Astronomy

2006/2007 Topic : Variable Stars

Variable Stars
A variable star is defined as a star that undergoes changes in luminosity. In Science Olympiad, you will be tested on your knowledge of various DSOs (Deep Sky Objects), types of variable stars, and other related things.

Variable stars are generally placed into two categories; Intrinsic variable stars and Extrinsic variable stars.

Intrinsic Variable Stars
Intrinsic Variable Stars are stars that change in luminosity due to physical changes in the star; there are three types of Intrinsic Variable Stars.

The first type is pulsating variable stars. These are stars that periodically swell and shrink. Pulsating variable stars that have short periods are called Cepheids; cepheids usually have more regular periods than Longer Period Variables. Important examples include the Delta Cepheid variables, W Virginis variables, RR Lyrae variables, and Beta Cephei variables. The other type of pulsating variable are long period variables. They have longer periods than cepheids (such as a year or longer) and have a much less regular period. These variables are also sometimes bigger and cooler than cepheids. A good example is the Mira Variables. Long period Variables can be split into three subgroups. The First is Irregular variables; these are usually red super giants and have almost no regular period. Second is the Semi regular variables, these sometimes go through regular periods, then change and go on an un-regular period. And third are Nonradial pulsators; variables that change to shapes other than the sphere to change magnitude.

The second major type of intrinsic variable star are eruptive variable stars. These are stars are mostly pre-main sequence stars, but an exceptional few main sequence stars are eruptive variables. Pre-main sequence (also called protostars) are stars that have not completed the process of becoming a main-sequence star from a gas nebula and are not yet condensed. So while condensing they change in magnitude. Eruptive Main-sequence variables are usually extremely larger or extremely smaller than the average main sequence star. Some eruptive variables are red giants since they easily lose their gases. The last type of eruptive variable star is binary eruptive stars. These stars flare up and can remain that way for 1-4 years. Important eruptive variable stars are the Orion Variables, the Wolf-Rayet variables, and RS Canum Venaticorum variables.

The third major type of an intrinsic variable star is a cataclysmic or explosive variable star. The most dramatic type of these variable stars are called Supernovae. This only occurs in extremely massive and old stars. The outer layers of the star are expelled at high speeds creating a supernovae remnant or nebula. A white dwarf or pulsar is usually left behind. The second type of these variables are called Novae. They are dramatic explosions caused by a pair of close binaries but don�t cause the total destruction of the star. An other type are dwarf novae and are very similar to novae. Dwarf novae are just two binary white dwarfs that regularly have outburst. The fourth and final type of intrinsic variable are Z Andromedae Variables. These are a less common type of variable and are caused by a double star system containinga red giant and a hot blue star; enclosed in a cloud of dust and gas.

Extrinsic Variable Stars
Extrinsic variable stars can stars that change in luminosity due to external changes; the two major types are rotating and eclipsing.

Rotating variable stars are stars that change in luminosity due to the rotation of the star. This can happen because of such things as a sunspot on the surface as it rotates. This can also occur when two close binary stars change shape due to their mutual gravity. Fluctuations in magnetic fields can cause slight change in magnitude.

The second type of extrinsic variable star are eclipsing variable stars. These are caused when stars in double star systems eclipse, causing a lowing of magnitude. However, this variation can only be viewed from certain angles. Planetary transits can also cause a very small variation in luminosity only viewed with very accurate instruments.

In competition, this is the major topic tested. But you will have to have to interpret data and identify deep sky objects (such as M13; seen below). The test may ask you to read a light curve, know the composition of stars, or give examples of variables.

M13: A globular cluster seen here.

Useful Resources
American Association of Variable Star Observers