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Extra-galactic Region around R136a1
The supermassive star R136a1 lies outside the Milky Way in star cluster RMC 136a (now more often referred to as just R136). This cluster of young, massive and hot stars is located inside the Tarantula Nebula (30 Doradus or NGC 2070) within the Large Magellanic Cloud, a satellite galaxy of the Milky Way, some 165,000 light-years away from Sol. 30 Doradus is illuminated by the dense central compact cluster, R136, which was initially believed to be a single, supermassive star of around a thousand Solar-masses, given the amount of ultra-violet radiation ionizing the surrounding HII region (Rubio et al, 2009). Further and more finely detailed observations revealed that R136 was composed of three bright components (a, b, and c), of which R136a is the brightest (Weigelt et al, 1985; and Jorge Melnick, 1983). Today, it is believed that R136 contains some tens of thousands of stars within a few light-years of space, and hundreds of its stars (many of rare and massive, spectral type O) are so incredibly bright that a planet orbiting a star in the middle of the cluster would not experience darkness at night (ESO news release).
R136a contains several stars with surface temperatures over 40,000 degrees, more than seven times hotter than our Sun, Sol, that appear to be at least several tens of times larger and several million times brighter. Analysis and comparisons with high-mass stellar evolution models indicate that several of the cluster's stars were born relatively recently with more than 150 Solar-masses (the theoretical limit under conventional star formation models). Four stars (including a1, b, and c) in the cluster have estimated masses larger than or close to 150 Solar-masses, and they produce nearly half of the power of the stellar winds and radiation of the entire cluster, which is comprised of around 100,000 stars in total. Moreover, one star, R136a1, appears to be the most massive ever found (as of July 21, 2010). These findings raise the previous accepted upper limit to how massive stars in the current universe can get from around 150 to around 300 Solar-masses. Supermassive stars, however, are extremely rare, forming within the densest star clusters.
Larger and jumbo near
Young star cluster RMC 136a (R136a)
in the Large Magellanic Cloud
is host to four supermassive
stars, including R136a1 (more).
R136a1 appears to have a current mass of about 265 Solar-masses. When first born, however, it may have had as much as 320 times that of the Sun; whether R136a1 was originally born as a single star or from the merger of smaller stars, however, is not known. Since the most massive stars produce the most powerful outflows of mass and radiative energy, they shed tremendous amounts of mass as they age. Although only a little over a million years old, R136a has already become “middle-aged” and has already lost around a fifth of its initial mass (ESO news release; Jonathan Amos, BBC News, July 21, 2010; Rachel Courtland, New Scientist, July 21, 2010; Associated Press, New York Times, July 21, 2010; and Crowther et al, 2010).
Crowther et al, 2010; VLT, ESO
Larger near-infrared image.
R136a1, the biggest among four
supermassive stars in star
cluster R136a with more than
150 Solar-masses, may have
been born with as much as 320
The supermassive star has the highest known stellar luminosity (as of July 2010), close to 10 million times greater than Sol. If R136a1 replaced the Sun in the Solar System, it would outshine Sol by as much as the Sun currently outshines the full Moon. In addition, its much great mass mass would reduce the length of the Earth's year to three weeks, and its incredibly intense ultraviolet radiation would sterilize life on the planet.
R136a1 and the handful of other massive stars identified thus far could be candidates for a type of supernovae hypothesized to be triggered by the creation of electrons and their antimatter counterparts. These hypothesized "pair-instability supernovae" may explain the properties of some recently detected bright supernovae, such as SN 2006gy, which were detected as unusually bright and large supernovae. Such supernovae have been proposed as a model for a new class of supernovae associated with the most massive (and possibly the first) stars born in the universe
Stars that have at least eight to 10 Solar-masses destroy themselves after fusing hydrogen to helium, helium to carbon, and on to larger elements until they reach iron, when fusion fails. Towards the end of this process, the energy produced in the core of the star becomes insufficient to support the outer layers, which collapse inward under gravitational pressure, ending fusion after creating some even heavier elements, and crunching the core to a neutron star or black hole. The rebound from the core implosion blows away the outer layers of the star as a bright supernova.
Stars evolve at a rate
depending on their mass,
but only the largest stars
and some white dwarfs (with
companions donating mass)
die as supernovae (more).
For much more massive stars with 140 to some 260 Solar-masses, core temperature becomes so great at several billion degrees that, before the fusion progression theorized for less massive stars is complete, high-energy gamma rays in the core begin to annihilate one another and create matter-antimatter pairs (mostly electron-positron pairs). Hence, instead of mass being converted to energy in the star's core (via Albert Einstein's famous equation: E = mc2), energy is being converted to mass. Since gamma radiation provides the energy preventing gravitational collapse of the outer layers of the star onto the core, at some point the loss of this energy (through so-called "pair instability") causes violent pulsations that eject a large fraction of the outer layers of the star and eventually a star's outer layers to collapse inward to create a thermonuclear explosion that, in theory, would be brighter than previously detected supernova. In this type of pair-instability supernova, the star is blown to bits without creating a black hole. For stars with greater than around 260 Solar-masses, the pulsations would be overwhelmed by gravity, and so the star would collapse to form a black hole without an explosion. Currently, the favored explanation for the unusual features of SN 2006gy is derived from the pair-instability model for supernova creation, and this type of supernova may lie in store for R136a1 within the coming million and a half years, after it sheds more mass to shrink below the 260 Solar-masse limit.
Within a million and a half years
or so, R136ai may blow up in a
new class of powerful pair-
instability supernovae (as has
hypothesized for SN 2006gy,
possible for some supermassive
stars, shown with shock wave
blasting into a circumstellar
nebula of earlier ejecta (more).
In theory, pair-instability supernovae should produce a relatively greater abundance of heavy elements. For stars with initial masses above about 200 suns, pair-instability supernovae would produce an abundance of radioactive nickel. According to some astronomers, the radioactive decay of nickel-56 produces most of the light of a supernova, and SN 2006gy produced about 22 Solar-masses of nickel (Smith et al, 2007), compared to maybe 0.6 solar masses in a Type Ia supernova created by a white dwarf (and stolen mass from a companion star). Since astronomers believe that a large proportion of the universe's first stars were supermassive stars like SN 2006gy's progenitor, such supernovae should have dispersed large quantities of newly synthesized elements heavier than hydrogen, instead of collapsing into black holes. In addition, as these pair-instability supernovae are so bright, astronomers hope to detect similar explosions from the first stars in the universe over 13 billions years ago with more powerful observatories, such as the upcoming James Webb Space Telescope.
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