The actual appearant colors of the star types discussed below.

In astronomy, stellar classification is a way of grouping stars by temperature and luminosity. A star is a ball of superheated gas called plasma. Star temperature can be measured by looking at its spectrum, the type of light that the star shines. Stars are also grouped into spectral types or classes by color. In general, a star's temperature determines its color, from red to blue-white. Blue-white stars are the hottest, while Red ones are the coolest. Each class has their own subclass as well, with Hindu-Arabic Numerals. A G0 star would be the hottest type G and G9 with be the coolest type G.

Life Cycle of a Star

Stellar life cycles.

Stars are born in clouds of gas called a Nebula (plural: Nebulae) Nebulae are pulled together and become Protostars. Clouds of gas orbiting these stars then condense to form Planets, like how the Solar System was formed. The Star is then born and becomes a Main-Sequence Star. These stars live for up to 10 billion years and start to fuse hydrogen into the other elements of the Periodic Table. But as they reach heavier elements like Oxygen, they start cooling in temperature and become Red Giant stars, pushing their outer layers outward. Some stars expand even further into Red Supergiants by fusing metals. If the original post-giant star is more than 7 times larger than the Sun, then it will explode as a supernova for its death. Then it will become either a Neutron Star or a Black Hole, a bottomless pit that nothing can escape. If the original post-giant star is less than 7 times the size of the sun, it will become a white dwarf. White Dwarfs are small stars about the size of Earth, but they are hotter and were once the core of the progenitor star. Neutron Stars are the cores of original stars that have been crushed to the size of a city and they are packed up with Neutrons, the part of the atom that has no electrical charge. One 1 cm x 1 cm x 1 cm cube of their material would weigh as much as 5 fully-loaded cargo ships.

Stellar evolution phase	Temperature (Kelvin)	Diameter (D☉, or other when specified)
Nebula (P)	~0	1 to 2,000 ly
Main Sequence Star	3,000 to 50,000	0.08 to 20
Red Giant	3,000 to 5,000	20 to 200
Red Supergiants		200 to 2,600
White Dwarf	<4,000 to 150,000	8,000 to 100,000 km
Neutron Star	600,000	5 to 50 km
Black Hole	0.00000000000000000001	Any

Luminosity Class

Stars are also organized into another classification, this time for luminosity. This tells if the star is illegible to be called a dwarf, giant, supergiant, or hypergiant. It is also called the Yerkes spectral classification.

Temperature Class

The stellar classification used to classify stars by color or temperature has seven main classes and four other ones. It is called the Morgan-Keenan stellar classification. It was originally proposed by Italian astronomer Angelo Secchi, but has been improved by Harvard professors William Morgan and Philip Keenan.

The Harvard Computers were a group of women astronomers best known for their contributions to the modern classes for stars. The most prominent of them were Annie Jump Cannon, who dropped all unnecessary letters from the larger Draper system leaving it as "O, B, A, F, G, K, M", and Williamina Fleming, who classified most of the stars in the Draper system.

Jump Cannon left stellar classes P and Q, which are used for planetary nebulae and novae and pulsars respectively.

It was originally thought that for stellar classes G to M there was a significant difference in dwarfs and giants, in terms of size and temperature, and being beyond distinguishable for earlier stellar classes. However, with the discovery of subdwarf O and B stars, this has proved to apply to classes O and B as well. White dwarfs are the dwarf counterparts of classes A and F in terms of average temperature, though they now belong to an entirely different class (D).

"Late-type" stars refer to classes F, G, K, and M, and the cooler subclasses of any class (such as G5-G9). "Early-type" refers to classes O, B, and A and the hotter subclases of any class. Thus "early" means hotter, and "late" means cooler. These terms were derived from an obsolete model of stellar evolution based on the Kelvin-Helmholtz mechanism, where stars started as red giants, before contracting into blue main-sequence stars and finally into red dwarfs. This model was rendered obsolete by the discovery that stars are powered by nuclear fusion. However, the terms "early" and "late", based on the model's timescale, were carried on, beyond the demise of the theory on which they were based.

Class	Surface temperature (kelvin)	Conventional color description	Actual apparent color	Mass(solar masses)	Radius(solar radii)	Luminosity (bolometric)	Hydrogen lines	Fraction of all main-sequence stars
O	≥ 33,000 K	blue	blue	≥ 16 M☉	≥ 6.6 R☉	≥ 30,000 L☉	Weak	~0.00003%
B	10,000–33,000 K	blue white	deep blue white	2.1–16 M☉	1.8–6.6 R☉	25–30,000 L☉	Medium	0.13%
A	7,500–10,000 K	white	blue white	1.4–2.1 M☉	1.4–1.8 R☉	5–25 L☉	Strong	0.6%
F	6,000–7,500 K	yellow white	white	1.04–1.4 M☉	1.15–1.4 R☉	1.5–5 L☉	Medium	3%
G	5,200–6,000 K	yellow	yellowish white	0.8–1.04 M☉	0.96–1.15 R☉	0.6–1.5 L☉	Weak	7.6%
K	3,700–5,200 K	orange	pale yellow orange	0.45–0.8 M☉	0.7–0.96 R☉	0.08–0.6 L☉	Very weak	12.1%
M	2,000–3,700 K	red	light orange red	≤ 0.45 M☉	≤ 0.7 R☉	≤ 0.08 L☉	Very weak	76.45%
R	1,300–2,000 K	red	red	Unknown	Unknown	Unknown	Very weak	Unknown
N	1,300–2,000 K	red	red	Unknown	Unknown	Unknown	Very weak	Unknown
S	1,300–2,000 K	red	red	Unknown	Unknown	Unknown	Very weak	Unknown

Stellar Class (examples)		Secchi's original classification	Draper Classification Equivalent	Temperature (in Kelvin)	Percentage (%) of known stars	Relevant image
Early-type	LBV (luminous blue variable) Pistol Star	V (according to Edward Charles Pickering)	n/a	7,500 to 50,000	0.000033	Spectrum of an O5V star
	O Alnitak		O	30,000 to 50,000 (giants) 40,000 to 100,000 (dwarfs)
	B Rigel Be stars Gamma Cassiopeiae	I (Orion subtype), V (Be stars)	B	10,000 to 30,000 (giants) 20,000 to 40,000 (dwarfs)	0.13
	A Sirius A	I	A, C, D	7,500 to 10,000 (giants) 3,000 to 250,000 (dwarfs; see below)	0.7
	F Polaris		E, F	6,000 to 7,500 (giants) 3,000 to 250,000 (dwarfs; see below)	3
	G Sun	II	G, H, I	5,000 to 6,000 (giants) 5,700 to 6,400 (dwarfs)	7
	K Arcturus		K, L	4,000 to 5,000 (giants) 4,800 to 5,500 (dwarfs)	12.1
Late-type	M Betelgeuse	III	M	2,000 to 4,000 (giants) 3,400 to 4,000 (dwarfs)	77	VY Canis Majoris
	R S Camelopardalis	IV	N	3,000 to 5,000	unknown
	N Y Canum Venaticorum			4,000 to 2,000
	S Chi Cygni
	Objects included are the Sun, the red and brown dwarf companions of Gliese 229, Teide 1 and Jupiter. Brown Dwarfs are stellar objects that cannot fuse hydrogen with their interiors, and thus can only radiate in infrared light. The first such object discovered was Teide 1, located in the constellation Taurus. L T Y	n/a	n/a	200 to 3,000	> 100	Comparison between the three types of brown dwarfs.
Early-type	Wolf-Rayet Stars Gamma Velorum	V (according to Edward Charles Pickering)	O (later separated from O-type stars)	30,000 to 200,000	unknown
	D Sirius B		A, D?	3,000 to 250,000	common
	P core of the Calabash Nebula		P	unknown		The "Heart and Soul nebulae"
?	Q DN Geminorum	n/a	Q

Hertzsprung-Russell Diagram

Star types are arranged in the Hertzsprung-Russell Diagram. This scattergraph was invented by Danish and American astronomers Ejnar Hertzsprung and Henry Russell in 1908. Stars farther away on the right are cooler in temperature, while stars that are near the top are more luminous and bright.

The line going from the top left-hand side down to the bottom right-hand side contains all of the main-sequence stars. The giant branch, containing yellow, orange, red giants, supergiants, hypergiants, and Mira variables, is located just above and right of the main sequence stars. Blue and white supergiants are directly above the central main sequence. Brown dwarfs are at the very bottom of the main sequence, with white dwarfs directly below the central main-sequence. The yellow evolutionary void is located between the blue-white supergiants and the giant branch.

B-V Color Index

A graph comparing several values of the B-V Color Index, with other possible but non-black body colors for comparison

In astronomy, the B-V color index is a numerical expression that determines the exact "color" of a certain star. The smaller the color index, the more the star appears blue. For comparison, our Sun has an approximate B-V color index of 0.656, while Rigel has a color index of -0.03. Red Giants have color indexes from 0.81 to 1.4 .

References

• http://adsabs.harvard.edu/abs/1981A&AS...46..193H
• http://adsabs.harvard.edu/abs/2001JRASC..95...32L
• http://adsabs.harvard.edu/abs/1994ASPC...60....3G
• https://www.aanda.org/articles/aa/abs/2008/37/aa09936-08/aa09936-08.html
• http://adsabs.harvard.edu/abs/2000A&A...353..163N
• http://www.annualreviews.org/doi/10.1146/annurev.astro.45.051806.110615
• https://www.cfa.harvard.edu/~pberlind/atlas/htmls/note.html

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