Stellar classification
Overview
 
In astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

, stellar classification is a classification of star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s based on their spectral
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...

 characteristics. The spectral class of a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 is a designated class of a star describing the ionization
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...

 of its chromosphere
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....

, what atomic excitations
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....

 are most prominent in the light, giving an objective measure of the temperature in this chromosphere. Light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 from the star is analyzed by splitting it up by a diffraction grating
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...

, subdividing the incoming photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s into a spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...

 exhibiting a rainbow of colors interspersed by absorption lines
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...

, each line indicating a certain ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

 of a certain chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

.
Encyclopedia
In astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

, stellar classification is a classification of star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s based on their spectral
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...

 characteristics. The spectral class of a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 is a designated class of a star describing the ionization
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...

 of its chromosphere
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....

, what atomic excitations
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....

 are most prominent in the light, giving an objective measure of the temperature in this chromosphere. Light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 from the star is analyzed by splitting it up by a diffraction grating
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...

, subdividing the incoming photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s into a spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...

 exhibiting a rainbow of colors interspersed by absorption lines
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...

, each line indicating a certain ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

 of a certain chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

. The presence of a certain chemical element in such an absorption spectrum primarily indicates that the temperature conditions are suitable for a certain excitation of this element. If the star temperature has been determined by a majority of absorption lines, unusual absences or strengths of lines for a certain element may indicate an unusual chemical composition of the chromosphere.

Most stars are currently classified using the letters O, B, A, F, G, K, and M (usually memorized
Mnemonic
A mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...

 by astrophysicists as "Oh, be a fine girl/guy, kiss me"), where O stars are the hottest and the letter sequence indicates successively cooler stars up to the coolest M class. According to informal tradition, O stars are called "blue", B "blue-white", A stars "white", F stars "yellow-white", G stars "yellow", K stars "orange", and M stars "red", even though the actual star colors perceived by an observer may deviate from these colors depending on visual conditions and individual stars observed. The current non-alphabetical scheme developed from an earlier scheme using all letters from A to O; the original letters were retained but the star classes were re-ordered in the current temperature order when the connection between the stars' class and temperatures became clear. A few star classes were dropped as duplicates of others.

In the current star classification system, the Morgan-Keenan system, the spectrum letter is enhanced by a number from 0 to 9 indicating tenths of the range between two star classes, so that A5 is five tenths between A0 and F0, but A2 is two tenths of the full range from A0 to F0. Lower numbered stars in the same class are hotter. Another dimension that is included in the Morgan-Keenan system is the luminosity class expressed by the Roman numbers
Roman numerals
The numeral system of ancient Rome, or Roman numerals, uses combinations of letters from the Latin alphabet to signify values. The numbers 1 to 10 can be expressed in Roman numerals as:...

 I, II, III, IV and V, expressing the width of certain absorption lines in the star's spectrum. It has been shown that this feature is a general measure of the size of the star, and thus of the total luminosity output from the star. Class I are generally called supergiants, class III simply giants and class V either dwarfs or more properly main sequence stars. For example, our Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 has the spectral type G2V, which might be interpreted as "a 'yellow' two tenths towards 'orange' main sequence star". The apparently brightest star Sirius
Sirius
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...

 has type A1V.

Secchi classes

During the 1860s and 1870s, pioneering stellar spectroscopist Father Angelo Secchi
Angelo Secchi
-External links:...

 created the Secchi classes in order to classify observed spectra. By 1866, he had developed three classes of stellar spectra:
  • Class I: white and blue stars with broad heavy hydrogen line
    Hydrogen line
    The hydrogen line, 21 centimeter line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420.40575177 MHz, which is equivalent to the vacuum...

    s, such as Vega
    Vega
    Vega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus...

     and Altair. This includes the modern class A and early class F.
    Class I, Orion subtype: a subtype of class I with narrow lines in place of wide bands, such as Rigel
    Rigel
    Rigel is the brightest star in the constellation Orion and the sixth brightest star in the sky, with visual magnitude 0.18...

     and Bellatrix. In modern terms, this corresponds to early B-type stars.
  • Class II: yellow stars—hydrogen less strong, but evident metallic lines, such as the Sun
    Sun
    The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

    , Arcturus and Capella
    Capella (star)
    Capella is the brightest star in the constellation Auriga, the sixth brightest star in the night sky and the third brightest star in the northern celestial hemisphere, after Arcturus and Vega. Although it appears to be a single star to the naked eye, it is actually a star system of four stars in...

    . This includes the modern classes G and K as well as late class F.
  • Class III: orange to red stars with complex band spectra, such as Betelgeuse
    Betelgeuse
    Betelgeuse, also known by its Bayer designation Alpha Orionis , is the eighth brightest star in the night sky and second brightest star in the constellation of Orion, outshining its neighbour Rigel only rarely...

     and Antares
    Antares
    Antares is a red supergiant star in the Milky Way galaxy and the sixteenth brightest star in the nighttime sky . Along with Aldebaran, Spica, and Regulus it is one of the four brightest stars near the ecliptic...

    . This corresponds to the modern class M.

In 1868, he discovered carbon star
Carbon star
A carbon star is a late-type star similar to a red giant whose atmosphere contains more carbon than oxygen; the two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds,...

s, which he put into a distinct group:
  • Class IV: red stars with significant carbon
    Carbon
    Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

     bands and lines (carbon stars.)

In 1877, he added a fifth class:
  • Class V: emission-line stars, such as γ Cassiopeiae and Sheliak.

In the late 1890s, this classification began to be superseded by the Harvard classification, which is discussed in the remainder of this article.

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
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

s. 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:
Class Temperature
(kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

s)
Conventional color 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
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

≥ 6.6 R ≥ 30,000 L Weak ~0.00003%
B 10,000–33,000 K blue to blue white blue white 2.1–16 M
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

1.8–6.6 R 25–30,000 L Medium 0.13%
A 7,500–10,000 K white white to blue white 1.4–2.1 M
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

1.4–1.8 R 5–25 L Strong 0.6%
F 6,000–7,500 K yellowish white white 1.04–1.4 M
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

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
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

0.96–1.15 R 0.6–1.5 L Weak 7.6%
K 3,700–5,200 K orange yellow orange 0.45–0.8 M
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

0.7–0.96 R 0.08–0.6 L Very weak 12.1%
M ≤ 3,700 K red orange red ≤ 0.45 M
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

≤ 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
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

 portion of their lives and so are not appropriate for red giants. The spectral classes O through M are subdivided by Arabic numerals
Arabic numerals
Arabic numerals or Hindu numerals or Hindu-Arabic numerals or Indo-Arabic numerals are the ten digits . They are descended from the Hindu-Arabic numeral system developed by Indian mathematicians, in which a sequence of digits such as "975" is read as a numeral...

 (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.
Classifications in the Draper Catalogue of Stellar Spectra
Secchi Draper Comment
I A, B, C, D Hydrogen lines dominant.
II E, F, G, H, I, K, L
III M
IV N Did not appear in the catalogue.
  O Wolf-Rayet
Wolf-Rayet
Wolf-Rayet can mean:* Wolf-Rayet star, a type of evolved, massive star* Wolf-Rayet galaxy, which contains large numbers of WR stars* Wolf-Rayet nebula, which surrounds a Wolf-Rayet star...

 spectra with bright lines.
  P Planetary nebulae.
  Q Other spectra.

The reason for the odd arrangement of letters is historical. An early classification of spectra by Angelo Secchi in the 1860s divided stars into those with prominent lines from the hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 Balmer series
Balmer series
The Balmer series or Balmer lines in atomic physics, is the designation of one of a set of six different named series describing the spectral line emissions of the hydrogen atom....

 (group I, with a subtype representing many of the stars in Orion
Orion (constellation)
Orion, often referred to as The Hunter, is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky...

); those with spectra which, like the Sun, showed calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

 and sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

 lines (group II); colored stars whose spectra showed wide bands (group III); and carbon stars (group IV). In the 1880s, the astronomer Edward C. Pickering began to make a survey of stellar spectra at the Harvard College Observatory
Harvard College Observatory
The Harvard College Observatory is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Harvard University Department of Astronomy. It is located in Cambridge, Massachusetts, USA, and was founded in 1839...

, using the objective-prism method. A first result of this work was the Draper Catalogue of Stellar Spectra, published in 1890. Williamina Fleming
Williamina Fleming
-External links:* * * * from the Astronomical Society of the Pacific- Obituaries :*...

 classified most of the spectra in this catalogue. It used a scheme in which the previously used Secchi classes (I to IV) were divided into more specific classes, given letters from A to N. Also, the letters O, P and Q were used, O for stars whose spectra consisted mainly of bright lines, P for planetary nebulae, and Q for stars not fitting into any other class.

In 1897, another worker at Harvard, Antonia Maury
Antonia Maury
Antonia Caetana de Paiva Pereira Maury was an American astronomer who published an important early catalog of stellar spectra.-Early life:Antonia Maury was born in Cold Spring, New York...

, placed the Orion subtype of Secchi class I ahead of the remainder of Secchi class I, thus placing the modern type B ahead of the modern type A. She was the first to do so, although she did not use lettered spectral types, but rather a series of 22 types numbered from I to XXII. In 1901, Annie Jump Cannon
Annie Jump Cannon
Annie Jump Cannon was an American astronomer whose cataloging work was instrumental in the development of contemporary stellar classification. With Edward C...

 returned to the lettered types, but dropped all letters except O, B, A, F, G, K, and M, used in that order, as well as P for planetary nebulae and Q for some peculiar spectra. She also used types such as B5A for stars halfway between types B and A, F2G for stars one-fifth of the way from F to G, and so forth. Finally, by 1912, Cannon had changed the types B, A, B5A, F2G, etc. to B0, A0, B5, F2, etc. This is essentially the modern form of the Harvard classification system.

The fact that the Harvard classification of a star indicated its surface temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 was not fully understood until after its development. In the 1920s, the Indian physicist Meghnad Saha
Meghnad Saha
Meghnad Saha FRS was an Indian astrophysicist best known for his development of the Saha equation, used to describe chemical and physical conditions in stars.-Early life:...

 derived a theory of ionization by extending well-known ideas in physical chemistry pertaining to the dissociation of molecules to the ionization of atoms. First applied to the solar chromosphere, he then applied it to stellar spectra. The Harvard astronomer Cecilia Helena Payne (later to become Cecilia Payne-Gaposchkin
Cecilia Payne-Gaposchkin
-Further reading:*Rubin, Vera , "Cecilia Payne-Gaposchkin" in OUT OF THE SHADOWS: Contributions of 20th Century Women to Physics, Nina Byers and Gary Williams, ed., Cambridge University Press ....

) then demonstrated that the OBAFGKM spectral sequence is actually a sequence in temperature. Because the classification sequence predates our understanding that it is a temperature sequence, the placement of a spectrum into a given subtype, such as B3 or A7, depends upon (largely subjective) estimates of the strengths of absorption features in stellar spectra. As a result, these subtypes are not evenly divided into any sort of mathematically representable intervals.

O, B, and A stars are sometimes misleadingly called "early type", while K and M stars are said to be "late type". This stems from an early 20th century model of stellar evolution in which stars were powered by gravitational contraction via the Kelvin–Helmholtz mechanism in which stars start their lives as very hot "early-type" stars, and then gradually cool down, thereby evolving into "late-type" stars. This mechanism provided ages of the Sun that were much smaller than what is observed, and was rendered obsolete by the discovery that stars are powered by nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

. However, brown dwarfs, whose energy comes from gravitational attraction alone, cool as they age and so progress to later spectral types. The highest-mass brown dwarfs start their lives with M-type spectra and will cool through the L, T, and Y spectral classes.

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 table colors used are D65 standard colors, which is what one would see if the star light would be intensely magnified and projected onto a white paper, then observed in ordinary daylight. Most stars in the sky, except the brightest ones, appear white or bluish white to the unaided eye because they are too dim for color vision to work.

Our Sun itself is white. It is sometimes called a yellow star (spectroscopically, relative to Vega), and may appear yellow or red (viewed through the atmosphere), or appear white (viewed when too bright for the eye to see any color). Astronomy images often use a variety of exaggerated colors (partially founded in faint-light conditions observations, partially in conventions). But the Sun's own intrinsic color is white (aside from sunspots), with no trace of color, and closely approximates a black body
Black body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...

 of 5780 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

 (see color temperature
Color temperature
Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics, and other fields. The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of...

). 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. The Sun is known as a G-type star.

Yerkes spectral classification



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
William Wilson Morgan
William Wilson Morgan was an American astronomer.The principal theme in Morgan's work was the study of stellar and galaxy classification. Along...

, Philip C. Keenan
Philip Childs Keenan
Philip Childs Keenan was an American astronomer.Keenan was an American spectroscopist who collaborated with William Wilson Morgan and Edith Kellman to develop the MKK stellar spectral classification system between 1939 and 1943...

, and Edith Kellman from Yerkes Observatory
Yerkes Observatory
Yerkes Observatory is an astronomical observatory operated by the University of Chicago in Williams Bay, Wisconsin. The observatory, which calls itself "the birthplace of modern astrophysics," was founded in 1897 by George Ellery Hale and financed by Charles T. Yerkes...

. This two-dimensional (temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 and luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...

) classification scheme is based on spectral line
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...

s sensitive to stellar temperature and surface gravity
Surface gravity
The surface gravity, g, of an astronomical or other object is the gravitational acceleration experienced at its surface. The surface gravity may be thought of as the acceleration due to gravity experienced by a hypothetical test particle which is very close to the object's surface and which, in...

 which is related to luminosity (whilst the Harvard classification is based on surface temperature only). Later, in 1953, after some revisions of list of standard stars and classification criteria, the scheme was named MK (by William Wilson Morgan and Phillip C. Keenan initials).

Since the radius of a giant star
Giant star
A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Stars still more luminous than giants are...

 is much greater than a dwarf star
Dwarf star
The term dwarf star refers to a variety of distinct classes of stars.* Dwarf star alone generally refers to any main sequence star, a star of luminosity class V.** Red dwarfs are low-mass main sequence stars....

 while their masses are roughly comparable, the gravity and thus the gas density and pressure on the surface of a giant star are much lower than for a dwarf. These differences manifest themselves in the form of luminosity effects which affect both the width and the intensity of spectral lines which can then be measured. Denser stars with higher surface gravity will exhibit greater pressure broadening of spectral lines.
A number of different luminosity classes are distinguished:
  • 0 hypergiant
    Hypergiant
    A hypergiant is a star with a tremendous mass and luminosity, showing signs of a very high rate of mass loss.-Characteristics:...

    s
  • I supergiant
    Supergiant
    Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram. In the Yerkes spectral classification, supergiants are class Ia or Ib . They typically have bolometric absolute magnitudes between -5 and -12...

    s
    • Ia-0 (hypergiant
      Hypergiant
      A hypergiant is a star with a tremendous mass and luminosity, showing signs of a very high rate of mass loss.-Characteristics:...

      s or extremely luminous supergiant
      Supergiant
      Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram. In the Yerkes spectral classification, supergiants are class Ia or Ib . They typically have bolometric absolute magnitudes between -5 and -12...

      s (later addition)), Example: Eta Carinae (spectrum-peculiar)
    • Ia (luminous supergiants), Example: Deneb
      Deneb
      Deneb is the brightest star in the constellation Cygnus and one of the vertices of the Summer Triangle. It is the 19th brightest star in the night sky, with an apparent magnitude of 1.25. A blue-white supergiant, Deneb is also one of the most luminous nearby stars...

       (spectrum is A2Ia)
    • Iab (intermediate luminous supergiants) Example: Betelgeuse
      Betelgeuse
      Betelgeuse, also known by its Bayer designation Alpha Orionis , is the eighth brightest star in the night sky and second brightest star in the constellation of Orion, outshining its neighbour Rigel only rarely...

       (spectrum is M2Iab)
    • Ib (less luminous supergiants)
  • II bright giant
    Bright giant
    The luminosity class II in the Yerkes spectral classification is given to bright giants. These are stars which straddle the boundary between giants and supergiants, and the classification is in general given to giant stars with exceptionally high luminosity, but which are not sufficiently bright...

    s
    • IIa, Example: β Scuti
      Beta Scuti
      Beta Scuti is a binary star in the constellation Scutum. It is approximately 690 light years from Earth.The primary component is a yellow G-type bright giant with an apparent magnitude of +4.22. It has a magnitude +8.5 companion classified as spectral type B9.5. The orbital period of the binary...

       (HD 173764) (spectrum is G4 IIa)
    • IIab Example: HR 8752 (spectrum is G0Iab:)
    • IIb, Example: HR 6902 (spectrum is G9 IIb)
  • III normal giants
    Giant star
    A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Stars still more luminous than giants are...

    • IIIa, Example: ρ Persei
      Rho Persei
      Rho Persei is a star in the constellation Perseus. It has the traditional name Gorgonea Tertia.Rho Persei is a semiregular variable star of the μ Cephei type, whose apparent magnitude varies between 3.3 and 4.0 with periods of 50 days and 1100 days. It is of spectral class M4 and is approximately...

       (spectrum is M4 IIIa)
    • IIIab Example: δ Reticuli (spectrum is M2 IIIab)
    • IIIb, Example: Pollux
      Pollux (star)
      Pollux is an orange giant star approximately 34 light-years from the Earth in the constellation of Gemini . Pollux is the brightest star in the constellation, brighter than Castor...

       (spectrum is K2 IIIb)
  • IV subgiants
    Subgiant star
    A subgiant star is a star that is slightly brighter than a normal main-sequence star of the same spectral class, but not as bright as true giant stars...

    • IVa, Example: ε Reticuli
      Epsilon Reticuli
      Epsilon Reticuli is a binary system approximately 59 light-years away in the constellation of Reticulum. The primary star is an orange subgiant star, while the secondary star is a white dwarf star. The primary star should be easily visible without optical aid under dark skies in the southern...

       (spectrum is K1-2 IVa-III)

    • IVb, Example: HR 672 A (spectrum is G0.5 IVb)
  • V main sequence
    Main sequence
    The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

     stars (dwarfs)
    • Va, Example: AD Leonis (spectrum M4Vae)
    • Vab
    • Vb, Example: 85 Pegasi A (spectrum G5 Vb)
    • "Vz", Example: LH10 : 3102 (spectrum O7 Vz), located in the Large Magellanic Cloud
      Large Magellanic Cloud
      The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...

      .
  • VI subdwarfs
    Subdwarf star
    A subdwarf star, sometimes denoted by "sd", is luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type...

    . Subdwarfs are generally represented with a prescript of sd
    Subdwarf star
    A subdwarf star, sometimes denoted by "sd", is luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type...

     or esd (extreme subdwarf) in front of the spectra.
    • sd, Example: SSSPM J1930-4311 (spectrum sdM7)
    • esd, Example: APMPM J0559-2903 (spectrum esdM7)
  • VII (uncommon) white dwarf
    White dwarf
    A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...

    s. White dwarfs are represented with a prescript wD or WD.


Marginal cases are allowed; for instance a star classified as Ia-0 would be a very luminous supergiant, verging on hypergiant. Examples are below. The spectral type of the star is not a factor.
Marginal Symbols Example Explanation
- G2 I-II A star is between supergiant and bright giant.
+ O9.5 Ia+ A star is a hypergiant star.
/ M2 IV/V A star is either a subgiant or a dwarf star.

Spectral types

The following illustration represents star classes with the colors very close to those actually perceived by the human eye. The relative sizes are for main-sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

 or "dwarf" stars.

Class O

Class O stars are very hot and extremely luminous, being bluish in color; in fact, most of their output is in the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 range. These are the rarest of all main-sequence stars. About 1 in 3,000,000 of the main-sequence stars in the solar neighborhood are Class O stars.These proportions are fractions of stars brighter than absolute magnitude 16; lowering this limit will render earlier types even rarer while generally adding only to the M class. Some of the most massive stars lie within this spectral class. Type-O stars are so hot as to have complicated surroundings which make measurement of their spectra difficult.
O-stars shine with a power over a million times our Sun's output. These stars have dominant lines of absorption and sometimes emission for He
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 II lines, prominent ionized (Si
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 IV, O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 III, N
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 III, and C
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 III) and neutral helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 lines, strengthening from O5 to O9, and prominent hydrogen Balmer lines, although not as strong as in later types. Because they are so massive, class O stars have very hot cores, thus burn through their hydrogen fuel very quickly, and so are the first stars to leave the main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

. Recent observations by the Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...

 indicate that planetary formation does not occur around other stars in the vicinity of an O class star due to the photoevaporation effect.

When the MKK classification scheme was first described in 1943, the only subtypes of class O used were O5 to O9.5. The MKK scheme was extended to O4 in 1978, and new classification schemes have subsequently been introduced which add types O2, O3 and O3.5. O3 stars are the hottest currently known stars of conventional structure.
Examples: Zeta Orionis, Zeta Puppis
Zeta Puppis
Zeta Puppis is a star in the constellation of Puppis. It is also known by the traditional names Naos and Suhail Hadar in Arabic....

, Lambda Orionis
Lambda Orionis
Lambda Orionis is a star in the constellation Orion. It has the traditional names Meissa or Heka. "Meissa" derives from the Arabic "Al-Maisan" which means "The Shining One". This term was used for Gamma Gemini , but was somehow also mistakenly applied to λ Orionis and the name stuck...

, Delta Orionis
Delta Orionis
Delta Orionis , traditionally known as Mintaka , is a star some 900 light years distant in the constellation Orion. Together with Zeta Orionis and Epsilon Orionis , the three stars make up the belt of Orion, known by many names across many ancient cultures...

, Theta¹ Orionis C, HD 93129A
HD 93129A
HD 93129A is one of the most luminous stars in the Milky Way. This very young blue hypergiant is an O-type hypergiant located about 7500 light-years from Earth in the bright nebula NGC 3372, the same nebula that harbors other super luminous stars, like Eta Carinae.HD 93129A is actually the brighter...


Class B

Class B stars are very luminous and blue. Their spectra have neutral helium, which are most prominent at the B2 subclass, and moderate hydrogen lines. Ionized metal lines include Mg
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 II and Si
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 II. As O and B stars
OB star
OB stars are hot, massive stars of spectral types O or B which form in loosely organized groups called OB associations. They are short lived, and thus don't move very far from where they were formed within their life. During their lifetime, they will emit copious amounts of ultraviolet radiation...

 are so powerful, they only live for a very short time, and thus they do not stray far from the area in which they were formed.

These stars tend to cluster together in what are called OB associations, which are associated with giant molecular cloud
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....

s. The Orion OB1 association occupies a large portion of a spiral arm of our galaxy
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 and contains many of the brighter stars of the constellation Orion. About 1 in 800 of the main-sequence stars in the solar neighborhood are Class B stars.
Examples: Rigel
Rigel
Rigel is the brightest star in the constellation Orion and the sixth brightest star in the sky, with visual magnitude 0.18...

, Spica
Spica
Spica is the brightest star in the constellation Virgo, and the 15th brightest star in the nighttime sky. It is 260 light years distant from Earth...

, the brighter Pleiades
Pleiades (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky...

, VV Cephei B
VV Cephei
VV Cephei, also known as HD 208816, is an eclipsing binary star system located in the constellation Cepheus, approximately 2,400 light years from Earth.Size, mass and luminosity estimates are all considerably uncertain due to insufficient knowledge of the Cephei star system: Professor Kaler writes...

, Algol A

Class A

Class A stars are amongst the more common naked eye stars, and are white or bluish-white. They have strong hydrogen lines, at a maximum by A0, and also lines of ionized metals (Fe
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 II, Mg
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 II, Si
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 II) at a maximum at A5. The presence of Ca
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

 II lines is notably strengthening by this point. About 1 in 160 of the main-sequence stars in the solar neighborhood are Class A stars.
Examples: Sirius
Sirius
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...

, Deneb
Deneb
Deneb is the brightest star in the constellation Cygnus and one of the vertices of the Summer Triangle. It is the 19th brightest star in the night sky, with an apparent magnitude of 1.25. A blue-white supergiant, Deneb is also one of the most luminous nearby stars...

, Altair, Vega
Vega
Vega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus...


Class F

Class F stars have strengthening H and K lines of Ca
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

 II. Neutral metals (Fe
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 I, Cr
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

 I) beginning to gain on ionized metal lines by late F. Their spectra are characterized by the weaker hydrogen lines and ionized metals. Their color is white. About 1 in 33 of the main-sequence stars in the solar neighborhood are Class F stars.
Examples: Alrakis
Mu Draconis
Mu Draconis is a binary star with a combined magnitude of 4.92m located approximately 85 light years from the Solar System, near the head of the constellation Draco. The component stars are nearly identical yellow-white stars in a tight orbit...

, Canopus
Canopus
Canopus |Alpha]] Carinae) is the brightest star in the southern constellation of Carina and Argo Navis, and the second brightest star in the night-time sky, after Sirius. Canopus's visual magnitude is −0.72, and it has an absolute magnitude of −5.53.Canopus is a supergiant of spectral...

, Dubhe B, Polaris
Polaris
Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....

, Procyon
Procyon
Procyon is the brightest star in the constellation Canis Minor. To the naked eye, it appears to be a single star, the seventh brightest in the night sky with a visual apparent magnitude of 0.34...

, Wezen
Delta Canis Majoris
Delta Canis Majoris is a star in the constellation Canis Major. It has the traditional name Wezen or Wesen...


Class G


Class G stars are probably the best known, if only for the reason that the Sun is of this class. About one in thirteen of the main-sequence stars in the solar neighborhood are Class G stars.

Most notable are the H and K lines of Ca
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

 II, which are most prominent at G2. They have even weaker hydrogen lines than F, but along with the ionized metals, they have neutral metals. There is a prominent spike in the G band of CH molecules. G is host to the "Yellow Evolutionary Void". Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the G classification as this is an extremely unstable place for a supergiant to be.
Examples: The Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, Alpha Centauri A, Capella
Capella (star)
Capella is the brightest star in the constellation Auriga, the sixth brightest star in the night sky and the third brightest star in the northern celestial hemisphere, after Arcturus and Vega. Although it appears to be a single star to the naked eye, it is actually a star system of four stars in...

, Tau Ceti
Tau Ceti
Tau Ceti is a star in the constellation Cetus that is spectrally similar to the Sun, although it has only about 78% of the Sun's mass. At a distance of just under 12 light-years from the Solar System, it is a relatively close star. Tau Ceti is metal-deficient and so is thought to be less likely to...


Class K

Class K are orangish stars that are slightly cooler than our Sun. Some K stars are giants and supergiants, such as Arcturus, while orange dwarfs, like Alpha Centauri
Alpha Centauri
Alpha Centauri is the brightest star in the southern constellation of Centaurus...

 B, are main-sequence stars. They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals (Mn
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...

 I, Fe
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 I, Si
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 I).

By late K, molecular bands of titanium oxide
Titanium oxide
Titanium oxide may refer to:* Titanium dioxide , TiO2* Titanium oxide , TiO, a non-stoichiometric oxide* Titanium oxide , Ti2O3* Ti3O* Ti2O* δ-TiOx...

 become present. About one in eight of the main-sequence stars in the solar neighborhood are Class K stars. There is a suggestion that K Spectrum stars are very well suited for biology.
Examples: Alpha Centauri B, Epsilon Eridani
Epsilon Eridani
Epsilon Eridani is a star in the southern constellation Eridanus, along a declination 9.46° south of the celestial equator. This allows the star to be viewed from most of the Earth's surface. At a distance of 10.5 light years , it has an apparent magnitude of 3.73...

, Arcturus, Aldebaran
Aldebaran
Aldebaran is a red giant star located about 65 light years away in the zodiac constellation of Taurus. With an average apparent magnitude of 0.87 it is the brightest star in the constellation and is one of the brightest stars in the nighttime sky...

, Algol B

Class M

Class M is by far the most common class. About 76% of the main-sequence stars in the Solar neighborhood are Class M stars.This rises to 78.6% if we include all stars. (See the above note.)

Although most Class M stars are red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....

s, the class also hosts most giants and some supergiants such as Antares
Antares
Antares is a red supergiant star in the Milky Way galaxy and the sixteenth brightest star in the nighttime sky . Along with Aldebaran, Spica, and Regulus it is one of the four brightest stars near the ecliptic...

 and Betelgeuse
Betelgeuse
Betelgeuse, also known by its Bayer designation Alpha Orionis , is the eighth brightest star in the night sky and second brightest star in the constellation of Orion, outshining its neighbour Rigel only rarely...

, as well as Mira
Mira
Mira also known as Omicron Ceti , is a red giant star estimated 200-400 light years away in the constellation Cetus. Mira is a binary star, consisting of the red giant Mira A along with Mira B. Mira A is also an oscillating variable star and was the first non-supernova variable star discovered,...

 variables
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...

. The late-M group holds hotter brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

s that are above the L spectrum. This is usually in the range of M6.5 to M9.5. The spectrum of an M star shows lines belonging to molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s and all neutral metals but hydrogen lines are usually absent. Titanium oxide
Titanium oxide
Titanium oxide may refer to:* Titanium dioxide , TiO2* Titanium oxide , TiO, a non-stoichiometric oxide* Titanium oxide , Ti2O3* Ti3O* Ti2O* δ-TiOx...

 can be strong in M stars, usually dominating by about M5. Vanadium oxide
Vanadium oxide
Vanadium oxide may refer to:* Vanadium oxide , VO* Vanadium oxide , V2O3* Vanadium oxide , VO2* Vanadium oxide , V2O5...

 bands become present by late M.

Example: VY Canis Majoris
VY Canis Majoris
VY Canis Majoris is the largest known star and also one of the most luminous. Located in the constellation Canis Major, it is a red hypergiant, between 1800 and 2100 solar radii, 8.4–9.8 astronomical units in radius, about 3.0 billion km or 1.9 billion miles in diameter, and about 1.5 kiloparsecs ...

 (hypergiant
Hypergiant
A hypergiant is a star with a tremendous mass and luminosity, showing signs of a very high rate of mass loss.-Characteristics:...

)
Examples: Betelgeuse
Betelgeuse
Betelgeuse, also known by its Bayer designation Alpha Orionis , is the eighth brightest star in the night sky and second brightest star in the constellation of Orion, outshining its neighbour Rigel only rarely...

, Antares
Antares
Antares is a red supergiant star in the Milky Way galaxy and the sixteenth brightest star in the nighttime sky . Along with Aldebaran, Spica, and Regulus it is one of the four brightest stars near the ecliptic...

 (supergiant
Supergiant
Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram. In the Yerkes spectral classification, supergiants are class Ia or Ib . They typically have bolometric absolute magnitudes between -5 and -12...

s)
Examples: Rasalgethi
Alpha Herculis
Alpha Herculis is a multiple star system in the constellation Hercules. It has the traditional name Rasalgethi or Ras Algethi , and the Flamsteed designation 64 Herculis...

, Beta Pegasi
Beta Pegasi
Beta Pegasi is a star in the constellation Pegasus. Its traditional name is Scheat, a name that has also been used for Delta Aquarii....

 (giant
Giant star
A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Stars still more luminous than giants are...

s)
Examples: Proxima Centauri
Proxima Centauri
Proxima Centauri is a red dwarf star about 4.2 light-years distant in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa, and is the nearest known star to the Sun, although it is too faint to be seen with the naked eye...

, Barnard's star
Barnard's star
Barnard's Star, also known occasionally as Barnard's "Runaway" Star, is a very low-mass red dwarf star approximately six light-years away from Earth in the constellation of Ophiuchus . In 1916, the American astronomer E.E...

, Gliese 581
Gliese 581
Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star system to the Sun. Observations suggest that the star has at least six planets:...

 (red dwarf)
Examples: LEHPM 2-59, SSSPM J1930-4311 (subdwarf)
Example: APMPM J0559-2903 (extreme subdwarf)
Examples: Teide 1
Teide 1
Teide 1 was the first brown dwarf to be verified in 1995. This brown dwarf is located in the Pleiades open star cluster located approximately 400 light years from Earth....

 (field brown dwarf), GSC 08047-00232 B  (companion brown dwarf)

Extended spectral types

A number of new spectral types have been taken into use from newly discovered types of stars.

Hot blue emission star classes

Spectra of some very hot and bluish stars exhibit marked emission lines from carbon or nitrogen, or sometimes oxygen.

Class W: Wolf-Rayet

Class W or WR represents the superluminous Wolf-Rayet stars, notably unusual since they have mostly helium in their atmospheres instead of hydrogen. They are thought to be dying supergiants with their hydrogen layer blown away by hot stellar wind
Stellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...

s caused by their high temperatures, thereby directly exposing their hot helium shell. Class W is subdivided into subclasses
WN (WNE early-type, WNL late-type) and WC (WCE early-type, WCL late-type, and extend class WO), according to the dominance of nitrogen and carbon emission lines in their spectra (and outer layers).
  • WR spectra range is listed below:
WN
WNE (WN2 to WN5 with some WN6)
WNL (WN7 to WN9 with some WN6)
Extended WN class (WN10 to WN11), was created to encompass the Ofpe/WN9 stars.
WN/C, and intermediate class between the nitrogen-rich and carbon-rich WR stars.
WC
WCE (WC4 to WC6)
WCL (WC7 to WC9)
WO (WO1 to WO4)

  • W: Up to 70,000 K
Example: WR124 (WN)
Example: Gamma Velorum A
Gamma Velorum
Gamma Velorum is a star system in the constellation Vela. At magnitude +1.7, it is one of the brightest stars in the night sky. It has the traditional names Suhail and Suhail al Muhlif, which confusingly also apply to Lambda Velorum...

 (WC)
Example: WR93B (WO)

Classes OC, ON, BC, BN: Wolf-Rayet related O and B stars

Intermediary between the genuine Wolf-Rayets and ordinary hot stars of classes O and early B, there are OC, ON, BC and BN stars. They seem to constitute a short continuum from the Wolf-Rayets into the ordinary OBs.
Example: HD 152249 (OC)
Example: HD 105056 (ON)
Example: HD 2905 (BC)
Example: HD 163181 (BN)

The "Slash" stars

The slash stars are stars with O-type spectra and WN sequence in their spectra. The name slash comes from their spectra having a slash.
Example spectra: Of/WNL


There is a secondary group found with this spectra, a cooler, "intermediate" group. They are found in the Large Magellanic Cloud and have a designation of Ofpe/WN9.

The "class" OB

In lists of spectra, the "spectrum OB" may occur. This is in fact not a spectrum, but a marker which means that "the spectrum of this star is unknown, but it belongs to an OB association, so probably either a class O or class B star, or perhaps a fairly hot class A star."

Cool red and brown dwarf classes

The new spectral types L and T were created to classify infrared spectra of cool stars. This included both red dwarf
Red dwarf
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type....

s and brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

s which are very faint in the visual spectrum. The hypothetical spectral type Y has been reserved for objects cooler than T dwarfs which have spectra that are qualitatively distinct from T dwarfs.

Class L

Class L dwarfs get their designation because they are cooler than M stars and L is the remaining letter alphabetically closest to M. L does not mean lithium dwarf; a large fraction of these stars do not have lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...

 in their spectra. Some of these objects have masses large enough to support hydrogen fusion, but some are of substellar
Substellar object
A substellar object, sometimes called a lump, is an astronomical object whose mass is smaller than the smallest mass, approximately 0.08 solar masses, at which a star can sustain hydrogen fusion...

 mass and do not, so collectively these objects should be referred to as L dwarfs, not L stars. They are a very dark red in color and brightest in infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

. Their atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 is cool enough to allow metal hydrides and alkali metal
Alkali metal
The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...

s to be prominent in their spectra. Due to low gravities in giant stars, TiO- and VO-bearing condensates never form. Thus, larger L-type stars can never form in an isolated environment. It may be possible for these L-type supergiants to form through stellar collisions, however, an example of which is V838 Monocerotis
V838 Monocerotis
V838 Monocerotis is a red variable star in the constellation Monoceros about 20,000 light years from the Sun, and possibly one of the largest known stars. The previously unknown star was observed in early 2002 experiencing a major outburst. Originally believed to be a typical nova eruption, it...

.
  • L: 1,300–2,000 K, dwarfs (some stellar, some substellar) with metal hydrides and alkali metal
    Alkali metal
    The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...

    s prominent in their spectra.
Example: VW Hyi
Example: 2MASSW J0746425+2000321 binary
Component A is an L dwarf star
Component B is an L brown dwarf
Example: LSR 1610-0040 (subdwarf)
Example: V838 Monocerotis
V838 Monocerotis
V838 Monocerotis is a red variable star in the constellation Monoceros about 20,000 light years from the Sun, and possibly one of the largest known stars. The previously unknown star was observed in early 2002 experiencing a major outburst. Originally believed to be a typical nova eruption, it...

 (supergiants)



Class T: methane dwarfs

Class T dwarfs are cool brown dwarfs with surface temperatures between approximately 700 and 1,300 K. Their emission peaks in the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

. Methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 is prominent in their spectra.
  • T: ~700-1,300 K, cooler brown dwarf
    Brown dwarf
    Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

    s with methane
    Methane
    Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

     in the spectrum
Examples: SIMP 0136
SIMP J013656.5+093347
SIMP J013656.5 +093347 is a brown dwarf in the constellation Pisces. It belongs to the spectral class T2.5 and its position shifts due to its proper motion annually by about 1.24 arcsec with a position angle of about 90°....

 (the brightest T dwarf discovered in northern hemisphere)
Examples: Epsilon Indi
Epsilon Indi
Epsilon Indi is a K-type main-sequence star approximately 12 light-years away in the constellation of Indus. Two brown dwarfs, found in 2003, orbit the star.- Observation :...

 Ba & Epsilon Indi Bb


Class T and L could be more common than all the other classes combined if recent research is accurate. From studying the number of proplyds
Protoplanetary disk
A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...

 (protoplanetary discs, clumps of gas in nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...

e from which stars and solar systems are formed) then the number of stars in the galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 should be several orders of magnitude
Order of magnitude
An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the exponent being applied to this amount...

 higher than what we know about. It is theorized that these proplyds are in a race with each other. The first one to form will become a proto-star, which are very violent objects and will disrupt other proplyds in the vicinity, stripping them of their gas. The victim proplyds will then probably go on to become main sequence stars or brown dwarf stars of the L and T classes, but quite invisible to us. Since they live so long, these smaller stars will accumulate over time.

Class Y

The spectral class Y has been proposed for brown dwarfs that are cooler than T dwarfs and have qualitatively different spectra from them. Although such dwarfs have been modelled, there is no well-defined spectral sequence yet with prototypes, and six Y-class bodies have recently (As of August 26, 2011) been detected within 40 light years with the Wide-field Infrared Survey Explorer
Wide-field Infrared Survey Explorer
Wide-field Infrared Survey Explorer is a NASA infrared-wavelength astronomical space telescope launched on December 14, 2009, and decommissioned/hibernated on February 17, 2011 when its transmitter was turned off...

  • Y: < 600 K, ultra-cool brown dwarf
    Brown dwarf
    Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

    s (theoretical)


The coolest known brown dwarfs have estimated effective temperatures between 500 and 600 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

, and have been assigned the spectral class T9. Three examples are the brown dwarfs CFBDS J005910.90-011401.3, ULAS J133553.45+113005.2
ULAS J133553.45+113005.2
ULAS J133553.45+113005.2 is a T-type brown dwarf in the constellation of Virgo. It was discovered in data from the UK Infrared Telescope Infrared Deep Sky Survey Large Area Survey...

, and ULAS J003402.77−005206.7
ULAS J003402.77−005206.7
ULAS J003402.77-005206.7 is a T-type brown dwarf in the constellation of Cetus.ULAS J0034-00 is one of the coolest brown dwarfs known. It was first identified in data from the UK Infrared Telescope Infrared...

. The absolute coolest known brown dwarfs are CFBDSIR 1458+10
CFBDSIR 1458+10
CFBDSIR 1458+10 is a binary system of two brown dwarfs orbiting each other, located 75 light-years away from Earth discovered by the Canada-France Brown Dwarf Survey using the Canada-France-Hawaii Telescope, and first found to be a binary by the Keck II Observatory...

 which has a surface temperature of 370±40K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

 and WISE 1828+2650, which has a surface temperature of just 300 K (80 F, or 25 Celsius
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...

). The spectra of these objects display absorption around 1.55 micrometers. Delorme et al. has suggested that this feature is due to absorption from ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 and that this should be taken as indicating the T-Y transition, making these objects of type Y0. However, the feature is difficult to distinguish from absorption by water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 and methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

, and other authors have stated that the assignment of class Y0 is premature.

Carbon-related late giant star classes

Carbon-related stars are stars whose spectra indicate production of carbon by helium triple-alpha
Triple-alpha process
The triple alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei are transformed into carbon.Older stars start to accumulate helium produced by the proton–proton chain reaction and the carbon–nitrogen–oxygen cycle in their cores...

 fusion. With increased carbon abundance, and some parallel s-process
S-process
The S-process or slow-neutron-capture-process is a nucleosynthesis process that occurs at relatively low neutron density and intermediate temperature conditions in stars. Under these conditions the rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta-minus decay...

 heavy element production, the spectra of these stars become increasingly deviant from the usual late spectral classes G, K and M. The giants among those stars are presumed to produce this carbon themselves, but not too few of this class of stars are believed to be double stars whose odd atmosphere once was transferred from a former carbon star companion that is now a white dwarf.

Class C: carbon stars

Originally classified as R and N stars, these are also known as 'carbon stars'. These are red giants, near the end of their lives, in which there is an excess of carbon in the atmosphere. The old R and N classes ran parallel to the normal classification system from roughly mid G to late M. These have more recently been remapped into a unified carbon classifier C, with N0 starting at roughly C6. Another subset of cool carbon stars are the J-type stars, which are characterized by the strong presence of molecules of 13CN in addition to those of 12CN. A few dwarf (that is, main sequence) carbon stars are known, but the overwhelming majority of known carbon stars are giants or supergiants.
  • C: Carbon stars, e.g. R CMi
    • C-R: Formerly a class on its own representing the carbon star equivalent of late G to early K stars. Example: S Camelopardalis
    • C-N: Formerly a class on its own representing the carbon star equivalent of late K to M stars. Example: R Leporis
      R Leporis
      R Leporis , sometimes called Hind's Crimson Star, is a well-known variable star in the constellation Lepus, near its border with Eridanus. It is designated "R" in the chart to the right....

    • C-J: A subtype of cool C stars with a high content of 13C. Example: Y Canum Venaticorum
      La Superba
      La Superba is a star in the constellation Canes Venatici, well-known for its strikingly red appearance.- Physical characteristics :...

    • C-H: Population II analogues of the C-R stars. Examples: V Ari, TT CVn
    • C-Hd: Hydrogen-Deficient Carbon Stars, similar to late G supergiants with CH and C2 bands added. Example: HD 137613

Class S

Class S stars have zirconium oxide lines in addition to (or, rarely, instead of) those of titanium oxide
Titanium oxide
Titanium oxide may refer to:* Titanium dioxide , TiO2* Titanium oxide , TiO, a non-stoichiometric oxide* Titanium oxide , Ti2O3* Ti3O* Ti2O* δ-TiOx...

, and are in between the Class M stars and the carbon stars. S stars have excess amounts of zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...

 and other elements produced by the s-process
S-process
The S-process or slow-neutron-capture-process is a nucleosynthesis process that occurs at relatively low neutron density and intermediate temperature conditions in stars. Under these conditions the rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta-minus decay...

, and have their carbon and oxygen abundances closer to equal than is the case for M stars. The latter condition results in both carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 being locked up almost entirely in carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 molecules. For stars cool enough for carbon monoxide to form that molecule tends to "eat up" all of whichever element is less abundant, resulting in "leftover oxygen" (which becomes available to form titanium oxide) in stars of normal composition, "leftover carbon" (which becomes available to form the diatomic carbon
Diatomic carbon
Diatomic carbon is a diatomic molecule of carbon , which occurs in electric arcs; in comets, stellar atmospheres and the interstellar medium; and in blue hydrocarbon flames.-Chemistry:...

 molecules) in carbon stars, and "leftover nothing" in the S stars. The relation between these stars and the ordinary M stars indicates a continuum of carbon abundance. Like carbon stars, nearly all known S stars are giants or supergiants.
Examples: S Ursae Majoris, HR 1105
BD Camelopardalis
BD Camelopardalis is an S star and symbiotic star in the constellation Camelopardalis. It was recognized as a spectroscopic binary star in 1922, and its orbital solution published in 1984; it has a 596-day orbital period...


Classes MS and SC: intermediary carbon-related classes

In between the M class and the S class, border cases are named MS stars. In a similar way border cases between the S class and the C-N class are named SC or CS. The sequence M → MS → S → SC → C-N is believed to be a sequence of increased carbon abundance with age for carbon stars in the asymptotic giant branch
Asymptotic Giant Branch
The asymptotic giant branch is the region of the Hertzsprung-Russell diagram populated by evolving low to medium-mass stars. This is a period of stellar evolution undertaken by all low to intermediate mass stars late in their lives....

.
Examples: R Serpentis, ST Monocerotis (MS)
Examples: CY Cygni, BH Crucis (SC)

White dwarf classifications

The class D (for Degenerate) is the modern classification used for white dwarfs - low-mass stars that are no longer undergoing nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

 and have shrunk to planetary size, slowly cooling down. Class D is further divided into spectral types DA, DB, DC, DO, DQ, DX, and DZ. The letters are not related to the letters used in the classification of other stars, but instead indicate the composition of the white dwarf's visible outer layer or atmosphere.
Examples: Sirius B (DA2), Procyon B (DA4), Van Maanen's star
Van Maanen's star
Van Maanen's star is a white dwarf star. Out of the white dwarfs known, it is the third closest to the Sun, after Sirius B and Procyon B, in that order, and the closest known solitary white dwarf...

 (DZ7), Table 1


The white dwarf types are as follows:
  • DA: a hydrogen
    Hydrogen
    Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

    -rich atmosphere or outer layer, indicated by strong Balmer hydrogen spectral lines.
  • DB: a helium
    Helium
    Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

    -rich atmosphere, indicated by neutral helium, He I
    Spectroscopic notation
    Spectroscopic notation provides various ways to specify atomic ionization states, as well as atomic and molecular orbitals.-Ionization states:Spectroscopists customarily refer to the spectrum arising from a given ionization state of a given element by the element's symbol followed by a Roman numeral...

    , spectral lines.
  • DO: a helium-rich atmosphere, indicated by ionized helium, He II
    Spectroscopic notation
    Spectroscopic notation provides various ways to specify atomic ionization states, as well as atomic and molecular orbitals.-Ionization states:Spectroscopists customarily refer to the spectrum arising from a given ionization state of a given element by the element's symbol followed by a Roman numeral...

    , spectral lines.
  • DQ: a carbon
    Carbon
    Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

    -rich atmosphere, indicated by atomic or molecular carbon lines.
  • DZ: a metal-rich atmosphere, indicated by metal spectral lines (a merger of the obsolete white dwarf spectral types, DG, DK and DM).
  • DC: no strong spectral lines indicating one of the above categories.
  • DX: spectral lines are insufficiently clear to classify into one of the above categories.


The type is followed by a number giving the white dwarf's surface temperature. This number is a rounded form of 50400/Teff, where Teff is the effective surface temperature
Effective temperature
The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation...

, measured in kelvins. Originally, this number was rounded to one of the digits 1 through 9, but more recently fractional values have started to be used, as well as values below 1 and above 9.

Two or more of the type letters may be used to indicate a white dwarf which displays more than one of the spectral features above. Also, the letter V is used to indicate a variable white dwarf
Pulsating white dwarf
A pulsating white dwarf is a white dwarf star whose luminosity varies due to non-radial gravity wave pulsations within itself. Known types of pulsating white dwarfs include DAV, or ZZ Ceti, stars, with hydrogen-dominated atmospheres and the spectral type DA, pp. 891, 895; DBV, or V777 Her,...

.

Extended white dwarf spectral types:
  • DAB: a hydrogen- and helium-rich white dwarf displaying neutral helium lines.
  • DAO: a hydrogen- and helium-rich white dwarf displaying ionized helium lines.
  • DAZ: a hydrogen-rich metallic white dwarf.
  • DBZ: a helium-rich metallic white dwarf.


Variable star designations:
  • DAV or ZZ Ceti: a hydrogen-rich pulsating white dwarf
    Pulsating white dwarf
    A pulsating white dwarf is a white dwarf star whose luminosity varies due to non-radial gravity wave pulsations within itself. Known types of pulsating white dwarfs include DAV, or ZZ Ceti, stars, with hydrogen-dominated atmospheres and the spectral type DA, pp. 891, 895; DBV, or V777 Her,...

    ., pp. 891, 895
  • DBV or V777 Her: a helium-rich pulsating white dwarf., p. 3525
  • GW Vir, sometimes divided into DOV and PNNV: a hot helium-rich pulsating white dwarf (or pre-white dwarf.), §1.1, 1.2; These stars are generally PG 1159 star
    PG 1159 star
    A PG 1159 star, often also called a pre-degenerate, is a star with a hydrogen-deficient atmosphere which is in transition between being the central star of a planetary nebula and being a hot white dwarf...

    s, although some authors also include non-PG 1159 stars in this class.

Non-stellar spectral types: Class P & Q

Finally, the classes P and Q are occasionally used for certain non-stellar objects. Type P objects are planetary nebula
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...

e and type Q objects are nova
Nova
A nova is a cataclysmic nuclear explosion in a star caused by the accretion of hydrogen on to the surface of a white dwarf star, which ignites and starts nuclear fusion in a runaway manner...

e.

Spectral peculiarities

Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum.
Code Spectral peculiarities for stars
: Blending and/or uncertain spectral value
... Undescribed spectral peculiarities exist
Special peculiarity
comp Composite spectrum
e Emission lines present
[e] "Forbidden" emission lines present
er "Reversed" center of emission lines weaker than edges
ep Emission lines with peculiarity
eq Emission lines with P Cygni
P Cygni
P Cygni is a variable star in the constellation Cygnus. The designation "P" was originally assigned by Johann Bayer in Uranometria as a nova....

 profile
ev Spectral emission that exhibits variability
f N III and He II emission (for element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

 name followed by roman numeral see spectral line
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...

)
f* NIV λ4058Å is stronger than the NIII λ4634Å, λ4640Å, & λ4642Å lines
f+ SiIV λ4089Å & λ4116Å are emission in addition to the NIII line
(f) Weak emission lines of He
((f)) Displays strong HeII absorption accompanied by weak NIII emissions
h WR stars with emission lines due to hydrogen.
ha WR stars with hydrogen emissions seen on both absorption and emission.
He wk Weak He lines
k Spectra with interstellar absorption features
m Enhanced metal features
n Broad ("nebulous") absorption due to spinning
nn Very broad absorption features due to spinning very fast
neb A nebula's spectrum mixed in
p Unspecified peculiarity, peculiar star
Peculiar star
In astrophysics, peculiar stars have distinctly unusual metal abundances, at least in their surface layers.Chemically peculiar stars are common among hot main sequence stars...

.
pq Peculiar spectrum, similar to the spectra of novae
q Red & blue shifts line present
s Narrowly "sharp" absorption lines
ss Very narrow lines
sh Shell star
Shell star
A shell star, also termed Gamma Cassiopeiae variable , is a star having a spectrum that exhibits features indicating a circumstellar disc of gas surrounding the star at the equator. They exhibit irregular variations in their luminosity due to the outflow of matter...

 features
v Variable spectral feature (also "var")
w Weak lines (also "wl" & "wk")
d Del Type A and F giants with weak calcium H and K lines, as in prototype Delta Delphini
Delta Delphini
Delta Delphini is a giant star in the constellation Delphinus.-References:*...

d Sct Type A and F stars with spectra similar to that of short-period variable Delta Scuti
Delta Scuti
Delta Scuti is a white, F-type giant star in the constellation Scutum. It is approximately 187 light years from Earth. Delta Scuti is the prototype of the Delta Scuti type variable stars. It is a high-amplitude δ Scuti type pulsator with light variations of about 0.15 minutes...

Code If spectrum shows enhanced metal features
Ba Abnormally strong Barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...

Ca Abnormally strong Calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

Cr Abnormally strong Chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

Eu Abnormally strong Europium
Europium
Europium is a chemical element with the symbol Eu and atomic number 63. It is named after the continent of Europe. It is a moderately hard silvery metal which readily oxidizes in air and water...

He Abnormally strong Helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

Hg Abnormally strong Mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

Mn Abnormally strong Manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...

Si Abnormally strong Silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

Sr Abnormally strong Strontium
Strontium
Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...

Tc Abnormally strong Technetium
Technetium
Technetium is the chemical element with atomic number 43 and symbol Tc. It is the lowest atomic number element without any stable isotopes; every form of it is radioactive. Nearly all technetium is produced synthetically and only minute amounts are found in nature...

Code Spectral peculiarities for white dwarfs
: Uncertain assigned classification
P Magnetic white dwarf with detectable polarization
E Emission lines present
H Magnetic white dwarf without detectable polarization
V Variable
PEC Spectral peculiarities exist


For example, Epsilon Ursae Majoris
Epsilon Ursae Majoris
Epsilon Ursae Majoris is the brightest star in the constellation Ursa Major , and at magnitude 1.76 is the thirty-first brightest star in the sky...

 is listed as spectral type A0pCr, indicating general classification A0 with strong emission lines of the element chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

. There are several common classes of chemically peculiar star
Peculiar star
In astrophysics, peculiar stars have distinctly unusual metal abundances, at least in their surface layers.Chemically peculiar stars are common among hot main sequence stars...

s, where the spectral lines of a number of elements appear abnormally strong.

Photometric classification

Stars can also be classified using photometric data from any photometric system
Photometric system
In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.The first known...

. For example, we can calibrate color index
Color index
In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature...

 diagrams of U−B and B−V in the UBV system according to spectral and luminosity classes. Nevertheless, this calibration is not straightforward, because many effects are superimposed in such diagrams: interstellar reddening, color changes due to metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...

, and the blending of light from binary
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...

 and multiple
Multiple star
A multiple star consists of three or more stars which appear from the Earth to be close to one another in the sky. This may result from the stars being physically close and gravitationally bound to each other, in which case it is physical, or this closeness may be merely apparent, in which case...

 stars.

Photometric systems with more colors and narrower passbands allow a star's class, and hence physical parameters, to be determined more precisely. The most accurate determination comes of course from spectral measurements, but there is not always enough time to get qualitative spectra with high signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...

.

See also

  • Astrograph
    Astrograph
    An astrograph is a telescope designed for the sole purpose of astrophotography. Astrographs are usually used in wide field surveys of the night sky as well as detection of objects such as asteroids, meteors, and comets.-Design:...

  • Hertzsprung–Russell diagram
    Hertzsprung–Russell diagram
    The Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their spectral types or classifications and effective temperatures. Hertzsprung–Russell diagrams are not pictures or maps of the locations of the stars...

  • Metallicity
    Metallicity
    In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...

  • Stellar associations
  • Stellar evolution
    Stellar evolution
    Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...

  • Star count
    Star count
    Star Counts are bookkeeping surveys of stars and the statistical and geometrical methods used to correct the survey data for bias. The surveys are most often made of nearby stars in the Milky Way Galaxy....

    , survey of stars

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