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 .
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
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 The designations early, medium and late come from the now outdated assumption that the spectral class says something about the stage of development of a star.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.
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)
( K )
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)||7500 9750||3.2||Wega , Sirius , Altair|
|F.||Calcium (Ca II), appearance of metals||White yellow||6000- 7350||1.7||Prokyon , Canopus , Pole Star|
|G||Calcium (Ca II), iron and other metals||yellow||5000- 5900||1.1||Tau Ceti , Sun , Alpha Centauri A|
|K||strong metal lines, later titanium (IV) oxide||orange||3500- 4850||0.8||Arcturus , Aldebaran , Epsilon Eridani , Albireo A|
|M.||Titanium oxide||Red orange||2000- 3350||0.3||Betelgeuse , Antares , Kapteyns Stern , Proxima Centauri|
|L.||red||1300- 2000||VW Hyi|
|T||red (maximum in IR)||600- 1300||ε Indi Ba|
|Y||infrared (IR)||200- 600||WISEP J041022.71 + 150248.5|
|Carbon classes of the red giants ( carbon stars )|
|R.||Cyan (CN), carbon monoxide (CO), carbon||Red orange||3500- 5400||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||2000- 3500||T Cam, U Cas|
|S.||Zirconium oxide||red||1900- 3500||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:
Prefixes and suffixes
|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")|
|sd||Under dwarf (Engl. Sub dwarf )|
|v, var||variable spectrum|
- 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 .