luminosity class

Two stars can be exactly the same colour and temperature, yet one is a small, ordinary star like the Sun and the other a swollen giant the size of Earth's orbit. They look identical on a colour chart. The luminosity class is the label that tells them apart — it sorts stars of the same temperature by how large and luminous they are, from puffed-up supergiants down to compact dwarfs.

Luminosity classes are written with Roman numerals appended to the spectral type. Class I marks the supergiants (often split into Ia for the most luminous and Ib for slightly less), II bright giants, III ordinary giants, IV subgiants, and V the main-sequence dwarfs where most stars, including the Sun, live. There are even classes VI for subdwarfs and the symbol 'wd' or VII for white dwarfs. The astonishing trick is that the class can be read directly from the spectrum: a giant's vast, low-density atmosphere has lower pressure, which makes certain absorption lines narrower and sharper than in a dense dwarf of the same temperature.

Because temperature plus luminosity class together pin a star to a region of the Hertzsprung-Russell diagram, and the diagram links that position to a true luminosity, the luminosity class lets astronomers estimate a star's intrinsic power from its light alone. Compare that to its apparent brightness and out pops the distance — the method called spectroscopic parallax. The luminosity class is thus the second half of the Morgan-Keenan label and a quiet workhorse of measuring the galaxy.