Classification ofstars is based primarily on their temperatures. TheHarvard spectral classification scheme assigns eachstar aspectral type which is further divided into 10 sub-classes depending on theabsorption features present in the spectrum. For example, ourSun has a temperature of about 5,700Kelvin and is classified as a G2 star.
However, this classification scheme does not completely describe the star as it cannot distinguish between stars with the same temperature but differentluminosities. In other words, it cannot distinguish between main sequence (dwarf) stars, giant stars and supergiant stars.
For this reason, theMorgan-Keenan luminosity class (MK orMKK) was established. Originally containing roman numerals between I (supergiant star) and V (main sequence), these days, class I stars have been subdivided into Ia-O, Ia and Ib, and classes VI (sub-dwarf) and D (white dwarf) have been added.
To completely describe the star, the MK luminosity class is appended to the original Harvard classification for the star. For example, our Sun is a main sequence G2 star, therefore its full classification isG2V.
The following table summarises the MK Luminosity Classes:
| Class | Star |
|---|---|
| Ia-O | extremely luminous supergiants |
| Ia | luminous supergiants |
| Ib | less luminous supergiants |
| II | bright giants |
| III | normal giants |
| IV | subgiants |
| V | main sequence dwarf stars |
| VI, or sd | subdwarfs |
| D | white dwarfs |
See also:Hertzsprung-Russell Diagram.
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