Scientists use a star's temperature to classify it into one of seven main groups referred to by the letters O, B, A, F, G, K and M, in descending order. Within those groups, stars receive a number classification based upon their temperature as compared to other stars within the same group. An additional classification denotes the luminosity of the star through a Roman numeral between I and VIII.
In addition to temperature variation, stars in different classifications also display different color properties, masses, radii and luminosity. Stars in the hottest classification, O, generally appear blue and measure over 53,000 degrees Fahrenheit. Moving down the classification scale, the colors go from blue to white to yellow to orange to red. Due to the color variation, stars in the M classification are also referred to as red giants or dwarfs.
Beginning in the late 20th century, scientists have posited additional classifications for stars to describe newly identified types, such as the slash stars, methane dwarfs, sub-brown dwarfs, carbon stars, white dwarfs, neutron stars, exotic stars and black holes. The most common type of star in the Milky Way Galaxy is the red dwarf, classified under M. The Earth's sun is classified as a G2V star.