Triple star systems could re-write the cosmic playbook

A team of researchers at the University of Leeds say they have made a discovery that may change our understanding of some of the biggest stars in the Universe, so-called “B” stars…

René - My name is René Oudmaijer and I'm currently the head of astrophysics here at the University of Leeds. Since about a hundred years we know that stars coming all kinds of colours and sizes. The heavier, the more massive the stars are, the rarer they are as well. And the particular type of stars that we've been looking at are B stars that are quite massive stars. They're three to ten times the mass of the song. And since 150 years ago, people found that quite a big proportion of them are quite special. They have some sort of emission line from hot gas around it. And basically ever since then, people have been trying to work out what it was and people found that the gas surrounding the star in a kind of a disc. But theories that we have to form the disc have been very few and far between. One of the things that we know, it probably has to do with the fact that these stars rotate very rapidly. But what we still don't know is why would these 'BE' stars, The E stands emission, rotate so rapidly. And that was basically the underlying problem that we wanted to solve.

Chris - When one looks up in the night sky, what proportion of the stars that we're seeing would fall into this category of being these so-called 'BE' stars.

René - The B stars that I'm talking about are perhaps less than 1% of all the stars that we know of. But when I'm looking up in the sky, many more of them are B stars and that's because they're so much brighter than the sun. So we can look at them much further away. So if you want me to make a guesstimate, I think about 10 to 20% of all the stars that we can see with our own eyes might be B stars.

Chris - The quandary then is why they've got this glowing disc of gas around these stars. So how did you try and probe that then? What have you done that people have scratched their heads about for a hundred years plus to try and solve?

René - People have been thinking about how a star can rotate so rapidly. And sometimes stars can just be born that way, but what people have found over the last couple of decades is that a lot of stars are formed in binary stars or double stars. And if these stars are very close to each other, they can actually interact with each other. One star can actually steal material from the other one and in. The process, it'll speed up itself as well. That would be a nice explanation for stars to actually spin up and because they rotate so much quicker can form that disc around them.

Chris - I suppose it's a bit like a couple of ballroom dancers twirling round the dance floor, isn't it? That's what you're saying. And you might have one bigger partner and one smaller partner with one feeding off the other almost. That doesn't go on in Strictly Come Dancing, thank goodness, but that's the sort of concept, isn't it? It's a pair twirling round together.

René - It's a pair twirling around together. And yeah, it depends whether you're doing the jive or the chacha, where you're a bit further away from each other, or whether you're doing the Argentine tango, when you're closer to each other, because you want to be closer to each other for these stars to interact.

Chris - Why stop at two though? Why not have more stars than two? Or does that happen?

René - Oh well Chris, that's actually quite an interesting question because what happened so far is that people have been thinking about the fact that double stars, binary stars, could explain this phenomenon of the BE stars because if they're very close to each other then they can interact and the star can spin up and therefore get you a nice disc. But no one had been looking at many stars and at the difference between normal B stars and BE stars, whether that might be the case. We managed to be able to do that by using the results of a fantastic new satellite called GAIA. We looked at more than a thousand objects and what we found was that there was really something different between B stars and BE stars, when you look very up close. For all distances between the stars, the number of double stars that we found was basically the same. But very close in, there was a big difference between the B stars and the BE stars. And we found that that's probably accounting for the fact that indeed stars are eating up their companion star and they become so dim that you don't detect them as a binary. That was a very counterintuitive result. And then we started thinking about what needs to be done for a binary companion to be eaten up. It needs to be close to the main star, the BE star. And it turns out that if you have a third star in the neighbourhood, then the chances of two stars moving towards each other are so large that interaction almost becomes inevitable. So what we found is that at first there were ideas that binary stars might explain the idea that you can have B star, BE star with a disc around it. And we confirm that, but it turns out it's necessary, almost, to get a third star to make that happen. So in the idea of dancing, you probably need a dancing teacher to tell you what to do a little bit further away and looking at you.

Chris - And how does this change, apart from accounting for the observation and giving us a neat explanation for why we see what we see, how does this change the big picture of what we understand about the evolution of stars and so on?

René - I think it changes the picture big time to be honest Chris, because one of the main things that has come out of astrophysics in the last 10 years is the detection of gravitational waves. Huge explosions that actually make the space time continuum vibrate a little bit and nowadays we are able to actually detect these very tiny vibrations in the fabric of space. These gravitational waves are now found to be the result of double neutron stars or even double black holes. Well also dancing together, getting very close to each other and merging. So now we're at the stage where we know that these gravitational waves are due to binary stars, the remnants of stellar revolutions, and neutron stars, the result of a supernova for example. But if we want to understand how we got there, we need to understand the formation of double stars and the evolution, the life cycle, of a double star. And I think finding now that these BE stars that could become neutron stars at the end of their lives are due to the interaction with the third star. It means that we now not only need to take into go binary star evolution, but probably triple star evolution to understand, for example, these gravitational waves.