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Stars differ widely in mass, size, temperature, and total energy output, or luminosity. The sun, a typical star, has a mass of about 2 × 1033 grams, a radius of about 7 × 1010 cm, a surface temperature of about 6,000°C, and a luminosity of about 4 × 1033 erg/sec. More than 90% of all stars have masses between one tenth and 50 times that of the sun. Other stellar quantities vary over a much larger range. The most luminous stars (excluding supernovas) are about a million times more powerful than the sun, while the least luminous are only one hundredth as powerful red giants, the largest stars, are hundreds of times greater in size than the sun; if one were placed at the sun's position, it would engulf Mars. At the opposite extreme, white dwarfs are no larger than the earth, and neutron stars are only a few kilometers in radius.
The visible stars are divided into six classes according to apparent brightness; the brightest are first magnitude and the faintest are sixth magnitude. The stars differ in apparent brightness both because they lie at different distances from us and because they vary in actual or intrinsic brightness. Variable stars do not shine steadily but fluctuate in either a regular or irregular fashion. The supernova, or exploding star, is the most spectacular variable star; the eclipsing binary, where the two stars alternately hide and then reinforce each other's light, is not a true variable.
Light received from a star consists of a spectrum of wavelengths; the hotter the star, the shorter the wavelength at which the light is most intense. The color of a star is closely related to its surface temperature. Red stars have surface temperatures around 3,000°C and blue-white stars have surface temperatures above 20,000°C (see spectral class).
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