness -- high much light the star actually radiates into space. It is defined as the apparent magnitude at the standard distance of 32.6 light-years (10 parsecs). Absolute magnitude depends only on the star's luminosity, the total amount of energy that a star emits into space. Both apparent magnitude and absolute magnitude are specified using a numerical scale, with lower numbers indicating bright stars. The numerical scale is such that two consecutive units of magnitude differ by a factor of 2.5 in brightness, that is, a magnitude +1.0 star is 2.5 times brighter than a magnitude +2.0 star. Most of the brightest stars have apparent magnitudes between 0 and +1.

Spectral Type classifies a star on the basis of its spectrum, which depends primarily on the star's temperature and color. There are seven main spectral types: O, B, A, F, G, K, M. For greater precision, astronomers divide each spectral type up into ten classes, labeled 0, 1, 2, 8, 9. Thus a G0 star is slightly hotter than a G1, etc

The Seven Main Spectral Types

Spectral Type
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.

-- giants and supergiants, which are many times brighter than the Sun

Types Of Stars And Related Objects Dwarf Stars
  Brown Dwarf: A star with too little mass (~0.08 the mass of the Sun or less) to collapse under gravity to the point where fusion can begin. Brown dwarfs are not really brown. When young, they glow red as they convert gravitational energy into heat. As they age, they fade, cool, and turn black. The matter in faded brown dwarfs is degenerate.

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.

Giant Stars
  Subgiant: A star that is making the transition from the main sequence to the red giant phase.

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 red.

Special Cases
  Supernova: A huge explosion that destroys a star. Most supernovae arise from supergiants that run out of fuel. They collapse and then explode, leaving behind only a neutron star or a black hole. A few supernovae arise from white dwarfs that become unstable due to the in-falling of matter pulled from an adjacent star.

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.