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49-490: Mira is a star in the constellation Cetus Mira may also refer to: Mira ο Ceti is a binary stellar system , consisting of a variable red giant (Mira A) along with a white dwarf companion ( Mira B ). Mira A is a pulsating variable star and was the first non- supernova variable star discovered, with the possible exception of Algol . It is the prototype of the Mira variables . ο Ceti ( Latinised to Omicron Ceti )

98-516: A bright red giant with a luminosity ranging up to thousands of times greater than the Sun. Its interior structure is characterized by a central and largely inert core of carbon and oxygen, a shell where helium is undergoing fusion to form carbon (known as helium burning ), another shell where hydrogen is undergoing fusion forming helium (known as hydrogen burning ), and a very large envelope of material of composition similar to main-sequence stars (except in

147-523: A decade or more, and an amount of time on the order of 10,000 years passes between each pulse. With every pulse cycle Mira increases in luminosity and the pulses grow stronger. This is also causing dynamic instability in Mira, resulting in dramatic changes in luminosity and size over shorter, irregular time periods. The overall shape of Mira A has been observed to change, exhibiting pronounced departures from symmetry. These appear to be caused by bright spots on

196-510: A factor of four times in luminosity. The total swing in brightness from absolute maximum to absolute minimum (two events which did not occur on the same cycle) is 1,700 times. Mira emits the vast majority of its radiation in the infrared , and its variability in that band is only about two magnitudes. The shape of its light curve is of an increase over about 100 days, and the return to minimum taking twice as long. Contemporary approximate maxima for Mira: From northern temperate latitudes, Mira

245-418: A few years. The shell flash causes the star to expand and cool which shuts off the hydrogen shell burning and causes strong convection in the zone between the two shells. When the helium shell burning nears the base of the hydrogen shell, the increased temperature reignites hydrogen fusion and the cycle begins again. The large but brief increase in luminosity from the helium shell flash produces an increase in

294-407: A lighter orange as the star brightens. Within the next few million years, Mira will discard its outer layers and become a planetary nebula, leaving behind a white dwarf. This binary star system consists of a red giant (Mira, designated Mira A) undergoing mass loss and a high-temperature white dwarf companion (Mira B) that is accreting mass from the primary. Such an arrangement of stars is known as

343-417: A previously unremarked third-magnitude star nearby. By August 21, however, it had increased in brightness by one magnitude , then by October had faded from view. Fabricius assumed it was a nova, but then saw it again on February 16, 1609. In 1638 Johannes Holwarda determined a period of the star's reappearances, eleven months; he is often credited with the discovery of Mira's variability. Johannes Hevelius

392-494: A regular star. There are three observations from Chinese and Korean archives, in 1596, 1070 and the same year when Hipparchus would have made his observation (134 BC) that are suggestive. An estimate obtained in 1925 from interferometry by Francis G. Pease at the Mount Wilson Observatory gave Mira a diameter of 250-260 million miles (402 to 418 million km, or approximately 290-300  R ☉ ), making it

441-537: A symbiotic system and this is the closest such symbiotic pair to the Sun . Examination of this system by the Chandra X-ray Observatory shows a direct mass exchange along a bridge of matter from the primary to the white dwarf. The two stars are currently separated by about 70  astronomical units . Mira A is currently an asymptotic giant branch (AGB) star, in the thermally pulsing AGB phase. Each pulse lasts

490-473: A thermal pulse occurs and the star quickly returns to the AGB, becoming a helium-burning, hydrogen-deficient stellar object. If the star still has a hydrogen-burning shell when this thermal pulse occurs, it is termed a "late thermal pulse". Otherwise it is called a "very late thermal pulse". The outer atmosphere of the born-again star develops a stellar wind and the star once more follows an evolutionary track across

539-478: A trail of material from the outer envelope, leaving a tail 13 light-years in length, formed over tens of thousands of years. It is thought that a hot bow wave of compressed plasma/gas is the cause of the tail; the bow wave is a result of the interaction of the stellar wind from Mira A with gas in interstellar space, through which Mira is moving at an extremely high speed of 130 kilometres per second (290,000 miles per hour). The tail consists of material stripped from

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588-445: A transition to the more massive supergiant stars that undergo full fusion of elements heavier than helium. During the triple-alpha process , some elements heavier than carbon are also produced: mostly oxygen, but also some magnesium, neon, and even heavier elements. Super-AGB stars develop partially degenerate carbon–oxygen cores that are large enough to ignite carbon in a flash analogous to the earlier helium flash. The second dredge-up

637-555: A white dwarf as originally thought. However, in 2010 further research indicated that Mira B is, in fact, a white dwarf. Asymptotic giant branch The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars . This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses ) late in their lives. Observationally, an asymptotic-giant-branch star will appear as

686-424: Is a maximum value since the wind material will start to mix with the interstellar medium at very large radii, and it also assumes that there is no velocity difference between the star and the interstellar gas . These envelopes have a dynamic and interesting chemistry , much of which is difficult to reproduce in a laboratory environment because of the low densities involved. The nature of the chemical reactions in

735-469: Is almost aligned with its previous red-giant track, hence the name asymptotic giant branch , although the star will become more luminous on the AGB than it did at the tip of the red-giant branch. Stars at this stage of stellar evolution are known as AGB stars. The AGB phase is divided into two parts, the early AGB (E-AGB) and the thermally pulsing AGB (TP-AGB). During the E-AGB phase, the main source of energy

784-494: Is calculated to be 9,360 ± 3,140  L ☉ at phase 0.13 and 8,400 ± 2,820  L ☉ at phase 0.26. The pulsations of Mira have the effect of expanding its photosphere by around 50% compared to a non-pulsating star. In the case of Mira, if it was not pulsating it is modelled to have a radius of only around 240  R ☉ . Ultraviolet studies of Mira by NASA 's Galaxy Evolution Explorer ( GALEX ) space telescope have revealed that it sheds

833-607: Is considerable speculation as to whether Mira had been observed prior to Fabricius. Certainly Algol 's history (known for certain as a variable only in 1667, but with legends and such dating back to antiquity showing that it had been observed with suspicion for millennia) suggests that Mira might have been known, too. Karl Manitius , a modern translator of Hipparchus ' Commentary on Aratus , has suggested that certain lines from that second-century text may be about Mira. The other pre-telescopic Western catalogs of Ptolemy , al-Sufi , Ulugh Beg and Tycho Brahe turn up no mentions, even as

882-414: Is generally not visible between late March and June due to its proximity to the Sun. This means that at times several years can pass without it appearing as a naked-eye object. The pulsations of Mira variables cause the star to expand and contract, but also to change its temperature. The temperature is highest slightly after the visual maximum, and lowest slightly before minimum. The photosphere, measured at

931-470: Is helium fusion in a shell around a core consisting mostly of carbon and oxygen . During this phase, the star swells up to giant proportions to become a red giant again. The star's radius may become as large as one astronomical unit (~215  R ☉ ). After the helium shell runs out of fuel, the TP-AGB starts. Now the star derives its energy from fusion of hydrogen in a thin shell, which restricts

980-485: Is the star's Bayer designation . It was named Mira ( Latin for 'wonderful' or 'astonishing') by Johannes Hevelius in his Historiola Mirae Stellae (1662). In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by

1029-506: Is very strong in this mass range and that keeps the core size below the level required for burning of neon as occurs in higher-mass supergiants. The size of the thermal pulses and third dredge-ups are reduced compared to lower-mass stars, while the frequency of the thermal pulses increases dramatically. Some super-AGB stars may explode as an electron capture supernova, but most will end as oxygen–neon white dwarfs. Since these stars are much more common than higher-mass supergiants, they could form

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1078-423: The Hertzsprung–Russell diagram . However, this phase is very brief, lasting only about 200 years before the star again heads toward the white dwarf stage. Observationally, this late thermal pulse phase appears almost identical to a Wolf–Rayet star in the midst of its own planetary nebula . Stars such as Sakurai's Object and FG Sagittae are being observed as they rapidly evolve through this phase. Mapping

1127-473: The Rosseland radius , is smallest just before visual maximum and close to the time of maximum temperature. The largest size is reached slightly before the time of lowest temperature. The bolometric luminosity is proportional to the fourth power of the temperature and the square of the radius, but the radius varies by over 20% and the temperature by less than 10%. In Mira, the highest luminosity occurs close to

1176-475: The photosphere of the stars which are 2,000 – 3,000 K . Chemical peculiarities of an AGB CSE outwards include: The dichotomy between oxygen -rich and carbon -rich stars has an initial role in determining whether the first condensates are oxides or carbides, since the least abundant of these two elements will likely remain in the gas phase as CO x . In the dust formation zone, refractory elements and compounds ( Fe , Si , MgO , etc.) are removed from

1225-498: The 2007 reduction suggest a distance of 299 light-years, with a margin of error of 11%. The age of Mira is suspected to be about 6 billion years old. Its gaseous material is scattered, as much as one-thousandth as thin as the air around us. Mira is also among the coolest known bright stars of the red giant class, with a temperature ranging from 3,000 to 4,000 degrees Fahrenheit (1,600 to 2,200 degrees Celsius). As with other long-period variables, Mira's deep red color at minimum pales to

1274-493: The AGB phase. The mass-loss rates typically range between 10 and 10 M ⊙ year , and can even reach as high as 10 M ⊙ year ; while wind velocities are typically between 5 and 30 km/s. The extensive mass loss of AGB stars means that they are surrounded by an extended circumstellar envelope (CSE). Given a mean AGB lifetime of one Myr and an outer velocity of 10  km/s , its maximum radius can be estimated to be roughly 3 × 10   km (30 light years ). This

1323-489: The WGSN, which included Mira for this star. Evidence that the variability of Mira was known in ancient China , Babylon or Greece is at best only circumstantial. What is certain is that the variability of Mira was recorded by the astronomer David Fabricius beginning on August 3, 1596. Observing what he thought was the planet Mercury (later identified as Jupiter ), he needed a reference star for comparing positions and picked

1372-554: The case of carbon stars ). When a star exhausts the supply of hydrogen by nuclear fusion processes in its core, the core contracts and its temperature increases, causing the outer layers of the star to expand and cool. The star becomes a red giant, following a track towards the upper-right hand corner of the HR diagram. Eventually, once the temperature in the core has reached approximately 3 × 10   K , helium burning (fusion of helium nuclei) begins. The onset of helium burning in

1421-399: The circumstellar magnetic fields of thermal-pulsating (TP-) AGB stars has recently been reported using the so-called Goldreich-Kylafis effect . Stars close to the upper mass limit to still qualify as AGB stars show some peculiar properties and have been dubbed super-AGB stars. They have masses above 7  M ☉ and up to 9 or 10  M ☉ (or more ). They represent

1470-442: The core halts the star's cooling and increase in luminosity, and the star instead moves down and leftwards in the HR diagram. This is the horizontal branch (for population II stars ) or a blue loop for stars more massive than about 2.3  M ☉ . After the completion of helium burning in the core, the star again moves to the right and upwards on the diagram, cooling and expanding as its luminosity increases. Its path

1519-415: The core regions remain, they evolve further into short-lived protoplanetary nebula . The final fate of the AGB envelopes are represented by planetary nebulae (PNe). Physical samples, known as presolar grains, of mineral grains from AGB stars are available for laboratory analysis in the form of individual refractory presolar grains . These formed in the circumstellar dust envelopes and were transported to

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1568-432: The direction of Mira B. The companion's orbital period around Mira is approximately 400 years. In 2007, observations showed a protoplanetary disc around the companion, Mira B. This disc is being accreted from material in the solar wind from Mira and could eventually form new planets. These observations also hinted that the companion was a main-sequence star of around 0.7 solar mass and spectral type K, instead of

1617-493: The dust no longer completely shields the envelope from interstellar UV radiation and the gas becomes partially ionized. These ions then participate in reactions with neutral atoms and molecules. Finally as the envelope merges with the interstellar medium, most of the molecules are destroyed by UV radiation. The temperature of the CSE is determined by heating and cooling properties of the gas and dust, but drops with radial distance from

1666-409: The early Solar System by stellar wind . A majority of presolar silicon carbide grains have their origin in 1–3 M ☉ carbon stars in the late thermally-pulsing AGB phase of their stellar evolution. As many as a quarter of all post-AGB stars undergo what is dubbed a "born-again" episode. The carbon–oxygen core is now surrounded by helium with an outer shell of hydrogen. If the helium is re-ignited

1715-418: The envelope changes as the material moves away from the star, expands and cools. Near the star the envelope density is high enough that reactions approach thermodynamic equilibrium. As the material passes beyond about 5 × 10   km the density falls to the point where kinetics , rather than thermodynamics, becomes the dominant feature. Some energetically favorable reactions can no longer take place in

1764-484: The first few, so third dredge-ups are generally the deepest and most likely to circulate core material to the surface. AGB stars are typically long-period variables , and suffer mass loss in the form of a stellar wind . For M-type AGB stars, the stellar winds are most efficiently driven by micron-sized grains. Thermal pulses produce periods of even higher mass loss and may result in detached shells of circumstellar material. A star may lose 50 to 70% of its mass during

1813-524: The first time, has been shed over the past 30,000 years. The companion star is 0.487 ± 0.006  arcseconds away from the main star. It was resolved by the Hubble Space Telescope in 1995, when it was 70 astronomical units from the primary; and results were announced in 1997. The HST ultraviolet images and later X-ray images by the Chandra space telescope show a spiral of gas rising off Mira in

1862-414: The formation of carbon stars . All dredge-ups following thermal pulses are referred to as third dredge-ups, after the first dredge-up, which occurs on the red-giant branch, and the second dredge up, which occurs during the E-AGB. In some cases there may not be a second dredge-up but dredge-ups following thermal pulses will still be called a third dredge-up. Thermal pulses increase rapidly in strength after

1911-788: The gas phase and end up in dust grains . The newly formed dust will immediately assist in surface catalyzed reactions . The stellar winds from AGB stars are sites of cosmic dust formation, and are believed to be the main production sites of dust in the universe. The stellar winds of AGB stars ( Mira variables and OH/IR stars ) are also often the site of maser emission . The molecules that account for this are SiO , H 2 O , OH , HCN , and SiS . SiO, H 2 O, and OH masers are typically found in oxygen-rich M-type AGB stars such as R Cassiopeiae and U Orionis , while HCN and SiS masers are generally found in carbon stars such as IRC +10216 . S-type stars with masers are uncommon. After these stars have lost nearly all of their envelopes, and only

1960-409: The gas, because the reaction mechanism requires a third body to remove the energy released when a chemical bond is formed. In this region many of the reactions that do take place involve radicals such as OH (in oxygen rich envelopes) or CN (in the envelopes surrounding carbon stars). In the outermost region of the envelope, beyond about 5 × 10   km , the density drops to the point where

2009-545: The head of the bow wave, which is also visible in ultraviolet observations. Mira's bow shock will eventually evolve into a planetary nebula , the form of which will be considerably affected by the motion through the interstellar medium (ISM). Mira’s tail offers a unique opportunity to study how stars like our sun die and ultimately seed new solar systems. As Mira hurls along, its tail drops off carbon, oxygen and other important elements needed for new stars, planets, and possibly even life to form. This tail material, visible now for

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2058-405: The inner helium shell to a very thin layer and prevents it fusing stably. However, over periods of 10,000 to 100,000 years, helium from the hydrogen shell burning builds up and eventually the helium shell ignites explosively, a process known as a helium shell flash . The power of the shell flash peaks at thousands of times the observed luminosity of the star, but decreases exponentially over just

2107-531: The particular case of Mira, its increases in brightness take it up to about magnitude 3.5 on average, placing it among the brighter stars in the Cetus constellation . Individual cycles vary too; well-attested maxima go as high as magnitude 2.0 in brightness and as low as 4.9, a range almost 15 times in brightness, and there are historical suggestions that the real spread may be three times this or more. Minima range much less, and have historically been between 8.6 and 10.1,

2156-505: The surface that evolve their shape on time scales of 3–14 months. Observations of Mira A in the ultraviolet band by the Hubble Space Telescope have shown a plume-like feature pointing toward the companion star. Mira A is a variable star , specifically the prototypical Mira variable . The 6,000 to 7,000 known stars of this class are all red giants whose surfaces pulsate in such a way as to increase and decrease in brightness over periods ranging from about 80 to more than 1,000 days. In

2205-428: The then-second largest star known and comparable to historical estimates of Betelgeuse , surpassed only by Antares . On the contrary, Otto Struve thought of Mira as a red supergiant with an approximate radius of 500  R ☉ , while modern consensus accepts Mira to be a highly evolved asymptotic giant branch star. Pre- Hipparcos estimates centered on 220 light-years ; while Hipparcos data from

2254-401: The time when the star is hottest and smallest. The visual magnitude is determined both by the luminosity and by the proportion of the radiation that occurs at visual wavelengths. Only a small proportion of the radiation is emitted at visual wavelengths and this proportion is very strongly influenced by the temperature ( Planck's law ). Combined with the overall luminosity changes, this creates

2303-508: The very big visual magnitude variation with the maximum occurring when the temperature is high. Infrared VLTI measurements of Mira at phases 0.13, 0.18, 0.26, 0.40 and 0.47, show that the radius varies from 332 ± 38  R ☉ at phase 0.13 just after maximum to 402 ± 46  R ☉ at phase 0.40 approaching minimum. The temperature at phase 0.13 is 3,192 ± 200  K and 2,918 ± 183 K at phase 0.26 about halfway from maximum to minimum. The luminosity

2352-553: The visible brightness of the star of a few tenths of a magnitude for several hundred years. These changes are unrelated to the brightness variations on periods of tens to hundreds of days that are common in this type of star. During the thermal pulses, which last only a few hundred years, material from the core region may be mixed into the outer layers, changing the surface composition, in a process referred to as dredge-up . Because of this dredge-up, AGB stars may show S-process elements in their spectra and strong dredge-ups can lead to

2401-420: Was observing it at the same time and named it Mira in 1662, for it acted like no other known star. Ismail Bouillaud then estimated its period at 333 days, less than one day off the modern value of 332 days. Bouillaud's measurement may not have been erroneous: Mira is known to vary slightly in period, and may even be slowly changing over time. The star is estimated to be a six-billion-year-old red giant . There

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