Types of star

Astronomers classify stars into a number of distinct types.

Single stars

These are stars that move through the galaxy on their own, have a fairly steady brightness and are not linked by gravitational attraction to any other. Our Sun is an example of this type of star.

Binary stars or double stars

These are a pair of stars that move together, rotating about the common centre of mass. Binaries are divided into three groups
(a) visual binaries – ones that you can see as double using naked eye observation or a telescope
(b) spectroscopic binaries – ones that require spectroscopic observation to see as two stars
(c) eclipsing variables (see below) – ones where one star eclipses the other when seen from the Earth as they orbit each other

Stars that are part of a system of more than two components are known as multiple stars.

Variable stars

(a) Cepheid variables
A type of star that varies in brightness in the same manner as delta Cephei. The period of the brightness variation is directly related to the brightness of the star. This connection has been very useful in the determination of stellar distances. The variation is shown in Figure 1.
(See Distance measurement in Astronomy)





(b) Eclipsing binaries
These stars vary in brightness because as the stars orbit each other they eclipse each other. One type of double star is Algol (b Lyrae) which is composed of two stars of roughly the same size but different intensities. The observed variation in the light coming from Algol is shown in Figure 2.

As one star orbits another one or other of the stars may be eclipsed by its companion and this affects the total observed brightness of the pair. The larger dip in the observed brightness-time curve is when the dimmer star moves in front of the brighter one. The smaller dip is where the brighter one moves in front of the dimmer star.

Red giants

Red giants are very large stars with surface temperatures less than about 4700 K. They have diameters between 10 and 200 times that of the Sun. In spite of their relatively low surface temperature their enormous surface area means that they have luminosities between 100 and 10000 times that of the Sun. e Aurigae (diameter about one hundred times that of the Sun) is an example of such a star.

Red supergiants

These are stars with masses more than ten times that of the Sun. They are very cool with surface temperatures between about 3500K and 4500K. Betelgeuse in Orion is an example of this type of star. It has a diameter of about 500 million km – large enough to contain the whole of the Earth’s orbit! Further examples of these enormous stars are KW Sagitarii, S Persei, VX Sagitarii, VY Canis Majoris and NML Cygni.

Their enormous size means a very low density – of the order of 0.000 005 kgm^-3, so low that they have been described by astronomers as merely a ‘hot vacuum’.

Supergiants usually end their lives in the cataclysmic explosion of a supernova.

White dwarfs

Unlike the supergiants white dwarfs are small stars with a high density. They are likely to be the final stages of the

majority of stars with masses similar to that of the Sun. A typical white dwarf would have a mass of around one and a half times that of the Sun but a volume comparable with that of the Earth!

This results in a density of about 10¹⁸ kgm^-3!

They have a huge range of surface temperature from over 100 000 K to less than 4000 K.

40 Eridani and Sirius B (the small companion of the bright star we call Sirius A) are both examples of white dwarfs.