A very big star has been seen, confounding emerging ideas on how large stars may get.
Recent years has seen a few tos and fros in the world of massive star formation and evolution. The initial mass function, which is essentially the mass distribution of stars in a given galaxy, never seemed at any point to stray much above 130 solar masses. Eventually, it was decided this must be because such stars could not form as stable entities. An absolute limit of around 150 solar masses was assumed. At this point, astronomers assumed, the brightness of the star would be such that the pressure from radiation leaving it would exceed the star’s ability to hold onto its outer layers – it would shine so powerfully, it would blow itself apart. This luminosity limit is called the Eddington Limit.
Similarly in star formation at lower masses, it was getting hard enough to describe how anything above ten solar masses could form (even though we could see they do). Collapse of a cloud of gas and dust would be halted when this amount of material had formed a protostar as the luminosity would blow away the stuff falling towards it. There were workarounds – maybe the star intersected a magnetic flux tube and got fed material, maybe discs could concentrate stuff and deliver it for longer than a cloud, maybe protostars merged in high density clusters. As it happened, not long ago, a disc was seen around a massive protostar proving that at least one of those methods does indeed work. However, even with a mechanism to form a star, there still needs to be a corner of the universe with sufficient gas and dust to form one. This isn’t like the very olden days when massive stars formed and then collapsed directly into a black hole, producing the gamma ray bursts that missions like Swift pick up once a day or so.
Just being there doesn’t mean a monster star will be seen – they have very short lifetimes relative to other stars, with lifetimes decreasing with age. However, during their lives, they do advertise themselves well. They are incredibly bright. Even if the surface of one star is the same intrinsic brightness as that of a smaller star, the increase in surface area is sufficient to ensure the star as a whole shines brighter.
However, despite the odds, it appears not one but four stars born above the 150 solar mass limit have been found, all happily coexisting in one place. 165,000 light years away in a star cluster, RCM 136a, residing in the Tarantula Nebula, itself within the Large Magellanic Cloud, a clump of leftover galaxy slowly being consumed by the Milky Way, visible in the skies of the Southern hemisphere.
The largest of the stars, discovered by a team led by Paul Crowther (who tweets here) of Sheffield University, has been designated RCM 136a1. It is presently around 265 solar masses in size and was likely born at 320 solar masses, more than twice the assumed limit. Its maximum radius would’ve been thirty times that of the Sun, about 42 million km (compared to Mercury’s orbital distance of 46-69 million km). Its temperature, at 40,000 degrees Kelvin, is seven times that of the Sun. Its luminosity would’ve been such that should it be in the centre of this solar system, it would outshine the Sun by around the same factor as the Sun outshines the Full Moon (400,000 times or so brighter).
However, there would be a downside for the stars. The massive star is so enormous in radius because of how close to the Eddington Limit it is (55 percent at birth, compared to 40 percent for 150 solar mass stars). As stars get older, they swell and become more luminous, bringing them closer to the Limit. The result is not that a sudden point is reached and the star goes from a well defined point to a nebula, but rather a slow progression that as the outward pressure increases, the wind of particles escaping the hot outside of the star (the solar wind in our Sun’s case) becomes a torrent and the change from a position when you are below the surface of a star to one when you’re in these stellar winds becomes less well defined. This is why the star has lost so much mass it has gone from 320 to 265 solar masses in one million years, half its assumed lifetime, whereas the Sun has lost a few percent of its mass over the course of 4.6 billion years, half of its own lifetime. The Sun is so long lived compared to these monsters, it has watched as the cluster it was born in slowly diffused and became individuals, blazing their own trails in the murky morass of the Milky Way.
Extraordinary claims require extraordinary evidence. Crowther and his team made use of the European Southern Observatory‘s Very Large Telescope in Chile as well as archive data from the Hubble Space Telescope to study the star cluster. The cluster contains 100,000 stars, but the radiation put out by the four big ones is equivalent to that put out by all the rest combined.
They looked at the movement of the stars to determine mass, they looked at the spectrum, which provides Doppler shifts giving velocities, spectral lines, which are the fingerprints of chemicals in the stars, black body radiation curves, which provide temperatures. They made sure they weren’t seeing two or more objects superimposed, both by using the spectrum of each star and making sure it didn’t have extra lines superimposed on it, looking for tell-tale x-ray signatures that result from stellar winds blowing into one another in close binary systems and also looking at the high-resolution pictures they’d obtained.
No doubt other groups will look carefully at the results and attempt to reproduce them. Other large stars have turned out in the past to be clusters pretending to be single objects, though the careful spectral studies should help mitigate that. Either way, future observations should ensure we know whether the largest star ever seen is indeed 265 solar masses.