Morgan-Keenan classification
/ MOR-gan KEE-nan /
Knowing a star's temperature alone is like knowing a person's age but not whether they are short or tall — useful, but incomplete. Two stars can be the exact same yellow temperature yet be wildly different beasts: one a modest dwarf like the Sun, the other a bloated giant a hundred times wider. The Morgan-Keenan classification is the two-part naming scheme that captures both facts at once: how hot a star is, and how big and luminous.
An MK type has two pieces. The first is the spectral type (O, B, A, F, G, K, M, with a digit 0 to 9 for fine temperature steps), which encodes temperature. The second is a Roman-numeral luminosity class, from I for supergiants down through II, III (giants), IV (subgiants), to V for ordinary main-sequence dwarfs. The trick is that giants and dwarfs of the same temperature have subtly different absorption lines — a giant's thinner, lower-pressure atmosphere makes some lines narrower and sharper — so a skilled eye can read luminosity straight from the spectrum. The system was set out by William Wilson Morgan and Philip Keenan at Yerkes Observatory in 1943.
The MK system matters because those two labels together place a star almost exactly on the Hertzsprung-Russell diagram, and from that position you can estimate its true luminosity. That makes the spectrum a distance tool: compare the luminosity implied by the MK type with how bright the star actually looks, and you get its distance — a method called spectroscopic parallax, which reaches far beyond ordinary parallax. The Sun's full MK type is G2V.
Aldebaran is classified K5III: 'K5' marks it as a cool orange star, and 'III' marks it as a giant. The same K5 temperature in luminosity class V would be a small, dim orange dwarf instead — same colour, utterly different star.
Spectral type plus luminosity class: temperature and size in one short code.
The luminosity class is read from subtle line shapes, not from the star's apparent brightness — that is the clever part. It works because atmospheric pressure, which differs between giants and dwarfs of the same temperature, leaves its own mark on the spectrum.