R136a1

A near infrared image of the R136 cluster, obtained at high resolution with the MAD adaptive optics instrument at ESO’s Very Large Telescope. R136a1 is resolved at the center with R136a2 close by, R136a3 below right, and R136b to the left. Credit: ESO/VLT
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Dorado
Right ascension	5h 38m 42.43s
Declination	−69° 06′ 02.2″
Apparent magnitude (V)	12.28[1]
Characteristics
Evolutionary stage	Wolf-Rayet star
Spectral type	WN5h[2]
B−V color index	+0.17[1]
Astrometry
Distance	157,000 ly (48,500 pc)
Absolute magnitude (MV)	-7.10[2]
Absolute bolometric magnitude (Mbol)	-12.3
Details
Mass	256[2] M☉
Radius	28.8[2] R☉
Luminosity	7,400,000[2] L☉
Luminosity (visual, LV)	59,000[2] L☉
Temperature	56,000[2] K
Metallicity	0.4 XH
Age	1.7[3] Myr
Other designations
BAT99 108, RMC 136a1, HSH95 3, WO84 1b, Cl* NGC 2070 MH 498, CHH92 1, P93 954.
Database references
SIMBAD	data

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 R136. Because of its large mass, it is one of the candidates for a potential supernova or hypernova in the astronomically near future.

Observations

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_☉.^[4]

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. 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.^[5]

In the early 1980s, R136a was first resolved using speckle interferometry into 8 components.^[6] 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. ^[7][8] Instead it was eventually found that it consisted of a few extremely luminous stars accompanied by a larger number of hot O stars.^[1]

Visibility

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 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.

Distance

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%.^{[9][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.

Surroundings

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.^[3] R136a2 itself is among the most massive and luminous stars known, at 179 M_☉ and 4,900,000 L_☉.

Characteristics

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_☉.

Classification

R136a1 is classified as a Wolf-Rayet star due to its emission of ionized nitrogen and hydrogen 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.^[10]

Composition

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.

Mass

The masses of stars are difficult to measure except by determination of a binary orbit. R136a1 is not a binary system, so its mass is not known accurately. However, measurements of its atmosphere and stellar evolutionary models indicate that it has a mass in the range of 260 M_☉, making it the most massive star known. It has clearly lost a large amount of mass since it formed and was initially 320 M_☉.

Stellar atmosphere models and stellar evolutionary models, used to calculate the present and initial mass for R136a1, may have systematic uncertainties. However, direct (model-independent) stellar masses can be obtained if the star is a member of a binary system. NGC 3603 A1, another system that has been analysed using identical techniques to R136a1, is an eclipsing binary system whose components have been accurately measured. Comparisons between the direct, dynamical masses for the components of NGC 3603 A1 and its theoretically deduced masses are in excellent agreement, arguing against systematic uncertainties for R136a1.^[11]

Reliable stellar masses requires knowledge of the temperature and bolometric (total) luminosity of a star, its evolutionary state, plus application of theoretical models describing how stars change their physical properties as they age. For R136a1 the stellar temperature was derived from stellar atmosphere fits to infrared and optical spectroscopic observations. Its stellar luminosity then followed from its infrared apparent magnitude, accounting for the 50 kpc (165,000 light year) distance to the LMC and modest interstellar extinction. The deduced stellar properties, plus its surface hydrogen abundance matched evolutionary models calculated for rotating, main-sequence LMC-metallicity stars.^[11]

Mass loss

R136a1's current rate of mass-loss was calculated through its stellar wind. This rate of 0.00005 solar masses per year (3.15 × 10¹⁸ kg/s) is corrected for wind clumping (structure) and agrees closely with theoretical wind calculations obtained for the mass, temperature and luminosity derived for this star that were adopted in the evolutionary model calculations. R136a1 has an estimated age of 1.7 million years, suggesting a loss of 75 solar masses, which is reduced to 64 solar masses after accounting for the anticipated lower rate of mass loss at earlier evolutionary phases. It has a stellar wind of 2,400 km/s.

The process through which this large mass loss rate 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.

Luminosity

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_☉.^[2] 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.^[3] However, it is predicted that a 300 M_☉ would be around 55% of its Eddington limit at birth, implying that R136a1 was significantly less luminous at its birth.

Temperature

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's 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).

Size

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.

Future

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.

Evolution

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.^[12]

A typical ZAMS appearance would be a 320 M_☉ star with a spectral type of O2If ^[13] 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 traveling 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.^[10]

WNh stars are an early stage in the evolution of very massive stars. They are defined as extremely massive and luminous stars that are burning hydrogen in their core and show strong hydrogen emission. They burn hydrogen in their core and move off the WNh sequence once the hydrogen at their core becomes depleted. They then start burning helium and expand into blue supergiants and possibly with a short time as a luminous blue variable. If mixing is efficient enough (e.g. through rapid rotation) they may progress directly to WNL stars without hydrogen. In either situation, the star would end up as a WNL (late type) nitrogen burning star. Further evolution would strip off more of the star's outer layers and it would move from WNL to WNE. After nitrogen burning the star would begin to burn carbon and the carbon core would be exposed. Therefore, the star would progress to the WC stage. However, only a short while would be spent on the WC sequence, with strong mass loss stripping off the star's carbon layers until the oxygen core is revealed. 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. Only a few thousand years would be spent as a WO star, which starts with core oxygen burning and continues to iron. 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.^[14] In total, the mass lost by R136a1 will be large, and the star will be a fraction of its initial mass at core collapse.

Supernova

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,^[11] 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.^[15]

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.

See also

• List of most massive stars
• List of most luminous stars
• R136
• R136a2

References