Talking Binary Evolution with Associate Professor Dr Jan Eldridge

When it comes to theorising about the dynamic interactions of binary stellar systems, the lives and deaths of stars in our Universe, and what the Daleks are doing on Dr Who, the first person that should come to your mind is Associate Professor Dr Jan J. Eldridge.

Dr Eldridge (Jan), who is also Head of the Department of Physics at the University of Auckland has focused her career on studying the particulars of how these binary star systems evolve over time, and in turn, drive the rates of supernovae and gravitational wave events that astronomers detect from Earth. This has been achieved through a powerful tool that Jan (along with Dr Elizabeth Stanway from the University of Warwick) have developed – the Binary Population and Spectral Synthesis (BPASS) code, which models stellar evolution in binary systems.

Last year (2020), Jan was awarded the Anne Green Prize by the Astronomical Society of Australia for BPASS having a high impact in a wide range of fields across astronomy, from the study of high-redshift galaxies and reionisation to nearby galaxies and stellar evolution.

Additionally, Jan (who identifies as a non-binary trans woman) has been an inspiring and powerful voice for the LGBTIQA+ community online and throughout academia, ensuring it becomes a more inclusive and equitable space for all, pioneering and leading the way for many other people to follow.

I was lucky enough to have a chat with Jan, covering a wonderful career in astronomy, inclusivity in academia, and what’s around the corner that they’re most excited about.

A chat with Jan …

First of all, congratulations on winning the 2020 Anne Green Prize at the ASA presentations, such an exciting and well-deserved prize for your work. How does that make you feel?

Well even though it was some time ago now it is still quite exciting. Especially when I had a photo with some of my team members it made me realise how much any individual effort is a team enterprise and I'm really lucky to have been able to work with so many people from around the world.

Why are binary stars so important to our understanding of the Universe, and what made you go down the path of researching them?

The question to answer before that one is why did I go into making computer models of stars? This was mainly because I've been using computers since I was 5 and when I started my PhD, I knew I wanted to do something in numerical simulations and so I started making models of stars. 

My project was looking at stars that exploded in core-collapse supernovae and after I'd been working on the project for some time it became apparent that the only way to match the observations of the supernovae was by including binaries. This is effective because in a binary when stars attempt to grow into red giants, they get so big, with a close companion they get in each other's way and interact in interesting ways or merge to become a "super star".

But since starting to look at them around supernovae I ended up looking into how binaries have an impact on all aspects of stars and galaxies, while others had looked into this before no one has done it quite as broadly as we have, nor made their results available for others to use.

Given the importance of binary stellar systems out there in the Universe (for a range of reasons – like the development of life, the potential for binary compact mergers resulting in nucleosynthesis of metals, or developing our understanding of the laws of the Universe) where would we expect to find binary stars in the Universe and why? (i.e., in every type of galaxy? Globular clusters? Etc.)

At least 70% of stars are in binaries, i.e., 3 of every 2 stars is in a binary. Although it does depend on the mass of the star, massive stars seem to all be in binaries while those like our Sun it is only 20-40%. Globular clusters certainly have many binaries, but this is because they store energy in the binaries which is why they're still bound and clusters today otherwise they would have dispersed around the galaxy and not be a cluster anymore. 

There is one problem however if we see a single star, how do we know it's been single its entire life? It might be a companion whose star died and ejected it or the result of a merger. So even answering this question is a bit difficult.

What is BPASS and what kind of science can astronomers do with it? What does BPASS tell us about our current understanding of stellar systems?

BPASS is a collection of codes and models that allow us to predict what a population of binaries, similar to those born in the Universe across a range of initial conditions, will "look like" if we observe them. A key feature of BPASS that sets it apart is that we don't just model the interior but also work out what it would look like with an optical telescope and now even gravitational wave telescopes.

What are some of the challenges of trying to write a platform (BPASS) that covers the evolution of binary star systems over a long period of time?

The main problem is that as we try to cover such a large range of initial conditions dealing with how things vary over that "parameter" space is tricky and means a lot of troubleshooting as what makes one star evolve from birth to death won't work for another one. It means we learn a lot about stars but that isn't necessarily useful to those who just want to use stars to understand the history of a galaxy.

What’s the link between exploding binary stars and gravitational waves?

To make a gravitational wave transient you need a binary system of two dead stars. Which means that we need to have two supernovae! But also, those supernovae have to not disrupt the binary nor the other star, so it is quite restricted what the eventual binary is like, especially if it needs to be close enough for gravitational radiation to be able to drive the two stars together to make the gravitational-wave transient. So, the two are closely linked. This is why when we try to model the rate of GW transients through the Universe, we also try to model the supernovae rates to make sure we're getting both these things correct at the same time.

What emerging areas of space science, astronomy, astrophysics are you most excited about?

"short time-domain astronomy" - not all stars are constant, but many vary, it's understanding that variation of stars on timescales of minutes/hours that we're only just starting to do that will allow us to find many new discoveries in the next few years.

You’re very well-published and respected across the astronomy and astrophysics communities. What got you into astrophysics in the first place?

Thanks! And science fiction. Reading too many sci-fi novels and watching Star Wars, Dr Who and Star Trek. Studying astrophysics is as close as I can get to being the Doctor. Travelling across time and space while also getting to show young people its wonders. As well as with lots of running.

What advice would you give to young people who want to become astrophysicists in the future?

Work hard, but not too hard, it's a difficult subject and other people might suggest it's easy but it's not. Solving the difficult problems takes effort. Also do lots of reading around the subject, at University you can study the subject in detail but before then getting a grand picture of the Universe is going to be most useful. Also, helpful to work out what you find interesting and exciting yourself.

You’ve been an inspiring voice for diversity across astrophysics, science communication and social media – how does that make you feel?

It's hard to process sometimes. I'm really just trying to be myself and get on with my life but to know that others find that positive and helps them be themselves is great. It does give me some motivation to keep being myself but also, I worry that sometimes I might let others down by making mistakes.

There are still many challenges ahead faced by minorities in science, what are some that you think we can easily solve and how would we do this?

It's not easy. And can't be, we need to do the hard difficult things because it's the only way to make changes. And that means everyone buying into diversity and thinking about it around every decision. Also looking around when decisions are being made and wondering who isn't in the room so whose thoughts and ideas are missing. Only when you've heard all ideas can you find the best solution for everyone.

Given the success of the latest version of BPASS, and some of the results it is already producing – what exciting science can we expect next from you and your team?

We're really trying to work more on gravitational wave emission from binary stars before they merge for the upcoming LISA space mission. But one thing we are grappling with now is how to constrain all the different uncertain physics in our models and constrain them with all the different observations that we have now, to make sure we understand the physics and it's tricky.

We hope you enjoyed this interview with one of astronomy’s leading figures, and highly recommend you follow Jan’s work through her website or following Jan on Twitter. Additionally, learn more about BPASS here.

Video credit: University of Auckland/YouTube.
Portrait image of Jan from MiNDFOOD website.