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Variable Stars - Introduction

A star is classified as variable if its apparent brightness (i.e. that seen from Earth) changes over time. The variation in brightness can result from internal changes to the amount of energy being emitted by the star, or from an external process, such as some of the light from a star being stopped from reaching Earth; for example, through being blocked by an orbiting companion star.

We have known about the existence of variable stars for nearly four hundred years. The first documented case was in 1638, when a star called Mira was found to pulsate in a regular 11 month cycle. By 1786 we knew of ten variable stars, but this has increased over the centuries to the point where we now know of more than 65,000 variables.

The most common method of studying variable stars is to measure their brightness over time and plot those measurements on a graph. The result is known as a light-curve, and is used to graphically show the variable nature of the target star.

There are many different kinds of variable star, however, we shall cover just four of them here.

Cepheid Variables

Cepheid Variable Light-curve
Light-curve for Cepheid Variable
Credit: Roger Pickard

One of the most important types of variable star is the Delta Cephei variable, which is more commonly known as a Cepheid Variable. These are yellow giant stars that pulsate on a very regular basis. In other words, they swell and shrink in both brightness and radius in a very regular and repeating fashion. Cepheid variables tend to have short periods (days to weeks) and are important because the period of the pulsation is directly related to the actual brightness of the star - not just the apparent brightness as seen from Earth. This relationship, known as the period-luminosity relationship, allows us to calculate how far away these stars are. This relationship was used back in the 1920s to figure out that the "spiral nebula" that astronomers could see, were in fact distant spiral galaxies, and not nearby clouds of gas as many had thought.

The mechanism behind the variation appears to be a variable opacity (i.e. how see-through it is) layer of molecular helium, that can block the outflow of energy from the star, causing it to inflate a bit like a balloon, and thus increase in brightness. At some point, however, the helium layer suddenly allows energy to flow through it, a bit like a release valve, and the star starts to shrink again. The whole process then repeats periodically.

RR Lyrae Variables

RR Lyrae Variable Light-curve
Light-curve for RR Lyrae Variable
Credit: University of Washington

These stars are similar to Cepheid variables but are far dimmer, and tend to be associated with globular clusters. Because they also have a well-established period-luminosity relationship, they are useful for working out the distance to these clusters. Their brightness varies by just 20% in some cases to over 500% in others, and the cycle usually lasts from just a few hours to a day or two.

Note from the graph shown to the right, that the increase in brightness occurs at a much faster rate than the subsequent decrease. This gives the RR Lyrae light-curve a very distinctive shape, and allows us to distinguish it from a Cepheid variable. Make a mental note of the different shapes of the light-curves shown here, as it will prove useful later on in the project.

Mira Variables

Mira Variable Light-curve
Light-curve for Mira Variable

Mira variables are very cool supergiant stars that are undergoing extremely large pulsations. They can vary anything between five times and 30,000 times their minimum brightness over the period of many months. The mechanism behind the variation is similar to that of the Cepheid variable, except that the variable opacity layer is made of molecular hydrogen.

The rise and fall in brightness is much more balanced than in other cases, with both being of fairly similar durations. This allows us to distinguish them from Cepheid and RR Lyrae variables. However, their brightness cycles remain very regular.

Eclipsing Binaries

Eclipsing Binary Light-curve
Light-curve for Eclipsing Binary

An eclipsing binary star is a system with two stars, where the orbital plane lies in our line of sight. This means that both of the stars will block out light from the other at some point during the orbit. As a result, we get periodic dimmings in the light-curve due to two eclipses, which differ in strength due to the relative sizes of the two stars. Because a larger star will block out more light, there will be a greater corresponding change in brightness.

Eclipsing Binary Animation
Eclipsing binary star animation
Credit: Wikimedia Commons

Eclipsing binary stars are very important in astronomy, because they allow us to calculate the relative sizes of the two stars involved. Where the two stars can also be observed using a spectrograph, we can calculate the mass and density of both components. The resulting information has taught us a great deal about the different types of stars that exist.

Now that you know a little bit about variable stars, you can proceed to the next stage of the project, where you will learn how to measure the brightness of variable star targets and use your results to create light-curves. OK , let's move on.