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| − | ==Classification==
| + | =Classification= |
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| | Variable stars may be either ''intrinsic'' or ''extrinsic''. | | Variable stars may be either ''intrinsic'' or ''extrinsic''. |
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| | + | ==Intrinsic Variable Stars== |
| | *'''Intrinsic variable stars''': stars where the variability is being caused by changes in the physical properties of the stars themselves. This category can be divided into three subgroups. | | *'''Intrinsic variable stars''': stars where the variability is being caused by changes in the physical properties of the stars themselves. This category can be divided into three subgroups. |
| | + | **Eruptive variables, stars who experience eruptions on their surfaces like flares or mass ejections. |
| | **Pulsating variables, stars whose radius alternately expands and contracts as part of their natural evolutionary ageing processes. | | **Pulsating variables, stars whose radius alternately expands and contracts as part of their natural evolutionary ageing processes. |
| − | **Eruptive variables, stars who experience eruptions on their surfaces like flares or mass ejections.
| + | **Cataclysmic or explosive variables, stars that undergo a cataclysmic change in their properties like [[nova]]e and [[supernova]]e. Interacting binary systems with white dwarfs or stars showing large amplitude outbursts. |
| − | **Cataclysmic or explosive variables, stars that undergo a cataclysmic change in their properties like [[nova]]e and [[supernova]]e. | + | |
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| − | *'''Extrinsic variable stars''': stars where the variability is caused by external properties like rotation or eclipses. There are two main subgroups.
| + | 1. Eruptive |
| − | **Eclipsing binaries, [[double star]]s where, as seen from [[Earth]]'s vantage point the stars occasionally eclipse one another as they orbit.
| + | FU, GCAS, I, IA, IB, IN, INA, INB, INT, IT, IN(YY), IS, ISA, ISB, RCB, RS, SDOR, UV, UVN, WR |
| − | **Rotating variables, stars whose variability is caused by phenomena related to their rotation. Examples are stars with extreme "sunspots" which affect the apparent brightness or stars that have fast rotation speeds causing them to become ellipsoidal in shape.
| + | 2. Pulsating |
| | + | ACYG, BCEP, BCEPS, CEP, CEP(B), CW, CWA, CWB, DCEP, DCEPS, DSCT, DSCTC, GDOR, L, LB, LC, M, PVTEL, RPHS, RR, RR(B), RRAB, RRC, RV, RVA, RVB, SR, SRA, SRB, SRC, SRD, SXPHE, ZZ, ZZA, ZZB |
| | + | 3. Cataclysmic (explosive and novalike) variables |
| | + | N, NA, NB, NC, NL, NR, SN, SNI, SNII, UG, UGSS, UGSU, UGZ, ZAND |
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| − | These subgroups themselves are further divided into specific types of variable stars that are usually named after their prototype. For example, dwarf novae are designated ''U Geminorum'' stars after the first recognized star in the class, ''U Geminorum''.
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| − | ==Intrinsic variable stars== | + | ==Extrinsic Variable Stars== |
| | + | *'''Extrinsic variable stars''': stars where the variability is caused by external properties like rotation or eclipses. There are two main subgroups. |
| | + | **Eclipsing binaries, double stars where, as seen from Earth's vantage point the stars occasionally eclipse one another as they orbit. |
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| − | [[Image:HR-vartype.svg|right|thumb|400px|Intrinsic variable types in the [[Hertzsprung–Russell diagram]]]]
| + | 4. Eclipsing binary systems |
| − | Examples of types within these divisions are given below.
| + | E, EA, EB, EW, GS, PN, RS, WD, WR, AR, D, DM, DS, DW, K, KE, KW, SD |
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| − | ===Pulsating variable stars===<!-- This section is linked from [[Rotation-powered pulsar]] -->
| + | **Rotating variables, stars whose variability is caused by phenomena related to their rotation. Examples are stars with extreme "sunspots" which affect the apparent brightness or stars that have fast rotation speeds causing them to become ellipsoidal in shape. |
| − | {{Main|Stellar pulsations}}
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| − | <ref name=asteroseismology>{{cite doi|10.1146/annurev.aa.32.090194.000345}}</ref><ref>Cox, John P., ''Theory of Stellar Pulsation'', Princeton, (1980)</ref><ref name=pulsating>{{cite web|url=http://www.star.ac.za/course-resources/local/david-buckley/var2.pdf|title=Variable Stars II - Pulsating Stars|last1=Kilkenny|first1=Dave}}</ref> The pulsating stars swell and shrink, affecting the brightness and spectrum. Pulsations are generally split into: [[radial pulsations|radial]], where the entire star expands and shrinks as a whole; and [[non-radial]], where one part of the star expand while another part shrinks. Some scientists consider non-radial pulsations to encompass everything, with radial pulsations as a special case, but considering them as mutually exclusive is convenient for variable stars since they generally vary with one type or the other.
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| − | Depending on the type of pulsation and its location within the star, there is a natural or [[fundamental frequency]] which determines the period of the star. Stars may also pulsate in a [[harmonic]] or [[overtone]] which is a higher frequency, corresponding to a shorter period. Pulsating variable stars sometimes have a single well-defined period, but often they pulsate simultaneously with multiple frequencies and complex analysis is required to determine the separate [[Interference (wave propagation)|interfering]] periods. In some cases, the pulsations do not have a defined frequency, causing random variation, referred to as [[stochastic]]. The study of stellar interiors using their pulsations is [[asteroseismology]].
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| − | A pulsation in a star must be caused by an unbalanced driving force with a feedback mechanism. In pulsating variable stars the driving force is the internal energy of the star, usually from [[nuclear fusion]], but in some cases just from stored energy, always attempting to propagate outwards. At certain locations on the HR diagram, corresponding to particular combinations of temperatures, size, and internal chemistry, the outward flow of energy by radiation varies strongly with the density or temperature of the material it is passing through. When the [[opacity (optics)|opacity]] of a layer is high that layer expands, and if the decrease of temperature or pressure causes the opacity to drop again, there is a feedback mechanism to create regular pulsations. This generally occurs as the ionisation level of the material changes, for example the ionisation of helium in yellow stars on the instability strip.
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| − | The expansion phase of a pulsation is caused by the blocking of the internal energy flow by material with a high opacity, but this must occur at a particular depth of the star to create visible pulsations. If the expansion occurs below a convective zone then nothing will be visible at the surface. If the expansion occurs too close to the surface then there may be no restoring force to create a pulsation. The restoring force to create the contraction phase of a pulsation can be pressure if the pulsation occurs in a non-degenerate layer deep inside a star, and this is called an [[Acoustics|acoustic]] or [[pressure]] mode of pulsation, abbreviated to [[P-mode star|p-mode]]. In other cases, the restoring force is simple [[gravity]] and this is called a [[gravity wave|g-mode]]. Pulsating variable stars typically pulsate in only one of these modes.
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| − | ==== Cepheids and cepheid-like variables ====
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| − | {{Main|Cepheid variable}}
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| − | This group consists of several kinds of pulsating stars, all found on the [[instability strip]], that swell and shrink very regularly by the star's own mass [[resonance]], generally by the [[fundamental frequency]]. Generally the [[κ mechanism|Eddington valve]] mechanism for pulsating variables is believed to account for cepheid-like pulsations. Each of the subgroups on the instability strip has a fixed relationship between period and absolute magnitude, as well as a relation between period and mean density of the star. The period-luminosity relationship was first established for Delta Cepheids by [[Henrietta Swan Leavitt]], and makes the high luminosity Cepheids very important for determining distances to galaxies within the [[Local Group]] and beyond. [[Edwin Hubble]] used this method to prove that the so-called spiral nebulae are in fact distant galaxies.
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| − | Note that the Cepheids are named only for [[Delta Cephei]], while a completely separate class of variables is named after [[Beta Cephei]].
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| − | =====Classical Cepheid variables=====
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| − | {{Main|Classical Cepheid variable}}
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| − | Classical Cepheids (or Delta Cephei variables) are population I (young, massive, and luminous) yellow supergiants which undergo pulsations with very regular periods on the order of days to months. On September 10, 1784 [[Edward Pigott]] detected the variability of [[Eta Aquilae]], the first known representative of the class of Cepheid variables. However, the namesake for classical Cepheids is the star [[Delta Cephei]], discovered to be variable by [[John Goodricke]] a few months later.
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| − | =====Type II Cepheids=====
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| − | {{Main|Type II Cepheids}}
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| − | Type II Cepheids (historically termed W Virginis stars) have extremely regular light pulsations and a luminosity relation much like the δ Cephei variables, so initially they were confused with the latter category. Type II Cepheids stars belong to [[Population II]], older stars than the type I Cepheids. They have somewhat lower [[metallicity]], much lower mass, somewhat lower luminosity, and a slightly offset period luminosity relationship, so it is always important to know which type of star is being observed.
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| − | =====RR Lyrae variables=====
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| − | {{Main|RR Lyrae variable}}
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| − | These stars are somewhat similar to Cepheids, but are not as luminous and with shorter periods. They are older than type I Cepheids, belonging to [[Population II]], but of lower mass than type II Cepheids. Due to their common occurrence in [[globular cluster]]s, they are occasionally referred to as ''cluster Cepheids''. They also have a well established period-luminosity relationship, and so are also useful distance indicators. These spectral type A stars vary by about 0.2 – 2 magnitudes (20% to over 500% change in luminosity) over a period of several hours to a day or more.
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| − | =====Delta Scuti variables=====
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| − | {{Main|Delta Scuti variable}}
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| − | Delta Scuti (δ Sct) variables are similar to Cepheids but much fainter and with much shorter periods. They were once known as ''Dwarf Cepheids''. They often show many superimposed periods, which combine to form an extremely complex light curve. The typical δ Scuti star has an amplitude of 0.003 – 0.9 magnitudes (0.3% to about 130% change in luminosity) and a period of 0.01 – 0.2 days. Their [[stellar classification|spectral type]] is usually between A0 and F5.
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| − | =====SX Phoenicis variables=====
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| − | {{Main|SX Phoenicis variable}}
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| − | These stars of spectral type A2 to F5, similar to δ Scuti variables, are found mainly in globular clusters. They exhibit fluctuations in their brightness in the order of 0.7 magnitude (about 100% change in luminosity) or so every 1 to 2 hours.
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| − | =====Rapidly oscillating Ap variables=====
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| − | {{Main|Rapidly oscillating Ap star}}
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| − | These stars of spectral type A or occasionally F0, a sub-class of δ Scuti variables found on the main sequence. They have extremely rapid variations with periods of a few minutes and amplitudes of a few thousandths of a magnitude.
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| − | ====Long period variables====
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| − | {{Main|Long period variable}}
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| − | The long period variables are cool evolved stars that pulsate with periods in the range of weeks to several years.
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| − | =====Mira variables=====
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| − | {{Main|Mira variable}}
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| − | Mira variables are AGB red giants. Over periods of many months they fade and brighten by between 2.5 and 11 [[apparent magnitude|magnitude]]s, a sixfold to 30 thousandfold change in luminosity. [[Mira]] itself, also known as Omicron Ceti (ο Cet), varies in brightness from almost 2nd magnitude to as faint as 10th magnitude with a period of roughly 332 days. The very large visual amplitudes are mainly due to the shifting of energy output between visual and infra-red as the temperature of the star changes. In a few cases, Mira variables show dramatic period changes over a period of decades, thought to be related to the thermal pulsing cycle of the most advanced AGB stars.
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| − | =====Semiregular variables=====
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| − | {{Main|Semiregular variable}}
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| − | These are [[red giants]] or [[red supergiant|supergiants]]. Semiregular variables may show a definite period on occasion, but more often show less well-defined variations that can sometimes be resolved into multiple periods. A well-known example of a semiregular variable is [[Betelgeuse]], which varies from about magnitudes +0.2 to +1.2 (a factor 2.5 change in luminosity). At least some of the semi-regular variables are very closely related to Mira variables, possibly the only difference being pulsating in a different harmonic.
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| − | =====Slow irregular variables=====
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| − | {{Main|Slow irregular variable}}
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| − | These are [[red giants]] or [[red supergiant|supergiants]] with little or no detectable periodicity. Some are poorly studied semiregular variables, often with multiple periods, but others may simply be chaotic.
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| − | ====Beta Cephei variables====
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| − | {{Main|Beta Cephei variable}}
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| − | Beta Cephei (β Cep) variables (sometimes called [[Beta Canis Majoris]] variables, especially in Europe)<ref>[http://www.aavso.org/vstar/vsots/winter05.shtml Variable Star Of The Season, Winter 2005: The Beta Cephei Stars and Their Relatives], John Percy, [[AAVSO]]. Accessed October 2, 2008.</ref> undergo short period pulsations in the order of 0.1 – 0.6 days with an amplitude of 0.01 – 0.3 magnitudes (1% to 30% change in luminosity). They are at their brightest during minimum contraction. Many stars of this kind exhibits multiple pulsation periods.
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| − | ====Slowly pulsating B stars====
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| − | {{Main|Slowly pulsating B star}}
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| − | Slowly pulsating B (APB) stars are hot main sequence stars slightly less luminous than the Beta Cephei stars, with longer periods and larger amplitudes.<ref name=spb>{{bibcode|2002ASPC..259..196D}}</ref>
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| − | ====PV Telescopii variables====
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| − | {{Main|PV Telescopii variable}}
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| − | Stars in this class are type Bp supergiants with a period of 0.1 – 1 day and an amplitude of 0.1 magnitude on average. Their spectra are peculiar by having weak [[hydrogen]] while on the other hand [[carbon]] and [[helium]] lines are extra strong, a type of [[Extreme helium star]].
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| − | ====RV Tauri variables====
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| − | {{Main|RV Tauri variable}}
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| − | These are yellow supergiant stars (actually low mass post-AGB stars at the most luminous stage of their lives) which have alternating deep and shallow minima. This double-peaked variation typically has periods of 30–100 days and amplitudes of 3 – 4 magnitudes. Superimposed on this variation, there may be long-term variations over periods of several years. Their spectra are of type F or G at maximum light and type K or M at minimum brightness. They lie near the instability strip, cooler than type I Cepheids more luminous than type II Cepheids. Their pulsations are caused by the same basic mechanisms related to helium opacity, but they are at a very different stage of their lives.
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| − | ====Alpha Cygni variables====
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| − | {{Main|Alpha Cygni variable}}
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| − | Alpha Cygni (α Cyg) variables are nonradially pulsating supergiants of [[spectral class]]es B<sub>ep</sub> to A<sub>ep</sub>Ia. 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 constellation of [[Cygnus (constellation)|Cygnus]] is the prototype of this class.
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| − | ====Gamma Doradus variables====
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| − | {{Main|Gamma Doradus variable}}
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| − | Gamma Doradus (γ Dor) variables are non-radially pulsating main sequence stars of [[spectral classes]] F to late A. Their periods are around one day and their amplitudes typically of the order of 0.1 magnitudes.
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| − | ====Pulsating white dwarfs====
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| − | {{Main|Pulsating white dwarf}}
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| − | These non-radially pulsating stars have short periods of hundreds to thousands of seconds with tiny fluctuations of 0.001 to 0.2 magnitudes. Known types of pulsating white dwarf (or pre-white dwarf) include the ''DAV'', or ''ZZ Ceti'', stars, with hydrogen-dominated atmospheres and the spectral type DA;<ref name="physrev">pp. 891, 895, [http://adsabs.harvard.edu/abs/1990RPPh...53..837K Physics of white dwarf stars], D. Koester and G. Chanmugam, ''Reports on Progress in Physics'' '''53''' (1990), pp. 837–915.</ref> ''DBV'', or ''V777 Her'', stars, with helium-dominated atmospheres and the spectral type DB;<ref name="wden">p. 3525, White dwarfs, Gilles Fontaine and François Wesemael, in ''Encyclopedia of Astronomy and Astrophysics'', ed. Paul Murdin, Bristol and Philadelphia: Institute of Physics Publishing and London, New York and Tokyo: Nature Publishing Group, 2001. ISBN 0-333-75088-8.</ref> and ''GW Vir'' stars, with atmospheres dominated by helium, carbon, and oxygen. GW Vir stars may be subdivided into ''DOV'' and ''PNNV'' stars.<ref name="quirion">§1.1, 1.2, [http://adsabs.harvard.edu/abs/2007ApJS..171..219Q Mapping the Instability Domains of GW Vir Stars in the Effective Temperature-Surface Gravity Diagram], Quirion, P.-O., Fontaine, G., Brassard, P., ''Astrophysical Journal Supplement Series'' '''171''' (2007), pp. 219–248.</ref><ref>§1, [http://adsabs.harvard.edu/abs/2004A%26A...426L..45N Detection of non-radial g-mode pulsations in the newly discovered PG 1159 star HE 1429-1209], T. Nagel and K. Werner, ''Astronomy and Astrophysics'' '''426''' (2004), pp. L45–L48.</ref>
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| − | ==== Solar-like oscillations ====
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| − | The [[Sun]] oscillates with very low amplitude in a large number of modes having periods around 5 minutes. The study of these oscillations is known as [[helioseismology]]. Oscillations in the Sun are driven stochastically by [[convection]] in its outer layers. The term [[solar-like oscillations]] is used to describe oscillations in other stars that are excited in the same way and the study of these oscillations is one of the main areas of active research in the field of [[asteroseismology]].
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| − | ===Eruptive variable stars===
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| − | Eruptive variable stars show irregular or semi-regular brightness variations caused by material being lost from the star, or in some cases being accreted to it. Despite the name these are not explosive events, those are the cataclysmic variables.
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| − | ====Protostars====
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| − | {{Main|Pre–main sequence star}}
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| − | Protostars are young objects that have not yet completed the process of contraction from a gas nebula to a veritable star. Most protostars exhibit irregular brightness variations.
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| − | ===== Herbig Ae/Be stars =====
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| − | {{Main|Herbig Ae/Be stars}}
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| − | Variability of more massive (2–8 [[Sun|solar]] mass) [[Herbig Ae/Be stars]] is thought to be due to gas-dust clumps, orbiting in the circumstellar disks.
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| − | =====Orion variables=====
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| − | {{Main|Orion variable}}
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| − | Orion variables are young, hot [[pre–main sequence star]]s usually embedded in nebulosity. They have irregular periods with amplitudes of several magnitudes. A well-known subtype of Orion variables are the [[T Tauri star|T Tauri]] variables. Variability of [[T Tauri star]]s is due to spots on the stellar surface and gas-dust clumps, orbiting in the circumstellar disks.
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| − | =====FU Orionis variables=====
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| − | {{Main|FU Orionis star}}
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| − | These stars reside in reflection nebulae and show gradual increases in their luminosity in the order of 6 magnitudes followed by a lengthy phase of constant brightness. They then dim by 2 magnitudes (six times dimmer) or so over a period of many years. ''V1057 Cygni'' for example dimmed by 2.5 magnitude (ten times dimmer) during an eleven-year period. FU Orionis variables are of spectral type A through G and are possibly an evolutionary phase in the life of ''[[T Tauri star|T Tauri]]'' stars.
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| − | ====Giants and supergiants====
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| − | Large stars lose their matter relatively easily. For this reason variability due to eruptions and mass loss is fairly common among giants and supergiants.
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| − | =====Luminous blue variables=====
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| − | {{Main|Luminous blue variable}}
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| − | Also known as the [[S Doradus]] variables, the most luminous stars known belong to this class. Examples include the [[hypergiant]]s [[Eta Carinae|η Carinae]] and [[P Cygni]]. They have permanent high mass loss, but at intervals of years internal pulsations cause the star to exceed its Eddington limit and the mass loss increases hugely. Visual brightness increases although the overall luminosity is largely unchanged. Giant eruptions observed in a few LBVs do increase the luminosity, so much so that they have been tagged [[supernova impostor]]s, and may be a different type of event.
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| − | =====Yellow hypergiants=====
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| − | {{Main|Yellow hypergiant}}
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| − | These massive evolved stars are unstable due to their high luminosity and position above the instability strip, and they exhibit slow but sometimes large photometric and spectroscopic changes due to high mass loss and occasional larger eruptions, combined with secular variation on an observable timescale. The best known example is [[Rho Cassiopeiae]].
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| − | =====R Coronae Borealis variables=====
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| − | {{Main|R Coronae Borealis variable}}
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| − | 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 at irregular intervals they suddenly fade by 1 – 9 magnitudes (2.5 to 4000 times dimmer) before recovering to their initial brightness over months to years. Most are classified as yellow supergiants by luminosity, although they are actually post-AGB stars, but there are both red and blue giant R CrB stars. [[R Coronae Borealis]] (R CrB) is the prototype star. [[DY Persei variable]]s are a subclass of R CrB variables that have a periodic variability in addition to their eruptions.
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| − | ====Wolf–Rayet variables====
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| − | {{Main|Wolf–Rayet star}}
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| − | Wolf–Rayet stars are massive hot stars that sometimes show variability, probably due to several different causes including binary interactions and rotating gas clumps around the star. They exhibit broad emission line spectra with [[helium]], [[nitrogen]], [[carbon]] and [[oxygen]] lines. Variations in some stars appear to be stochastic while others show multiple periods.
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| − | ====Gamma Cassiopeiae variables====
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| − | {{Main|Gamma Cassiopeiae variable}}
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| − | [[Gamma Cassiopeiae]] (γ Cas) variables are non-supergiant fast-rotating B class emission line type stars that fluctuate irregularly by up to 1.5 magnitudes (fourfold change in luminosity) due to the ejection of matter at their [[equator]]ial regions caused by the rapid rotational.
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| − | ====Flare stars====
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| − | {{Main|Flare star}}
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| − | In main-sequence stars major eruptive variability is exceptional. It is common only among the [[flare star]]s, also known as the [[UV Ceti]] variables, very faint main-sequence stars which undergo regular flares. They increase in brightness by up to two magnitudes (six times brighter) in just a few seconds, and then fade back to normal brightness in half an hour or less. Several nearby red dwarf stars are flare stars, including [[Proxima Centauri]] and [[Wolf 359]].
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| − | ====RS Canum Venaticorum variables====
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| − | {{Main|RS Canum Venaticorum variable}}
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| − | These are close binary systems with highly active chromospheres, including huge sunspots and flares, believed to be enhanced by the close companion. Variability scales ranges from days, close to the orbital period and sometimes also with eclipses, to years as sunspot activity varies.
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| − | ===Cataclysmic or explosive variable stars===
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| − | [[Image:V838 Monocerotis expansion.jpg|300px|right|thumb|Images showing the expansion of the light echo of a red variable star, the [[V838 Monocerotis]]]]
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| − | {{Main|Cataclysmic variable star|Supernova|Symbiotic variable star}}
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| − | ====Supernovae====
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| − | [[Supernova]]e 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]], brightening by more than 20 magnitudes (over one hundred million times brighter). The supernova explosion is caused by a white dwarf or a star core reaching a certain mass/density limit, the [[Chandrasekhar limit]], causing the object to collapse in a fraction of a second. This collapse "bounces" and causes the star to explode and emit this enormous energy quantity. The outer layers of these stars are blown away at speeds of many thousands of kilometers an [[hour]]. The expelled matter may form nebulae called ''[[supernova remnant]]s''. A well-known example of such a nebula is the [[Crab Nebula]], left over from a supernova that was observed in [[China]] and [[North America]] in 1054. The core of the star or the white dwarf may either become a [[neutron star]] (generally a [[pulsar]]) or disintegrate completely in the explosion.
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| − | Supernovae can result from the death of an extremely massive star, many times heavier than the Sun. At the end of the life of this massive star, a non-fusible iron core is formed from fusion ashes. This iron core is pushed towards the Chandrasekhar limit till it surpasses it and therefore collapses.
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| − | A supernova may also result from mass transfer onto a [[white dwarf]] from a star companion in a double star system. The Chandrasekhar limit is surpassed from the infalling matter. 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. One of the most studied supernovae is [[SN 1987A]] in the [[Large Magellanic Cloud]].
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| − | ====Novae====
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| − | {{Main|Nova}}
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| − | [[Nova]]e are also the result of dramatic explosions, but unlike supernovae do not result in the destruction of the progenitor star. Also unlike supernovae, novae ignite from the sudden onset of thermonuclear fusion, which under certain high pressure conditions ([[degenerate matter]]) accelerates explosively. They form in close [[binary system (astronomy)|binary system]]s, one component being a white dwarf accreting matter from the other ordinary star component, and may recur over periods of decades to centuries or millennia. Novae are categorised as ''fast'', ''slow'' or ''very slow'', depending on the behaviour of their light curve. Several [[naked eye]] novae have been recorded, [[Nova Cygni 1975]] being the brightest in the recent history, reaching 2nd magnitude.
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| − | ====Dwarf novae====
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| − | {{Main|Dwarf nova}}
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| − | Dwarf novae are double stars involving a [[white dwarf]] star in which matter transfer between the component gives rise to regular outbursts. There are three types of dwarf nova:
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| − | *[[U Geminorum star]]s, which have outbursts lasting roughly 5–20 days followed by quiet periods of typically a few hundred days. During an outburst they brighten typically by 2 – 6 magnitudes. These stars are also known as [[SS Cygni variable]]s after the variable in [[Cygnus (constellation)|Cygnus]] which produces among the brightest and most frequent displays of this variable type.
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| − | *[[Z Camelopardalis star]]s, in which occasional plateaux of brightness called ''standstills'' are seen, part way between maximum and minimum brightness.
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| − | *[[SU Ursae Majoris star]]s, which undergo both frequent small outbursts, and rarer but larger ''superoutbursts''. These binary systems usually have orbital periods of under 2.5 hours.
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| − | ====Z Andromedae variables====
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| − | These symbiotic binary systems are composed of a red giant and a hot blue star enveloped in a cloud of gas and dust. They undergo nova-like outbursts with amplitudes of some 4 magnitudes.
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| − | ==Extrinsic variable stars==
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| − | There are two main groups of extrinsic variables: rotating stars and eclipsing stars.
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| − | ===Rotating variable stars===
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| − | Stars with sizeable [[sunspot]]s may show significant variations in brightness as they rotate, and brighter areas of the surface are brought into view. Bright spots also occur at the magnetic poles of magnetic stars. Stars with ellipsoidal shapes may also show changes in brightness as they present varying areas of their surfaces to the observer.
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| − | ==== Non-spherical stars ====
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| − | =====Ellipsoidal variables=====
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| − | These are very close binaries, the components of which are non-spherical due to their mutual gravitation. As the stars rotate the area of their surface presented towards the observer changes and this in turn affects their brightness as seen from Earth.
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| − | ====Stellar spots====
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| − | The surface of the star is not uniformly bright, but has darker and brighter areas (like the sun's [[Sun spot|solar spots]]). The star's [[chromosphere]] too may vary in brightness. As the star rotates we observe brightness variations of a few tenths of magnitudes.
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| − | =====FK Comae Berenices variables=====
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| − | These stars rotate extremely fast (~100 km/s at the [[equator]]); hence they are [[ellipsoidal]] in shape. They are (apparently) single giant stars with [[spectral type]]s G and K and show strong [[chromosphere|chromospheric]] [[emission line]]s. Examples are FK Com, HD 199178 and UZ Lib. A possible explanation for the rapid rotation of FK Comae stars is that they are the result of the merger of a [[contact binary|(contact) binary]].
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| − | ===== [[BY Draconis variable]] stars =====
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| − | BY Draconis stars are of spectral class K or M and vary by less than 0.5 magnitudes (70% change in luminosity).
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| − | ====Magnetic fields====
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| − | ===== Alpha-2 Canum Venaticorum variables =====
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| − | {{Main|Alpha-2 Canum Venaticorum variable}}
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| − | Alpha-2 Canum Venaticorum (α<sup>2</sup> CVn) variables are [[main sequence]] stars of spectral class B8 – A7 that show fluctuations of 0.01 to 0.1 magnitudes (1% to 10%) due to changes in their magnetic fields.
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| − | =====SX Arietis variables=====
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| − | Stars in this class exhibit brightness fluctuations of some 0.1 magnitude caused by changes in their magnetic fields due to high rotation speeds.
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| − | =====Optically variable pulsars=====
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| − | {{Main|Pulsar}}
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| − | Few [[pulsar]]s have been detected in [[visible light]]. These [[neutron star]]s change in brightness as they rotate. Because of the rapid rotation, brightness variations are extremely fast, from milliseconds to a few seconds. The first and the best known example is the [[Crab Pulsar]].
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| − | ===Eclipsing binaries===
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| − | [[Image:Light curve of binary star Kepler-16.jpg|right|thumb|400px|How eclipsing binaries vary in brightness]]
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| − | {{Main|Binary star#Eclipsing binaries}}
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| − | 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 (β Per).
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| − | ====Algol variables====
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| − | {{Main|Algol variable}}
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| − | Algol variables undergo eclipses with one or two minima separated by periods of nearly constant light. The prototype of this class is [[Algol]] in the [[constellation]] [[Perseus (constellation)|Perseus]].
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| − | ====Beta Lyrae variables====
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| − | {{Main|Beta Lyrae variable}}
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| − | Beta Lyrae (β Lyr) variables are extremely close binaries, named after the star [[Beta Lyrae|Sheliak]]. The light curves of this class of eclipsing variables are constantly changing, making it almost impossible to determine the exact onset and end of each eclipse.
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| − | ====W Ursae Majoris variables====
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| − | {{Main|W Ursae Majoris variable}}
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| − | The stars in this group show periods of less than a day. The stars are so closely situated to each other that their surfaces are almost in contact with each other.
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| − | ===Planetary transits===
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| − | Stars with [[extrasolar planet|planets]] 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. Examples include [[HD 209458]] and [[GSC 02652-01324]], and all of the planets and planet candidates detected by the [[Kepler Mission]].
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| − | =Variable Star Resources=
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| − | [http://www.aavso.org American Association of Variable Star Observers] - (AAVSO) is a non-profit worldwide scientific and educational organization of amateur and professional astronomers who are interested in stars that change in brightness—variable stars.
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| − | [http://arxiv.org/archive/astro-ph arXiv.org Astrophysics] - An e-print service in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance and statistics maintained Cornell University.
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| − | [http://www.adsabs.harvard.edu Astrophysics Data System] - The ADS maintains three bibliographic databases containing more than 10.8 million records: Astronomy and Astrophysics, Physics, and arXiv e-prints.
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| − | [http://www.britastro.org/vss The British Astronomical Association: Variable Star Section] - The VSS was formed in 1890, the year the BAA was founded, with the aim of collecting and analysing observations of variable stars.
| + | 5. Rotating |
| | + | ACV, ACVO, BY, ELL, FKCOM, PSR, SXARI |
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| − | [http://http://www.konkoly.hu/IBVS/IBVS.html Konkoly Observatory, Hungary - Variable Stars] - The Information Bulletin on Variable Stars is a bulletin of the Commissions 27 and 42 of the International Astronomical Union, published by the Konkoly Observatory, Budapest, Hungary.
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| − | [http://www.rasnz.org.nz Royal Astronomical Society of New Zealand] - The RASNZ has links to [http://www.variablestarssouth.org Variable Stars South], a group focused on Variable Stars in the Southern skies.
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| − | [http://simbad.u-strasbg.fr/Simbad The SIMBAD astronomical database] provides basic data, cross-identifications, bibliography and measurements for astronomical objects outside the solar system.
| + | 6. intense variable X-ray sources (X, XB, XF, XI, XJ, XND, XNG, XP, XPR, |
| | + | XPRM, XM), |
| | + | 7. other symbols (BLLAC, CST, GAL, L:, QSO, S, *, +, :). |
| | + | 8. the new variability types (ZZO, AM, R, BE, LBV, BLBOO, EP, SRS, LPB) |
1. Eruptive
2. Pulsating
3. Cataclysmic (explosive and novalike) variables
4. Eclipsing binary systems
5. Rotating
6. intense variable X-ray sources (X, XB, XF, XI, XJ, XND, XNG, XP, XPR,
7. other symbols (BLLAC, CST, GAL, L:, QSO, S, *, +, :).
8. the new variability types (ZZO, AM, R, BE, LBV, BLBOO, EP, SRS, LPB)
