Luminosity classes and populations

Besides spectral types, two further classifications are common. The first one are the luminosity classes. These go as follows:

Ia bright supergiants (>10000 solar luminosities)
Ib supergiants (5000-10000 s.l.)
II bright giants (200-5000 s.l.)
III normal giants (50-200 s.l.)
IV subgiants (5-50 s.l.)
V main sequence stars
VI subdwarfs
VII white dwarfs

These luminosity classes are appended to the spectral type, e.g. M0Ib, K2III, F5IV, G2V, K3VI. VII is rarely used; it's much more common to denote white dwarfs by a prefix D (e.g. DA5).

The stellar populations are defined by the age of the stars; they differ mainly in chemical composition. Population I stars contain much more heavy elements than Population II stars. The reason is that Population II stars are first-generation stars; they are as old as the galaxy (about 15000 million years) and were formed of pre-stellar matter, while Population I stars are younger and contain reasonable amounts of the ashes of stars that died before. All main sequence stars belong to Population I, as well as supergiants. The Population II stars are subdwarfs or some giants and subgiants. While Population I stars are concentrated in the galactic disk, Population II stars are found in the central region of the galaxy and in the galactic halo. Globular clusters consist solely of Population II stars, as well as elliptical galaxies. The difference in composition strongly affects the probability of planets and life: Population II stars have at best hydrogen giant planets without mineral cores. These are gloomy, pale and colourless globes because there is nothing that could form coloured clouds, just hydrogen and a small proportion of helium (primarily in their core) -- and no life!