steltyper

steltyper is a term that refers to the different categories or types of stars based on their spectral characteristics. These classifications are primarily determined by the temperature of the star's surface, which in turn influences the elements that are ionized or excited in its atmosphere, and thus the spectral lines observed. The most common stellar classification system is the Morgan-Keenan (MK) system, which assigns letters from O, B, A, F, G, K, and M, in order of decreasing surface temperature. O-type stars are the hottest and bluest, while M-type stars are the coolest and reddest.

Each of these spectral types is further subdivided by a number from 0 to 9, with 0 being the hottest within that type and 9 being the coolest. For example, a G2 star is a specific type of G-type star, hotter than a G3 but cooler than a G1. In addition to the letter and number, the MK system also includes luminosity classes, denoted by Roman numerals (I for supergiants, II for bright giants, III for giants, IV for subgiants, V for main-sequence or dwarf stars, and VI for subdwarfs). The most common stars, like our Sun, are G2V, indicating a G2 spectral type and a main-sequence luminosity class.

Other, less common spectral types exist, such as L, T, and Y, which are used for very cool objects like brown dwarfs. These types are even cooler than M-type stars and are characterized by the presence of molecules like water and methane in their atmospheres. The study of steltyper is crucial for understanding stellar evolution, as a star's spectral type provides significant clues about its age, mass, and temperature.