Stars come in different colours and sizes. There are bright stars and faint stars, there are blue, white, yellow and red stars. While brightness is a measurable quantity, colour is a subjective impression. However, the astronomers have found a way to make the fuzzy term `colour' into a measurable quantity. This is the spectral type. They classify stars by characteristics of their visual spectrum, and as colour is the visible outcome of the spectrum, each spectral type corresponds to a colour. The spectral types are O (blue), B (blue-white), A (white), F (yellow-white), G (yellow), K (orange) and M (red). (Note that these apparent colours are merely a delusion of the eye. Against the blue straylight at night, the stars appear much redder than they are. Actually, the `reddest' stars are no redder than a light bulb!) Each of these types is subdivided into 10 subtypes denoted by digits 0 to 9. However, not all colour/brightness combinations are equally likely. If you draw a diagra showing the relation of colour and luminosity (the Hertzsprung Russell Diagram, HRD -- fig.1), you will see that most stars are aligned on a narrow band stretching from bright blue to faint red stars. This is called the main sequence; among these, the faint red stars are the most common. (An typical area of 100x100x100 light-years contains a few hundred G-type stars, but several thousand M-type stars.) Stars above the main sequence (brighter than main sequence stars of their colour) are called giants. Most areas below the main sequence are virtually empty; most stars below the main sequence fall into the class of the very faint white dwarfs, but there is another group of stars below the main sequence, the subdwarfs. Despite their name, these are not fainter than white dwarfs, but only half an order of magnitude fainter than main sequence stars of the same colour. Subdwarfs are very rare (except in galactic centre, globular clusters and elliptical galaxies); they only occur in the spectral types F, G, K and M (the O, B and A subdwarfs all have died long ago -- see section Luminosity classes and populations).

The colour of a star depends on its surface temperature: blue stars are the hottest, red ones the coolest. The hotter a star is, the more light it irradiates per surface area. This means that of two equally-sized stars, the more blueish one will be the brighter, of two different-coloured, equally-bright stars, the more reddish one will be larger. This means that bright red supergiants are HUGE, with diameters of up to several thousand million kilometers, while white dwarfs are only planet-sized. Along the main sequence, the differences in diameter are within one order of magnitude or two (the blue stars are bigger than the red ones).

However, the stellar masses differ much less than diameter and luminosity. The biggest blue stars have about 50 solar masses, the smallest red ones about one tenth of the mass of our good old sun. Giants usually are less massive than equally-bright main sequence stars; white dwarfs, though planet-sized, have masses comparable to that of our sun. For sun-like main sequence stars, the luminosity approximately grows proportionally to the fourth power of the star's mass.

This means that stellar densities span a wide range. Of the main sequence stars, the cool red ones are the densest. These reach a density of about 5 grams per cubic centimetre, while blue main sequence stars are about 0.05 g/cm3. However, red giant stars are very tenuous; a cubic metre of a red giant contains only a few milligrams. This is much less than Earth's atmosphere! Astrophysicists often speak of a `red vacuum'. On the other hand, white dwarfs are incredibly dense: a cubic centimetre taken out of a white dwarf weighs about a ton. The mass of an entire car compressed into a thimble!