Epoch J2000.0 Equinox J2000.0
|Right ascension||5h 38m 42.43s|
|Declination||−69° 06′ 02.2″|
|Apparent magnitude (V)||12.28|
|Evolutionary stage||Wolf-Rayet star|
|B−V color index||+0.17|
|Absolute magnitude (MV)||-7.10|
|Luminosity (visual, LV)||59,000 L☉|
BAT99 108, RMC 136a1, HSH95 3, WO84 1b, Cl* NGC 2070 MH 498, CHH92 1, P93 954.
R136a1 (RMC 136a1) is a Wolf-Rayet star located near the center of R136, the central condensation of stars of the large NGC 2070 open cluster in the Tarantula Nebula, a giant H II region in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. It has the highest confirmed mass and bolometric luminosity of any known star, at 256 M☉ and 7.4 million L☉, as well as one of the highest surface temperatures of any main sequence star, at more than 55,000 K. It lies at a distance of about 48 kiloparsecs (157,000 light-years) based on the currently accepted distance to the Tarantula Nebula. Because of its large mass, it is one of the candidates for a potential supernova or hypernova in the astronomically near future.
It was only in 2010 that the star was recognized as the most massive and luminous star known. Previous estimates placed the luminosity as low as 1,500,000 L☉.
In 1960, a group of astronomers working at the Radcliffe Observatory in Pretoria made systematic measurements of the brightness and spectra of bright stars in the Large Magellanic Cloud. Among the objects cataloged was RMC 136, (Radcliffe observatory Magellanic Cloud catalog number 136) the central "star" of 30 Doradus. Subsequent observations showed that R136 was located in the center of a giant H II region that was a center of intense star formation in the immediate vicinity of the observed stars.
In the early 1980s, R136a was first resolved using speckle interferometry into 8 components. R136a1 was marginally the brightest found within 1 arc-second at the centre of the R136 cluster. Previous estimates that the brightness of the central region would require as many as 30 hot O class stars within half a parsec at the centre of the cluster had led to speculation that a star several thousand times the mass of the sun was the more likely explanation.  Instead it was eventually found that it consisted of a few extremely luminous stars accompanied by a larger number of hot O stars.
R136a1 is visible below the southern half of the equator within the magnitude 0.9 Large Magellanic Cloud in the constellation of Dorado. It is located at the center of the Tarantula Nebula on the south-east corner of the Large Magellanic Cloud. It lies within R136, the central condensation of stars in the NGC 2070 open cluster at the heart of the Tarantula Nebula. The star itself is only magnitude 12, so it is not visible by binoculars. However, most stars within R136a are extremely luminous and are also packed together so most instruments cannot resolve them, with the exception of speckle interferometry. However, if viewed through speckle interferometry (or other equipment similar to it) , the star would be the brightest object in R136a by far, and would also appear to be merged with the star R136a2, since they are so luminous and close together.
The distance of R136a1 is not very well known but is assumed to be 157 kly (48.5 kpc). Determining a precise distance to it is challenging due to many factors. Its home galaxy, the LMC, is too remote for its distance to be measured using parallax methodology. Measurement of the star itself is also unsuitable due to the fact that it is surrounded by several bright stars and cannot be resolved. However, the distance can be obtained by measuring other objects close to R136a1 and that are assumed to be at the same distance. Cepheid variables, known for their regular pulsating period, can provide an unusual method to measure the distance to the LMC and, accordingly, the distance to R136a1 itself. However, the Cepheids in the LMC differ from the Milky Way's due to the fact that the metallicity of the LMC is different from the metallicity of our galaxy, making this method unsuitable. However, the Cepheids were recently recalibrated using Cepheids from the galaxy Messier 106, whose Cepheid metallicities vary between that of the Milky Way and of the LMC. Using this newly corrected value, a team of astronomers obtained a distance of 48 kpc to the LMC.
In March 2013, a study used Algol variables to yield a distance of 49.97 kpc to the LMC. The distance of R136 itself was measured to be about 48.2 kpc using Algol variables in the cluster with a margin of error of 5%.[verification needed]
The distances to stars can be measured by using the star's position on the H-R diagram or color-color diagram to figure the star's absolute magnitude, for example fitting the main sequence or identifying features such as a horizontal branch, and hence their distance from Earth. It is also necessary to know the amount of interstellar extinction to the star and this is difficult in regions such as the LMC.
R136a1 is located 157,000 light years away from Earth in the Large Magellanic Cloud. It is positioned on the south-east corner of the galaxy in a cluster called R136 at the centre of the Tarantula Nebula, also known as 30 Doradus. R136 itself is just the central condensation of the much larger NGC 2070 open cluster.
R136a1 is a component of the R136a system at the core of R136. This dense luminous knot of stars contains 24 resolved components, (the most dominant being R136a1, R136a2, R136a3, and R136c) all of which are extremely luminous and massive. R136a1 is separated from R136a2, the second brightest star in the cluster, by 5,000 AU. R136a2 itself is among the most massive and luminous stars known, at 179 M☉ and 4,900,000 L☉.
The star is classified as a WN5h Wolf-Rayet star. It has the surface temperature of 56,000 K; the same as that of a lightning bolt. It has a mass of 256 M☉, a luminosity of 7.4 million L☉, and a radius of 28.8 R☉.
X-ray emission was detected from R136a1 using the Chandra satellite. This instrument cannot resolve R136a1 due to the crowded environment, so it is treated as a single star, although the X-rays could indicate colliding winds in a binary system.
R136a1, as well as any stars in R136 over 150 M☉, are not claimed to be single. However, if the 150 M☉ limit were to (approximately) remain valid, an equal-mass binary system would have to be adopted for R136a1. Rapid doppler radial velocity variations would be expected from a pair of equal mass stars in a close orbit. However, infrared observations did not detect this, ruling out a pair of equal mass stars in a close orbit. However, models show that a pair of equal mass stars in a intermediate orbit would each possess very powerful stellar winds, which would collide and produce bright X-rays. Calculations show that a pair of equal mass stars in a wide orbit would interact with other high mass stars within the centre of R136a which would reduce the separation of the stars and cause them to spiral together into an intermediate separation leading to the emission of X-rays. However, no X-rays were detected in R136a1, ruling out a equal-mass binary system.  If R136a1 were a binary system, the primary's mass would be reduced by 10%.
R136a1 is classified as a Wolf-Rayet star due to its emission of ionized nitrogen and helium lines. This type of massive stars is characterized by the ionized lines of nitrogen, carbon or oxygen and the strong, broad emission lines in their spectra, mostly with helium, nitrogen, carbon, silicon, and oxygen, but with hydrogen lines usually weak or absent.
R136a1 is not a typical Wolf-Rayet star. It is classified as "WNh", which shows a spectrum similar to WN plus the enhancement of hydrogen. Despite the similar spectra, the WNh stars differ much from the "normal" WN stars. They are much younger, enriched in hydrogen, and are some of the most massive and luminous stars known, They still show strong lines of hydrogen in their spectra and are much younger than any other Wolf-Rayet star, sometimes even being called the main sequence.
WNh stars show lines of hydrogen and nitrogen their spectra, the nitrogen being the product of CNO cycle fusion in the core. It appears at the surface of the most massive stars due to rotational and convectional mixing while still in the core hydrogen burning phase, rather than after the outer envelope is lost during core helium fusion. Their hydrogen in their spectra is due to the fact that these stars are actually much less evolved than other Wolf-Rayet stars and so still retain much of their hydrogen. This holds true for R136a1, whose surface hydrogen abundance accounts for 30-40% by mass.
Atmospheric and stellar evolutionary models used to calculate the mass of R136a1 may have systematic uncertainties. However, model-independent techniques, such as a binary star measurement, exist. R136a1 is not a binary system, so its mass cannot be measured accurately. However, the atmospheric and stellar evolutionary models used to calculate R136a1’s mass was tested using a binary system known as NGC 3603 A1. The masses measured for NGC 3603 A1 using the atmospheric and stellar evolutionary models closely agree with the binary star measurement, arguing against systematic uncertainties for the atmospheric and stellar evolutionary models.
The acquiring of reliable stellar masses requires knowledge of certain physical properties; the temperature, bolometric luminosity, and evolutionary state of the star. To determine if the masses are reliable, models are needed that describe how stars change their physical properties as they age. The deduced stellar properties, plus its surface hydrogen abundance matched evolutionary models calculated for rotating, main-sequence LMC-metallicity stars.
R136a1 is undergoing extreme mass loss through a fast stellar wind. The process through which this occurs is that its large and dense wind region surrounding its photosphere creates a large amount of UV radiation which causes the star’s outer layers to be stripped away in succession. It loses 5 × 10-5 M☉/year (3.15 × 1018 kg/s) and has a stellar wind of 2,400 km/s. It is expected to have shed over 60 M☉ since its formation. If its mass loss rate had stayed constant over the years, than it would have already shed 75 M☉. However, star's mass loss rates increase over age, so the amount of mass lost by R136a1 is valid.
The star's high mass compresses its core and ignites fusion using the CNO cycle which results in a tremendous amount of energy being released and the consumption of fuel at a large rate. Visually, the star is only 59,000 L☉. However, it has the very high surface temperature of 56,000 K (55,700 °C; 100,300 °F) so, in accordance with the Stefan-Boltzmann law most of the power output of the star is in the near ultraviolet region of the electromagnetic spectrum and not in the visible light. When the power output is measured in all wavelengths the star is 7,400,000 L☉ and it alone provides ~7% of the ionizing flux of the entire 30 Doradus region.
The star was described to have a temperature of 56,000 K (55,700 °C; 100,300 °F), nearly ten times hotter than the sun. Because of its extreme surface temperature, its energy temperature (Boltzmann constant multiplied by its kelvin temperature) is about 4.57 eV, so most of the star's color is in the near ultraviolet range. and not in the blue part of the visible light. However, if viewed in the visual band, the star would appear to be dark blue (+0.17 on the B-V color index).
The size of R136a1 was derived to be 28.8 times the size of the sun (20,030,400 km; 12,446,300 mi) which corresponds to a diameter that is 3168 times larger than Earth and a volume of 24,000 suns, or approximately 31,000,000,000 Earths (3.366 × 1022 km3). It is approximately 94 times less dense than the sun and has an average density of 14.89 kg/m3.
R136a1 is a rare object, with no other stars being close to its mass and luminosity. A star like this is bound to explode as a supernovae or hypernovae, with no other possible outcome, but will exhibit extreme mass loss before undergoing a large explosion. However, current models do not fully reproduce stars as massive as this, so its future is highly uncertain.
It is thought that no star can be formed over 150 M☉ so stars like R136a1 are thought to be "impossible". However, recent computer simulations suggest that instead of forming through the accretion of gas in a nebula, stars like this one were formed through the collision and merger of multiple stars.
A typical ZAMS appearance would be a 320 M☉ star with a spectral type of O2If  and an extremely hot surface, although the temperature is not expected to have differed much from now. Rapid fusion of hydrogen through the CNO cycle would result in a strongly convective star, with most of the elements fused in its core travelling up to the surface. This process is enhanced by rotation, or possibly by differential rotation where the core is spun up to a faster rotation than the surface. This would result in a strong nitrogen enhancement at a very early age as well as strong stellar winds which would lead to increasing levels of nitrogen at the star's surface. Because of this process, the star would eventually transition from O to Of. If the star was sufficiently hot, it would have developed an Of* spectrum, which would quickly transition to WNh. The total amount of time spent as a O star before showing a nitrogen appearance would be a few thousand years, even before it became visible through the surrounding gas nebula. R136a1 is currently on the top of the main sequence and shows a nitrogen emission line profile.
WNh stars are an early stage in the evolution of very massive stars. If mixing is efficient enough (e.g. through rapid rotation) they may progress directly to WNL stars without hydrogen. This process would continue with heavier elements being fused until an iron core develops, at which point the core collapses and the star is destroyed. Subtle differences in initial conditions, in the models themselves, and most especially in the rates of mass loss, produce different predictions for the final state of the most massive stars. They may survive to become a bare carbon-oxygen core with a WO spectrum or they may collapse at an earlier stage while the star retains more of its outer layers. In total, the mass lost by R136a1 will be large, and the star will be a fraction of its initial mass at core collapse.
Fundamental properties of supernovae depend on the initial mass of their progenitors. For a sufficiently massive star, the radiation-driven mass loss will have a large effect on its evolution. Mass loss would certainly have a large effect on a star with an initial mass of ~300 M☉, as with R136a1, which has already shed 60 M☉. Mass would also have a large effect, as it governs the lifetime and properties of a star. A 260 M☉ star with an age of ~1.7 Myr would be undergoing hydrogen fusion through the CNO cycle with several stages of nuclear fusion to undergo before it explodes as a supernova or hypernova. The total lifetime would be approximately 2.5-3 Myr, with the star shedding a large amount of mass through its stellar wind. However, the fate of massive stars like this is highly uncertain, but evolutionary tracks can be estimated due to the current properties of the star.
Supernovae properties are expected to depend on the mass of the star at core collapse, which, in the case of R136a1, is expected to be below 100 M☉. The final stage of R136a1 would probably be a WO type star with a temperature of 200,000 K.
Gamma-ray bursts are unlikely when this type of star reaches core collapse because rotation rates decrease significantly due to mass loss and envelope inflation. At low metallicity, many massive stars will collapse directly to a black hole with no visible explosion or a sub-luminous supernova, and a small fraction will produce a pair instability supernova, but at solar metallicity and above there is expected to be sufficient mass loss before collapse to allow a visible supernova of type Ib or Ic.
According to independent calculations R136a1 may be composed of material that is too rich in `metals' (elements other than hydrogen or helium) to undergo a pair-instability supernova.
Most Wolf-Rayet stars explode as type Ib or Ic supernovae, which are divided into to two categories; the luminous type, at M = -22; and the normal, at M = -16 to -17. R136a1 is most likely to explode as the luminous type and leave behind a black hole.
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