Variable star

Most stars are of nearly constant luminosity. Our own Sun is a good example of one which goes through relatively little variation in brightness (usually about 0.1% over an 11 year solar cycle). Many stars, however, undergo significant variations in luminosity, and these are known as variable stars. Variable stars may be either intrinsic or extrinsic.

Intrinsic variables

These are stars which have intrinsic variations in brightness, that is the star itself gets brighter and dimmer. This category can itself be divided into two subgroups: pulsating and cataclysmic (or eruptive) variables. Examples of types within these divisions are given below.

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Pulsating variables

Cepheid variables

One of the most important types of variables star are Cepheid variables, yellow giant stars which undergo pulsations with very regular periods. They are named after Delta Cephei, the first of the class to be discovered, and have periods ranging from about a day to several weeks.

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Cepheids are important because they are a type of standard candle. Their luminosity is directly related to their period of variation, with a slight dependence on metallicity as well. The longer the pulsation period, the more luminous the star. Once this period-luminosity relationship is calibrated, this means that by observing the periods of Cepheids, their luminosity can be established. Their distance is then easily found from their apparent brightness. Observations of Cepheid variables are very important for determining distances to galaxies within the Local Group.

Related Topics:
Standard candle - Metallicity - Period-luminosity relationship - Local Group

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Edwin Hubble used that method to prove that the so-called spiral nebulae are in fact distant galaxies.

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W Virginis stars

These are very similar to Cepheids, but belong to Population II and so have a lower metallicity and hence a slightly different period-luminosity relationship.

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δ Scuti stars

These are similar to Cepheids but rather fainter, and with shorter periods. They were once known as Dwarf Cepheids. They often show many superimposed periods, which combine to form an extremely complex light curve.

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RR Lyrae stars

These stars are somewhat similar to Cepheids, but are not as luminous. They are older than cepheids, belonging to Population II. They are common in globular clusters, and were occasionally referred to in the past as cluster Cepheids. They also have a well established period-luminosity relationship, and so are also useful distance indicators.

Related Topics:
Population II - Globular cluster

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RS Canum Venaticorum stars

These are close binary systems with a longer period chromospheric activity, including flares, that typically last 1-4 years. This activity cycle is comparable to the solar cycle of the Sun. The type is often abbreviated RS CVn.

Related Topics:
Solar cycle - Sun

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RV Tauri stars

These are yellow supergiant stars which have alternating deep and shallow minima. This double-peaked variation typically has periods of 30-100 days. Superimposed on this variation, there may be long-term variations over periods of several years.

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Alpha Cygni stars

These are nonradially pulsating supergiants of spectral classes Bep to AepIa. Their periods range from several days to several weeks, and their amplitudes of variation are typically of the order of 0.1 magnitudes. The light changes, which often seem irregular, are caused by the superposition of many oscillations with close periods. Deneb, in the constellaton of Cygnus is the prototype of this class; the GCVS code is ACYG.

Related Topics:
Deneb - Cygnus - GCVS

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Mira variables

Mira variables are very cool red supergiants, which are undergoing very large pulsations. Over periods usually of many months, they may brighten by several magnitudes before fading again. Mira itself, also known as Omicron Ceti, varies in brightness from almost 2nd magnitude to as faint as 9th magnitude.

Related Topics:
Magnitude - Mira

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Semiregular variables

These are usually red supergiants. Semiregular variables may show a definite period on occasion, but also go through periods of irregular variation. The best known example of a semiregular variable is Betelgeuse, which varies from about magnitudes 0.2 to 1.2.

Related Topics:
Supergiant - Betelgeuse

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Irregular variables

These are usually red supergiants.

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Cataclysmic variables

Supernovae

Supernovae are the most dramatic type of cataclysmic variable, being some of the most energetic events in the universe. A supernova can briefly emit as much energy as an entire galaxy. Supernovae can result from the death of an extremely massive star, many times heavier than the sun. They may also result from the transfer of matter onto a white dwarf. The absolute luminosity of this latter type is related to properties of its light curve, so that these supernovae can be used to establish the distance to other galaxies.

Related Topics:
Galaxy - Sun - White dwarf

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Novae

Novae are also the result of dramatic explosions, but unlike supernovae do not result in the destruction of the progenitor star. They form in close binary systems, and may recur over periods of decades to centuries or millenia. Nova Cygni 1975 was the most recent very bright nova in our galaxy, reaching the second magnitude. Novae are categorised as fast, slow or very slow, depending on the behaviour of their light curve.

Related Topics:
Binary system - Nova Cygni 1975

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Dwarf novae

Dwarf novae are double stars in which matter transfer between the component gives rise to regular outbursts. There are three types of dwarf nova: U Geminorum stars, which have outburst lasting roughly 5-20 days followed by quiet periods of typically a few hundred days; Z Camelopardalis stars, in which occasional plateaux of brightness called standstills are seen, lasting the duration of several normal periods, part way between maximum and minimum brightness; and SU Ursae Majoris stars, which undergo both frequent small outbursts, and rarer but larger outbursts.

Related Topics:
U Geminorum - Z Camelopardalis - SU Ursae Majoris

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R Coronae Borealis stars

While classed as eruptive variables, these stars do not undergo periodic increases in brightness; instead, they spend most of their time at maximum brightness but after irregular intervals, suddenly fade by many magnitudes. They slowly recover to maximum brightness over months to years. This variation is thought to be caused by episodes of dust formation in the atmosphere of the star. As dust is formed and moves away from the star, it will eventually cool to below the dust condensation temperature, at which point a cloud of opaque dust will form, causing the star's observed brightness to drop.

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R Coronae Borealis (R CrB) is the prototype star. Other examples include Z Ursae Minoris (Z UMi) and SU Tauri (SU Tau).

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Flare stars

Also known as UV Ceti stars, these are very faint main sequence stars, which undergo regular flares. They increase in brightness by up to two magnitudes in just a few seconds, and then fade back to normal brightness in half an hour or less.

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