# Do three-star systems exist in space?

How big can star systems get?
05 December 2017

## Question

You can have binary systems in space - are there ever TRINARY systems?

This stellar suspicion was sent to The Naked Scientists from Caitlin. Chris Smith asked Astrophysicist, Matt Middleton, from the University of Southampton to give it a try. Starting with, what IS a trinary system?

Matt - Absolutely Chris, and Caitlin thank you for a lovely question. A binary system is where you have two stars that are orbiting around a common centre of mass. The way to think of that is imagine you’ve got a friend - let’s hope you all have a friend - and you hold their hands and you spin around. You are basically pivoting around that common thing. It’s not the greatest analogy but if you imagine that there are these two stars that are going round, orbiting around that common centre of mass.

Now, can you get trinary systems or higher order systems? Absolutely you can, absolutely. Most of the stars that we see, and if you look out on a clear night, they’re going to be binary systems. There are lots of individual, faint stars called red dwarfs, but we’re not going to be able to see those. But it turns out that loads, and loads, and loads of these are also trinary systems. In fact, you can also get quaternary, quinternary, etc, etc.

Chris - Just to get this right, you’re saying when I look at the stars in the sky, I’m seeing not one source of that light, but that’s actually a pair or maybe three, or even four little stars twiddling round each other?

Matt - Absolutely, and I’m going to blow your mind here. Probably the most famous star that we look at, the pole star, that’s a trinary system.

Chris - Oh goodness! Kate?

Kate - What’s the biggest system?

Matt - You can extend all the way to star clusters where we have thousands upon thousands upon thousands of stars who are all sort of interact in a gravitational way, so you can play that game endlessly. The problem is that the more you have, the more complicated those orbits become.

In a trinary system, what you quite often find is you’ve got a binary, so you’ve got your two friends zipping around, and then you’ve got another star that’s further round on a bigger orbit, and that’s nice and stable and nice and easy. But the more stars you chuck in there, it becomes quite complicated and quite a messy thing. What you’ll find is that there are a lot of trinaries, fewer with four stars, fewer with five, fewer with six, etc., etc., etc.

Chris - I just want to bring in Phil, but before I do, how do you know that this is the case, where we used to think it was just a star, now we’re calling in binary and trinary, and quaternary systems?

Matt - There’s this wonderful thing called the Hubble Space Telescope. Some of you may have heard of, I hope all of the listeners have, and it was able to resolve out these points of light. You can also look at how they move so you can get an idea from looking at the light itself that tells you about some of the motion. This is how you look for exoplanets and you can reveal ah, there must be another object in there somewhere.

Phillip - As an astrophysicist, if you’re looking at these kinds of trinary systems, does the mathematics get exponentially harder the more stars you start to involve?

Matt - That’s quite an understatement. There are these people who work on n-body simulations and those get extremely complicated. The most extreme version is where we have these simulations of the universe as it formed, so those are these incredible n-body simulations about dark matter, and gas, and supermassive black holes. So yeah, it gets ugly very, very quickly.