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ASTRONOMY - SPECTRAL CLASSES

The Solar System | Star Formation | Celestial Bodies | Spectral Classes

Spectral Class	Tempterature - Colour	Principal Characteristics
O	25,000 K - Blue	This stage is characterized by lines of helium, oxygen, and nitrogen in the spectrum of the photosphere. O stars are extremely hot, very bright stars that emit large amounts of ultraviolet radiation.
B	11,000 K - 25,000 K - White-blue	In this group the lines of helium in the spectrum reach a maximum intensity and then fade. The intensity of the hydrogen lines regularly increases in all the subdivisions of stage B. Type B stars are typically represented by the star Epsilon Orionis.
A	7500 K - 11,000 K - White	This group comprises the so-called hydrogen stars. The spectra of these stars is dominated by absorption lines of hydrogen. Sirius, the Dog star, is a typical type A star.
F	6000 K - 7500 K - Yellow-white	This group is composed of stars characterized by an elevated intensity of the H and K lines of calcium and of lines characteristic of hydrogen. A notable star in this category is Delta Aquilae.
G	5000 K - 6000 K - Yellow	This group is composed of stars with prominent H and K calcium lines and less prominent hydrogen lines. The spectra of numerous metals, in particular iron, are also present. The Sun belongs to this group, and therefore G stars are frequently called solar stars.
K	K 3500 K - 5000 K - Orange-yellow	This group comprises stars having strong calcium lines and lines indicating the presence of other metals in their spectra.The violet light of class K stars is less intense than the stars' red light. This group is typically represented by Arcturus.
M	3500 K - Red	This group is composed of stars whose spectra are dominated by bands resulting from the presence of metallic-oxide molecules, notably those of titanium oxide. The violet end of the spectra is less intense than that of K stars. The star Orionis is typical of this group.

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