How to Classify Stars

Classifying by surface temperature. In the classification system, stars are numbered 0 through 9, 0 being the hottest. This can be hard to classify stars that are far away. The coldest stars are between 2000 to 3500 degrees Kelvin, and the hottest are between 30,000 and 60,000 degrees Kelvin. Our Sun is a 2 inside of its class.

Classifying by color. Common names like "red giant" show that stars are classified by color as well. The hottest stars are described as a pure blue. Ones lower in temperature are more of a bluish white or pure white. The coldest stars are classified as red, going up to orange and yellow. Regarding color, these are the colors that the stars look like when magnified, not the color we see with our naked eye (which is usually always white).

Classifying by size. Stars classified by size go from White Dwarfs (smallest) to Supergiants (largest), with other larger dwarfs and smaller giants in between. To find this absolute magnitude, scientists take into mind both the mass and the radius of the stars. In general, hotter stars are usually larger, though any color star can be considered a dwarf star.

Classifying by spectral lines. This classification system was invented out of the Yerkes Observatory in 1943. Spectral lines are affected by the surface gravity of stars, so this is how they're detected. The heavier stars that exhibit more gravity have heavier spectral lines. Spectral lines show up as bright lines on an otherwise undisturbed color spectrum.

Classifying by the Secchi classes. This is the most archaic form of classification. In the 1870s, Angelo Secchi came up with four classes for stars: white and blue stars that had big hydrogen lines, yellow stars with more metallic lines than strong hydrogen, orange and red stars with "complex band spectra" and red stars that had heavy carbon bands. By 1900, this technique was replaced by newer theories.