Variable star

From Academic Kids

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 either intrinsic or extrinsic.

Contents

Variable star observations

Variable stars are generally analysed using photometry and photospectrometry. Observations of their brightness compared to non-variable stars of known magnitude can be used to establish a light curve. For regular variables, the period of variation and its amplitude can be very well established; for many variable stars, though, these quantities may vary slowly over time, or even from one period to the next. Peak brightnesses in the light curve are known as maxima, while troughs are known as minima.

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.

Pulsating variables

Cepheid variables

Main article: Cepheid variable

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.

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.

W Virginis stars

Main article: W Virginis variable

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.

δ Scuti stars

Main article: Delta Scuti variable

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.

RR Lyrae stars

Main article: RR Lyrae variable

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.

RV Tauri stars

Main article: RV Tauri variable

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.

Mira variables

Main article: Mira variable

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.

Semiregular variables

Main article: Semiregular variable

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.

Irregular variables

Main article: Irregular variable

These are usually red supergiants.

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.

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.

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.

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.

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.

Extrinsic variables

Eclipsing binaries

Extrinsic variables have variations in their brightness, as seen by terrestrial observers, due to some external source. One of the most common reasons for this is the presence of a binary companion star, so that the two together form a binary star. When seen from certain angles, one star may eclipse the other, causing a reduction in brightness. One of the most famous eclipsing binaries is Algol, or Beta Persei.

Planetary eclipses

Stars with planetary systems may also show brightness variations if their planets pass between the earth and the star. These variations are much smaller than those seen with stellar companions, and are only detectable with extremely accurate observations.

Rotating stars

Stars with sizable sunspots may show significant variations in brightness as they rotate, and brighter areas of the surface are brought into view.

Variable star nomenclature

Main article: Variable star designation

In a given constellation, the first variable stars discovered were designated with letters R through Z, e.g. R Andromedae. (This system of nomenclature was developed by Friedrich W. Argelander, who noted that many variable stars were red and so began the naming with the letter R for rot (Red).) Letters RR through RZ, SS through SZ, up to ZZ are used for the next discoveries, e.g. RR Lyrae. Later discoveries used letters AA through AZ, BB through BZ, and up to QQ through QZ (with J omitted). Once those 334 combinations are exhausted, variables are numbered in order of discovery, and prefixed with V, e.g. V1500 Cygni.

See also

External link

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