A star is classed as variable if it appears to change brightness (seen from Earth) over time. The changes can come from the star itself varying the amount of energy it emits. Changes can also be outside the star, for example, through light being blocked by an orbiting companion.
We have known about variable stars for nearly four hundred years. The first one, Mira, was found in 1638, to pulsate in an 11-month cycle. Over a century later, by 1786 we knew of only ten variable stars. Today there are more than 65,000!
Most variable stars are studied by measuring their brightness over time. This is plotted on a graph known as a light-curve.
There are many kinds of variable star, but four of the most common are described here.
An important type of variable star is called a Cepheid. These are yellow giant stars which pulsate. This is caused by the star swelling and shrinking in a regular, repeating way. Each cycle of pulsation is called a period. Cepheid's have short periods (days to weeks) which are related to the brightness of the star. Each time the star swells it gets cooler and fainter, then as it shrinks it gets hotter and brighter. The connection is called the period-luminosity relationship. It allows astronomers to calculate how far away these stars are. This is complicated by how much light the star releases at each stage. The star changes its opacity (how see-through it is) during the process, changing the amount of light we see. This is due to a layer of helium in the stars' envelope. In the1920s Cepheid's were used to prove that there were other galaxies in the Universe.
RR Lyrae Variables
These stars are like Cepheid's but are far dimmer. They can usually be seen in globular clusters. They have a period-luminosity relationship and are used to work out the distance to these clusters. The periods of RR Lyrae stars are much shorter (a few hours to a day or 2). Their brightness varies by just 20% in some cases to over 500% in others!
The increase in brightness of these stars occurs at a much faster rate than the decrease. This gives the light-curve a distinctive shape, separating it from a Cepheid variable.
Mira variable stars are cool supergiants that are also pulsating. In Mira stars the pulsations are large. They can vary between 5 times and 30,000 times their brightness over many months. The variation is produced in the same way as Cepheid variables. The difference is from the opacity changing due to a layer of hydrogen.
The rise and fall in brightness is much more balanced tin Mira stars. This allows us to distinguish them from Cepheid and RR Lyrae variables.
An eclipsing binary is a system with two stars which orbits in our line of sight. This means that both stars will block out light from the other at some point during the orbit. Each time a star passes in front of the other we see a dip light-curve. The size of the dip relates to the relative sizes of the two stars. A larger star will block out more light, so there will be a greater change in brightness.
Astronomers can use the size of the dip to calculate the relative sizes of the two stars. This can be combined with other observations to find the mass and density of both stars. This has taught us a lot about the different types of stars that exist.