Stars

=DESCRIPTION OF A STAR= A star is a massive, luminous ball 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. Other stars are visible from Earth during the night when they are not outshone by the Sun or blocked by atmospheric phenomena. Historically, the most prominent stars on the celestial sphere were grouped together into constellations and asterisms, and the brightest stars gained proper names. Extensive catalogues of stars have been assembled by astronomers, which provide standardized star designations.

For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen in its core releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium were created by stars, either via stellar nucleosynthesis during their lifetimes or by supernova nucleosynthesis when stars explode. Astronomers can determine the mass, age, chemical composition and many other properties of a star by observing its spectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung-Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.

A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, some of the hydrogen is steadily converted into helium through the process of nuclear fusion. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective processes. The star's internal pressure prevents it from collapsing further under its own gravity. Once the hydrogen fuel at the core is exhausted, those stars having at least 0.4 times the mass of the Sun expand to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves into a degenerate form, recycling a portion of the matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements.

Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a cluster or a galaxy.

=STAR CLASSIFICATION= In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure of the temperature in this chromosphere. Light from the star is analyzed by splitting it up by a diffraction grating, subdividing the incoming photons into a spectrum exhibiting a rainbow of colors interspersed by absorption lines, each line indicating a certain ion of a certain chemical element. 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 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 old 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 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 has the spectral type G2V, which might be interpreted as "a 'yellow' two tenths towards 'orange' main sequence star". The apparently brightest star Sirius has type A1V.

Harvard spectral classification
The Harvard classification system is a one-dimensional classification scheme. Stars vary in surface temperature from about 2 to 40 kK (2,000 to 40,000 kelvins). 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: The mass, radius, and luminosity listed for each class are appropriate only for stars on the main sequence portion of their lives and so are not appropriate for red giants. The spectral classes O through M are subdivided by Arabic numerals (0–9). For example, A0 denotes the hottest stars in the A class and A9 denotes the coolest ones. The Sun is classified as G2. The 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 Balmer series (group I, with a subtype representing many of the stars in Orion); those with spectra which, like the Sun, showed calcium and sodium 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, using the objective-prism method. A first result of this work was the Draper Catalogue of Stellar Spectra, published in 1890. Williamina Fleming 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, 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 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 Hertzsprung-Russell diagram relates stellar classification with absolute magnitude, luminosity, and surface temperature.The fact that the Harvard classification of a star indicated its surface temperature was not fully understood until after its development. In the 1920s, the Indian physicist Meghnad Saha 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) 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. 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 of 5780 K (see color temperature). 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.

Spectral types
The Morgan-Keenan spectral classificationThe 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 or "dwarf" stars.

Class O
Main article: O-type main sequence starClass O stars are very hot and extremely luminous, being bluish in color; in fact, most of their output is in the ultraviolet 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. 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. Spectrum of an O5 V starO-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 II lines, prominent ionized (Si IV, O III, N III, and C III) and neutral helium 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. Recent observations by the Spitzer Space Telescope 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, Lambda Orionis, Delta Orionis, Theta¹ Orionis C, HD 93129A

Class B
Main article: B-type main sequence star Proper motion of stars spectral classes B and A in -/+ 200 000 years 3D viewing (for red-green or red-blue glasses) of proper motionClass 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 II and Si II. As O and B stars 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 clouds. The Orion OB1 association occupies a large portion of a spiral arm of our galaxy 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, Spica, the brighter Pleiades, VV Cephei B, Algol A

Class A
Main article: A-type main sequence starClass A Vega (left) compared to the Sun (right).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 II, Mg II, Si II) at a maximum at A5. The presence of Ca 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. Two Class F stars: Supergiant Polaris A and its distant companion Polaris B

Examples: Sirius, Deneb, Altair, Vega

Class F
Main article: F-type main sequence starClass F stars have strengthening H and K lines of Ca II. Neutral metals (Fe I, Cr 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, Canopus, Dubhe B, Polaris, Procyon, Wezen

Class G
"G star" redirects here. For other uses, see G star (disambiguation).Main article: G-type main sequence star The most important class G star to humanity: our Sun. The dark area visible in the lower left is a large sunspot.The movement of stars of spectral class G around the apex (left) and antapex (right) in -/+ 200 000 years The movement of stars of spectral class G for 3D glasses (red-green or red-blue).Class G stars are probably the best known, if only for the reason that our Sun is of this class. About 1 in 13 of the main sequence stars in the solar neighborhood are Class G stars.

Most notable are the H and K lines of Ca 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".[34] 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: Sun, Alpha Centauri A, Capella, Tau Ceti

Class K
Main article: K-type main sequence star Comparison between Class K star Arcturus, Class M Antares, and the Sun.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 B, are main sequence stars. They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals (Mn I, Fe I, Si I).

By late K, molecular bands of titanium oxide become present. About 1 in 8 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 life.
 * Examples: Alpha Centauri B, Epsilon Eridani, Arcturus, Aldebaran, Algol B

Class M
Main articles: Red giant and Red dwarfBetelgeuse is a red supergiant, one of the largest stars known. Image from the Hubble Space Telescope.Class M is by far the most common class. About 76% of the main-sequence stars in the solar neighborhood are Class M stars.

Although most Class M stars are red dwarfs, the class also hosts most giants and some supergiants such as Antares and Betelgeuse, as well as Mira variables. The late-M group holds hotter brown dwarfs 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 molecules and all neutral metals but hydrogen lines are usually absent. Titanium oxide can be strong in M stars, usually dominating by about M5. Vanadium oxide bands become present by late M.
 * Example: VY Canis Majoris (hypergiant)
 * Examples: Betelgeuse, Antares (supergiants)
 * Examples: Rasalgethi, Beta Pegasi (giants)
 * Examples: Proxima Centauri, Barnard's star, Gliese 581 (red dwarf)
 * Examples: LEHPM 2-59,[36] SSSPM J1930-4311 (subdwarf)
 * Example: APMPM J0559-2903 (extreme subdwarf)
 * Examples: Teide 1 (field brown dwarf), GSC 08047-00232 B [37] (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
Main article: Wolf-Rayet starArtist's impression of a Wolf-Rayet starClass 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 winds 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 (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 Magnetic O stars
They are O stars with strong magnetic fields. Designation is Of?p

The "class" OB
Main article: OB starIn 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 dwarfs and brown dwarfs 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.[39]

Class L
Artist's vision of an L-dwarfClass 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 in their spectra. Some of these objects have masses large enough to support hydrogen fusion, but some are of substellar 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. Their atmosphere is cool enough to allow metal hydrides and alkali metals 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.
 * L: 1,300–2,000 K, dwarfs (some stellar, some substellar) with metal hydrides and alkali metals 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 (supergiants)

Class T: methane dwarfs
Artist's vision of a T-dwarfClass T dwarfs are cool brown dwarfs with surface temperatures between approximately 700 and 1,300 K. Their emission peaks in the infrared. Methane is prominent in their spectra.
 * T: ~700-1,300 K, cooler brown dwarfs with methane in the spectrum
 * Examples: SIMP 0136 (the brightest T dwarf discovered in northern hemisphere)
 * Examples: Epsilon Indi 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 discs, clumps of gas in nebulae from which stars and solar systems are formed) then the number of stars in the galaxy should be several orders of magnitude 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
See also: Sub-brown dwarf and Substellar objectThe 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 no certain example of class Y has yet been seen. The coolest known brown dwarfs have estimated effective temperatures between 500 and 600 K, and have been assigned the spectral class T9. Three examples are the brown dwarfs CFBDS J005910.90-011401.3, ULAS J133553.45+113005.2, and ULAS J003402.77−005206.7. The absolute coolest known brown dwarf is CFBDSIR 1458+10 which has a surface temperature of 370±40K. 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 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 and methane, and other authors have stated that the assignment of class Y0 is premature.
 * Y: < 600 K, ultra-cool brown dwarfs (theoretical)

Carbon related late giant star classes
Carbon related stars are stars whose spectra indicate production of carbon by helium triple-alpha fusion. With increased carbon abundance, and some parallel s-process 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
Main article: Carbon starOriginally 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
 * C-J: A subtype of cool C stars with a high content of 13C. Example: Y Canum Venaticorum
 * 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
Main article: S-type starClass S stars have zirconium oxide lines in addition to (or, rarely, instead of) those of titanium oxide, and are in between the Class M stars and the carbon stars. S stars have excess amounts of zirconium and other elements produced by the s-process, and have their carbon and oxygen abundances closer to equal than is the case for M stars. The latter condition results in both carbon and oxygen being locked up almost entirely in carbon monoxide 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 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

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

White dwarf classifications
Main article: White dwarf spectroscopySirius A and B (a white dwarf of type DA2) resolved by HSTThe class D (for Degenerate) is the modern classification used for white dwarfs - low-mass stars that are no longer undergoing nuclear fusion 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 (DZ7)Table 1

The white dwarf types are as follows: 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, 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.
 * DA: a hydrogen-rich atmosphere or outer layer, indicated by strong Balmer hydrogen spectral lines.
 * DB: a helium-rich atmosphere, indicated by neutral helium, He I, spectral lines.
 * DO: a helium-rich atmosphere, indicated by ionized helium, He II, spectral lines.
 * DQ: a carbon-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.

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.

Extended white dwarf spectral types: Variable star designations:
 * 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.
 * DAV or ZZ Ceti: a hydrogen-rich pulsating white dwarf.
 * DBV or V777 Her: a helium-rich pulsating white dwarf
 * 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 stars, 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 nebulae and type Q objects are novae.