Giants and Supergiants
As indicated in the adjacent HR diagram, some stars are much more luminous than main sequence stars
of the same spectral class. These are called giants and supergiants.
Large Surface Areas
From the theory of blackbody radiation, the luminosity L of a star
should be proportional to its surface area A multiplied by
its temperature T raised
to the fourth power,
L = kAT4
where k is a constant.
A main sequence star and giant or supergiant of the same spectral classification have about the
same surface temperature. Thus, this formula tells us that
the much larger luminosity of giants and supergiants can come only from their
having much larger surface areas than main sequence stars. That is
why they are called giants and
supergiants: they are
much larger stars than the corresponding main sequence stars.
For example,
a giant star
like Arcturus is almost twenty times larger than the Sun,
and
the supergiant star Antares is more than 300 times larger than the Sun.
If these stars were placed at the center of the Solar System,
Arcturus would extend almost a quarter of the way to
Mercury's orbit and Antares would engulf the Earth and
extend to the orbit of Mars.
Densities
Although giants and supergiants occupy much larger volumes than main sequence stars, they usually are
comparable to them in mass (within say a factor of ten).
It follows that the density of giants and supergiants must be very low. A star
like Antares occupies a huge volume, but the gas in that volume is very thin. For Antares, the
average density
is about ten million times less than that of the Sun, so much of the star is of such low density
that it is a very good vacuum.
Origin
As we shall discuss in the chapter on the death of stars, giants and supergiants represent a later stage
of stellar evolution, after a time spent on the main sequence. Thus, all giant and supergiant stars were
once main sequence stars. We shall see also
that the giant or supergiant phase of a star's life is very
short compared with its main sequence life. This is the main reason that only a few percent of the stars
that we see are giants and supergiants but the vast majority are main sequence stars.