tähtityyppi

A star's type refers to its classification based on its spectral characteristics, primarily its surface temperature. This spectral classification system, often called the Morgan-Keenan (MK) system, categorizes stars into spectral types denoted by letters, with the most common sequence being O, B, A, F, G, K, and M. These letters represent a temperature sequence from hottest (O-type stars, with surface temperatures above 30,000 Kelvin) to coolest (M-type stars, with surface temperatures below 3,700 Kelvin). Each letter can be further subdivided using numbers from 0 to 9, with 0 being the hottest within that class and 9 being the coolest. For example, a G2 star is hotter than a G9 star.

Beyond the primary spectral type, the MK system also includes luminosity classes, denoted by Roman numerals. These classes indicate the star's size and luminosity, ranging from I (supergiants) to V (main-sequence or dwarf stars) and VI (subdwarfs) and VII (white dwarfs). The most common stars, like our Sun, are main-sequence stars, designated as luminosity class V. Therefore, a star's full spectral type might be something like G2V, indicating a G2-type star that is also a main-sequence dwarf. This detailed classification helps astronomers understand a star's physical properties, evolutionary stage, and potential characteristics.