R136a1 (short for RMC 136a1) is the most massive and luminous star known, at around 291 solar masses (M☉) and around 7.2 million times the Sun's luminosity (L☉). It is a Wolf–Rayet star at the center of R136, the central concentration of stars of the large NGC 2070 open cluster in the Tarantula Nebula (30 Doradus) in the Large Magellanic Cloud. The cluster can be seen in the far southern celestial hemisphere with binoculars or a small telescope, at magnitude 7.25. R136a1 itself is 100 times fainter than the cluster and can only be resolved using speckle interferometry.
Discovery
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 (LMC). Among the objects cataloged was RMC 136 (Radcliffe observatory Magellanic Cloud catalog number 136), the central "star" of the Tarantula Nebula, which the observers concluded was probably a multiple star system. Subsequent observations showed that R136 was located in the middle of a giant region of ionized interstellar hydrogen, known as an H II region, which was a center of intense star formation in the immediate vicinity of the observed stars. In 1979, ESO's 3.6 m telescope was used to resolve R136 into three components; R136a, R136b, and R136c. The exact nature of R136a was unclear and a subject of intense discussion. Estimates that the brightness of the central region would require as many as 100 hot O class stars within half a parsec at the centre of the cluster led to speculation that a star 3,000 times the mass of the Sun was the more likely explanation. The first demonstration that R136a was a star cluster was provided by Weigelt and Beier in 1985. Using the speckle interferometry technique, R136a was shown to be made up of 8 stars within 1 arcsecond at the centre of the cluster, with R136a1 being the brightest. Final confirmation of the nature of R136a came after the launch of the Hubble Space Telescope. Its Wide Field and Planetary Camera (WFPC) resolved R136a into at least 12 components and showed that R136 contained over 200 highly luminous stars. The more advanced WFPC2 allowed the study of 46 massive luminous stars within half a parsec of R136a and over 3,000 stars within a 4.7 parsec radius.
Visibility
In the night sky, R136 appears as a 10th magnitude object at the core of the NGC 2070 cluster embedded in the Tarantula Nebula in the Large Magellanic Cloud. It required a 3.6 metre telescope to detect R136a as a component of R136 in 1979, and resolving R136a to detect R136a1 requires a space telescope or sophisticated techniques such as adaptive optics or speckle interferometry. South of about the 20th parallel south, the LMC is circumpolar, meaning that it can be seen (at least in part) all night every night of the year, weather and light pollution permitting. In the Northern Hemisphere, it can be visible south of the 20th parallel north. This excludes North America (except southern Mexico), Europe, northern Africa and northern Asia.
Surroundings The R136a system at the core of R136 is a dense luminous knot of stars containing at least 12 stars, the most prominent being R136a1, R136a2, and R136a3, all of which are extremely luminous and massive WN5h stars. R136a1 is separated from R136a2, the second brightest star in the cluster, by 5,000 AU. R136 is located approximately 157,000 light-years from Earth in the Large Magellanic Cloud, positioned on the south-east corner of the galaxy 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. For such a distant star, R136a1 is relatively unobscured by interstellar dust. The reddening causes the visual brightness to be reduced by about 1.8 magnitudes, but only around 0.22 magnitudes in the near infrared.
Distance The distance to R136a1 cannot be determined directly, but is assumed to be at the same distance as the Large Magellanic Cloud at around 50 kiloparsecs or 163,000 light years.
Properties
Binary A possible binary companion to R136a1, HSH95-17, has been resolved, although there is a 25% possibility that it is a chance alignment. X-ray emission was detected from R136 using the Chandra X-ray Observatory. R136a and R136c were both clearly detected, but R136a could not be resolved. Another study separated the R136a1/2 pair from R136a3. R136a1/2 showed relatively soft x-rays not thought to indicate a colliding winds binary. Rapid Doppler radial velocity variations would be expected from a pair of equal mass stars in a close orbit, but this has not been seen in the R136a1 spectrum. A high orbital inclination, a more distant binary, or a chance alignment of two distant stars cannot be completely ruled out but is thought to be unlikely. Highly unequal binary components are possible, but would not affect the modelling of R136a1's properties.
Classification
R136a1 is a high-luminosity WN5h star, placing it on the extreme top left corner of the Hertzsprung–Russell diagram. A Wolf–Rayet star is distinguished by the strong, broad emission lines in its spectrum. This includes ionized nitrogen, helium, carbon, oxygen and occasionally silicon, but with hydrogen lines usually weak or absent. A WN5 star is classified on the basis of ionised helium emission being considerably stronger than the neutral helium lines, and having roughly equal emission strength from NIII, NIV, and NV. The "h" in the spectral type indicates significant hydrogen emission in the spectrum, and hydrogen is calculated to make up 40% of the surface abundance by mass. WNh stars as a class are massive luminous stars still burning hydrogen at their cores. The emission spectrum is produced in a powerful dense stellar wind, and the enhanced levels of helium and nitrogen arise from convectional mixing of CNO cycle products to the surface.
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