Q&A: a star modeller
How do gamma-ray bursts occur in massive stars when they die? The answer could tell us more about how galaxies – and even life – evolve
What is a gamma-ray burst (GRB)? When a massive star exhausts all of its fuel, it collapses under its own weight and dies in a supernova. When the star is very, very massive and is spinning rapidly at the same time as collapsing, it forms a black hole at its core and sends out jets of energy. A gamma-ray burst occurs when a very massive star collapses like this and we are looking along the line of the jet. We’ve got this beam of radiation pointing directly at us. It is tens or hundreds of times brighter than a normal supernova. GRBs can outshine entire galaxies.
What are the main questions about how GRBs are formed?
The big question has been how you get a star that is that big and is still spinning really fast when it dies. The work I have been doing with my colleagues Ashley Chrimes and Dr JJ Eldridge has been to model this as a form of binary star interaction. It’s saying the reason these massive stars are still spinning so fast when they collapse is because the companion star is stopping them from slowing down – it’s spinning them up over time.
What did you do in your recent study in the Monthly Notices of the Royal Astronomical Society?
We have a set of stellar evolution models, which include the idea that stars could be transferring material from one star in a binary to a companion. A normal star will blow out winds and material off its surface over time, reducing its rotation. But if that material is preferentially going to a neighbour, and that is exerting a tide on the star, then it will get that star spinning faster again. We’ve calculated the effects of tides in these binary systems – it’s the same effect the Moon has on the Earth and its oceans.
We were also looking specifically at binary stars. Given what we know about the prevalence of binaries and how stars are formed in the history ▲ of the Universe, does this set of models confirm observations?
It was consistent; our model could explain the data.
What was the key result? We have a quarter of a million individual stellar evolution models, and for each one we can determine what’s going to happen at the end: maybe only about 20,000 of them will explode. We looked to see whether the tidal effects in binaries were more or less likely to make these stars explode as GRBs.
The key thing is that until now, the previous models we had that keep these stars spinning only worked when we had metal-poor stars.
Our model, with the tidal effect, allows this spinning to happen in stars with higher levels of metals. It explains some of the observations that have suggested that these events are happening in galaxies quite rich in metals.
What are the wider implications of your study?
A very small fraction of stars become GRBs, only stars more than about 20 to 25 times bigger than our Sun do this. Though extremely rare, when a GRB goes off it irradiates a very large volume
– so they affect nearby stars and they can certainly affect any potential life in neighbouring star systems. Studying these systems may be important, as it helps us understand the areas in the Universe where life could potentially survive rather than being wiped out; you really don’t want to be near a GRB.
Although we now live in a fairly quiet planetary system, on the edge of a fairly quiet Galaxy, in the past the Universe was forming stars at a much higher rate and these massive stars were impacting their surroundings in many ways. They were driving radiation and blowing gas and dust out of entire galaxies with the force of their radiation and supernovae.
So our research into the lives and deaths of these stars is important for our understanding of how galaxies have evolved over time.