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In the 1920s, the Bengali physicist Meghnad Saha derived a theory of ionization by extending well-known ideas in physical chemistry pertaining to the dissociation of molecules to the ionization of atoms.

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This means that the simplified assignment of colors of the spectrum can be misleading.In astronomy, stellar classification is the classification of stars based on their spectral characteristics.Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines.The Yerkes spectral classification, also called the MKK system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by William Wilson Morgan, Philip C. This two-dimensional (temperature and luminosity) classification scheme is based on spectral lines sensitive to stellar temperature and surface gravity, which is related to luminosity (whilst the Harvard classification is based on just surface temperature).Later, in 1953, after some revisions of list of standard stars and classification criteria, the scheme was named the Morgan–Keenan classification, or MK (by William Wilson Morgan and Philip C.The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars.

In the MK system, a luminosity class is added to the spectral class using Roman numerals.

Excluding color-contrast illusions in dim light, there are no green, indigo, or violet stars.

Red dwarfs are a deep shade of orange, and brown dwarfs do not literally appear brown, but hypothetically would appear to be a dim grey to a nearby observer.

Other modern stellar classification systems, such as the UBV system, are based on color indexes—the measured differences in three or more color magnitudes.

Those numbers are given labels such as "U-V" or "B-V", which represent the colors passed by two standard filters (e.g. The Harvard system is a one-dimensional classification scheme by astronomer Annie Jump Cannon, who re-ordered and simplified a prior alphabetical system.

For example, A0 denotes the hottest stars in class A and A9 denotes the coolest ones.