Stellar classification

spectral typeK-typeG-typespectral classluminosity classB-typeF-typeA-typeM-typespectral classification
In astronomy, stellar classification is the classification of stars based on their spectral characteristics.wikipedia
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Sun

solarSolThe Sun
The full spectral class for the Sun is then G2V, indicating a main-sequence star with a temperature around 5,800 K.
The Sun is a G-type main-sequence star (G2V) based on its spectral class.

Red dwarf

redred dwarf starsred dwarfs
Red dwarfs are a deep shade of orange, and brown dwarfs do not literally appear brown, but hypothetically would appear dim grey to a nearby observer.
A red dwarf (or M dwarf) is a small and cool star on the main sequence, of M spectral type.

Annie Jump Cannon

Annie J. CannonCannonCannon, Annie Jump
The Harvard system is a one-dimensional classification scheme by astronomer Annie Jump Cannon, who re-ordered and simplified a prior alphabetical system.
Annie Jump Cannon (December 11, 1863 – April 13, 1941) was an American astronomer whose cataloging work was instrumental in the development of contemporary stellar classification.

Brown dwarf

brown dwarfsbrown dwarvesPlanetar
Red dwarfs are a deep shade of orange, and brown dwarfs do not literally appear brown, but hypothetically would appear dim grey to a nearby observer. Although most class M stars are red dwarfs, most of the largest ever supergiant stars in the Milky Way are M stars, such as VY Canis Majoris, Antares and Betelgeuse, which are also class M. Furthermore, the larger, hotter brown dwarfs are late class M, usually in the range of M6.5 to M9.5.
Stars are categorized by spectral class, with brown dwarfs designated as types M, L, T, and Y. Despite their name, brown dwarfs are of different colors.

Hypergiant

red hypergiantblue hypergianthypergiants
There are also giant K-type stars, which range from hypergiants like RW Cephei, to giants and supergiants, such as Arcturus, whereas orange dwarfs, like Alpha Centauri B, are main-sequence stars.
A hypergiant (luminosity class 0 or Ia + ) is among the very rare kinds of stars that typically show tremendous luminosities and very high rates of mass loss by stellar winds.

Hertzsprung–Russell diagram

color-magnitude diagramHR diagramcolor magnitude diagram
The fact that the Harvard classification of a star indicated its surface or photospheric temperature (or more precisely, its effective temperature) was not fully understood until after its development, though by the time the first Hertzsprung–Russell diagram was formulated (by 1914), this was generally suspected to be true.
The Hertzsprung–Russell diagram, abbreviated as H–R diagram, HR diagram or HRD, is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their stellar classifications or effective temperatures.

Subgiant

subgiant staryellow subgiantsubgiant branch
A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as true giant stars.

Subdwarf

blue-white subdwarfsdcool subdwarf
A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the Yerkes spectral classification system.

Carbon star

carbon starsCCarbon
The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars.
Carbon stars have quite distinctive spectral characteristics, and they were first recognized by their spectra by Angelo Secchi in the 1860s, a pioneering time in astronomical spectroscopy.

HD 93129

HD 93129 AHD 93129 B
An example star is HD 93129 B.
HD 93129 is a triple star system in the Carina Nebula, with all three components being hot O class stars amongst the most luminous stars in the Milky Way.

Bright giant

bright giant stargiant
The luminosity class II in the Yerkes spectral classification is given to bright giants.

Giant star

giantorange giantgiants
There are also giant K-type stars, which range from hypergiants like RW Cephei, to giants and supergiants, such as Arcturus, whereas orange dwarfs, like Alpha Centauri B, are main-sequence stars. The gravity, and hence the pressure, on the surface of a giant star is much lower than for a dwarf star because the radius of the giant is much greater than a dwarf of similar mass. Therefore, differences in the spectrum can be interpreted as luminosity effects and a luminosity class can be assigned purely from examination of the spectrum.
They lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III.

White dwarf

white dwarfswhite dwarf starcentral star
The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars.
In 1910, Henry Norris Russell, Edward Charles Pickering and Williamina Fleming discovered that, despite being a dim star, 40 Eridani B was of spectral type A, or white.

Betelgeuse

α OriBetelg'''euseBetelgeuse mass loss
Although most class M stars are red dwarfs, most of the largest ever supergiant stars in the Milky Way are M stars, such as VY Canis Majoris, Antares and Betelgeuse, which are also class M. Furthermore, the larger, hotter brown dwarfs are late class M, usually in the range of M6.5 to M9.5.
Classified as a red supergiant of spectral type M1-2, the star is one of the largest stars visible to the naked eye.

UBV photometric system

V bandVJohnson
Other modern stellar classification systems, such as the UBV system, are based on color indexes—the measured differences in three or more color magnitudes.
The UBV photometric system (Ultraviolet, Blue, Visual), also called the Johnson system (or Johnson-Morgan system), is a wide band photometric system for classifying stars according to their colors.

Altair

αα AqlAlpha
Altair is an A-type main sequence star with an apparent visual magnitude of 0.77 and is one of the vertices of the Summer Triangle asterism (the other two vertices are marked by Deneb and Vega).

Antares

Antares Bα Scorpionisα
Although most class M stars are red dwarfs, most of the largest ever supergiant stars in the Milky Way are M stars, such as VY Canis Majoris, Antares and Betelgeuse, which are also class M. Furthermore, the larger, hotter brown dwarfs are late class M, usually in the range of M6.5 to M9.5.
Classified as a red supergiant of spectral type M1.5Iab-Ib, Antares is the brightest, most massive, and most evolved stellar member of the nearest OB association, the Scorpius–Centaurus Association.

Dwarf star

dwarfDSdwarfs
The gravity, and hence the pressure, on the surface of a giant star is much lower than for a dwarf star because the radius of the giant is much greater than a dwarf of similar mass. Therefore, differences in the spectrum can be interpreted as luminosity effects and a luminosity class can be assigned purely from examination of the spectrum.
The term was originally coined in 1906 when the Danish astronomer Ejnar Hertzsprung noticed that the reddest stars—classified as K and M in the Harvard scheme could be divided into two distinct groups.

Achernar

AchenarAchernar Three
Of the ten apparent brightest stars in the night-time sky, Alpha Eridani is the hottest and bluest in color, due to Achernar being of spectral type B. Achernar has an unusually rapid rotational velocity, causing it to become oblate in shape.

Shell star

γ Cassiopeiae variableBe shell starShell
Shell stars have spectral types O7.5 to F5, with rotation velocities of 200–300 km/s, not far from the point when the rotational acceleration would disrupt the star.

William Wilson Morgan

MorganWilliam MorganMorgan, W. W.
The Yerkes spectral classification, also called the MKK system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by William Wilson Morgan, Philip C. Keenan, and Edith Kellman from Yerkes Observatory.
Along with Philip Keenan he developed the MK system for the classification of stars through their spectra.

Edith Kellman

The Yerkes spectral classification, also called the MKK system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by William Wilson Morgan, Philip C. Keenan, and Edith Kellman from Yerkes Observatory.
Edith Kellman (April 4, 1911, Walworth, Wisconsin – May 11, 2007, Walworth, Wisconsin ) was a noted American astronomer who is known for her work on the Yerkes system of stellar classification, also called the MKK system.

Rigel

β Ori (Rigel)B2 IaeBeta Orionis
B8Ia – Rigel
Since 1943, the spectrum of this star has served as the spectral reference, for class B8Ia, by which other stars are classified.

B(e) star

B[e] starB[e] starsA[e
Objects known as [[B(e) star|"B(e)" or "B[e]" stars]] possess distinctive neutral or low ionisation emission lines that are considered to have 'forbidden mechanisms', undergoing processes not normally allowed under current understandings of quantum mechanics.
A B[e] star, frequently called a B[e]-type star, is a B-type star with distinctive forbidden neutral or low ionisation emission lines in its spectrum.

B-type main-sequence star

Bblue main-sequence starblue-white main sequence star
About 1 in 800 (0.125%) of the main-sequence stars in the solar neighborhood are B-type main-sequence stars.
A B-type main-sequence star (B V) is a main-sequence (hydrogen-burning) star of spectral type B and luminosity class V. These stars have from 2 to 16 times the mass of the Sun and surface temperatures between 10,000 and 30,000 K.