Some 165,000 light years from Earth, the heaviest star in the known universe lies at the heart of a brilliant star cluster – a rare type of blue hypergiant with a mass of around 230 Suns
Just how big can a star get? The biggest stars in terms of diameter are red supergiants J that swell to enormous sizes as their lives come to an end. As they begin to exhaust their fuel and go through internal changes, these stars brighten, swelling in size as their surfaces become cooler and redder. But if by defining the biggest star you simply mean the most massive, the answer is very different. The most monstrous stars of all are hypergiants, with many times the mass of the Sun. The most massive of all was discovered in a neighbouring galaxy of the Milky Way in 2010 – a hypergiant star with up to 230 times the mass of the Sun, called R136a1.
Fittingly, this stellar bruiser is a resident of the largest star-forming nebula in our Local Group – the Tarantula Nebula in the Large Magellanic Cloud (LMC). Looking at first glance like a detached portion of the Milky Way in far southern skies, the LMC is one of the largest and brightest of several satellite galaxies trapped in billion-year orbits around the Milky Way. Huge tidal forces are compressing its copious reserves of gas and dust to trigger the birth of new stars at a much faster rate than in our own galaxy, giving rise to the Tarantula Nebula.
Within this region, some 650 light years across, radiation from newborn stars excites the surrounding gas to glow intensely. The Tarantula Nebula is famously so large and luminous that if it were transplanted to the location of the Orion Nebula, just 1,350 light years away, it would cover an area several times that of the full Moon and be bright enough to cast shadows at night. Inside the nebula, stars are born in waves as huge knots of gas condense in a runaway chain reaction. Radiation from the first newborn stars in any given region creates shock waves that ripple through the surrounding nebulosity and trigger its collapse into more stars. At the same time, the pressure of radiation blows away any material left unclaimed by other nascent stars. This produces distinctive caverns of star formation with newborn open star clusters at their centre.
The Tarantula Nebula has already gone through this process at least twice, producing two generations of young stars. The older group is about 25 million years old and now lies 150 light years from the centre of the nebula. It contains a couple hundred stars – about 40 of which have already exploded as supernovae. The younger R136 cluster at the very centre of the nebula, meanwhile, is thought to be just 2 million years old. This means that even its heaviest stars have not yet had time to age and die, and it’s this youth that makes R136 a rich hunting ground for astronomers in search of the heaviest stars in
