Variable Stars

by Fraser Cain on February 12, 2009

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The variable star Mira. Image credit: Galex

The variable star Mira. Image credit: Galex


Ancient astronomers thought that the stars were unchanging, perfect spheres in the heavens. But thanks to the telescope, modern astronomers have learned that stars can change in brightness significantly. These changing stars are known as variable stars, and there can be many different reasons whey they’re variable.

The first variable star ever discovered was in 1572, and then again in 1604, when astronomers recorded the eruption of supernovae. Although, these don’t really qualify as variable stars in the current thinking. In 1638, Johannes Holwarda discovered that the star Omicron Ceti (aka Mira), pulsated in a regular pattern over the course of 11 months. Then the eclipsing variable Algol was discovered in 1669, and soon many others were found. Astronomers now publish a list of 40,000 known variable objects in the Milky Way alone.

Let’s take a look at the different kinds of variable stars.

Cepheid Variables
The Cepheid variables are a group of stars known to pulse in a very specific pattern. Astronomers now know that these stars expand and shrink dramatically over a period of time. A helium layer in the star expands and contracts, and as it does, it changes the opacity of the star, which changes its brightness. This period can last days, or take a few weeks to complete. There’s a very important connection between the period of a Cepheid’s brightening and its luminosity. This allows astronomers to determine the distance to a Cepheid just by measuring the period of its brightening.

Cataclysmic Variable Stars
These are stars that have a brief brightening because of some kind of explosion on the surface of the star. The most violent example of this are supernovae, which can indicate the death of a star. But regular novae can erupt from the surface of many stars, and can indicate that a star is consuming material from a binary partner.

Eclipsing Binaries
These are stars that change in brightness because two stars are in a binary system. The stars orbit around one another, and can line up from time to time so that one star blocks off the light from the other from our perspective.

We have written many articles about stars here on Universe Today. Here’s an article about how variable stars can cloak themselves from view, and here’s an article about Polaris, a well known variable star.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
SEDS.org
NASA: Cepheid Variables
NASA: Cataclysmic Variables
University of Tennessee – Knoxville

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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