As there are different varieties of nebulae so there are different varieties of stars. Each variety has a distinct lifecycle, and that in turn is determined by its mass at birth. There are over a dozen distinct varieties of stars, though some are theoretical. By far the most popular category are the main sequence stars, which include our own sun and account for about 90% of all stars in the universe.

The infinite variety of stars
Star	Star type	Luminosity class	Absolute magnitude
Rigel β Orionis	B8 Ia	Blue-white supergiant	-7.84
Sirius α Canis Majoris	A1 v	White, main sequence	+1.42
Procyon α Canis Minoris	F5 v	Yellow sub-giant	+2.6
Alpha Centauri A	G2 v	Yellow (sun-like)	+4.38
Aldebaran α Tauri	K5 III	Orange giant	-0.63
Betelgeuse α Orionis	M2 lab	Red supergiant	-6.0
Barnard's Star	M5	Red dwarf	+13.4

The  following is a list of all the known types of stars, a few of which are a matter of hypothesis on the part of astronomers. They are briefly described in two sections: Giants and Dwarfs.

The next table reveals the typical diameters of some of the star types.

Diameter of star types
Red hypergiant	1 977 million kilometres
Red giant	1 376 million kilometres
Blue supergiant	110 million kilometres
Sun	1 392 000 kilometres
White dwarf	13 830 kilometres
Neutron star	2 kilometres

Spectral classification

First known as the Harvard Stellar Classification the table below was devised by astronomer Annie Jump Cannon (1863 – 1941). She was a contemporary and colleague at Harvard Observatory of Henrietta Leavitt (1868-1921).

The stars are now classified under the Morgan-Keenan (MK) system, using the letters O (the hottest) to M (the coolest). Roughly speaking, O is blue, B is blue-white, A is white, F is yellow-white, G is yellow, K is orange, and M is red. Harvard’s classification agrees until K which it labels “light orange” and M as “light orange red”.

There is a degree of correlation. However, colours can be subjective, especially when you consider they depend as much on temperature as they do on spectral lines, which in turn depend on the relative portion of elements that make up a star. In any event, the human eye is unable to resolve colours at astronomical distances which is why most stars appear white to the naked eye.

Main sequence stars vary in temperature from 2 000°C to 50 000°C. The temperatures are those for the surface of the bodies.

Spectral classification and temperature
Class	Effective temperature	Vega-relative chromaticity	Chromaticity
O	>30 000°C	Blue
B	10 000°C - 30 000°C	Blue-white	Blue
A	7 500°C - 10 000°C	White	Deep blue-white
F	6 000°C - 7 500°C	Yellow white	White
G	5 200°C - 6 000°C	Yellow	Yellowish white
K	3 700°C - 5 200°C	Light orange	Pale yellow orange
M	2 400°C - 3 700°C	Orange red	Light orange red
Source: Harvard Spectral Classification

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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar.

Last updated: Saturday, 20 March 2021