Epoch J2000.0 Equinox J2000.0
|Right ascension||5h 38m 42.43s|
|Declination||−69° 06′ 02.2″|
|Apparent magnitude (V)||12.28|
|B−V color index||+0.17|
|Absolute magnitude (MV)||-7.10|
|Luminosity||7,400,000  L☉|
|Luminosity (visual, LV)||59,000  L☉|
|Temperature||56,000  K|
|Age||~1.7  Myr|
BAT99 108, RMC 136a1, HSH95 3, WO84 1b, Cl* NGC 2070 MH 498, CHH92 1, P93 954.
R136a1 is a Wolf Rayet star in the Large Magellanic Cloud, some 165,000 light years away. It is located near the core of the super star cluster R136 in the Tarantula Nebula. It is the most massive and the most luminous star known, at 256M☉ and 7,400,000 L☉ respectively.
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. In 1992, R136a was resolved into 12 components. Only three of them (including R136a1) were of the W-R type.  The next year, a survey conducted by Joel W. Parker observed 2400 stars within 50 arcseconds. The survey's goal was to resolve and study the OB associations in the LMC. Since R136 was known to have a number of massive stars, it was targeted. All of the stars in R136a were resolved, including R136a1. The star was cataloged as a binary consisting of a OB and a WN4.5 star.  In 1994, astronomers used the HST to derive UBV photometry of the stars in the 30 Doradus region. R136 was further divided into 24 components. Over 800 stars were cataloged, among them R136a1.  In 1995, the newly refurbished HST and the Wide Field/Planetary Camera were used to observe R136 to measure the age of the cluster and the stellar population. The survey also cataloged the stars in the cluster, including R136a1.  In 1999 the fourth catalogue of Population I Wolf-Rayet stars in the Large Magellanic Cloud was published. R136a1’s luminosity was cataloged to be 1.5 million L☉, its temperature 45,000 K, and its size 20R☉. In 2010, a team of British astronomers led by Paul Crowther, professor of astrophysics at the University of Sheffield, used European Southern Observatory's Very Large Telescope (VLT) in Chile, as well as data from the Hubble Space Telescope, to study two star clusters: NGC 3603 and R136. The team found several stars with surface temperatures exceeding 40,000–56,000 K, more than seven times that of the Sun, and which were several million times brighter. At least two of the stars (R136a1 and R136a2) were over 150 M☉.
The star is classified as a Very Massive Star (VMS) with a mass of 256M☉. Its luminosity is 7,400,000L☉ which translates to 73% of its Eddington luminosity. It is so bright that it provides 7% of the light of R136. If it were to be placed 10 pc away, it would shine at magnitude -7.10.  Visually the star is only 59,000 L☉ (Absolute magnitude -7.10)  because of the high temperature. Most of the radiation is emitted primarily in the ultraviolet and other high-energy electromagnetic waves. The luminosity and high temperature result in a stellar wind that moves at 2400km/s and a mass loss rate that sheds 5.1 × 10-5 solar masses per year. The star is thought to have had a birth mass in excess of 320 M☉ but has since shed 50M☉ over the past 1.7 Myr. Current theories suggest that no stars can be born above 150 M☉ but massive stars like this one formed through mergers of multiple stars.
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, 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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