The stars show a tremendous range in luminosity, though much less in mass. Some known stars are millions of times more luminous than the Sun, while others are remarkably feeble. At its peak, in the 1840s, the erratic variable η Carinæ was estimated to be 6 000 000 times as powerful as the Sun; S Doradûs, in the Large Magellanic Cloud, has an absolute magnitude of –8.9, so that it is at least a million times as luminous as the Sun – yet because of its great distance (170 000 light-years) it cannot be seen with the naked eye. At the other end of the scale is MH18, discovered in 1990 by M. Hawkins at what was then the Royal Observatory, Edinburgh, from plates taken with the UK Schmidt telescope in Australia. It has 1/20 000 the luminosity of the Sun, and is presumably a brown dwarf (see below). Its mass is 5% that of the Sun, and its distance is 68 light-years.

The first attempt to classify the stars according to their spectra was made by the Italian Jesuit astronomer, Angelo Secchi, in 1863–7. He divided the stars into four main types:

1. (1) White or bluish stars: with broad, dark lines of hydrogen but obscure metallic lines. Example: Sirius.

2. (2) Yellow stars: hydrogen lines less prominent, metallic lines more so. Examples: Capella, the Sun.

3. (3) Orange stars: complicated, banded spectra. Examples: Betelgeux, Mira. The class included many long-period variables.

4. (4) Red stars: with prominent carbon lines; all below magnitude 5. Example: R Cygni. This class also included many variables.