Mira variables /ˈmaɪrə/ ("Mira", Latin, adj. - feminine form of adjective "wonderful"), named for the prototype star Mira, are a class of pulsating variable stars characterized by very red colours, pulsation periods longer than 100 days, and amplitudes greater than one magnitude in infrared and 2.5 magnitude at visual wavelengths. They are red giants in the very late stages of stellar evolution, on the asymptotic giant branch, that will expel their outer envelopes as planetary nebulae and become white dwarfs within a few million years.
Mira variables are stars massive enough that they have undergone helium fusion in their cores but are less than two solar masses, stars that have already lost about half their initial mass. However, they can be thousands of times more luminous than the Sun due to their very large distended envelopes. They are pulsating due to the entire star expanding and contracting. This produces a change in temperature along with radius, both of which factors cause the variation in luminosity. The pulsation depends on the mass and radius of the star and there is a well-defined relationship between period and luminosity (and colour). The very large visual amplitudes are not due to large luminosity changes, but due to a shifting of energy output between infra-red and visual wavelengths as the stars change temperature during their pulsations.
Early models of Mira stars assumed that the star remained spherically symmetric during this process (largely to keep the computer modelling simple, rather than for physical reasons). A recent survey of Mira variable stars found that 75% of the Mira stars which could be resolved using the IOTA telescope are not spherically symmetric, a result which is consistent with previous images of individual Mira stars, so there is now pressure to do realistic three-dimensional modelling of Mira stars on supercomputers.
Mira variables may be oxygen-rich or carbon-rich. Carbon-rich stars such as R Leporis arise from a narrow set of conditions that override the normal tendency for AGB stars to maintain a surplus of oxygen over carbon at their surfaces due to dredge-ups. Pulsating AGB stars such as Mira variables undergo fusion in alternating hydrogen and helium shells, which produces periodic deep convection known as dredge-ups. These dredge-ups bring carbon from the helium burning shell to the surface and would result in a carbon star. However, in stars above about 4 M☉, hot bottom burning occurs. This is when the lower regions of the convective region are hot enough for significant CN cycle fusion to take place which destroys much of the carbon before it can be transported to the surface. Thus more massive AGB stars do not become carbon-rich.
A small subset of Mira variables appear to change their period over time: the period increases or decreases by a substantial amount (up to a factor of three) over the course of several decades to a few centuries. This is believed to be caused by thermal pulses, where the helium shell reignites the outer hydrogen shell. This changes the structure of the star, which manifests itself as a change in period. This process is predicted to happen to all Mira variables, but the relatively short duration of thermal pulses (a few thousand years at most) over the asymptotic giant branch lifetime of the star (less than a million years), means we only see it in a few of the several thousand Mira stars known, possibly in R Hydrae. Most Mira variables do exhibit slight cycle-to-cycle changes in period, probably caused by nonlinear behaviour in the stellar envelope including deviations from spherical symmetry.
Mira variables are popular targets for amateur astronomers interested in variable star observations, because of their dramatic changes in brightness. Some Mira variables (including Mira itself) have reliable observations stretching back well over a century.
|S Coronae Borealis||5.8||14.1||360||431+60
|R Leonis Minoris||6.3||13.2||372||347+653
|R Ursae Majoris||6.5||13.7||302||489+54
|R Canum Venaticorum||6.5||12.9||329||661+65
|T Ursae Majoris||6.6||13.5||257||1337+218
|V Coronae Borealis||6.9||12.6||358||843+43
|LL Pegasi||9.6 K||11.6 K||696||1300|||
|CW Leonis||11.0 R||14.8 R||640||95+22
|TX Camelopardalis||11.6 B||17.7 B||557||333+42
Horologium is a faint constellation in the southern sky. It was devised by French astronomer Nicolas Louis de Lacaille in 1752, and it remains one of the 88 modern constellations. The constellation's brightest star is Alpha Horologii, an orange giant. R Horologii is a red giant Mira variable with one of the widest ranges in brightness known. Three star systems have exoplanets, while Nu Horologii has a debris disk.II Lupi
II Lupi (IRAS 15194-5115) is a Mira variable and carbon star located in the constellation Lupus. It is the brightest carbon star in the Southern Hemisphere at 12 μm.
In 1987, the infrared source IRAS 15194-5115 was identified as an extreme carbon star. It was seen to be strongly variable at optical and infrared wavelengths. It is very faint visually, 15th or 16th magnitude in a red filter and below 21st magnitude in a blue filter, but at mid-infrared wavelengths (N band) it is the third-brightest carbon star in the sky. A star at the location had earlier been catalogued as WOS 48, a possible S-type star, on the basis of strong LaO bands in its spectrum.On the basis of infrared photometry, IRAS 15194-5115 was given the variable star designation II Lupi in 1995, although the variability type was still unknown. More detailed infrared photometry confirmed that II Lupi was a Mira variable and showed regular variations with a period of 675 days over 18 years. The mean magnitude also dimmed and brightened during that time and has been characterised as a 6,900-day secondary period although less than a full cycle was observed. The secondary period could be interpreted as an isolated or irregular obscuration event in a dust shell surrounding the star.IK Tauri
IK Tauri is a Mira variable star located about 280 parsecs (910 ly) from the Sun in the zodiac constellation of Taurus.LL Pegasi
LL Pegasi (AFGL 3068) is a Mira variable star surrounded by a pinwheel-shaped nebula, IRAS 23166+1655, thought to be a preplanetary nebula. It is a binary system that includes an extreme carbon star. The pair is hidden by the dust cloud ejected from the carbon star and is only visible in infrared light.LP Andromedae
LP Andromedae (often abbreviated to LP And) is a carbon star in the constellation Andromeda. It is also a Mira variable whose mean apparent visual magnitude is 15.12 and has pulsations with an amplitude of 1.50 magnitudes and a period of 614 days.In 1974 LP Andromedae, known then as IRC+40540, was identified as a carbon star and also shown to be variable. It had previously been suspected of variability during the 2 Micron All Sky Survey (2MASS). A detailed study of its spectrum showed an unusually cool star with a basic class of C8, and Swan band strength of 3.5. It also showed strong C13 isotopic bands. The period was narrowed down to around 614 days, one of the longest periods known for a Mira variable.This star has a dusty envelope with an estimated mass of 3.2 M☉, fueled by the star itself which is losing mass at a rate 1.9×10−5 M☉/yr. Such a high mass loss rate should place LP Andromedae close to the end of its asymptotic giant branch evolution. The envelope extends to a distance of 3 parsec from the star, and is mainly made of silicon carbide and carbon particles.List of variable stars
There are over 41,638 known variable stars (2008), with more being discovered regularly, so a complete list of every single variable is impossible at this place (cf. GCVS). The following is a list of variable stars that are well-known, bright, significant, or otherwise interesting.R Aquilae
R Aquilae is a Mira variable in the constellation Aquila. Over a period of approximately 270 days, the apparent magnitude of this star varies from 5.5 to 12. The period was over 300 days when first observed, and has declined steadily since.R Boötis
R Boötis is a Mira variable in the constellation Boötes. It ranges between magnitudes 6.2 and 13.1 over a period of approximately 223 days.R Cancri
R Cancri is a Mira variable in the constellation Cancer. Located approximately 633 parsecs (2,060 ly) distant, it varies between magnitudes 6.07 and 11.9 over a period of approximately 357 days.R Canum Venaticorum
R Canum Venaticorum is a Mira variable star in the constellation Canes Venatici. It ranges between magnitudes 6.5 and 12.9 over a period of approximately 329 days.R Centauri
R Centauri (R Cen) is a Mira variable star in the constellation Centaurus. It is approximately 1,300 light years from Earth.
R Centauri is a Mira variable and its brightness varies from magnitude +5.2 to +11.5 with a period of about 500 days. It used to have an unusual double-peaked light curve, but by 2001 this had reverted to an almost normal single-peaked curve. Prior to 1950 the period was about 550 days, but since then has decreased to about 500 days. A 2016 analysis of ASAS data derived a period of 498.84 days.It is thought that the unusual behaviour of R Centauri is caused by a flash in the helium shell around its core, which occurs periodically in asymptotic giant branch (AGB) stars as the mass of the helium shell increases with helium from the outer hydrogen shell.It is also an H2O maser.R Virginis
R Virginis is a Mira variable in the constellation Virgo. Located approximately 530 parsecs (1,700 ly) distant, it varies between magnitudes 6.1 and 12.1 over a period of approximately 146 days. Its variable nature was discovered by Karl Ludwig Harding in 1809.S Boötis
S Boötis is a Mira variable in the constellation Boötes. It ranges between magnitudes 7.8 and 13.8 over a period of approximately 270 days.S Coronae Borealis
S Coronae Borealis (S CrB) is a Mira variable star in the constellation Corona Borealis. Its apparent magnitude varies between 5.8 and 14.1, with a period of 360 days—just under a year. Within the constellation, it lies to the west of Theta Coronae Borealis, and around 1 degree southeast of the eclipsing binary star U Coronae Borealis.S Pegasi
S Pegasi (S Peg) is a Mira variable star in the constellation Pegasus. It varies between magnitude 7 and 13 with a period of 319.22 days. It is believed to be pulsating in the first overtone. First overtone pulsators have masses less than 1.8 M☉ at a temperature of 2,107 K, and less than 1.4 M☉ at the luminosity of S Pegasi.Southern Crab Nebula
The Southern Crab Nebula or Hen 2-104 is a nebula in the constellation Centaurus. The nebula is several thousand light years from Earth, and its central star is a symbiotic Mira variable - white dwarf pair. It is named for its resemblance to the Crab Nebula, which is in the northern sky.
The nebula had already been observed using Earth-based telescopes, but images taken with the Hubble Space Telescope (shown) in 1999 have provided much more detail, revealing that at the center of the nebula are a pair of stars, a red giant and a white dwarf.Symbiotic nova
Symbiotic novae are slow irregular eruptive variable stars with very slow nova-like outbursts with an amplitude of between 9 and 11 magnitudes. The symbiotic nova remains at maximum for one or a few decades, and then declines towards its original luminosity. Variables of this type are double star systems with one red giant, which probably is a Mira variable, and one white dwarf, with markedly contrasting spectra and whose proximity and mass characteristics indicate it as a symbiotic star. The red giant fills its Roche lobe so that matter is transferred to the white dwarf and accumulates until a nova-like outburst occurs, caused by ignition of thermonuclear fusion. The temperature at maximum is estimated to rise up to 200,000 K, similar to the energy source of novae, but dissimilar to the dwarf novae. The slow luminosity increase would then be simply due to time needed for growth of the ionization front in the outburst.It is believed that the white dwarf component of a symbiotic nova remains below the Chandrasekhar limit, so that it remains a white dwarf after its outburst.One example of a symbiotic nova is V1016 Cygni, whose outburst in 1971–2007 clearly indicated a thermonuclear explosion. Other examples are HM Sagittae and RR Telescopii.U Andromedae
U Andromedae is a variable star in the constellation of Andromeda. It is a star of spectral type M6e, then it is classified as a Mira variable, and varies from an apparent visual magnitude of 9.0 at maximum brightness to a magnitude of 15.0 at minimum brightness, with a period of approximately 347 days. It was first observed to be variable by Thomas D. Anderson during 1894 and 1895.W Andromedae
W Andromedae is a variable star in the constellation of Andromeda. It is classified as a Mira variable of S-type star, and varies from an apparent visual magnitude of 14.6 at minimum brightness to a magnitude of 6.7 at maximum brightness, with a period of approximately 397.3 days. The star is losing mass due to stellar winds at a rate of 2.79×10−7 M☉/yr.