# Special Stars

[Key Words] [General] [Binary Stars] [Variable Stars]

## Key Words

 Binary Star Variable Star Intrinsic Variable Long-period Variables Spectral Type Centrifugal Force Extrinsic Variable Cepheids Novae Eclipse

General
A star is not just a star. In the previous section, you have learned that as star goes through many stages in its life, it becomes a different type of star. These are not the only types of stars. This section will explain about more types of stars, such as binary stars and variable stars.

Binary Stars
A binary star is actually two or more stars. The gravity produced by each star keeps them together. Centrifugal force keeps these two stars from crashing in to one another. There are different types of binary stars. Sometimes binary stars have equal masses, but usually the stars have different masses. An example of this would be a binary star consisting of a white dwarf and a red giant. Other binary stars are composed of more than two stars. An example of this is two pairs of binary stars revolving around each other. These types of binary stars usually have highly elliptical orbits.

Variable Stars
Variable stars are stars where the total light we see from earth changes. This can happen because of one of two reasons. Either an eclipse is taking place in a binary star system between two stars (known as an extrinsic variable), or the star is actually giving off more or less light (known as an intrinsic variable). Observations of the magnitude of variable stars over time yield a graph called a light curve.

Extrinsic Variable:
In a binary star system, two stars revolve around each other. Sometimes, earth is right along their plane of orbit. When this is the case, one star will eclipse another star, preventing it's light form getting to earth. Although the total output of light from the star is the same, when this happens, the binary star looks dimmer because of the eclipse.

Intrinsic Variable:
These variable stars actually change in brightness. There are three different types of these, cepheids, long-period variables, and novae.
Cepheids are the most numerous and well known variable stars. Cepheids are supergiants that pulsate, which changes surface temperature and spectral type. This happens over a period of a few days to a few months. There is a numerical relationship between the period of pulsation and the star's absolute magnitude. By finding out the star's absolute magnitude by its pulsation rate and comparing it to its apparent magnitude, an astronomer can then determine how far away the star is. This is why cepheids are know as astronomical yardsticks.
Long-period variables are red supergiants with very long periods of pulsation and extreme magnitudes.
Novae are explosions on binary stars. A nova occurs usually between a red giant and a white dwarf. About every 50 years, matter is transferred from the larger star to the smaller star. A nuclear chain reaction begins to take place on the surface of the smaller star. The reaction ceases, and material gets blown off of the surface of the star. The star glows brightly, and days or weeks later, after the star fades, the process begins again.

[Key Words] [General] [Binary Stars] [Variable Stars]