MK system

astronomy

Also known as: MKK system, Yerkes classification system

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June 30, 2026

Quick Summary

The MK system, also known as the Yerkes system, is a method used in astronomy to classify stars. It combines two classification schemes: the Harvard system, which categorizes stars by their surface temperature, and the MK system itself, which classifies stars by their luminosity.1

The spectral sequence, ordered from hottest to coolest stars, is O, B, A, F, G, K, M. This sequence is supplemented by R, N (carbon stars), S, L, T, and Y types, which include brown dwarfs and cooler stars. Each spectral type is further subdivided by numbers 0 through 9, with higher numbers indicating cooler stars. The MK system adds a luminosity class to the spectral type, using Roman numerals (I for supergiants, II for bright giants, III for normal giants, IV for subgiants, and V for main-sequence or dwarf stars). For example, our Sun is classified as G2 V.12

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Classification of spectral types
…of spectral classification, called the MK system (after the American astronomers William W. Morgan and Philip C. Keenan, who introduced it), luminosity class is assigned to the star along with the Draper spectral type. For example, the star Alpha Persei is classified as F5 Ib, which means that it falls…
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…of spectral classification, called the MK system (after the American astronomers William W. Morgan and Philip C. Keenan, who introduced it), luminosity class is assigned to the star along with the Draper spectral type. For example, the star Alpha Persei is classified as F5 Ib, which means that it falls…
Read More

parallax
Indirect measurement
The system, called the MK system, assigns a precise system of Draper classes and five luminosity classes, using the Roman numerals I to V. The system divides the majority of stars into supergiants, bright giants, subgiants, and main sequence (dwarf) stars, depending upon their intrinsic brightness, as determined from…
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The system, called the MK system, assigns a precise system of Draper classes and five luminosity classes, using the Roman numerals I to V. The system divides the majority of stars into supergiants, bright giants, subgiants, and main sequence (dwarf) stars, depending upon their intrinsic brightness, as determined from…
Read More

stellar classification
The MK, or Yerkes, system is the work of the American astronomers W.W. Morgan, P.C. Keenan, and others. It is based on two sets of parameters: a refined version of the Harvard O-M scale, and a luminosity scale of grades I (for supergiants), II (bright giants),…
Read More
The MK, or Yerkes, system is the work of the American astronomers W.W. Morgan, P.C. Keenan, and others. It is based on two sets of parameters: a refined version of the Harvard O-M scale, and a luminosity scale of grades I (for supergiants), II (bright giants),…
Read More