Astronomy
The Spectral Classification of Stars Using the Morgan-Keenan System
Quick fact
The Morgan-Keenan system classifies stars into seven main types (O, B, A, F, G, K, M) based on their spectra, with O stars being the hottest and M the coolest. The sequence is memorized by astronomy students with the phrase 'Oh, Be A Fine Girl/Guy, Kiss Me!'
Why this is interesting
Every star tells a story through its rainbow of light. But why do some stars blaze blue while others glow red, and what secrets hide in the dark lines of their spectra?
Read the full explanation
Understanding The Spectral Classification of Stars Using the Morgan-Keenan System
When you look at a star's spectrum, you see a rainbow crossed by dark lines—these are absorption lines, fingerprints of the elements in the star's atmosphere. The temperature of a star dictates which lines appear strongest. In hot O stars, hydrogen lines are weak because hydrogen is ionized; in cooler A stars, hydrogen lines are prominent; in even cooler K and M stars, molecular bands like titanium oxide appear. The MK system uses the sequence OBAFGKM—from hottest to coolest—along with a luminosity class (I for supergiants, V for main-sequence stars) to pinpoint a star's temperature and size on the Hertzsprung-Russell diagram. This two-dimensional classification allows astronomers to locate a star's evolutionary stage.
A deeper explanation
The MK system works because a star's spectrum is a direct consequence of its surface temperature and atmospheric pressure, which in turn depend on its mass and evolutionary stage. Surface temperature controls the balance of ionized, neutral, and molecular species, producing characteristic absorption lines. For example, the Balmer lines of hydrogen peak at about 10,000 K, so A stars show the strongest hydrogen lines. Meanwhile, luminosity class affects the starkness of certain lines—supergiants have broader, stronger lines for some elements due to lower surface gravity compared to main-sequence dwarfs. By combining the spectral type (temperature) and luminosity class (surface gravity), the MK system reveals a star's position on the HR diagram, which is the cornerstone of stellar evolution studies, distance estimation via spectroscopic parallax, and mapping the Milky Way.