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 the R136 super star cluster, which is located near the center of the Tarantula Nebula (Caldwell 103), near the southeast corner of the Large Magellanic Cloud satellite galaxy. It is the most massive star known, at over 250 solar masses (M☉), and the most luminous, at some seven million times the luminosity of the Sun (L☉).
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 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. 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.
The star is classified as a Wolf-Rayet star, which means that it is extremely hot and shows surface enhancement of heavy elements. 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☉.
R136a1 is positioned 165,000 light years away from Earth in the R136 cluster in the Tarantula Nebula, also known as 30 Doradus.
R136a1 is located in R136a, the core of R136. This dense luminous knot of stars contains 24 resolved components, all of which are extremely luminous and massive. R136a1 is positioned 5000 AU away from R136a2, the second brightest star in the cluster. R136a2 itself is among the most massive and luminous stars known, at 179 M☉ and 4,900,000 L☉
R136a1 currently has the largest known mass for a star at 256 M☉, and it is calculated that R136a1 had a birth mass around 320M☉. However, in the formation of stars over 150 M☉ a number of obstacles arise. One theory to explain rare ultra-massive stars that exceed this limit is the collision and merger of multiple stars in a close binary system.
Mass loss is one of the most intensively studied aspects of massive star research. Put simply, using observed mass loss rates in the best models of stellar evolution do not reproduce the observed distribution of evolved massive stars such as Wolf-Rayets, the number and types of core collapse supernovae, or their progenitors. To match those observations, the models require much higher mass loss rates. R136a1 has a mass loss rate of 5.1 × 10-5 M☉/year (3.2×1018 kg per second) and a stellar wind of 2,400km/s. Since its birth, the star is thought to have shed over 60 M☉.
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. The star is only 59,000 L☉ in the visible light. However, the star has the very high surface temperature of 56,000 K (55,700 °C; 100,300 °F) that gives it a the B-V color index of +0.17 so, in accordance with the Stefan-Boltzmann law most of the power output of the star is in the ultraviolet region of the electromagnetic spectrum and not in 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), more than ten times hotter than the sun.
Because of the star's extreme luminosity and high temperature, it can be described as a "ultraviolet star", meaning that the color is primarily in the ultraviolet and not in the blue. 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 volume of 25,000 suns. Although this volume seems to be quite large, some red supergiants and hypergiants such as UY Scuti, NML Cygni, and VV Cephei A have volumes exceeding a billion times the sun.
Despite the star's high mass, it has an average density of only 0.06612 kg/m3.
Although the star’s metallicity has not been measured, the average metallicity of 30 Doradus has been described as 0.4 Z☉ so the star’s metallicity is expected be similar.
The star is classified as a WN5h Wolf-Rayet star. The spectral type of WN signifies that the spectrum is dominated by nitrogen emission lines. The numeral 5 indicates the presence of NIII, NIV, and NV lines all at similar intensities. The spectral suffix “h” denotes the presence of hydrogen emission lines.
The star's spectrum and location on the H-R diagram suggests that it is approximately 1.7 Myr old. Its high mass creates such a high pressure and temperature in the core that hydrogen is fused at a very high rate, resulting in the extreme luminosity. Because of the high rate of fusion, it is expected that R136a1 will run out of fuel within a million years or so and then will explode as a supernova or hypernova.
Stars more than about 8 M☉ explode at the end of their lives as supernovae, leaving behind neutron stars or black holes. The future of R136a1 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 or a hypernova. 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 and leave behind a black hole.
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