SEARCHING FOR MONSTER STARS
In 2010, Crowther and his team discovered R136a1, the most massive single star known at the time
What drew you to the R136 star cluster?
It’s probably the prime target for anyone looking for the most massive stars – it’s the most obvious place to look really because it’s the most massive young star cluster in our part of the universe. It’s about the same size as the Orion Nebula, but while that’s got a couple of thousand stars, R136 probably contains 100,000 stars or more, if you could see them all. It’s been known about for a long time, but the exciting thing is that now, with Hubble and large groundbased telescopes, we can resolve separate stars and look at them individually.
There’s a really tight knot of stars at the cluster’s centre called R136a?
There were claims that R136a was a single supermassive star thousands of times more massive than the Sun. But about 25 years ago astronomers confirmed that it was actually a cluster and now, thanks to technological advances, we can finally analyse the individual stars within it. When our team looked at it with the European Southern Observatory’s Very Large Telescope in Chile, we were actually looking for binary stars, hoping we could use them to measure the masses of stars directly. We didn’t find any binaries, but we did find that the individual stars in the cluster, and the brightest one in particular, are far more exceptional than anyone thought.
How do you work out the mass of a giant single star like R136a1?
First, you work out the star’s luminosity, but that’s a problem in itself. If you’re looking at a yellow star with the same surface temperature as the Sun, then it’s fairly straightforward – you’re seeing most of the radiation in the optical and can work out the total energy output quite easily. Red stars such as cool supergiants emit only a tiny fraction of their energy as visible light, but you can still measure them in the infrared. The challenge with hot stars like R136a1 is that the energy’s coming out in the ultraviolet, at wavelengths that get soaked up by the interstellar medium on their way to Earth. We can’t measure the star’s peak energy directly, so we have to infer it through other features of its light. Even once you’ve got an idea of the star’s temperature and its overall luminosity, you still have to go an extra leg to get a mass. On the main sequence there’s a clean relationship: the more luminous a star, the more massive it is. So for R136a1, where we came up with a luminosity not far off 10 million times that of the Sun, we asked our colleagues to work out evolutionary models for what the expected mass would be. That’s how we got the figure of 265 solar masses.
Is it possible to check that result?
You’re relying on one method to get a temperature, another to get a mass, and so on… The figures are not backed up by enough evidence to prove it, so we looked for another example of a similar star to prove the technique. Ideally we were looking for a star in a close eclipsing binary system, which would let us work out the mass. We found one in a cluster called
NGC 3603, about 25,000 light years from Earth. That’s now the most massive star system to be confirmed through the laws of orbital motion – it’s got two stars in an orbit of about four days, with masses of 120 and 90 Suns. Once we’d got those numbers for that system, we used them to test our temperature and luminosity-based method, and we got basically the right answer.