<P> The modern classification system is known as the Morgan--Keenan (MK) classification . Each star is assigned a spectral class from the older Harvard spectral classification and a luminosity class using Roman numerals as explained below, forming the star's spectral type . </P> <P> Other modern stellar classification systems, such as the UBV system, are based on color indexes--the measured differences in three or more color magnitudes . Those numbers are given labels such as "U-V" or "B-V", which represent the colors passed by two standard filters (e.g. Ultraviolet, Blue and Visual). </P> <P> The Harvard system is a one - dimensional classification scheme by astronomer Annie Jump Cannon, who re-ordered and simplified a prior alphabetical system . Stars are grouped according to their spectral characteristics by single letters of the alphabet, optionally with numeric subdivisions . Main - sequence stars vary in surface temperature from approximately 2,000 to 50,000 K, whereas more - evolved stars can have temperatures above 100,000 K. Physically, the classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest . </P> <Table> <Tr> <Th> Class </Th> <Th> Effective temperature </Th> <Th> Vega - relative chromaticity </Th> <Th> Chromaticity (D65) </Th> <Th> Main - sequence mass (solar masses) </Th> <Th> Main - sequence radius (solar radii) </Th> <Th> Main - sequence luminosity (bolometric) </Th> <Th> Hydrogen lines </Th> <Th> Fraction of all main - sequence stars </Th> </Tr> <Tr> <Th> O </Th> <Td> ≥ 30,000 K </Td> <Td> blue </Td> <Td> blue </Td> <Td> ≥ 16 M </Td> <Td> ≥ 6.6 R </Td> <Td> ≥ 30,000 L </Td> <Td> Weak </Td> <Td> ~ 0.00003% </Td> </Tr> <Tr> <Th> </Th> <Td> 10,000--30,000 K </Td> <Td> blue white </Td> <Td> deep blue white </Td> <Td> 2.1--16 M </Td> <Td> 1.8--6.6 R </Td> <Td> 25--30,000 L </Td> <Td> Medium </Td> <Td> 0.13% </Td> </Tr> <Tr> <Th> </Th> <Td> 7,500--10,000 K </Td> <Td> white </Td> <Td> blue white </Td> <Td> 1.4--2.1 M </Td> <Td> 1.4--1.8 R </Td> <Td> 5--25 L </Td> <Td> Strong </Td> <Td> 0.6% </Td> </Tr> <Tr> <Th> </Th> <Td> 6,000--7,500 K </Td> <Td> yellow white </Td> <Td> white </Td> <Td> 1.04--1.4 M </Td> <Td> 1.15--1.4 R </Td> <Td> 1.5--5 L </Td> <Td> Medium </Td> <Td> 3% </Td> </Tr> <Tr> <Th> </Th> <Td> 5,200--6,000 K </Td> <Td> yellow </Td> <Td> yellowish white </Td> <Td> 0.8--1.04 M </Td> <Td> 0.96--1.15 R </Td> <Td> 0.6--1.5 L </Td> <Td> Weak </Td> <Td> 7.6% </Td> </Tr> <Tr> <Th> </Th> <Td> 3,700--5,200 K </Td> <Td> light orange </Td> <Td> pale yellow orange </Td> <Td> 0.45--0.8 M </Td> <Td> 0.7--0.96 R </Td> <Td> 0.08--0.6 L </Td> <Td> Very weak </Td> <Td> 12.1% </Td> </Tr> <Tr> <Th> </Th> <Td> 2,400--3,700 K </Td> <Td> orange red </Td> <Td> light orange red </Td> <Td> 0.08--0.45 M </Td> <Td> ≤ 0.7 R </Td> <Td> ≤ 0.08 L </Td> <Td> Very weak </Td> <Td> 76.45% </Td> </Tr> </Table>

Stars of spectral type a and f are considered