During antiquity, stars represented a symbol of consistency in the heavens, the exception being the planets, that was non-variable. This view started to come under scrutiny a few centuries A.D. by Arab astronomers when they gave the name "Algol", meaning "Demon Star", to the star once they had established that the star changes in brightness. During the 16th century, with the further discovery of variable stars, such as Mira, the view of stars as a steady point of light was finally overturned. There are over thirty variable stars that can be observed with the naked eye, two of those being Delta Cepheus and Betelgeuse.
Stars from their birth and throughout their life undergo periods of slow change and periods that dramatize quick and impressive changes. During quiet periods the star behaves as a normal star, but, during periods of strong variability, (period of change<<period of a stars life) as a variable star, the results can be dramatic. This variability is usually towards the brightening of the variable from Earth based observations.
So, variable stars are stars that display a brightness change in relation to time. The change in brightness can be measured from 1/1000 of a magnitude on up to 20 magnitudes on a time period of between a fraction of a second and up to one year, depending on the category of the variable star. There are 30,000 known variable stars that have been categorized and many thousands of others that are potential candidates. This large amount of variable star requires a lot consistent observations that cannot be achieved without the help of dedicated amateur astronomers.
There are various reasons why variable stars change in brightness. Pulsating variables, for example, swell and shrink due to various internal forces. Eclipsing variables dim due to a second star passing across the line of sight between the star and us and then brightening once again as the eclipsing star moves away from the star. The various types of variable stars have led astronomers to classify variable stars in their various categories.
Variable stars in general are classified as natural variables where their variability is created by natural changes in the pulse, an explosion in the star or an explosion in the stars system (pulsating variables and cataclysmic variables). Eclipsing variables, where the variability in the star is created by one star eclipsing the other or by the result of stellar rotation.
The list below is indicative of the various types of variable stars that exist.
Pulsating Variable Stars
I. Cepheid Variables
II. RR Lyrae Stars
III. RV Taurus Stars
IV. Long Period Variables (Mira)
V. Semiregular Variables
Cataclysmic Variable Stars
I. Supernova
II. Nova
III. Recurring Nova
IV. Dwarf Nova
U Geminorum type
Z Camelopardalis type
SU Ursa Major type
Symbiotic Variables
R Corona Borealis type
Eclipsing Double Stars
Rotating Stars
UV Cetus Variables (Flare Stars)
Irregular Variables
The naming system below uses the Latin alphabet in the naming of variable star and begins with the letter "R" for the first variable to be discovered in a constellation. An example being the variable star R Coronae Borealis. The second variable to be discovered is assigned the letter "S", the third is assigned the letter "T" until the letter "Z" is reached. The naming system then starts with "RR", "RS", up to "RZ". At this point we start with "SS", not "SR", until "SZ" is reached and continues in such a fashion until we reach "ZZ". After "ZZ" is reached, the naming configuration starts with "AA-AZ". Then it starts with "BB", not "BA", until "QZ" is reached. The letter "J" is not used as it can be easily confused with the letter "I".
This naming configuration allows for the naming of up to 334 variable stars per constellation. When a new variable is discovered that exceeds the 334 star allotment, it is designated as "V (VARIABLE)335, the next being "V (VARIABLE)336" and continues as such.
Naturally some variable stars with common names such as alpha Ursa Minoris (Polaris) and delta Cepheus, keep their given names.