Variable Stars
The Ancients thought the heavens were
unchanging, but we now know that there are many
variable stars
that vary their light output in a periodic fashion.
Types of Variable Stars
We have seen that eclipsing binary stars vary in brightness, but many other
kinds of stars vary their light output because of changes in their
internal properties. Variable stars may be classified broadly into three
categories:
| 1.
pulsating variables
|
| 2.
eclipsing variables
|
| 3.
eruptive variables
|
Among the
pulsating variables we may list Cepheid variables, RR Lyrae variables,
long-period red variables, and others. Examples of eruptive variables are
novae, supernovae, and X-ray bursters. In this module we will concentrate
on the pulsating variables. Eclipsing variables were discussed
in conjunction with multiple star systems in Chapter 19
and eruptive variables are discussed in other modules of the present chapter.
The following table lists some of the common classes of pulsating variable
stars, along with the stellar population in which they are commonly found
(Pop I or Pop II) and the typical range of periods.
|
Some Classes of Variable Stars
|
| Variable Type |
Population Type |
Range of Periods
|
| Long-Period Variables |
Pop I and Pop II |
100 - 700 days
|
| Classical Cepheids |
Pop I |
1 - 50 days
|
Type II Cepheids
(W Virginis Stars) |
Pop II |
2 - 45 days
|
| RR Lyrae Stars |
Pop II |
1.5 - 24 hours
|
| delta Scuti Stars |
Pop I |
1 - 3 hours
|
| beta Cephei Stars |
Pop I |
3 - 7 hours
|
| ZZ Ceti Stars |
Pop I |
100 - 1000 seconds |
|
|
|
|
*Adapted from B. W. Carroll and D. A. Ostlie, An
Introduction to Modern Astrophysics
|
(ZZ Ceti stars are pulsating white dwarfs and so are not
stars by the strict definition.) We see that the range of pulsation periods for these variables
ranges from minutes up to several years.
We shall discuss the Cepheid variables, RR Lyrae stars, and long-period variables explicitly.
Light Curves
The key quantity in observing variable stars is the light curve, which is a plot of the
variation in light output as a function of time. It is this plot that establishes that the star is a
variable, and the detailed nature of the curve often
allows us to classify it according to the different
types of variable stars that we are about to discuss.