Examples of types within these divisions are given below.
Pulsating variable stars Pulsating stars swell and shrink, affecting their brightness and spectrum. Pulsations are generally split into:
radial, where the entire star expands and shrinks as a whole; and non-radial, where one part of the star expands while another part shrinks. 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
interfering periods. In some cases, the pulsations do not have a defined frequency, causing a random variation, referred to as
stochastic. The study of stellar interiors using their pulsations is known as
asteroseismology. The expansion phase of a pulsation is caused by the blocking of the internal energy flow by material with a high opacity, 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
acoustic or
pressure mode of pulsation, abbreviated to
p-mode. In other cases, the restoring force is
gravity and this is called a
g-mode. Pulsating variable stars typically pulsate in only one of these modes. These stars swell and shrink very regularly, caused by the star's own mass
resonance, generally by the
fundamental frequency. The
Eddington valve mechanism for pulsating variables is believed to account for cepheid-like pulsations. Because of their high luminosity, Classical Cepheids can be viewed in nearby galaxies outside the Milky Way. 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. Type II Cepheids 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 are uncommon stars that belong to the older
Population II category, somewhat lower luminosity, and a slightly offset period versus luminosity relationship, so it is always important to know which type of star is being observed. They can be identified based on the shape of their light curve. Type II Cepheids are further sub-divided based on their pulsation periods as
BL Her stars for periods of days,
W Vir stars for days, and
RV Tau stars for longer periods of up to 100 days. These three subtypes correspond to consecutive states of stellar evolution after the star has exhausted the helium at its core. This double-peaked variation typically has periods of 30–150 days and amplitudes of up to 3 magnitudes. Superimposed on this variation, there may be long-term variations over periods of several years. They lie near the instability strip, forming a higher luminosity extension of the type II Cepheids, while being cooler than type I 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. RR Lyrae variables These relatively common variable stars are somewhat similar to Cepheids, but are not as luminous and have shorter periods. They are older than type I Cepheids, belonging to
Population II, but of lower mass than type II Cepheids. They also have a well established period-luminosity relationship in the infrared K-band, and so are also useful as distance indicators. These are low mass giants having an A- or F-type spectrum, and are currently on the
horizontal branch. They are radially pulsating and vary by about 0.2–2 in visual magnitude (20% to over 500% change in luminosity) over a period of several hours to a day or more. The category is divided into Bailey subtypes 'a', 'b', and 'c', depending on the shape of the light curve. Delta Scuti variables Delta Scuti (δ Sct) variables are similar to Cepheids but much fainter and with much shorter periods. They were once known as
Dwarf Cepheids. Delta Scuti variables display both radial and non-radial pulsations modes. They often show many superimposed periods, which combine to form a complex light curve. Their
spectral type is usually late A- and early F-type stars, and they lie on or near the
main sequence on the
H-R diagram. When metallicity is solar, they have masses ranging from about 1.6 times the Sun for slower periods up to 2.4 at higher pulsation rates. With rotation rates of , Delta Scuti stars show small amplitudes of magnitude with multiple pulsation modes, including many non-radial. For slower rotation rates under , the amplitude is magnitude or more, and they are often radial pulsators. Stars with Delta Scuti-like variations and an amplitude greater than 0.3 magnitude are known as AI Vel-type variables, after their prototype,
AI Velorum. SX Phoenicis variables These stars are metal-poor, population II analogues of δ Scuti variables and are mainly found in globular clusters. They exhibit fluctuations in their brightness in the order of 0.7 magnitude (about 100% change in luminosity) or so with short periods of 1 to 3 hours. They have masses in the range of solar. Within a cluster, they are referred to as pulsating
blue stragglers, presumably being formed from the merger of two ordinary stars in a close binary system. SX Phe variables are slow rotators and most pulsation modes are radial. Rapidly oscillating Ap variables The roAp variables are rapidly rotating, strongly magnetic,
chemically peculiar stars of spectral type A or occasionally F0, known as Ap stars. Their pulsatation behavior is much like those of Delta Scuti or Gamma Doradus 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. Unlike Delta Scuti stars, roAp stars pulsate with either a single high frequency or with multiple high frequencies that are closely spaced. However, the isolated high frequencies of roAp stars have also been observed in stars that are not chemically peculiar, and some Delta Scuti stars show pulsation in the roAp range. Thus the distinction is unclear.
Long period variables The long period variables are cool evolved stars that pulsate with periods in the range of weeks to several years. All giant stars cooler than spectral type K5 are variable because of radial pulsations. Mira variables of
Mira variable χ Cygni Mira variables are aging
red giant stars nearing the end of their active life on
asymptotic giant branch (AGB). They have radial pulsation periods that can range from under 100 to over 2,000 days, although most are in the day range. They fade and brighten over a range of 8
magnitudes, a thousand fold change in luminosity. 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. Semiregular variables These are long-period variables with shorter periods and smaller amplitudes than Miras, and their light curves are less regular. Types SRa and SRb are
red giants, with the latter type displaying a less regular periodicity. The visual amplitude is typically less than 2.5 magnitudes. 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 in brightness by half a magnitude with overlapping periods of and 5.75 years. 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. Slow irregular variables These are
red giants or
supergiants with little or no detectable periodicity. Some are poorly studied semiregular variables, often with multiple periods, but others may simply be chaotic. A prominent example of a slow irregular variable is
Antares, which is classified as an Lc type with a brightness that ranges from in
visual magnitude.
Beta Cephei variables Beta Cephei (β Cep) variables (sometimes called
Beta Canis Majoris variables, especially in Europe) 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.
Slowly pulsating B-type stars Slowly pulsating B (SPB) stars are hot main-sequence stars slightly less luminous than the Beta Cephei stars, with longer periods and larger amplitudes. They have masses in the range of , and non-radial pulsation periods from days. Many are rapid rotators, which can cause them to appear cooler and, in some cases, lie outside instability strip.
Very rapidly pulsating hot (subdwarf B) stars The prototype of this rare class is
V361 Hydrae, a 15th magnitude
subdwarf B star. They pulsate with periods of a few minutes and may simultaneous pulsate with multiple periods. They have amplitudes of a few hundredths of a magnitude and are given the GCVS acronym RPHS. They are
p-mode pulsators.
PV Telescopii variables Stars in this rare class are chemically peculiar type B (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 but extra strong
carbon and
helium lines, making this a type of
extreme helium star. The prototype for this category of variable is
PV Telescopii, which undergoes small but complex luminosity variations and radial velocity fluctuations.
Alpha Cygni variables Alpha Cygni (α Cyg) variables are nonradially pulsating supergiants of
spectral classes B to A. 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, may be caused by the superposition of many oscillations with close periods. The progenitors of these stars have at least 14 solar masses. At least for the brighter members, these variables appear to have returned to the blue supergiant region of the H–R diagram after losing considerable mass as red supergiants.
Deneb, in the constellation of
Cygnus is the prototype of this class.
Gamma Doradus variables Gamma Doradus (γ Dor) variables are non-radially pulsating main-sequence stars of
spectral classes F to late A, with
luminosity classes of IV-V or V. Their periods are 0.3 to 3 days and their amplitudes typically of the order of 0.1 magnitudes or less. This variable type occupies a narrow range near the low-luminosity part of the instability strip, which partially overlaps the range of Delta Scuti variables. The physical properties of Gamma Doradus variables are similar to long-period Delta Scuti variables. Their slow period and low amplitudes makes Gamma Doradus variables difficult to discover from the ground; most have been spotted by space missions.
Solar-like oscillations 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. Stars with surface convection layers that can produce solar-like oscillations are generally cooler than the right edge of the instability strip, which includes the lower main sequence along with subgiants and red giants. However, solar-like oscillations can also be excited by stellar pulsations, such as by Cepheids.
Fast yellow pulsating supergiants A fast yellow pulsating supergiant (FYPS) is a luminous yellow supergiant with pulsations shorter than a day. They are thought to have evolved beyond a red supergiant phase, but the mechanism for the pulsations is unknown. The class was named in 2020 through analysis of
TESS observations.
Pulsating white dwarfs 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;
DBV, or
V777 Her, stars, with helium-dominated atmospheres and the spectral type DB; and
GW Vir stars, with atmospheres dominated by helium, carbon, and oxygen. GW Vir stars may be subdivided into
DOV and
PNNV stars.
BLAP variables A Blue Large-Amplitude Pulsator (BLAP) is a very rare class of radially-pulsating star characterized by changes of 0.2 to 0.4 magnitudes with typical periods of 7 to 75 minutes. Alternatively, they may form from the merger of two low-mass white dwarfs. BLAP are effectively pre-
white dwarf bodies with an effective temperature between 20,000 and 35,000 K. Most of these objects are in the medium or late stage of helium fusion.
Eruptive variable stars Eruptive variable stars show unpredictable brightness variations caused by material being lost from the star, or in some cases being accreted to it. Despite the name, these are distinguished from cataclysmic variables because the eruptions are due to non-thermonuclear processes.
Young stellar object Protostars are young objects that have not yet completed the process of contraction from a gas nebula to a veritable star. During this phase, the object is deeply embedded in an
optically thick envelope, so that the variability induced by the rapid accretion process is primarily visible in the infrared. Once the object has expelled most of this nascent cocoon of gas and dust, it stabilizes in mass and becomes a
pre–main-sequence star that is contracting toward the
main sequence. The luminosity of this object is derived from
gravitational contraction. These objects often exhibit irregular brightness variations in association with strong magnetic fields. Orion variables Orion variables are young, hot pre–main-sequence stars usually embedded in nebulosity. They have irregular periods with amplitudes of several magnitudes. These irregular variables are so-named because many were first located in the
Orion Nebula. A well-known subtype of Orion variables are the
T Tauri variables. Variability of
T Tauri stars is due to
spots on the stellar surface and gas-dust clumps, orbiting in the circumstellar disks. This class of variables are subdivided into classical and weak-line T Tauri. The former display a typical
emission line spectra, while the latter do not show strong emission lines and lack a strong stellar wind or accretion disk. The third class, Herbig Ae/Be stars, are the more massive form. The fourth are the
RW Aurigae irregular variables that have similar properties but lack nearby nebulosity. These last irregular variables do display emission lines, providing evidence for circumstellar shells. Herbig Ae/Be stars
V1025 Tauri and surrounding molecular nebula Variability of more massive (2–8
solar mass)
Herbig Ae/Be stars is thought to be due to gas-dust clumps, orbiting in the
circumstellar disks. They can also occur due to cold spots on the photosphere or pulsations when crossing the instability strip. The optical variations are typically up to a magnitude in amplitude and occur on time scales of days to weeks. A particularly extreme example is
UX Orionis, which is the prototype of "UXORs"; these protostars vary by magnitudes. FU Orionis variables A small fraction of young stellar objects are eruptive. The two primary types are dubbed FUors and EXors, after their prototype stars,
FU Orionis and
EX Lupi. (There are also intermediate types and Fu Ori-like YSOs.) The two types differ in the amplitude and time scales of their outbursts. FUors reside in reflection nebulae and show sharp increases in their luminosity in the order of 5–6 magnitudes followed by a very slow decline. FU Orionis variables are of spectral type F or G and are possibly an evolutionary phase in the life of
T Tauri stars. EXors exhibit flares like a FUor, but their duration is much shorter. They can exhibit brief flashes up to 5 magnitudes. Its possible these are the next stage in evolution following the FUor phase. For this reason variability due to eruptions and mass loss is more common among giants and supergiants. Luminous blue variables Also known as the
S Doradus variables, luminous blue variables (LBV) are among the most luminous stars known. Examples include the
hypergiants
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 significantly. 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 impostors, and may be a different type of event. Yellow hypergiants 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. One of the best studied examples is
Rho Cassiopeiae. R Coronae Borealis variables While classed as eruptive variables, these stars do not undergo periodic increases in brightness. Instead they spend most of their time undergoing small amplitude, semi-regular changes in luminosity, probably due to pulsations. At irregular intervals, they suddenly decline by 1–9 magnitudes (2.5 to 4000 times dimmer) before recovering to their initial brightness over months to years. They are carbon dust-producing stars belonging to a category of carbon-rich, hydrogen deficient supergiants.
R Coronae Borealis (R CrB) is the prototype star. This dust production is the cause of the large declines in brightness. Two scenarios have been proposed for the formation of an R CrB star: either the merger of a carbon-oxygen white dwarf with a helium white dwarf, or the central stellar remnant from a
planetary nebula undergoes
helium flash, becoming a supergiant.
DY Persei variables are considered a subclass of cool R CrB variables. They are carbon-rich stars on the
asymptotic giant branch that display both pulsational and irregular patterns of variability. Their dust declines are shallower and more symmetric than typical R CrB variables. This may indicate the two types have different dust production methods. While evolving they underwent intense mass loss, leaving behind a hot helium core with little hydrogen in the outer layers. They exhibit broad emission line spectra with
helium,
nitrogen,
carbon and
oxygen lines. Variations in some WR stars appear to be stochastic while others show multiple periods. This is caused by the ejection of matter at their
equatorial regions due to the rapid rotational velocity. Gamma Cas variables are a
source of bright X-ray emission, which may be due to gas accretion onto a white dwarf companion.
Flare stars Flare stars are defined by the observation of a flare event, which is a brief but dramatic increase in stellar luminosity. In main-sequence stars major eruptive variability is uncommon. Flare activity is more likely among young stars that are spinning rapidly. They increase in brightness by several magnitudes in just a few seconds, and then fade back to normal brightness in half an hour or less. Several nearby red dwarfs are flare stars, including
Proxima Centauri and
Wolf 359. A
superflare is a class of energetic, short duration flare that has been observed on
Sun-like stars. It has a typical energy of at least , which is greater than the strongest observed
solar flare: the 1859
Carrington Event with an estimated energy of . The
Kepler space telescope light curves showed over 2,000 superflares on 250 G-type dwarfs. The occurrence rate is higher on younger, faster rotating stars.
RS Canum Venaticorum variables These are detached binary systems with at least one of the components having a highly active chromosphere, including huge sunspots and flares, believed to be enhanced by the close companion. The former is usually an evolved star, while the latter is a lower mass star, either main-sequence or a subdwarf. Tidal forces between the stars has
locked their
rotation period to the
orbital period, giving them a high rotation rate of a few days. They display emission lines from the chromosphere and X-ray output from the
corona. Variability scales ranges from days, close to the orbital period and sometimes also with eclipses, to years as sunspot activity varies.
Cataclysmic or explosive variable stars These variables display outbursts from thermonuclear bursts at the surface or near the core. The category also includes nova-like objects that display outbursts like a nova from a rapid release of energy, or because their spectrum resembles that of a nova at minimum light. 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, brightening by more than 20 magnitudes (over one hundred million times brighter). The expelled matter may form nebulae called
supernova remnants. A well-known example of such a nebula is the
Crab Nebula, left over from a supernova that was observed in
China and elsewhere in 1054. The progenitor object may either disintegrate completely in the explosion, or, in the case of a massive star, the core can become a
neutron star (generally a
pulsar) or a
black hole. 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. The mass of this iron core is pushed towards the Chandrasekhar limit until it is surpassed and therefore collapses. 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.
Luminous red nova , a luminous red nova that erupted in 2002 Luminous red novae are stellar explosions caused by the merger of two stars. They are not related to classical
novae. For a brief period prior to the merger event the two components share a
common envelope, which is followed by a mass ejection event that expels the envelope. They have a characteristic red appearance and a lengthy plateau phase following the initial outburst. The luminosity of these transient events lies between those of novae and supernovae, and their evolution lasts from several weeks to months. The galactic rate of these events is 0.2 per year. Several
naked eye novae have been recorded,
V1500 Cygni being the brightest in the recent history, reaching 2nd magnitude in 1975. Recurrent novae are defined as having undergone more than one such event in recorded history. These tend to occur on higher mass white dwarfs and have smaller ejecta mass.
M31N 2008-12a, a recurrent nova in the
Andromeda Galaxy, erupts as often as every 12 months. It has an estimated mass close to the Chandrasekhar limit, and thus is a
Type Ia supernova progenitor candidate. There are three types of dwarf nova: •
U Geminorum stars, 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 variables after the variable in
Cygnus which produces among the brightest and most frequent displays of this variable type.
AM CVn variables AM CVn variables are symbiotic binaries where a white dwarf is accreting helium-rich material from either another white dwarf, a helium star, or an evolved main-sequence star. They can undergo complex variations, or at times no variations, with ultrashort periods. The orbital periods of these systems are in the range of minutes, with those between minutes showing outburst behavior that increases the brightness by magnitudes. The last is due to instabilities in the accretion disk.
DQ Herculis variables DQ Herculis systems are interacting binaries in which a low-mass star transfers mass to a highly magnetic white dwarf. The white dwarf spin period is significantly shorter than the binary orbital period and can sometimes be detected as a photometric periodicity. An accretion disk usually forms around the white dwarf, but its innermost regions are magnetically truncated by the white dwarf. Once captured by the white dwarf's magnetic field, the material from the inner disk travels along the magnetic field lines until it accretes. In extreme cases, the white dwarf's magnetism prevents the formation of an accretion disk.
AM Herculis variables In these cataclysmic variables, the white dwarf's magnetic field is so strong that it synchronizes the white dwarf's spin period with the binary orbital period. Instead of forming an accretion disk, the accretion flow is channeled along the white dwarf's magnetic field lines until it impacts the white dwarf near a magnetic pole. Cyclotron radiation beamed from the accretion region can cause orbital variations of several magnitudes. BY Cam-type systems are known as
asynchronous polars due to a slight (1–2%) difference between the rotation period and the orbital period. This asynchronity is believed to be caused by flare activity on the accreting white dwarf.
X-ray binaries High mass X-ray binaries consist of a
Be star or a supergiant in a relatively close orbit with a
neutron star companion. Mass is being transferred to the accreting compact object from the donor star, which results in X-ray emission. In the case of a Be star, a gaseous disk orbiting the star at the equator is responsible for the optical variability, while the interaction of the companion is truncating the disk. ==Extrinsic variable stars==