Introduction to Astrophysics/Stars/Classification

Stellar classification is the classification of stars based on their spectral characteristics.

Currently, most stars are classified under the Morgan-Keenan system (MK)., where each star is assigned a spectral class from the older Harvard classification and a luminosity class using Roman numerals, forming the star's spectral type.

Conventional colour description
The conventional colour description only looks at the peak of the stellar spectrum. In actuality, though, stars radiate in all parts of the spectrum and because all spectral colours combined appear white, the colours the human eye would see are far lighter than the conventional colour descriptions would suggest. Meaning, the simplified assignment of colours of the spectrum can be misleading. For example, so-called red dwarfs are a deep shade of orange and brown dwarfs would hypothetically appear to a nearby observer as a dim grey.

Modern classification
As described above, the modern classification system used is the Morgan-Keenan system (MK). Stars are given labels such as G2 (the Sun) or K1.5 (Arcturus). They are composed of the Harvard spectral classification (O, B, A, F, G, K and M, ranging from the hottest to the coldest), and a Roman numeral also ranging from the hottest (0) to the coldest (9).

Other modern systems, (e.g. the UBV system) are based on colour indexes - the measured differences in three or more colour magnitudes. Those numbers are given labels such as "U-V" (Ultraviolet) or "B-V" (Blue and Visual), which represent the colours passed by two standard filters.

Harvard spectral classification
The Harvard system, which was developed by the astronomer Annie Jump Cannon, is a simple alphabetical classification. Physically, the classes indicate the temperature of the star's atmosphere (effective temperature) and are normally listed from hottest to coldest.