Variable star Guide, Meaning , Facts, Information and Description
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.
Double 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.
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.
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 Cepheii, 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.
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.
These are similar to Cepheids but rather fainter. They were once known as Dwarf Cepheids. They often show many superimposed periods, which combine to form an extremely complex light curve.
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.
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 stars are very cool red supergiants, which are undergoing very large pulsations. Over periods usually of many months, they may brighten by several magnitudess before fading again. Mira itself, also known as Omicron Ceti, varies in brightness from almost 2nd magnitude to as faint as 9th magnitude.
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.
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 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 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.
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 though 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 eventual 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.
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 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 Perseii.
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.
Stars with sizable sunspots may show significant variations in brightness as they rotate, and brighter areas of the surface are brought into view.
In a given constellation, the first variable stars discovered are designated with letters of the alphabet, e.g. P Cygni. Two letters are used for later discoveries, e.g. RR Lyrae. Once these 702 combinations are exhausted, variables are numbered in order of discovert, and prefixed with V, e.g. V1500 Cygni.
This is an Article on Variable star. Page Contains Information, Facts Details or Explanation Guide About Variable star Double star observations
Intrinsic variables
Pulsating variables
Cepheid variables
W Virginis stars
δ Scuti stars
RR Lyrae stars
RV Tauri stars
Mira variables
Semiregular variables and Irregular variables
Cataclysmic variables
Supernovae
Novae
Dwarf novae
R Coronae Borealis stars
Flare stars
Extrinsic variables
Eclipsing binaries
Planetary eclipses
Rotating stars
Variable star nomenclature
See also
External link