Types of Variable Stars
Variable Stars are stars that vary in their light output. The origins of these light variations define the classification system of variable stars.
There are two kinds of variable stars; intrinsic in which variation is due to physical changes in the star or stellar system and extrinsic in which variability is due to the eclipse of one star by another or the effects of stellar rotation.
There are four main classes of variable stars. Within the intrinsic group of variables there are two classes: pulsatingand eruptive. Within the extrinsicgroup there are two classes: eclipsing binary and rotating stars. Below is a more thorough investigation of these four classes of variable stars.
Pulsating Variables
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Pulsating Variables are stars that show periodic expansion and contraction of their surface layers. Pulsations may be radial or non-radial. A radially pulsating star remains spherical in shape, while a star experiencing non-radial pulsations may deviate from a sphere periodically. The following types of pulsating variables may be distinguished by the pulsation period, the mass and evolutionary status of the star, and the characteristics of their pulsations.
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Cepheids (Period: 1-70 days; Amplitude of variation: .1 to 2.0 mag.)
These massive stars have high luminosity and are of F spectral class at maximum, and G to K at minimum. The later the spectral class of a Cepheid, the longer is its period. Cepheids obey a strict period-luminosity relationship. An example of a Cepheid variable light curve is given below.
RR Lyrae stars (Period: .2 to 1.0 days; Amplitude of variation: .3 to 2 mag.)
These are short-period, pulsating, white giant stars, usually of spectral class A. They are older and less massive than Cepheids.
RV Tauri stars (Period: 30-100 days; Amplitude of variation: up to 3.0 mag)
These are yellow supergiants having a characteristic light variation with alternating deep and shallow minima. Their periods are defined as the interval between two deep minima. Some of these stars show long-term cyclic variations from hundreds to thousands of days. Generally, the spectral class ranges from G to K.
Long Period Variables (LPVs)
(Period: 80-1000 days; Amplitude of variation: 2.5 to 5.0 mag.)
These are giant red variables that show characteristic emission lines. The spectral classes range through M, C, and S. Also known as “Miras” after the prototype star.
Semiregular (Period: 30-1000 days; Amplitude of variation: 1.0 to 2.0 mag.)
These are giants and supergiants showing appreciable periodicity accompanied by intervals of irregular light variation.
Cataclysmic Variables
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Cataclysmic variables (also known as Eruptive variables), as the name implies, are stars that have occasional violent outbursts caused by thermonuclear processes either in their surface layers or deep within their interiors.
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Supernovae (Period: none; Amplitude of variation: 20+)
These massive stars show sudden, dramatic, and final magnitude increases as a result of a catastrophic stellar explosion.
Novae (Period: 1-300+days; Amplitude of variation: 7-16 mag.)
These close binary systems consist of a main sequence, Sun-like star and a white dwarf. They increase in brightness by 7 to 16 magnitudes in a matter of one to several hundred days. After the outburst, the star fades slowly to the initial brightness over several years or decades. Near maximum brightness, the spectrum is generally similar to that of an A or F giant star.
Recurrent Novae (Period: 1-200+days; Amplitude of variation: 7-16 mag.)
These objects are similar to novae, but have two or more slightly smaller-amplitude outbursts during their recorded history.
Dwarf Novae
These are close binary systems made up of a Sun-like star, a white dwarf, and an accretion disk surrounding the white dwarf. There are three sub-classes of dwarf novae:
U Geminorum (Period: 30-500 days: Amplitude range variation: 2-6 mag.)
After intervals of quiescence at minimum light, they suddenly brighten. The duration of outburst is generally from 5 to 20 days.
Z Camelopardalis
These systems show cyclic variations, interrupted by intervals of constant brightness called “standstills”. These standstills last the equivalent of several cycles, with the star “stuck” at the brightness approximately one-third of the way from maximum to minimum.
SU Ursae Majoris
These systems have two distinct kinds of outbursts: one is faint, frequent, and short, with a duration of 1 to 2 days; the other (“superoutburst”) is bright, less frequent, and long, with a duration of 10 to 20 days. During superoutbursts, small periodic modulations (“superhumps”) appear.
Symbiotic stars (Period: semi-periodic; Amplitude of variation: up to 3 mag.)
These close binary systems consist of a red giant and a hot blue star, both embedded in nebulosity. They show nova-like outbursts, up to three magnitudes in amplitude.
R Coronae Borealis (Period: irregular; Amplitude of variation: up to 9 mag.)
These are rare, luminous, hydrogen-poor, carbon-rich, variables that spend most of their time at maximum light, occasionally fading as much as nine magnitudes at irregular intervals. They then slowly recover to their maximum brightness after a few months to a year. Members of this group have F to K and R spectral types.
Eclipsing Binary Stars
These are binary systems of stars with an orbital plane lying near the line-of-sight of the observer. The components periodically eclipse one another, causing a decrease in the apparent brightness of the system as seen by the observer. The period of the eclipse, which coincides with the orbital period of the system, can range from minutes to years.
 Rotating Stars
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Rotating stars show small changes in light that may be due to dark or bright spots, or patches on their stellar surfaces (“starspots”). Rotating stars are often binary systems.
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Other Types of Variable Stars
The following types of stars are not recommended for observation by inexperienced observer due to either their irregularity, or the small amplitude of variation that they exhibit.
Flare stars
Also known as UV Ceti stars, these are intrinsically faint, cool, red, main-sequence stars that undergo intense outbursts from localized areas of the surface. The result is an increase in brightness of two or more magnitudes in several seconds, followed by a decrease to its normal minimum in about 10 to 20 minutes.
Irregular variables
These stars, which include the majority of red giants, are pulsating variables. As the name implies, these stars show luminosity changes with either no periodicity or with a very slight periodicity.
The GCVS (General Catalogue of Variable Stars) classification of variable stars provides a thorough description of the different types of variable stars.
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