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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.
Period
Brightness
(See Distance measurement in Astronomy)
Radius
Figure 1
(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 ( 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.
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.  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.