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What are Variable Stars?

     Quite simply, variable stars are stars which vary in brightness over long or short periods of time, with amplitudes of variation from a thousandth of a magnitude to as much as 20 magnitudes. Variable stars may be found to be periodic (with a single period or multiple periods at once) with little or no irregularity in their light curves (plots of brightness vs. time), semi-periodic with some irregularity and periodicity, and irregular with no apparent periodicity in their light curves whatsoever. Depending on the type of variable star being observed (see below), periods can range from a fraction of a second to years. In light of this, one can say that nearly all stars are variable stars to a certain extent since all stars possess constantly changing physical properties and features affecting their brightness either periodically or irregularly.

Types of Variable Stars

      Variable stars are classified according to the main cause of their variation in brightness. Most generally, variable stars are classified as either intrinsic or extrinsic. For intrinsic variables, variability is caused by physical changes intrinsic to the star (eg. radial or non-radial vibration or "pulsation"; eruptions in the star), while for extrinsic variables, variability is caused by changes which are external to the star as a whole (eg. the effects of stellar rotation; two stars eclipsing each other in a binary system, causing an apparent dimming in the brightness of the system).

     Intrinsic variables are classified as either pulsating or eruptive variables. In pulsating variables, variation in brightness is caused by periodic expansion and contraction in the star's surface layers. These pulsations may be radial (in which case the star retains its spherical shape) or non-radial (resulting in periodic deviations from a spherical shape). Pulsating variables themselves can be divided up into different types according to their pulsation period, mass, and evolutionary status:

Cepheids: These stars have periods of 1-70 days with amplitudes of variation from 0.1 to 2.0 magnitudes. Cepheids obey a strict period-luminosity relationship with Cepheids of higher luminosities having longer periods, both depending on the radius. Therefore, by measuring the period of a Cepheid variable, one can obtain its luminosity, and by measuring its apparent brightness one may deduce how far away it is.

RR Lyrae stars: These stars have periods of 0.2-1.2 days with amplitudes of variation from 0.3 to 2 magnitudes. These pulsating variables are white giant stars of spectral class A.

RV Tauri stars: These stars have periods of 30-150 days with amplitudes of variation up to 3.0 magnitudes. They are yellow supergiants generally of spectral classes from G to K.

Long Period Variables (Miras): These stars have periods of 80-1000 days with amplitudes of variation from 2.5 to 5.0 magnitudes. They are giant red variables with spectral classes ranging through M, C, and S.

Semiregular stars: These stars have periods of 30-1000 days with amplitudes of variation from 1.0 to 2.0 magnitudes. They are giants and supergiants displaying periodicity superimposed with intervals of irregular light variation.

Small-Amplitude Pulsating Red Giants (SAPRGs): These stars have periods of 5-100 days with amplitudes of variation from 0.05 to 1 magnitude. As their name (also called small-amplitude red variables) suggests, these stars are red giants. Due to their instability, a majority of red giants physically expand and contract (pulsate) periodically as a result of convective processes. Convection in red giants involves the cyclic motion of huge cells of hot gas. This results in periodic changes in luminosity with small amplitudes of variation. The pulsations may be radial or non-radial as mentioned above. These stars may also be multi-periodic.

     In eruptive variables, variation in brightness is caused by occasional violent eruptions as a result of complex processes deep within the interior of the star or in the surface layers. The most common types of eruptive variables are:

Supernovae: These stars show sudden, dramatic, and final magnitude increases as a result of a catastrophic stellar explosion. Thus, there is no period, and amplitudes of variation are 20+ magnitudes.

Novae: 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 its initial brightness over several years or decades. Near maximum brightness, the spectrum is generally similar to that of an A or F giant star. Periods are typically 1-300+ days, and amplitudes of variation are 7-16 magnitudes.

Recurrent Novae: These objects are similar to novae, but have two or more slightly smaller-amplitude outbursts during their recorded history. Periods are 1-200+ days, and amplitudes of variation are 7-16 magnitudes.

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. The accretion disk "erupts" every few weeks.

Symbiotic Stars: 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, and are semi-periodic.

R Coronae Borealis Stars: 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.

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.

      Extrinsic variables are classified as either eclipsing binary or rotating variables. In eclipsing binaries, the object in question is a binary star system with an orbital plane lying near the line-of-sight of the observer, and light variation is due to the two stars making up the system eclipsing each other. The system appears to dim when one star passes in front of the other (in relation to the observer on earth) and subsequently brightens when the star moves out of the way so that both stars are exposed and contributing to the overall brightness of the system. The period of variation coincides with the orbital period of the system and can range from a few minutes to several years.

     In rotating variable stars, variation in brightness is usually small and results in the rotation of the star exposing dark or bright spots, or patches ("starspots") on its surface. Rotating variable stars are often binary systems.

For general information on variable stars, www.aavso.org/vstar/ has a lot of useful topics. For more detailed information about specific stars the variable star of the month archive has an excellent description of many stars.

For a review of Astronomical Terms, click here.
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Site created by JoAnne Hosick and Vince Velocci, and extended by Akos Bakos and Artur Chudolinski. Last updated July 19, 2004.