Stellar classification

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

Contents 1 Secchi classes 2 Morgan-Keenan spectral classification 3 Spectral types 3.1 Class O 3.2 Class B 3.3 Class A 3.4 Class F 3.5 Class G 3.6 Class K 3.7 Class M 4 Extended spectral types 4.1 Hot blue emission star classes 4.1.1 Class W: Wolf-Rayet's 4.1.2 Classes OC, ON, BC, BN: Wolf-Rayet related O and B stars 4.1.3 The "class" OB 4.2 Cool red and brown dwarf classes 4.2.1 Class L 4.2.2 Class T: methane dwarfs 4.2.3 Class Y: ultra-cool dwarfs 4.3 Carbon related late giant star classes 4.3.1 Class C: carbon stars 4.3.2 Class S 4.3.3 Classes MS and SC: intermediary carbon related classes 4.4 White dwarf classifications 4.5 Non-stellar spectral types: Class P & Q 5 Spectral peculiarities 6 Yerkes spectral classification 7 Photometric classification 8 See also 9 References 10 External links

[edit] Secchi classes

Pioneering stellar spectroscopy Pater Angelo Secchi created and consolidated the Secchi classes during 1863–1867, they were:

• Class I: white and blue stars with broad heavy hydrogen lines and metallic lines, (modern class A)
• Class II: yellow stars – hydrogen less strong, but evident metallic lines, (modern classes G and K)
• Class III: orange stars with complex band spectra, (modern class M)
• Class IV: red stars with significant carbon bands and lines (carbon stars),
• Class V: emission lines (f.ex. Be, Bf etc).

This classification was superseded by the Harvard classification in the late 1890s, which is what the rest of this article is about. ^[1]

[edit] Morgan-Keenan spectral classification

This stellar classification is the most commonly used. It is also known as the Yerkes classification. 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.^[2] The colors in this table are greatly exaggerated for illustration. The actual colors of the listed stars are mostly white with a faint tint of the color indicated; stars' colors are often too subtle to notice, particularly when they are near the horizon.

Class	Temperature	Star color	Mass	Radius	Luminosity	Hydrogen 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")	3.1	2.1	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 J. Cannon and Antonia Maury, based on the work of Williamina Fleming. Spectral classes are further subdivided by Arabic numerals (0–9); because the classification sequence predates our understanding that it is a temperature sequence; the precise values of these digits depend upon (largely subjective) estimates of the strengths of absorption features in stellar spectra. As a result, the subclasses are not evenly divided into any sort of mathematically representable intervals. 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).

[edit] Spectral types

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

[edit] 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 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 05 to 09, and prominent hydrogen Balmer lines, although not as strong as in later types. 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 around other stars in the vicinity of an O class star due to the photo evaporation effect.^[3]

Examples: Zeta Puppis, Lambda Orionis, Delta Orionis

[edit] Class B

Class B stars are extremely 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 OB1 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. They constitute about 0.13% of main sequence stars—rare, but much more common than those of class O. (LeDrew)

Examples: Rigel, Spica, the brighter Pleiades

[edit] 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, 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. They comprise perhaps 0.63% of all main sequence stars. (LeDrew)

Examples: Vega, Sirius, Deneb

[edit] Class F

Class F stars are still quite powerful but they tend to be main sequence stars. These stars have strengthening H&K lines of Ca II. Neutral metals (Fe I, Cr I) beginning to gain on ionized metal lines by late F. 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

[edit] Class G

Class G stars are probably the best known, if only for the reason that our Sun is of this class. Most notable is the H&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".^[1] 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, Tau Ceti

[edit] 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 (Mn I, Fe I, Si I). By late K, molecular bands of titanium oxide become present. These make up some 13% of main sequence stars.(LeDrew)

Examples: Epsilon Eridani, Arcturus, Aldebaran

[edit] 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 Late-M group hold 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 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: Betelgeuse (giant)

Examples: Proxima Centauri, Barnard's star (dwarf)

Example: LEHPM 2-59 ^[2] (subdwarf)

Examples: Teide 1, GSC 08047-00232 B ^[3] (brown dwarf)

[edit] Extended spectral types

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

[edit] Hot blue emission star classes

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

[edit] Class W: Wolf-Rayet's

Main article: Wolf-Rayet stars

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 WC (WCE early-type, WCL late-type), WN (WNE early-type, WNL late-type), and WO according to the dominance of carbon, nitrogen, or oxygen in their spectra (and outer layers).

• W: Up to 70,000 K

Example: Gamma Velorum A (WC)

Example: WR124 (WN)

Example: WR93B (WO)

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

Intermediary between the genuine Wolf-Rayet's 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-Rayet's into the ordinary OB:s.

Example: HD 152249 (OC)

Example: HD 105056 (ON)

Example: HD 2905 (BC)

Example: HD 163181 (BN)

[edit] The "class" OB

Main article: OB star

In lists of spectra, the "spectrum OB" may occur. This is in fact not a spectrum, but a marker "that the spectrum of this star is unknown, but it belongs to an OB association, so probably an early type star".

[edit] Cool red and brown dwarf classes

The novel spectral types L and T were erected to classify infrared spectra of cool stars and brown dwarfs very faint in visual light.

[edit] 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.^[4][5] Due to low gravities in giant stars, TiO- and VO-bearing condensates never form. Thus, "L giant" can never form.

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

Example: L-giants are not possible

Example: VW Hyi

Example: 2MASSW J0746425+2000321 binary^[4]

Component A is an L-Dwarf Star

Component B is an L Brown Dwarf

[edit] Class T: methane dwarfs

Class T stars 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.^[4][5]

• T: 1,000 K - Cooler brown dwarfs with methane in the spectrum

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 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.

[edit] Class Y: ultra-cool dwarfs

Class Y stars are expected to be much cooler than T-dwarfs. None have been found as of yet, but they have been modelled.^[6]

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

[edit] 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 are becoming 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.

[edit] Class C: carbon stars

Main article: Carbon star

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.^[7] 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^[8]
  • C-Hd: Hydrogen-Deficient Carbon Stars, similar to late G supergiants with CH and C₂ bands added. Example: HD 137613

[edit] Class S

Class 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.^[9] 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

[edit] 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 – 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)

[edit] White dwarf classifications

Main article: White dwarf

The class D is the modern classification used for white dwarfs, lighter stars that have consumed all their fusable matter and now have shrunk to planetary size, slowly cooling down. 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".

• Example: Sirius B (DA)

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 T_eff = 50,400 K.)^[10]

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

[edit] 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.

[edit] Spectral peculiarities

Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum.^[11]

Code	Spectral peculiarities for stars
:	bleeding 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//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^
Code	If spectrum shows enhanced metal features
Ba	abnormally strong Barium
Ca	abnormally strong Calcium
Cr	abnormally strong Chromium
Eu	abnormally strong Europium
He	abnormally strong Helium
Hg	abnormally strong Mercury
Mn	abnormally strong Manganese
Si	abnormally strong Silicon
Sr	abnormally strong Strontium
Code	Spectral peculiarities for white dwarfs
P	polarized light
H	magnetic field from Zeeman splitting
V	variable
PEC	spetral peculiarities exist

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. There are several common classes of chemically peculiar stars, where the spectral lines of a number of elements appear abnormally strong.

[edit] 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:

• I supergiants
  • Ia-0 (hypergiants or extremely luminous supergiants (later addition)), Example: HD 183143 (spectrum is B6-8 Ia-0)
  • Ia (luminous supergiants), Example: Deneb (spectrum is A2Ia)
  • Iab (intermediate luminous supergiants)
  • Ib (less luminous supergiants), Example: Betelgeuse (spectrum is M2Ib)
• II bright giants
  • IIa, Example: β Scuti (HD 173764) (spectrum is G4 IIa)
  • IIab Example: HR 8752 (spectrum is G0Iab:)
  • IIb, Example: HR 6902 (spectrum is G9 IIb)
• III normal giants
  • IIIa, Example: ρ Persei (spectrum is M4 IIIa)
  • IIIab Example: δ Reticuli (spectrum is M2 IIIab)
  • IIIb, Example: Pollux (spectrum is K2 IIIb)
• IV subgiants
  • IVa, Example: ε Reticuli (spectrum is K1-2 IVa-III)
  • IVb, Example: HR 672 A (spectrum is G0.5 IVb)
• V main sequence stars (dwarfs)
  • Va, Example: AD Leonis (spectrum M4Vae)
  • Vb, Example: 85 Pegasi A (spectrum G5 Vb)
• VI subdwarfs (rarely used)
• VII white dwarfs (rarely used)

Marginal cases are allowed; for instance a star classified as Ia0-Ia would be a very luminous supergiant, verging on hypergiant. Examples are below. The spectral type of the star are not a factor.

Marginal Symbols	Example	Explanation
-	G2 I-II	The star is between super giant and bright giant.
+	O9.5 Ia+	The star is a hypergiant star.
/	M2 IV/V	The star is either a subgiant or a dwarf star.

[edit] 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 a 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.

[edit] See also

• Stellar associations
• Metallicity
• Astrograph

[edit] References

1. ^ Classification of Stellar Spectra: Some History
2. ^ Charity, Mitchell. What color are the stars?. Retrieved on May 13, 2006.
3. ^ Planets Prefer Safe Neighborhoods
4. ^ ^{a b} Kirkpatrick et al, J. Davy (July 10, 1999). "Dwarfs Cooler than M: the Ddefinition of Spectral Type L Using Discovery from the 2-µ ALL-SKY Survey (2MASS)". Astrophysical Journal 519 (2): 802–833. ISSN: 0004-637X. 
5. ^ ^{a b} Kirkpatrick, J. Davy (2005). "New Spectral Types L and T". Annual Review of Astronomy and Astrophysics 43 (1): 195–246. ISSN: 0066-4146. 
6. ^ Y-Spectral class for Ultra-Cool Dwarfs, N.R.Deacon and N.C.Hambly, 2006
7. ^ Bouigue, R. 1954, Annales d'Astrophysique, Vol. 17, p.104
8. ^ Spectral Atlas of Carbon Stars (Barnbaum+ 1996)
9. ^ Keenan, P. C. 1954 Astrophysical Journal, vol. 120, p.484
10. ^ White Dwarf (wd) Stars
11. ^ SkyTonight: The Spectral Types of Stars

[edit] External links

• www.twcac.org/Tutorials/spectral_classes.htm
• Libraries of stellar spectra, D. Montes, UCM
• Webfooted Astronomer
• The rate of period change in pulsating DB white dwarf stars, A. H. Corsico, L. G. Althaus, has the DAV, DBV, & DOV explanation.
• The Close DAO+dM Binary RE J0720-318: A Stratified White Dwarf with a Thin H Layer and a Possible Circumbinary Disk DAO type White Dwarf.
• The Spatial Distribution and Kinematics of Cool Metallic Line White Dwarfs, has DAZ and DBZ spectrums
• A K-Band Spectral Atlas of Wolf-Rayet Stars, has WC, WN, and WO spectrums
• Properties of the WO Wolf-Rayet stars, has WO spectrum ranging from WO1 to WO5
• Spectral Types for Hipparcos Catalogue Entries
• [5], has the luminous subclasses.
• Discovery of a Very Young Field L Dwarf, 2MASS J01415823-4633574, J. Davy Kirkpatrick et al.