Some stars that we can see in the sky, vary in brightness quite noticably over time.
Their brightness may alter by several magnitudes over a period of days, weeks or months.
One of the most famous variable stars is Algol, in the constellation of Perseus (Beta Persei). Algol is markedly red in colour and varies by about one magnitude, over the course of a few days.
Algol’s behaviour was first recorded in the 1600′s, but almost certainly it was known in ancient times. The clue for this is its name Algol, which means “the Demon” in Arabic.
There are two main types of variable stars, “intrinsic” and “extrinsic”, and the cause of the brightness changes are very different.
Intrinsic variables alter because of an actual change in brightness of their surface.
Extrinsic variables alter because of some outside factor such as another star getting in the way and blocking out light.
Intrinsic Variable Stars
Intrinsic variable stars produce an increase in brightness as a result of some physical change within them.
A dramatic example are Flare stars. This is the name given to stars that suddenly and unpredictably get much brighter over a short space of time, perhaps only minutes. They then fade, often taking much more time than the initial flare.
Proxima Centauri, the closest star to Earth is a well known example of a flare star.
The other main type of intrinsic variables are those that have a regular, predictable, brightening cycle.
These pulsating variables live up to their name and actually pulsate in size. Most of this type are red giants but there is a particularly important class called Cepheids.
Cepheid variables are named after the original one of this type (Delta Cephei) that was discovered. They have the important property that the period of the pulsation is proportional to their absolute magnitude. Because of this, Cepheid variable stars have been used to estimate distances within the Universe.
Extrinsic Variable Stars
Variable stars of the extrinsic type are usually eclipsing binaries.
With an eclipsing binary pair, what appears to be a single star from Earth with the unaided eye, is actually a pair of stars that are orbiting around a centre of gravity. this can be seen through a telescope.
Algol, mentioned above, is a famous example of an eclipsing binary.
The stars of the pair orbit each other and when one star is in front of the other, as seen from Earth, the system is less bright then when the stars are side-by-side.
