Stjärntypen

Stjärntypen refers to the spectral classification of stars. This system categorizes stars based on their surface temperature, which is directly related to their color and the spectral lines observed in their light. The most common classification scheme uses letters: O, B, A, F, G, K, and M, ordered from hottest to coolest. O-type stars are the hottest and bluest, while M-type stars are the coolest and reddest.

Each letter is further subdivided by a number from 0 to 9, where 0 is the hottest and 9 is the coolest within that spectral class. For example, a G2 star is hotter than a G9 star. The Sun is a G2 star. This classification system was developed in the early 20th century, initially by astronomers at Harvard College Observatory, and is also known as the Harvard spectral classification.

Beyond temperature, spectral type also provides information about a star's chemical composition and its evolutionary stage. The presence and strength of specific absorption lines in a star's spectrum reveal the elements present in its atmosphere and their ionization states, which are temperature-dependent. Luminosity classes, indicated by Roman numerals (e.g., I for supergiants, V for main-sequence stars like our Sun, and VI for subdwarfs), are also often appended to the spectral type to further refine a star's characterization. Together, spectral type and luminosity class provide a comprehensive understanding of a star's fundamental properties.