In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics. Stellar temperatures can be classified by using Wien's displacement law; but this poses difficulties for distant stars. Stellar spectroscopy offers a way to classify stars according to their absorption lines; particular absorption lines can be observed only for a certain range of temperatures because only in that range are the involved atomic energy levels populated. An early schema (from the 19th century) ranked stars from A to P, which is the origin of the currently used spectral classes.
Morgan-Keenan spectral classification
This stellar classification is the most commonly used. The common classes are normally listed from hottest to coldest (with mass, radius and luminosity compared to the Sun) and are: 
|O||30,000 - 60,000 K||Bluish ("blue")||60||15||1,400,000|
|B||10,000 - 30,000 K||Bluish-white ("blue-white")||18||7||20,000|
|A||7,500 - 10,000 K||White with bluish tinge ("white")||3.2||2.5||80|
|F||6,000 - 7,500 K||White ("yellow-white")||1.7||1.3||6|
|G||5,000 - 6,000 K||Light yellow ("yellow")||1.1||1.1||1.2|
|K||3,500 - 5,000 K||Light orange ("orange")||0.8||0.9||0.4|
|M||2,000 - 3,500 K||Reddish orange ("red")||0.3||0.4||0.04|
A popular mnemonic for remembering the order is "Oh Be A Fine Girl, Kiss Me" (there are many variants of this mnemonic). This scheme was developed in the 1900s, by Annie J. Cannon and the Harvard College Observatory. The Hertzsprung-Russell diagram relates stellar classification with absolute magnitude, luminosity, and surface temperature. It should be noted that while these descriptions of stellar colors are traditional in astronomy, they really describe the light after it has been scattered by the atmosphere. The Sun is not in fact a yellow star, but has essentially the color temperature of a black body of 5780 K; this is a white with no trace of yellow which is sometimes used as a definition for standard white.
The reason for the odd arrangement of letters is historical. When people first started taking spectra of stars, they noticed that stars had very different hydrogen spectral lines strengths, and so they classified stars based on the strength of the hydrogen balmer series lines from A (strongest) to Q (weakest). Other lines of neutral and ionized species then came into play (H&K lines of calcium, sodium D lines etc). Later it was found that some of the classes were actually duplicates and those classes were removed. It was only much later that it was discovered that the strength of the hydrogen line was connected with the surface temperature of the star. The basic work was done by the "girls" of Harvard College Observatory, primarily Cannon and Antonia Maury, based on the work of Williamina Fleming. These classes are further subdivided by Arabic numerals (0-9). A0 denotes the hottest stars in the A class and A9 denotes the coolest ones. The sun is classified as G2.
O, B, and A spectra are sometimes misleadingly called "early spectra", while K and M stars are said to have "late spectra". This stems from an early 20th century theory, now obsolete, that stars start their lives as very hot "early type" stars, and then gradually cool down, thereby evolving into "late type" stars. We now know that this theory is entirely wrong (see: stellar evolution).
Class O stars are very hot and very luminous, being bluish in colour; in fact, most of their output is in the ultraviolet range. These are the rarest of all main sequence stars, constituting as few as 1 in 32,000. (LeDrew) O-stars shine with a power over a million times our Sun's output. These stars have prominent ionized and neutral helium lines and only weak hydrogen lines. Because they are so huge, Class O stars burn through their hydrogen fuel very quickly, and are the first stars to leave the main sequence.
Class B stars are extremely luminous and blue. Their spectra have neutral helium and moderate hydrogen lines. As O and B stars are so powerful, they live for a very short time. They do not stray far from the area in which they were formed as they don't have the time. They therefore tend to cluster together in what we call OB1 associations, which are associated with giant molecular clouds. The Orion OB1 association is an entire spiral arm of our Galaxy (brighter stars make the spiral arms look brighter, there aren't more stars there) and contains all of the constellation of Orion. They constitute about 0.13% of main sequence stars -- rare, but much more common than those of class O.(LeDrew)
Class A stars are amongst the more common naked eye stars. As with all class A stars, they are white or bluish-white. They have strong hydrogen lines and also ionized metals. They comprise perhaps 0.63% of all main sequence stars.(LeDrew)
Class F stars are still quite powerful but they tend to be main sequence stars. Their spectra is characterized by the weaker hydrogen lines and ionized metals, their colour is white with a slight tinge of yellow. These represent 3.1% of all main sequence stars.(LeDrew)
Class G stars are probably the best known, if only for the reason that our Sun is of this class. They have even weaker hydrogen lines than F, but along with the ionized metals, they have neutral metals. 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. These are about 8% of all main sequence stars. (LeDrew)
Class K are orangish stars which are slightly cooler than our Sun. Some K stars are giants and supergiants, such as Arcturus while others like Alpha Centauri B are main sequence stars. They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals. These make up some 13% of main sequence stars.(LeDrew)
Class M is by far the most common class if we go by the number of stars. All the red dwarfs go in here and they are plentiful; over 78% of stars are red dwarfs, such as Proxima Centauri.(LeDrew) M is also host to most giants and some supergiants such as Antares and Betelgeuse, as well as Mira variables. The spectrum of an M star shows lines belonging to molecules and neutral metals but hydrogen is usually absent. Titanium oxide can be strong in M stars. The red color is deceptive; it is because of the dimness of the star. When an equally hot object, a halogen lamp (3000 K) which is white hot is put at a few kilometers distance, it appears like a red star.
Spectral types for rare stars
A number of new spectral types have been taken into use for rare types of stars, as they have been discovered:
- W: Up to 70,000 K - Wolf-Rayet stars, e.g. WR124.
- L: 1,500 - 2,000 K - Stars with masses insufficient to run the regular hydrogen fusion process (brown dwarfs). Class L stars contain lithium which is rapidly destroyed in hotter stars.
- T: 1,000 K - Cooler brown dwarfs with methane in the spectrum, e.g. Epsilon Indi ba & Epsilon Indi bb.
- Y: Ultra-cool dwarfs, are brown dwarfs that are cooler than T-dwarfs, (theoretical).
- C: Carbon stars, e.g. R CMi.
- R: Formerly a class on its own representing the carbon star equivalent of Class K stars, e.g. S Camelopardalis.
- N: Formerly a class on its own representing the carbon star equivalent of Class M stars, e.g. R Leporis.
- S: Similar to Class M stars, but with zirconium oxide in addition to or replacing the regular titanium oxide.
- D: White dwarfs, e.g. Sirius B.
Class W or WR represents the superluminous Wolf-Rayet stars, being notably different since they have mostly helium 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 and WC according to the dominance of nitrogen or carbon in their spectra (and outer layers).
Class L, (lithium dwarfs), stars get their designation from the lithium present in their core. Any lithium would be destroyed in ongoing nuclear reactions in regular stars, which indicates that these objects have no ongoing fusion processes. They are a very dark red in colour and brightest in infrared. Their gas is cool enough to allow metal hydrides and alkali metals to be prominent in the spectrum.
Class T stars, (methane dwarfs), are very young and low density stars often found in the interstellar clouds they were born in. These are stars barely big enough to be stars and others that are substellar, being of the brown dwarf variety. They are black, emitting little or no visible light but being strongest in infrared. Their surface temperature is a stark contrast to the fifty thousand kelvins or more for Class O stars, being merely up to 1,000 K. Complex molecules can form, evidenced by the strong methane lines in their spectra.
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's theorised 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 (no star below 0.8 solar masses has ever died in the history of the galaxy) then these smaller stars will accumulate over time.
Class Y stars, (ultra-cool dwarfs), are much cooler than T-dwarfs. Also note, that none have been found as of yet.
Class R and N stars are carbon stars (red giants thought to reach the end of their life) which run 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.
Class S stars have ZrO lines in addition to (or, rarely, instead of) those of TiO, 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 C and O being locked up almost entirely in CO molecules. For stars cool enough for CO to form that molecule tends to "eat up" all of whichever element is less abundant, resulting in "leftover oxygen" (which becomes available to form TiO) 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.
In reality the relation between these stars and the traditional main sequence suggest a rather large continuum of carbon abundance and if fully explored would add another dimension to the stellar classification system.
White dwarf classifications
The class D is sometimes used for white dwarfs, the state most stars end their life in. Class D is further divided into classes DA, DB, DC, DO, DZ, and DQ. Note the letters are not related to the letters used in the classification of true stars, but instead indicate the composition of the white dwarf's outer layer or "atmosphere".
The white dwarf classes are as follows:
- DA: a hydrogen-rich "atmosphere" or outer layer, indicated by strong Balmer hydrogen spectral lines.
- DB: a neutral helium-rich "atmosphere" or outer layer, indicated by neutral helium spectral lines, (He I lines).
- DC: no strong spectral lines indicating one of the above categories.
- DZ: a 'metal'-rich "atmosphere" or outer layer, indicated by magnesium, calcium, and/or iron lines, (Ca I, Ca II H and K, Mg I, Fe I, Na I).
- DQ: a carbon-rich "atmosphere" or outer layer, indicated by atomic or molecular carbon lines.
- DO: an ionized helium-rich "atmosphere" or outer layer, indicated by ionized helim spectral lines, (He II lines).
- DX: spectral lines are insufficiently clear to classify into one of the above categories.
All class D stars use the same sequence from 1 to 9, with 1 indicating a temperature above 37,500 K and 9 indicating a temperature below 5,500 K. (The number is by definition equal to 50,400/T, where T is the effective temperature of the star.) 
Extended White Dwarf Class
- DAB: a hydrogen- and neutral helium-rich white dwarf.
- DAO: a hydrogen- and ionized helium-rich white dwarf.
- DAZ: a hydrogen-rich cool metallic white dwarf.
- DBZ: a helium-rich cool metallic white dwarf.
- DAV: a hydrogen-rich pulsating white dwarf.
- DBV: a helium-rich pulsating white dwarf
- DOV: a helium-rich pulsating white dwarf.
Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum.
|:||bleeding and/or uncertain spectral value|
|…||undescribed spectral pecularities exist|
|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//gr 304.446667, 38.032944^ profile|
|ev||spectral emission that exhibits variability|
|f||NIII and HeII emission|
|(f)||weak emission lines of He|
|((f))||no emission of He|
|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 spetrum mixed in|
|p||peculiar spectrum, strong spectral lines due to metal|
|pq||peculiar spectrum, similar to the spectra of novae|
|q||red & blue shifts line present|
|s||narrowly "sharp" absorption lines|
|ss||very narrow lines|
|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//gr 310.8647916, 15.074694^|
|d Sct||type A and F stars with spectra similar to that of short-period variable ^delta Scuti//gr 280.568417, -9.0525556^|
|Code||If has enhance metal features|
|Si||abnormally strong Silicon lines|
|Ba||abnormally strong Barium|
|Cr||abnormally strong Chromium|
|Eu||abnormally strong Europium|
|He||abnormally strong Helium|
|Hg||abnormally strong Mercury|
|K||abnormally strong Calcium line|
|Mg||abnormally strong Manganese|
|Sr||abnormally strong Strontium|
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, Phillip C. Keenan and Edith Kellman of Yerkes Observatory.
This classification is based on spectral lines sensitive to stellar surface gravity which is related to luminosity, as opposed to the Harvard classification which is based on surface temperature.
Since the radius of a giant star is much larger than a dwarf star 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 hypergiants (later addition)
- I supergiants
- Ia most luminous supergiants
- Ib less luminous supergiants
- II bright giants
- III normal giants
- IV subgiants
- V main sequence stars (dwarfs)
- VI subdwarfs (rarely used)
- VII white dwarfs (rarely used)
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 are not a factor.
|-||G I-II||The star is between super giant and bright giant.|
|+||O Ia+||The star is on verge of being a hypergiant star.|
|/||M IV/V||The star is either a subgiant or a dwarf star.|
The UBV system, also called the Johnson system, is a photometric system for classifying stars according to their magnitude. The letters U, B, and V stand for ultraviolet, blue, and visual magnitudes, which are measured for a star in order to classify it in the UBV system. The choice of colors on the blue end of the spectrum is because of the bias that photographic film has for those colors. It was introduced in the 1950s by American astronomers Harold Lester Johnson and William Wilson Morgan.
- Libraries of stellar spectra, D. Montes, UCM
- Y-Spectral class for Ultra-Cool Dwarfs, N.R.Deacon and N.C.Hambly
- Webfooted Astronomer
Marasama 00:30, 30 July 2006 (UTC)Sorry, I'm not sure how to link the references. Marasama 00:30, 30 July 2006 (UTC)http://www.ssl.berkeley.edu/~euve/sci/Resources_pubs_abstracts_j0720_burle.html (DAO) Marasama 00:30, 30 July 2006 (UTC)http://www.astro.indiana.edu/~classweb/a540_s05/notes/whitedwarf_pres.ppt (DO) Marasama 00:30, 30 July 2006 (UTC)http://www.aas.org/publications/baas/v31n3/aas194/688.htm (DAZ, DBZ) Marasama 00:30, 30 July 2006 (UTC)http://www.ias.ac.in/jaa/junsep2005/jaa446.pdf (DAV, DBV)