Variable stars are stars that vary in brightness over a period of time. Some stars vary by a hundredth of a magnitude, others by a factor of 15 or more. The time taken for each period can be seconds or years, with some variables having a periodic frequency, while others are irregular.
The “brightness” of a star should more accurately be called its magnitude. Luminosity is a measure of how much energy an object radiates per area per time, and is key to determining a stars magnitude. For a more in-depth explanation of these terms please read my article on Stellar Magnitudes Explained.
There are two types of variable star, the intrinsic variable and the extrinsic variable.
- Intrinsic variables actually change in luminosity, i.e. the star expands and contracts periodically.
- Extrinsic variables do not actually change luminosity, but they appear to do so from our viewpoint, usually because they are eclipsed.
Variable stars were first discovered in the mid 18th century when it was noticed that some stars periodically disappear. By the late 19th century there were 12 variables discovered, and by the dawn of the 20th century with the advent of photography the rate of discovery increased dramatically. In the latest NGC catalogue (2003) there are over 40,000 variable stars listed in our galaxy, and a further 20,000 outside.
Variable stars are analysed using a few methods, including spectroscopy, photometry and spectrophotometry. By comparing a variables brightness with a known fixed star, a light curve be established. The light curve can tell us if the variable is periodic or irregular, the period of fluctuation and the shape of the light curve.
By analysing the variables spectrum we can derive the stars temperature, luminosity class, if it is a single or binary star and if the spectral lines are shifted it can indicate the star expanding or contracting.
Intrinsic Variable Stars
About two thirds of variable stars are Intrinsic variables which vary in brightness due to physical changes in the star. In most cases the layers in the star will expand and contract, increasing or decreasing the surface area. Since luminosity and brightness are linked, the change in surface area affects the luminosity, hence its brightness.
As a stars outer layer expands, it cools down, which causes the degree of ionisation to also decrease. This makes the solar material more transparent, thus easier for the star to radiate its energy, which causes the star to start contract. As the layers contract, the effect of ionisation increases, causing energy to once again be trapped, leading to another expansion cycle.
The most famous intrinsic variables stars are the Cepheid variable stars that have a very predictable correlation between their period of variability and absolute luminosity. Because of this correlation a Cepheid variable can be used as a standard candle in order to calculate the distance to its host cluster or galaxy.
Extrinsic variable stars
Extrinsic variables are caused by external properties acting on our apparent view of the star.
These could take the form of an eclipsing binary, where two stars orbit each other and periodically they eclipse each other as seen from Earth. They could be rotating variables, i.e. a large sunspot will affect the luminosity as the star rotates. There could also be other objects passing between the star and Earth causing the magnitude to vary, i.e. a large orbiting planet or cosmic dust.
This guide is intended as an introduction only, the subject of variable stars is very complex and in depth area of study.
