Spectral class

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The spectral class , also known as the spectral type , is a classification of stars in astronomy according to the appearance of their light spectrum .

The system is based on the discovery of Joseph von Fraunhofer in 1813, who found dark absorption lines in the solar spectrum . Robert Wilhelm Bunsen and Gustav Robert Kirchhoff discovered in 1859 that these lines are positionally identical to emission lines that are given off by certain chemical elements .

It was easy to conclude that these elements must be present in the sun. The spectral analysis was justified. In addition to the analysis of materials on earth, the star spectra can also be analyzed.


The classification of a star in a spectral class of the MK system (according to W. Morgan and P. Keenan ) is based on a visual comparison of its spectrum with the spectra of standard stars. In order to exclude instrumental effects - such as a higher spectral resolution - on the classification, a standard instrumentation is given. In view of the advanced development of astronomical instruments, the classification resolution has meanwhile been increased several times. The original MK system also underwent changes in that new standard stars were included and others, recognized as unsuitable, were removed from the system. Because of the photographic emulsions used at the time, the spectral range to which the MK classification refers extends from about 390 nm to about 500 nm.

The MK classification explicitly does not include any classification according to secondary physical quantities, but makes use of the human brain's ability to recognize patterns. More recently, artificial neural networks have also been trained for the MK classification with some success. This ensures that the classification remains consistent even if the findings on stellar physics change.

Comparative examples
Spectra of early main sequence stars with marked classification features of He II (strong in O stars), He I (strong in early B stars), and Balmer lines (strong in late B / early A stars)
Luminosity sequence of early B-type stars - the width of the Balmer lines decreases sharply until Hβ at B1a + is even in emission, while the classification features for the temperature, here the He I / Mg II ratio, hardly change


Comparison of the spectral classes O – M for main sequence stars

It has become common practice to designate the spectral classes O to A as early spectral classes , the spectral classes F to G as middle spectral classes and the other spectral classes as late spectral classes . The designations early, medium and late come from the now outdated assumption that the spectral class says something about the stage of development of a star. Despite this erroneous classification, these terms are still in use today, and a star is considered sooner or later if its spectral class is closer to class O or class M compared to that of another.

There are the following seven basic classes, as well as three classes for brown dwarfs and three sub-classes for chemical peculiarities of red giant stars caused by nucleosynthesis .

For more precise classification, spectral classes can be divided into subclasses 0 to 9. Today there are several systems of spectral classification that use this notation of the spectral type and align their classes with this system. In the original Harvard system and its extension, the MK system , which also defines the luminosity classes, not all of these subtypes were also used. For example, B3 stars were immediately followed by B5 stars; class B4 was skipped. With increasingly better instruments it was possible to differentiate more finely over time, so that intermediate classes were defined, for example there are now even three additional classes between B0 and B1, which are called B0.2, B0.5, and B0.7

There were various forerunners of today's spectral classes, see: Classification of the stars # History (previous classifications)

class Characteristic colour Surface
( K )
typical dimensions
for main
row ( M )
Main row and giant branch
O ionized helium (He II) blue 30000-50000 60 Mintaka (δ Ori), Naos (ζ Pup)
B. neutral helium (He I), Balmer series hydrogen blue White 10000-28000 18th Rigel , Spica , Achernar
A. Hydrogen, calcium (Ca II) white (slightly bluish) 07500 09750 03.2 Wega , Sirius , Altair
F. Calcium (Ca II), appearance of metals White yellow 06000- 07350 01.7 Prokyon , Canopus , Pole Star
G Calcium (Ca II), iron and other metals yellow 05000- 05900 01.1 Tau Ceti , Sun , Alpha Centauri A
K strong metal lines, later titanium (IV) oxide orange 03500- 04850 00.8 Arcturus , Aldebaran , Epsilon Eridani , Albireo A
M. Titanium oxide Red orange 02000- 03350 00.3 Betelgeuse , Antares , Kapteyns Stern , Proxima Centauri
Brown dwarfs
L. red 01300- 02000 VW Hyi
T red (maximum in IR) 00600- 01300 ε Indi Ba
Y infrared (IR) 00200- 00600 WISEP J041022.71 + 150248.5
Carbon classes of the red giants ( carbon stars )
R. Cyan (CN), carbon monoxide (CO), carbon Red orange 03500- 05400 S Cam, RU Vir
N Similar to class R, with more carbon.
From this spectral class onwards, the spectrum has
practically no blue components.
Red orange 02000- 03500 T Cam, U Cas
S. Zirconium oxide red 01900- 03500 R Lep, Y CVn , U Hya

The spectral classes with their seven basic types (O, B, A, F, G, K, M) make up around 99% of all stars, which is why the other classes are often neglected.

The following sentences serve as a mnemonic for these spectral classes:

  • Main series (OBAFGKM):
" O pa B astelt A m F reitag G erne K leash M ännchen"
" O ffenbar B enutzen A stronomen F urchtbar G erne K omische M erksätze"
" O hne B ier a us'm F ass g ibt's k OA M ate"
  • Main sequence + red giants (OBAFGKM (RNS)):
" O h B e A F ine G irl / G uy K iss M e ( R ight N ow. S mack! )"
  • Main sequence + brown dwarfs (OBAFGKMLT):
" O h B e A F ine G irl / G uy K iss M y L ips T onight"
" O hne B ier a us F tabs g eht k a M ensch l ang t Drink"

There are a large number of other variants of corresponding memorandum sentences .

Classes outside the standard sequences

The following classes cannot be classified in the sequences described above:

Q Novae
Pv Planetary nebula
W. Wolf-Rayet stars
WN Nitrogen lines
WC Carbon lines

Prefixes and suffixes

The subdivision of the spectral classes can be further refined using suffixes and prefixes .


suffix meaning
c particularly sharp lines ( crisp )
comp composite (Engl. composite ) spectrum
d Dwarf (main sequence; Engl. Dwarf )
e, em Emission lines
G normal giant (Engl. giant )
k interstellar absorption lines
m strong metal lines
n, nn diffuse lines ( nebulous )
p, pec Special features of line intensity (English peculiar , "special")
s sharp lines
sd Under dwarf (Engl. Sub dwarf )
v, var variable spectrum
w White dwarf

In some cases, these additions are superfluous by specifying the luminosity class, which was introduced in 1943 by William Wilson Morgan and Philip Keenan (MK system).


prefix meaning
english (international) German
d dwarf dwarf
sd sub-dwarf Subdwarf
G giant giant


  • James B. Kaler: Stars and Their Spectra. An Introduction to the Spectral Sequence. Cambridge University Press 1997, ISBN 0-521-58570-8 .
  • James B. Kaler: Stars and their Spectra. Astronomical signals from light. Spectrum Academic Publishing House, Heidelberg u. a. 1994, ISBN 3-86025-089-2 .

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