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Star classification is the classification of stars.

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stellar classification.
stellar classification: Hertzsprung-Russell diagram.

In astronomy, star classification or 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 scheme (from the 19th century) ranked stars from A to Q, which is the origin of the currently used spectral classes.

Morgan-Keenan stellar 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 given in the following table. The colors in this table are greatly exaggerated for illustration. The actual color of the listed stars is mostly white with a very faint tint of the color indicated; often stars' colors are too subtle to notice and may be affected by their proximity to the horizon (from the perspective of the observer).

ClassTemperatureStar colourMassRadiusLuminosityHydrogen lines
O 30,000 - 60,000 K Bluish ("blue") 60 15 1,400,000 Weak
B 10,000 - 30,000 K Bluish-white ("blue-white") 18 7 20,000 Medium
A 7,500 - 10,000 K White with bluish tinge ("white") 3.2 2.5 80 Strong
F 6,000 - 7,500 K White ("yellow-white") 1.7 1.3 6 Medium
G 5,000 - 6,000 K Light yellow ("yellow") 1.1 1.1 1.2 Weak
K 3,500 - 5,000 K Light orange ("orange") 0.8 0.9 0.4 Very weak
M 2,000 - 3,500 K Reddish orange ("red") 0.3 0.4 0.04 Very weak

The sizes 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. 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. 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 Annie Jump Cannon and Antonia Maury, based on the work of Williamina Fleming. Spectral classes are further 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.

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).

Spectral types of stellar classification.

The following illustration represents star classes with the colors very close to those actually percieved by the the human eye. The relative sizes are for main sequence or "dwarf" stars.

spectral classification.
The Morgan-Keenan spectral classification.

Stellar classification: Class O

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. Recent observations by the Spitzer space telescope indicate that planetary formation does not occur within the vicinity of an O class star due to the Photo evaporation effect.

Examples of Stellar classification: Zeta Puppis, Lambda Orionis.

Class B

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)

Examples of Stellar classification: Rigel, Spica, the brighter Pleiades.

Class A

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 lines of ionized metals. They comprise perhaps 0.63% of all main sequence stars.(LeDrew)

Examples: Vega, Sirius

Class F

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)

Examples: Canopus, Procyon

Class G

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.

Examples: Sun, Capella

Class K

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)

Examples: Arcturus, Aldebaran

Class M

Class M is by far the most common class. 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 all neutral metals but hydrogen are usually absent. Titanium oxide can be strong in M stars.

Examples: Betelgeuse, Barnard's star

Extended Spectral types

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

Class W

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 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).

  • W: Up to 70,000 K - Wolf-Rayet stars, e.g. WR124.

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 in their spectra. Some of these objects are of substellar mass (do not support fusion) and some are not, so collectively this class of objects should be referred to as "L dwarfs", not "L stars." 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 their spectra.

  • L: 1,300 - 2,500 K - Dwarfs (some stellar, some substellar) with metal hydrides and alkali metals prominent in their spectra.

Class T

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.

  • T: 1,000 K - Cooler brown dwarfs with methane in the spectrum, e.g. 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'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, these smaller stars will accumulate over time.

Class Y

Class Y stars, (ultra-cool dwarfs), are much cooler than T-dwarfs. None have been found as of yet.

  • Y: Ultra-cool dwarfs, are brown dwarfs that are cooler than T-dwarfs, (theoretical).

Class C

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.

  • 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.

Class S

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.

  • S: Similar to Class M stars, but with zirconium oxide in addition to or replacing the regular titanium oxide.

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.

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.

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. 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".

  • D: white dwarfs, e.g. Sirius B.

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).
  • DO: an ionized helium-rich "atmosphere" or outer layer, indicated by ionized helium spectral lines, (He II lines).
  • DC: no strong spectral lines indicating one of the above categories.
  • DQ: a carbon-rich "atmosphere" or outer layer, indicated by atomic or molecular carbon lines.
  • 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).
  • 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 or zz Ceti: a hydrogen-rich pulsating white dwarf.
  • DBV or V777 Her: a helium-rich pulsating white dwarf.
  • DOV or PG 1159: a helium-rich pulsating white dwarf.

Stellar classification: Spectral peculiarities.

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
 ! 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//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 spectrum 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
sh shell star
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^
CodeIf 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

For example, Epsilon Ursae Majoris is listed as spectral type A0pCr, indicating general classification A0 with an unspecified peculiarity and strong emission lines of the element chromium.

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.
    • Iab.
    • Ib less luminous supergiants.
  • II bright giants
    • IIa.
    • IIab.
    • IIb.
  • III normal giants
    • IIIa.
    • IIIab.
    • IIIb.
  • IV subgiants
    • IVa.
    • IVab.
    • IVb.
  • V main sequence stars (dwarfs)
    • Va.
    • Vab.
    • Vb.
  • 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.

Marginal SymbolsExampleExplanation
- 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.

Photometric classification.

Stars can also be classified using photometric data from any photometric system. For example, we can calibrate colour index diagrams UB,BV in the UBV system according to spectral and luminosity classes. Nevertheless, this callibration is not straightforward, because many effects are superimposed in such diagrams: metallicity, interstellar reddening, binary and multiple stars.

The more colours and more narrow passbands in photometric systems we use, the more precisely we can derive star's class (and, hence, physical parameters). The best are, of course, spectral measurements, but we not always have enough time to get qualitative spectra with high signal-to-noise ratio.

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