Stellar classification

From Wikipedia, the free encyclopedia
  (Redirected from F-type star)
Jump to: navigation, search
Hertzsprung–Russell diagram
Spectral type
Brown dwarfs
White dwarfs
Red dwarfs
Main sequence
Bright giants


Most stars are currently classified under the Morgan–Keenan (MK) system using the letters O, B, A, F, G, K, and M, a sequence from the hottest (O type) to the coolest (M type). Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. A8, A9, F0, F1 form a sequence from hotter to cooler). The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such class D for white dwarfs and class C for carbon stars.

In the MK system a luminosity class is added to the spectral class using Roman numerals. This is based on the width of certain absorption lines in the star's spectrum which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class 0 or Ia+ stars for hypergiants, class I stars for supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main-sequence stars, class sd for sub-dwarfs, and class D for white dwarfs. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a temperature around 5,800K.

Modern classification[edit]

The modern classification system is known as the Morgan–Keenan (MK) classification. Each star is assigned a spectral class from the older Harvard spectral classification and a luminosity class using Roman numerals as explained below, forming the star's spectral type.

Harvard spectral classification[edit]


Class Effective temperature[1][2][3] Conventional color description[nb 1] Actual apparent color[4][5][6] Main-sequence mass[1][7]
(solar masses)
Main-sequence radius[1][7]
(solar radii)
Main-sequence luminosity[1][7]
Fraction of all
main-sequence stars[8]
Typical spectra of main-sequence stars of various spectral types.


The spectral classes O through M, as well as other more specialized classes discussed later, are subdivided by Arabic numerals (0–9), which can be further divided into half-subtypes. For example, A0 denotes the hottest stars in the A class and A9 denotes the coolest ones. The Sun is classified as G2.


Yerkes spectral classification[edit]

The Morgan–Keenan 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, Philip C. Keenan, and Edith Kellman from Yerkes Observatory.[9] This two-dimensional (temperature and luminosity) classification scheme is based on spectral lines sensitive to stellar temperature and surface gravity which is related to luminosity (whilst the Harvard classification is based on surface temperature only). Later, in 1953, after some revisions of list of standard stars and classification criteria, the scheme was named the Morgan–Keenan classification, or MK (by William Wilson Morgan and Philip C. Keenan's initials),[10] and this system remains the system in modern use today.


A number of different luminosity classes are distinguished[11]

Marginal cases are allowed; for example, a star may be either a supergiant or a bright giant, or may be in between the subgiant and main-sequence classifications. In these cases, two special symbols are used: a slash (/) means that a star is either one class or the other, and a dash (-) means that the star is in between the two classes. For example, a star classified as A3-4III/IV would be in between spectral types A3 and A4, while being either a giant star or a subgiant.

Spectral peculiarities[edit]


Code Spectral peculiarities for stars
 : uncertain spectral value[11]
... Undescribed spectral peculiarities exist
 ! Special peculiarity
comp Composite spectrum[22]
e Emission lines present[22]
[e] "Forbidden" emission lines present
er "Reversed" center of emission lines weaker than edges
eq Emission lines with P Cygni profile
f N III and He II emission[11]
f* NIV λ4058Å is stronger than the NIII λ4634Å, λ4640Å, & λ4642Å lines[23]
f+ SiIV λ4089Å & λ4116Å are emission in addition to the NIII line[23]
(f+) [24]
((f)) Displays strong HeII absorption accompanied by weak NIII emissions[25]
((f*)) [24]
h WR stars with emission lines due to hydrogen.[26]
ha WR stars with hydrogen emissions seen on both absorption and emission.[26]
He wk Weak He lines
k Spectra with interstellar absorption features
m Enhanced metal features[22]
n Broad ("nebulous") absorption due to spinning[22]
nn Very broad absorption features[11]
neb A nebula's spectrum mixed in[22]
p Unspecified peculiarity, peculiar star.[nb 3][22]
pq Peculiar spectrum, similar to the spectra of novae
q Red & blue shifts line present
s Narrowly "sharp" absorption lines[22]
ss Very narrow lines
sh Shell star features[22]
var Variable spectral feature[22] (sometimes abbreviated to "v")
wl Weak lines[22] (also "w" & "wk")
Abnormally strong spectral lines of the specified element(s)[22]
P Magnetic white dwarf with detectable polarization (white dwarf only)
E Emission lines present (white dwarf only)
H Magnetic white dwarf without detectable polarization (white dwarf only)
V Variable (white dwarf only)
PEC Spectral peculiarities exist (white dwarf only)

For example, 59 Cygni is listed as spectral type B1.5Vnne,[27] indicating a spectrum with the general classification B1.5V, as well as very broad absorption lines and certain emission lines.

Conventional and apparent colors[edit]

Just-saturated RGB-camera discs


The Sun itself appears white and approximates a black body of 5780 K (see color temperature). It is sometimes called a yellow star (spectroscopically relative to Vega), although it may appear yellow or red when viewed through the atmosphere, or appear white if viewed when too bright for the eye to see any color.[28]



Secchi classes[edit]

During the 1860s and 1870s, pioneering stellar spectroscopist Father Angelo Secchi created the Secchi classes in order to classify observed spectra. By 1866, he had developed three classes of stellar spectra:[29][30][31]


In 1868, he discovered carbon stars, which he put into a distinct group:[32]


In 1877, he added a fifth class:[33]

In the late 1890s, this classification began to be superseded by the Harvard classification, which is discussed in the remainder of this article.[34][35][36]

Draper system[edit]

Classifications in the Draper Catalogue of Stellar Spectra[37][38]
Secchi Draper Comment
I A, B, C, D Hydrogen lines dominant.
IV N Did not appear in the catalogue.
  O Wolf–Rayet spectra with bright lines.
  P Planetary nebulae.
  Q Other spectra.


Harvard system[edit]


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.[41][42] Finally, by 1912, Cannon had changed the types B, A, B5A, F2G, etc. to B0, A0, B5, F2, etc.[43][44] This is essentially the modern form of the Harvard classification system. A common mnemonic for remembering the spectral type letters is "Oh, Be A Fine Guy/Girl, Kiss Me".

Modern interpretation[edit]



Spectral types[edit]

Class O[edit]

Main article: O-type star
Artist's rendering of Zeta Puppis, an O4 supergiant



When the MKK classification scheme was first described in 1943, the only subtypes of class O used were O5 to O9.5.[48] The MKK scheme was extended to O9.7 in 1971[49] and O4 in 1978,[50] and new classification schemes have subsequently been introduced which add types O2, O3 and O3.5.[51]

Spectral standards:[52]

Class B[edit]

Artist's impression of Aludra, a B5 supergiant

Class B stars are very luminous and blue. Their spectra have neutral helium, which are most prominent at the B2 subclass, and moderate hydrogen lines. Ionized metal lines include Mg II and Si II. As O and B stars are so energetic, they only live for a relatively short time. Thus, due to the low probability of kinematic interaction during their lifetime, they do not, and are unable to, stray far from the area in which they were formed.


Spectral standards:[52]

Class A[edit]

Fomalhaut, an A3 main-sequence star.


Spectral standards:[52]

Class F[edit]

Canopus, an F-type supergiant and the second brightest star in the night sky


Spectral standards:[52]

Class G[edit]

The Sun, a prototypical G2 main-sequence star



Spectral standards:[52]

Class K[edit]

Arcturus, a K1.5 giant

Class K stars are orangish stars that are slightly cooler than the Sun. They make up about 12%, nearly one in eight, of the main-sequence stars in the solar neighborhood.[nb 4][8] There are also giant K-type stars, which range from hypergiants like RW Cephei, to giants and supergiants, such as Arcturus, whereas orange dwarfs, like Alpha Centauri B, are main-sequence stars.


Spectral standards:[52]

Class M[edit]

UY Scuti is a class M supergiant or hypergiant



Spectral standards:[52]

Extended spectral types[edit]

A number of new spectral types have been taken into use from newly discovered types of stars.[56]

Hot blue emission star classes[edit]

UGC 5797 is an emission-line galaxy where massive bright blue stars are formed.[57]


Class W: Wolf–Rayet[edit]

Main article: Wolf–Rayet star
Hubble Space Telescope image of the nebula M1-67 and the Wolf–Rayet star WR 124 in the center.


WR spectra range is listed below:[58][59]

  • WN,[26] spectrum dominated by NitrogenIII-V and HeliumI-II lines
    • Extended WN classes WN10 and WN11 sometimes used for the Ofpe/WN9 stars[26]
  • WN/C, WN stars plus strong CarbonIV lines, intermediate between WN and WC stars[26]
  • WC,[26] spectrum with strong CarbonII-IV lines

Although the central stars of most planetary nebulae (CSPNe) show O-type spectra,[60] around 10% are hydrogen-deficient and show WR spectra.[61] These are low mass stars and to distinguish them from the massive Wolf Rayet stars, their spectra are enclosed in square brackets: e.g. [WC]. Most of these show [WC] spectra, some [WO], and very rarely [WN].

Wolf–Rayet examples:

Example: R136a1 (WN5h)
Example: Gamma2 Velorum A (WC8)
Example: WR93B (WO3)

The "Slash" stars[edit]


Example spectra: Of/WNL[62]


Cool red and brown dwarf classes[edit]

Main articles: brown dwarf and red dwarf


Brown dwarfs, whose energy comes from gravitational attraction alone, cool as they age and so progress to later spectral types. Brown dwarfs start their lives with M-type spectra and will cool through the L, T, and Y spectral classes; faster the less massive they are—the highest-mass brown dwarfs cannot have cooled to Y or even T dwarfs within the age of the universe. Because this leads to a degeneracy between mass and age for a given effective temperature and luminosity, no unique values can be assigned to a given spectral type.[7]

Class L[edit]

Artist's impression of an L-dwarf


Example: VW Hyi
Example: 2MASSW J0746425+2000321 binary[68]
Component A is an L dwarf star
Component B is an L brown dwarf
Example: LSR 1610-0040 (subdwarf)[69]
Example: V838 Monocerotis (supergiants)

Class T: methane dwarfs[edit]

Artist's impression of a T-dwarf

Class 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.[65][66]

Examples: SIMP 0136 (the brightest T dwarf discovered in northern hemisphere)[70]
Examples: Epsilon Indi Ba & Epsilon Indi Bb
Examples: CFBDS 1448 a T dwarf with an exoplanet orbiting it


Class Y[edit]

Artist's impression of a Y-dwarf

Brown dwarfs of spectral class Y are cooler than those of spectral class T and have qualitatively different spectra from them. A total of 17 objects have been placed in class Y as of August 2013.[71] Although such dwarfs have been modelled[72] and detected within forty light years by the Wide-field Infrared Survey Explorer (WISE)[56][73][74][75][76] there is no well-defined spectral sequence yet with prototypes. Nevertheless, several objects have been assigned spectral classes Y0, Y1, and Y2.[77] The spectra of these objects display absorption around 1.55 micrometers.[78] 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.[78][79] In fact, this ammonia-absorption feature is the main criterion that has been adopted to define this class.[77] However, this feature is difficult to distinguish from absorption by water and methane,[78] and other authors have stated that the assignment of class Y0 is premature.[80]

  • Y: < 600 K, ultra-cool brown dwarfs
Example: WISE 0410+1502, class Y0
Example: WISE 0350-5658, class Y1
Example: WISE 1405+5534, class Y1

The brown dwarf with the latest assigned spectral type, WISE 1828+2650, is a >Y2 dwarf with an effective temperature originally estimated around 300 K, the temperature of the human body.[73][74][81] Parallax measurements have however since shown that its luminosity is inconsistent with it being colder than ~400 K; the likely coolest Y dwarf currently known is WD 0806−661B with approximately 350 K.[2]


Carbon-related late giant star classes[edit]


Class C: carbon stars[edit]

Main article: Carbon star
Image of the carbon star R Sculptoris and its striking spiral structure


  • C: Carbon stars, e.g. R CMi

Class S[edit]

Main article: S-type star


Examples: S Ursae Majoris, BD Camelopardalis



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

White dwarf classifications[edit]



Examples: Sirius B (DA2), Procyon B (DA4), Van Maanen's star (DZ7)[85], Table 1

The white dwarf types are as follows:[86][87]




Extended white dwarf spectral types:[86]

  • DAZ: a hydrogen-rich metallic white dwarf.
  • DBZ: a helium-rich metallic white dwarf.

Variable star designations:

  • DAV or ZZ Ceti: a hydrogen-rich pulsating white dwarf.[89], pp. 891, 895
  • DBV or V777 Her: a helium-rich pulsating white dwarf.[90], p. 3525
  • GW Vir, sometimes divided into DOV and PNNV: a hot helium-rich pulsating white dwarf (or pre-white dwarf.)[91], §1.1, 1.2;[92][93] These stars are generally PG 1159 stars, although some authors also include non-PG 1159 stars in this class.[91][94]



Degenerate and exotic stars[edit]

Main articles: Neutron star, Black hole and Exotic star



Further information: Planetary habitability



For these reasons NASA's Kepler Mission has been searching for habitable planets mainly at main-sequence stars that are less massive than spectral type A but more massive than type M—i.e. main-sequence stars of types F, G, and K, the most probable stars to host life.[96]

Variable star classification[edit]

Main article: Variable star


Photometric classification[edit]



See also[edit]


  2. ^ Use a different set of spectral types from element-burning stars
  3. ^ When used with A-type stars, this instead refers to abnormally strong metallic spectral lines


  2. ^ a b T. J. Dupuy & A. L. Kraus (2013). "Distances, Luminosities, and Temperatures of the Coldest Known Substellar Objects". Science. published online 5 September 2013 (6153): 1492–1495. arXiv:1309.1422. Bibcode:2013Sci...341.1492D. doi:10.1126/science.1241917. 
  7. ^ a b c d Baraffe; Chabrier; Barman; Allard & Hauschildt (2003). "Evolutionary models for cool brown dwarfs and extrasolar giant planets. The case of HD 209458". Astronomy and Astrophysics 402 (2): 701. arXiv:astro-ph/0302293. Bibcode:2003A&A...402..701B. doi:10.1051/0004-6361:20030252. 
  11. ^ a b c d "A note on the spectral atlas and spectral classification". Centre de données astronomiques de Strasbourg. Retrieved 2 January 2015. 
  12. ^ Caballero-Nieves, S. M.; Nelan, E. P.; Gies, D. R.; Wallace, D. J.; Degioia-Eastwood, K.; Herrero, A.; Jao, W. -C.; Mason, B. D.; Massey, P.; Moffat, A. F. J.; Walborn, N. R. (2014). "A High Angular Resolution Survey of Massive Stars in Cygnus Ob2: Results from Thehubble Space Telescopefine Guidance Sensors". The Astronomical Journal 147 (2): 40. doi:10.1088/0004-6256/147/2/40.  edit
  13. ^ Prinja, R. K.; Massa, D. L. (2010). "Signature of wide-spread clumping in B supergiant winds". Astronomy and Astrophysics 521: L55. doi:10.1051/0004-6361/201015252.  edit
  14. ^ Gray, D. F. (2010). "PHOTOSPHERIC VARIATIONS OF THE SUPERGIANT γ Cyg". The Astronomical Journal 140 (5): 1329. doi:10.1088/0004-6256/140/5/1329.  edit
  15. ^ a b Nazé, Y. (2009). "Hot stars observed by XMM-Newton". Astronomy and Astrophysics 506 (2): 1055. doi:10.1051/0004-6361/200912659.  edit
  16. ^ Lyubimkov, L. S. et al. (2010). "Accurate fundamental parameters for A-, F- and G-type Supergiants in the solar neighbourhood". Monthly Notices of the Royal Astronomical Society 402 (2): 1369–1379. arXiv:0911.1335. Bibcode:2010MNRAS.402.1369L. doi:10.1111/j.1365-2966.2009.15979.x.  edit
  17. ^ Gray, R. O.; Corbally, C. J.; Garrison, R. F.; McFadden, M. T.; Robinson, P. E. (2003). "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I". The Astronomical Journal 126 (4): 2048. doi:10.1086/378365.  edit
  18. ^ Shenavrin, V. I.; Taranova, O. G.; Nadzhip, A. E. (2011). "Search for and study of hot circumstellar dust envelopes". Astronomy Reports 55: 31. doi:10.1134/S1063772911010070.  edit
  19. ^ Cenarro, A. J.; Peletier, R. F.; Sanchez-Blazquez, P.; Selam, S. O.; Toloba, E.; Cardiel, N.; Falcon-Barroso, J.; Gorgas, J.; Jimenez-Vicente, J.; Vazdekis, A. (2007). "Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters". Monthly Notices of the Royal Astronomical Society 374 (2): 664. doi:10.1111/j.1365-2966.2006.11196.x.  edit
  20. ^ Sion, Edward M.; Holberg, J. B.; Oswalt, Terry D.; McCook, George P.; Wasatonic, Richard (2009). "The White Dwarfs Within 20 Parsecs of the Sun: Kinematics and Statistics". The Astronomical Journal 138 (6): 1681–1689. arXiv:0910.1288. Bibcode:2009AJ....138.1681S. doi:10.1088/0004-6256/138/6/1681.  edit
  21. ^ SkyTonight: The Spectral Types of Stars
  22. ^ a b c d e f g h i j k Allen, J. S. "The Classification of Stellar Spectra". UCL Department of Physics and Astronomy: Astrophysics Group. Retrieved 1 January 2014. 
  27. ^ Rountree Lesh, J. (1968). "The Kinematics of the Gould Belt: An Expanding Group?". The Astrophysical Journal Supplement Series 17: 371. doi:10.1086/190179.  edit
  32. ^ pp. 62–63, Hearnshaw 1986.
  33. ^ p. 60, Hearnshaw 1986.
  34. ^ Classification of Stellar Spectra: Some History
  38. ^ a b pp. 106–108, Hearnshaw 1986.
  39. ^ pp. 111–112, Hearnshaw 1986.
  42. ^ pp. 117–119, Hearnshaw 1986.
  44. ^ pp. 121–122, Hearnshaw 1986.
  52. ^ a b c d e f g Garrison, R. F. (1994). "A Hierarchy of Standards for the MK Process". Astronomical Society of the Pacific. Bibcode:1994ASPC...60....3G. 
  56. ^ a b Stars as Cool as the Human Body
  57. ^ "Galactic refurbishment". ESA/Hubble. Retrieved 29 April 2015. 
  59. ^ Bibcode1995A&A...295...75K
  62. ^ a b Multiwavelength Systematics of OB Spectra, Walborn, N. R., 2008
  68. ^ Ultra-cool Diminutive Star Weighs In
  70. ^ Discovery of the brightest T dwarf in the northern hemisphere, 2007
  74. ^ a b NASA spots chilled-out stars cooler than the human body
  75. ^ NASA'S Wise Mission Discovers Coolest Class of Stars
  83. ^ Spectral Atlas of Carbon Stars (Barnbaum+ 1996)
  95. ^ Digital Demo Room Stellar Structure and Evolution Simulator(date unknown)

External links[edit]