Variable Stars: What are They?

Paige Yarker. 21 November, 2016.

There are many ways to classify the stars that decorate our night skies. They can be grouped according to their size, colour, age or behaviours. One of the, arguably, most interesting type of star is the variable star. A variable star is a star whose brightness changes either regularly or irregularly due to either physical instability or external interference from our perspective on Earth. Variable stars have two main categories, extrinsic and intrinsic stars, and many more sub-categories, such as binary, pulsating and cataclysmic stars. For those interested, variable stars can be an invaluable insight into the mechanisms of some of the most fundamental bodies in our universe. 
An extrinsic variable star is caused by an external object blocking the light from a star periodically from our perspective. An example of an extrinsic variable star would be a star in an eclipsing binary (two) star system. This is where, as they move in front of each other, the light provided by each star as seen from Earth fluctuates or is obscured. It is also possible for these systems to have planets, which will add to the eclipsing effect. Algol, or Beta Persei, is a well-known eclipsing binary star. In the UK, you can observe Algol from around the end of autumn throughout winter, and Algol can be found in the constellation Perseus.

The Constellation: Perseus

An intrinsic variable star is caused by a physical instability in the star, which causes the random fluctuations in the brightness of the star. Some intrinsic stars are have regular fluctuations, whereas others are irregular and some are semiregular. An example of an intrinsic star is a Cepheid star, an incredibly bright pulsating star. Cepheid star brightness generally fluctuates regularly every 1–100 days depending on the star. There are also Cataclysmic Variables that include supernovae, and are caused by thermonuclear explosions within the stars. Delta Cephei is a renowned Cepheid star, located in the constellation Cepheus. You can view Delta Cephei in winter or late autumn on clear evenings.

Delta Cephei: within the constellation Cepheus

Bibliography:

Types of Variable Stars: Cepheid, Pulsating and Cataclysmic

The Top 12 Naked-Eye Variable Stars

Starlight Nights: Perseus

Delta Cephei is a Famous Variable Star

Further Reading:

Astronomy for Amateurs (James Muirde)

Understanding Variable Stars (John R. Percy)

Turn Left at Orion: A Hundred Night Sky Objects to See in a Small Telescope-and How to Find Them (Guy Consolmagno, Dan M. Davis)