When you gaze at the night sky, it's obvious that the various stars are different; some shine more brightly than others, and some have noticeable colors. Scientists classify stars according to these two characteristics, but they do it with sensitive instruments that provide much more detail than stargazers—even ones with powerful telescopes—could ever manage to discern. In other words, scientists classify stars according to their luminosity and temperature, as determined by spectral analysis.
Apparent and Absolute Magnitude
The Greek astronomer Hipparchus was the first person to catalog stars according to their visual magnitude, and today scientists use digital light sensors to get much more detailed measurements. However, the brightness of a star as it appears from Earth—its apparent magnitude—isn't useful for comparing stars to each other, so scientists compute another value that incorporates a star's distance from Earth. This is the absolute magnitude, or a measure of how bright the star would be if it were 10 parsecs from earth, which is 32.6 light-years. Stars with lower values of absolute magnitude are brighter. The sun's absolute magnitude is 4.2, whereas that of the supergiant star Deneb in the constellation Cygnus is –7.2.
Luminosity or Brightness
After calculating a star's absolute magnitude, which takes into account its distance from Earth, scientists can use that value to calculate luminosity, another measure of brightness. Luminosity is expressed in watts and can be compared to the luminosity of the the sun, which is approximately 400 trillion trillion watts. The relationship between absolute magnitude and luminosity is logarithmic. In general, a difference of five on the scale of absolute magnitudes corresponds to a difference of 100 on the luminosity scale. The brightest stars are also typically the largest. White dwarfs, which are smaller but brighter than the sun, and red giants, which are dying stars, are two exceptions to this trend.
Besides luminosity, scientists also classify stars by their temperatures, which they determine by examining the spectral signature of the star. There are seven spectral classifications corresponding to the intrinsic colors of stars, which also depend on temperature. The hottest stars, which are blue or blue-white, are O and B on the spectral classification scale. White, yellow-white and yellow stars, which are smaller and cooler, are classified as A, F and G, whereas the coolest stars, which are yellow-orange and red, are classified as K and M. O stars can burn out in a matter of millions of years, whereas some M stars, for example, red dwarfs, have lifetimes longer than the age of the universe.
The Morgan-Keenan System
When classifying a star, astronomers assign it a spectral classification letter, and they also give it a number that indicates how far it is on the scale from the next lowest letter. For example, the sun is a yellow G star, and because it is 20 percent of the way toward being a K star, it's more accurately classified as a G2 star. A Roman numeral then defines the size and luminosity of a star; I stars are super-bright supergiants, whereas V stars are main-sequence stars. This last classification is further subdivided by the letters a, b and c, such that a Ia star is brighter than a Ib star. In this system, the sun is a G2V star, and Sirius, the brightest star in Earth's sky, is an A1V star.
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