INSIDE THE GIANT
A high mass and compact size turn R136a1 into a superhot stellar behemoth
With a mass of up to 230 Suns, R136a1 is so huge that astronomers were shocked by its discovery – it’s almost twice as massive as any known star in the Milky Way. R136a1 is surprising because it comes close to breaking the rules of stellar physics. For over half a century we’ve understood that the heavier a star is, the brighter it shines during its main sequence lifetime. This is the stable period of its life where it generates energy by nuclear fusion of hydrogen to form helium in its core. This mass-luminosity relationship is driven by the two different processes involved in fusion. Even though R136a1 has much more hydrogen fuel than the Sun, it’s squandering it much more quickly and will reach the end of its life after around 5 million years.
Astronomers believe this puts a natural upper limit on stellar mass – a star with the mass of R136a1 should generate such fierce radiation that it would simply blow itself apart, even as it is formed. However, this giant has challenged conventional stellar theories, causing scientists to change the rules – it simply shouldn’t exist within the previous laws. Instead, it now seems that stellar behemoths up to 300 solar masses are capable of holding themselves together through their powerful gravity, which keeps
the outer layers close to the star rather than forcing them to balloon outwards.
This makes radical changes to the other properties of such hypergiants. The amount of energy blasting its way out through a relatively small surface area heats them to searing temperatures, and this creates a powerful stellar wind as hot gas from the surface blows away into space. This effect enables stars like R136a1 to shed an entire solar mass of material every few hundred thousand years. This exposes more of its hot internal layers at the surface, which only serves to strengthen the wind further.
The result is an extreme Wolf-Rayet star whose exposed surface exceeds
50,000 degrees Celsius (90,000 degrees Fahrenheit). Although astronomers have not yet obtained information about R136a1’s composition from a spectrum of its light, evolutionary models show that as R136a1 approaches the end of its life, perhaps 2 million years from now, it will develop a complex layered structure thanks to internal changes, which enable it to keep shining.
Ultimately, it will die in a huge supernova, but the precise details of this event are still uncertain. It may, like less massive stars, keep shining to the bitter end, becoming unstable as it burns heavier and heavier elements. Eventually, a doomed attempt to generate energy from the fusion of iron would tip it into a Type II supernova.
This cataclysm would be one of the most violent ever seen and would leave behind a massive black hole. Alternatively, R136a1’s intense energy could trigger a rare event known as a pair-instability supernova, in which the creation of antimatter in the star’s core triggers a sudden drop in pressure and a premature core collapse. This triggers runaway nuclear reactions that blow the star apart, scattering its material across surrounding space.