tähtiasteikot

Tähtiasteikot, also known as stellar classification or spectral types, are a system used by astronomers to categorize stars based on their surface temperature and spectral characteristics. The most common classification scheme is the Morgan-Keenan (MK) system, which uses letters to represent spectral types. The primary spectral types are O, B, A, F, G, K, and M, ordered from hottest to coolest. O-type stars are extremely hot, with surface temperatures exceeding 30,000 Kelvin, while M-type stars are much cooler, below 3,500 Kelvin.

Each spectral type is further subdivided by numbers from 0 to 9, with 0 being the hottest within that type. For example, a G2 star is hotter than a G9 star. The Sun, for instance, is a G2V star. The Roman numeral following the spectral type (e.g., V) indicates the luminosity class, which relates to the star's size and brightness. Common luminosity classes include I (supergiants), II (bright giants), III (giants), IV (subgiants), V (main-sequence or dwarf stars), and VI (subdwarfs). White dwarfs are often assigned the luminosity class D.

The spectral lines observed in a star's light are the key to determining its spectral type. Different elements and molecules absorb light at specific wavelengths, creating dark lines in the star's spectrum. The presence and strength of these lines are highly dependent on the star's surface temperature. For example, hydrogen lines are strongest in A-type stars, while molecular bands become prominent in cooler M-type stars. This system allows astronomers to infer crucial properties of stars, such as their temperature, size, and evolutionary stage, from a distance.