Epoch J2000 Equinox J2000
|Right ascension||05h 38m 42.45s|
|Declination||−69° 06′ 02.2″|
|Apparent magnitude (V)||12.34|
|Evolutionary stage||Wolf-Rayet star|
|B−V color index||0.23|
|Absolute magnitude (MV)||-6.50|
|Luminosity (visual, LV)||32,000 L☉|
Cl* NGC 2070 MH 511, RMC 136a2, HSH95 5, BAT99 109, CHH92 2
R136a2 (RMC 136a2) is a Wolf-Rayet star located near the center of the R136 super star cluster, which is located near the center of the Tarantula Nebula (Caldwell 103), near the SE corner of the Large Magellanic Cloud satellite galaxy. It is one of the most massive stars known, at almost 180 solar masses (M☉), and one of the most luminous, at some five million times the luminosity of the Sun (L☉). It is located near R136a1, an even more massive and more luminous star.
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 Catalogue, 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.  R136a2 was marginally the second 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.
Like all Wolf-Rayet stars, R136a2 is undergoing severe mass loss by a fast stellar wind. The star loses 4.6×10-5 solar masses per year through a stellar wind with a speed of 2400 km/s. The high mass of the star compresses the core and ignites fusion at a high rate using the CNO process, leading to an extremely high luminosity of 4,900,000L☉. The fusion rate is so great that in 10 seconds R136a2 produces more energy than the Sun does in a year. It may have been a 240 M☉ star at the time it was born and lost as much as 40 M☉ in the past 1 to 2 million years, but since current theories suggest that no stars can be born above 150 M☉, it may be a merger of two or more stars. Although the star is one of the most massive known, it has a volume of 13,000 suns and a radius of 23.4 R☉. It also does not emit much visual light, only 32,000 L☉ (mag -6.5).
The future of R136a2 depends on its mass loss. It is thought that stars this massive will never lose enough mass to avoid a catastrophic end. The result is likely to be a supernova, hypernova, gamma-ray burst, or perhaps almost no visible explosion, and leaving behind a black hole or neutron star. The exact details depend heavily on the timing and amount of the mass loss, with current models not fully reproducing observed stars, but the most massive stars in the local universe are expected to produce type Ib or Ic supernovae, sometimes with a gamma-ray burst, and leave behind a black hole.
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