Stellar classification

spectral typeK-typeG-typeluminosity classspectral classB-typeF-typeA-typeK-type starB-type star
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 surface temperature around 5,800 K.
The Sun is a G-type main-sequence star (G2V) based on its spectral class.

Annie Jump Cannon

Annie CannonCannonAnnie J. Cannon
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 dwarvesL dwarf
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 VV Cephei, 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.

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.

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.

Hertzsprung–Russell diagram

Hertzsprung-Russell 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.

Red dwarf

M dwarfredred dwarf stars
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.
When explicitly defined, it typically includes late K- and early to mid-M-class stars, but in many cases it is restricted just to M-class stars.

Star

starsstellarmassive star
In astronomy, stellar classification is the classification of stars based on their spectral characteristics.
In 1865, Secchi began classifying stars into spectral types.

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.

UBV photometric system

UBV systemV bandV
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.

HD 93129

HD 93129AHD 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.

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.

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 giant stars.

Bright giant

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

Subdwarf

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

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.

Dwarf star

dwarfDSDwarf star (disambiguation)
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 ThreeAlpha Eridani
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.

Betelgeuse

Alpha OrionisBételgeuseα Ori
Although most class M stars are red dwarfs, most of the largest ever supergiant stars in the Milky Way are M stars, such as VV Cephei, 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, Betelgeuse is one of the largest stars visible to the naked eye.

Cecilia Payne-Gaposchkin

Cecilia PayneSergei Gaposchkin[14
Harvard astronomer Cecilia Payne then demonstrated that the O-B-A-F-G-K-M spectral sequence is actually a sequence in temperature.
Payne was able to accurately relate the spectral classes of stars to their actual temperatures by applying the ionization theory developed by Indian physicist Meghnad Saha.

Spectrum

spectraenergy spectrumspectral
Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines.
Stellar classification is the categorisation of stars based on their characteristic electromagnetic spectra.

Altair

αAlpha Aquilaeα Aql
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 ScorpionisAlpha Scorpii
Although most class M stars are red dwarfs, most of the largest ever supergiant stars in the Milky Way are M stars, such as VV Cephei, 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 a red supergiant, a large evolved massive star.

Philip Childs Keenan

Philip C. KeenanPhilip KeenanKeenan
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.
Keenan was an American spectroscopist who collaborated with William Wilson Morgan and Edith Kellman (1911–2007) to develop the MKK stellar spectral classification system between 1939 and 1943.