Surveys have shown one third of Kiwis think science is too specialised to understand. In this series, scientists who tackle mind-bogglingly complex problems each day make it simple for us. Today, we talk to University of Auckland astrophysicist Dr JJ Eldridge.

How would you describe what you do to a peer at a conference? And then how would you to a stranger at a barbecue?

"I study binary stars by creating numerical models to understand what happens when the two stars interact. I then compare the models to different observations of stars from exploding stars to individual stars like our Sun to whole galaxies of stars at the edge of the observable Universe."



"I study binary stars, two stars that orbit around each other so closely they can get in each others' way. So their evolution is much more varied and uncertain than that of our own Sun. The two stars could exchange material, with one becoming more massive at the expense of the other. I look at these stars as they lead to interesting types of dramatic stellar explosions."

What project or projects are you currently working on at the moment and what's involved?

The core of my projects at the moment concern my "Binary Population and Spectral Synthesis" code.

Over the last few years we're beginning to understand that single stars like our Sun are quite rare.

Seventy-five per cent of stars in the galaxy are in a binary system, with some in triples, quadruples, and so on.

This is a problem as most astronomers treat every star as if it was single, the work in this project shows that if we don't account for binaries we can get the wrong answer when we look at galaxies.

University of Auckland astrophysicist Dr JJ Eldridge. Photo / Supplied
University of Auckland astrophysicist Dr JJ Eldridge. Photo / Supplied

So in collaboration with Associate Professor Elizabeth Stanway we've created this code as a tool for astronomers to take account of interacting binaries.

Similar codes exist but we're putting in as much effort to making our models - being referred to as the "Auckland" models by some - easy to use and freely available.


What is cool is that we've also been able to find out interesting things about supernovae from exploding stars, the first galaxies in the Universe as well as the merging black holes while making the code.

But it's just as interesting to see what other astronomers have been able to use the models for.

What are the trickiest questions facing your field and why is solving them so difficult?

The biggest problem is around stars interacting, or "getting in each others' way".

There is one phase we really don't understand well at all called "common-envelope evolution".

What happens here is one star completely engulfs the other, then the two stars either merge into one of end up in a much smaller orbit with the two stars orbiting around each other every few days.

While we have some ideas how this extreme interaction between the two stars occurs we don't understand the details.

It's difficult to simulate in computers as the stars are normally giants, the size of the solar system and the interaction last days so it is really extreme physics.

We have to understand it because we know that the closer stars are together at the end of their lives the quicker their remnants, black holes for example, are going to merge.

Since we know that many black holes do merge thanks to the detection by LIGO we need to make sure we understand common-envelope evolution to understand the life cycle of stars that gave rise to those black holes.

Also in these events a lot of the stellar material is thrown out into the galaxy, and gets incorporated into the next generation of stars and planets, so it's important to understand how the elements that go to making us came along in the universe.

What do you feel are the most interesting or fascinating aspects of your field?

That my field is still so observationally led.

While we try our best to make our best models with our current understanding of physics, the most exciting time is when a new exploding star or merging gravitational wave source is seen which we didn't expect.

We then have to furiously work to figure out what happened, piecing the explosion together to work out what type of stellar death throes we observed.

Why do you think the work is important and what could it help us understand?

When the universe was created there was only hydrogen and helium, over the last 13.8 billion years stars have created enough carbon, oxygen, iron, gold and other elements so that complex life such as we can exist.

Stars are the nuclear reactors that created everything we need to live

Trying to understand them, and how the various variety of stars and explosion created each of the different elements, is intimately linked with our story of where we came from and how we came to be.

We are star dust - or radioactive stellar waste if you're a bit more pessimistic.