Stellar classification

M stars are the coldest and O stars are the hottest in stellar classification. These stars are from the main sequence.

In astronomy, stellar classification is a way of grouping stars by temperature. 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. Spectral types are named with a letter. The seven main types are M, K, G, F, A, B and O. M stars are the coldest stars and O stars are the hottest.

Harvard spectral classification

The Harvard classification system is a one-dimensional classification scheme. Stars vary in surface temperature from about 2,000 to 40,000 kelvin. Physically, the classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest, as is done in the following table:

Note: The conventional color description describe only the peak of the stellar spectrum. However, the actual apparent colors the eye sees are lighter than the conventional color descriptions.

Class	Surface temperature (kelvin)	Conventional color description	Actual apparent color	Mass (solar masses)	Radius (solar radius)	Luminosity (bolometric)	Hydrogen lines	Fraction of all main sequence stars
O	≥ 33000 K	blue	blue	≥ 16 M☉	≥ 6.6 R☉	≥ 30,000 L☉	Weak	~0.00003%
B	10000–33000 K	blue white	deep blue white	2.1–16 M☉	1.8–6.6 R☉	25–30,000 L☉	Medium	0.13%
A	7500–10000 K	white	bluish-white	1.4–2.1 M☉	1.4–1.8 R☉	5–25 L☉	Strong	0.6%
F	6000–7500 K	yellowish-white	slightly whitish	1.04–1.4 M☉	1.15–1.4 R☉	1.5–5 L☉	Medium	3%
G	5200–6000 K	yellow	orangish-white	0.8–1.04 M☉	0.96–1.15 R☉	0.6–1.5 L☉	Weak	7.6%
K	3700–5200 K	orange	light orange	0.45–0.8 M☉	0.7–0.96 R☉	0.08–0.6 L☉	Very weak	12.1%
M	2500–3700 K	red	orange	≤ 0.45 M☉	≤ 0.7 R☉	≤ 0.08 L☉	Very weak	76.45%

The mass, radius, and luminosity listed for each class are appropriate only for stars on the main sequence portion of their lives and so are not appropriate for red giants and blue and red supergiants. The spectral classes O through M are subdivided by Arabic numerals (0–9). For example, A0 denotes the hottest stars in the A class and A9 denotes the coolest ones. The Sun is classified as G2.

The Hertzsprung-Russell diagram relates stellar classification with absolute magnitude, luminosity, and surface temperature.

The Hertzsprung-Russell diagram is more often used in astronomy. It relates the absolute magnitude and the surface temperature. It is as important to astronomy as the periodic table is to chemistry.

Luminosity classes

Luminosity classes also exist and stars can be assigned to one based on their spectrum. The conventional classes are called the Yerkes system.

Yerkes luminosity classes

Class	Description
0 or Ia+	Hypergiants
Ia	Very bright supergiants
Iab	Bright supergiants
Ib	Fainter supergiants
II	Bright giants
III	Giant stars
IV	Subgiants
V	Dwarf stars (main-sequence)
VI	Subdwarf stars
VII	White dwarfs

Our Sun, a yellow dwarf, is G2V. Red giants are K to M and have luminosity class III. Blue supergiants are O to A with II to Ia, while red supergiants are K to M with II to Ia.

Conventional and apparent colors

The conventional color descriptions are traditional in astronomy, and represent colors relative to the mean color of an A-class star which is considered to be white. The apparent color descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars.

The Sun itself is white, though it is sometimes called a yellow star. This is a natural consequence of the evolution of human optical senses: the response curve that maximizes the overall efficiency against solar illumination will by definition perceive the Sun as white, though there is some subjective variation between observers.

Class O

Type O stars are stars with temperatures of ~35,000 degrees Celsius or Kelvin. They are blue in color and can be more than 6.5 times bigger than the Sun. They are also very massive but, at the same time, very rare. Examples:

• Mu Columbae (main sequence)

• HD 269327 (hypergiant)

Class B

Type B stars are blue stars which have temperatures of ~25,000 degrees (Celsius or Kelvin). They are high in helium spectral lines and luminosity (brightness compared to the Sun), with moderately strong hydrogen lines. B type stars with Balmer lines in their spectra are part of the Be stars subtype. Examples:

• VV Cephei B (main sequence)
• Rho Leonis (giant)
• Rigel (supergiant)
• Pistol Star (hypergiant)

Class A

Type A stars are white stars which have the strongest hydrogen lines. They are some of the brightest stars in the sky. Temperatures ~10,000 degrees Celsius or Kelvin. Examples:

• Sirius A (main sequence star)
• Deneb (supergiant)

Class F

Type F stars are yellow stars that contain dominant calcium lines in their spectra. They have temperatures of ~7,500 degrees Celsius or Kelvin. Examples:

• Procyon (dwarf)
• Polaris (giant)
• Arneb (supergiant)
• Rho Cassiopeiae (hypergiant)

Class G

Type G stars are yellow stars with temperatures between 5,700-6,400 degrees Kelvin. Ionized calcium and iron are present in their spectra. Examples:

• The Sun (G2 dwarf)
• Capella (giant)
• HD 269953 (hypergiant)

Class K

Type K stars are orange to orange-red stars with temperatures between 4,100-5,100 degrees. Molecules begin to appear in weak spectral lines. Examples:

• Alpha Centauri B (dwarf)
• Pollux (giant)
• Zeta Cephei (supergiant)
• HR 5171 (hypergiant)

Class M

Type M stars are the most common type of star in the universe. Red dwarfs, smaller than the sun, temperatures 3,000-4,000 degrees Kelvin and more common than red giants, which are old 3,100 degree Celsius stars in the last stages of their life. Red hypergiants are also the biggest stars, while red dwarfs, having a fully convective structure, exhaust their hydrogen fuel very slowly compared to other stars. These stars are cool enough to allow molecules to exist intact in their atmosphere; these leave many dark spectral lines. Examples:

• Proxima Centauri (dwarf)
• Mira A (giant)
• Antares (supergiant)
• VY Canis Majoris (hypergiant)

Class S

S-type stars are red giant stars with equal quantities of carbon and oxygen in its atmosphere. Below is a size comparison of all the known S-type stars with known radii.

Name	Radius (Solar radii)
CIT 11	982
V384 Persei	937
S Cassiopeiae	930
R Cygni	825
IRAS 10176-5802	751.2
TT Centauri	744
Y Lyncis	580
WY Cassiopeiae	575
R Andromedae	476
NO Aurigae	439.4
R Geminorum	431
RR Andromedae	404
EP Vulpeculae	380
Chi Cygni	348
W Andromedae	332.47
T Ceti	275
CY Cygni	232.44
RS Cancri	225
BH Crucis	216
Omicron1 Orionis	214
DT Piscium	197
BQ Octantis	197
R Orionis	192.722
V597 Carinae	173.34
NGC 2682 FBC 3456 A	139.542
BD Camelopardalis A	135
57 Pegasi	126
HR Pegasi	125
V1139 Tauri	103.695
AA Camelopardalis	97.849
98 Herculis	85.39
HD 35155	80.61
TX Canum Venaticorum	50.92
HD 310	45.734
BS Cygni	44.94
Z Piscium	36.888
X Andromedae	33

Related pages

• Hertzsprung-Russell Diagram

Other websites

• Stellar classification (astrophysics) Citizendium