How do the Harvard and MK stellar classification systems differ?

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Open cluster NGC 290 — Star Stars in the open cluster NGC 290, as seen by the Hubble Space Telescope.

The Harvard and MK systems are distinct but complementary components of modern stellar classification. While the Harvard system focuses on a star's surface temperature, the MK system incorporates both temperature and luminosity to provide a more comprehensive profile.1

The Harvard System

Developed at the Harvard College Observatory, this system classifies stars based on their spectral lines, which correspond to their surface temperatures. The sequence, ordered from hottest to coolest, is O, B, A, F, G, K, and M (with L, T, and Y added for brown dwarfs). Each type is further subdivided using numbers from 0 to 9, where higher numbers indicate cooler temperatures.1

The MK System

Also known as the Yerkes system, the MK system refines the Harvard scale by adding a luminosity classification. This is expressed through Roman numerals ranging from I (supergiants) to V (main sequence or dwarf stars), with additional grades for subdwarfs and white dwarfs. By combining these, astronomers can precisely categorize stars; for example, the Sun is classified as G2 V, indicating its temperature (G2) and its status as a main-sequence star (V).1