In mid-July 2010, scientists announced the discovery of the largest star ever, in the Tarantula Nebula: R136a1 (until a more imaginative nickname is accepted), measured at several hundred times the mass and several million times the brightness of our own Sun. If our Sun were as large as R136a1, the heat and radiation would destroy everything on the surface of Earth – although such a star, according to our current models, can actually have no planets of its own. Paradoxically, a star as large as R136a1 would have a life expectancy so short that life could never evolve in orbit around it.
The discovery of R136a1 came out of a British project using the Very Large Telescope in Chile. The British team, under Paul Crowthier, was searching for new stars in two regions known as stellar nurseries, theorized to be areas of particularly dense gas and dust and therefore most prone to rapid star formation. Crowthier examined the NGC 3603 stellar nursery in our own Milky Way galaxy, about 22,000 light-years away, and then turned his attention to the stunningly beautiful Tarantula Nebula in our small companion galaxy (about 150,000 light-years away), the Large Magellanic Cloud. The Tarantula Nebula is believed to be forming new stars more quickly than any other region in the Local Group of galaxies.
It was when they turned the telescope to the Tarantula Nebula that the Crowthier team picked out the behemoth star R136a1, as well as several other stars smaller than their larger cousin but still around 150 times the mass of our own Sun. The discovery is especially significant because astronomers previously theorized that a star could not hold itself together above about 150 times the mass of our own Sun. Beyond that point, stars should stop growing because their radiation emissions grow more powerful than their own gravity and blow away the remainig nearby gas and dust. R136a proves that, while there may still be such a theoretical limit, it is higher than scientists previously believed.
At the same time, being so large is clearly taking a heavy toll on R136a1, along with its smaller cousins. Although it is now about 250 times the size of the Sun, astronomers believe that it might have been as much as 320 times the mass of our own Sun when it initially formed and ignited. In addition to the hydrogen fusion occurring in its core, which is occurring at a far faster rate than the same reactions in our Sun, the solar wind on R136a1 is probably jetting immense amounts of material into space on a continuous basis.
Stars as large as R136a1, the blue giants at the top of the main sequence of stars, live paradoxically short lifespans. Because they are so large, heat and pressure grow so intense that they burn through their hydrogen fuel very quickly, typically in a few million years. In contrast, yellow stars like our Sun will take billions of years for the hydrogen in their cores to be completely fused into helium – and red dwarf stars will take trillions. At an estimated one million years old, R136a1 is aging and may already be past its prime. It is unclear what will happen to a star this large when it runs out of hydrogen, but the current models suggest that very large stars undergo an explosive supernova, in which the outer layers are flung outwards to seed new generations of smaller stars, and the remnant of the core (potentially) undergoes gravitational collapse to become a black hole.
In the meantime, however, R136a1 will be alive for many more human lifespans, and Crowthier’s team believes that it gives us an important glimpse into the early universe. Billions of years ago, gas and dust would have been much more prevalent than they are now – and so very large stars like R136a1 would have formed much more frequently. These stars then exploded, shedding the debris which fueled the birth of new generations of stars, eventually including our own Sun.
