Dr Emily Hatt is a stellar physicist who studies the structure of distant stars. Her focus is
on magnetic fields in stars and how they impact stellar evolution. This is important for a wide range of astronomical topics, including understanding the Milky Way.

How does magnetism come up in your study of stars?

I measure stellar properties by observing the way they pulsate—a field called asteroseis- mology. The properties of these pulsations, including their frequencies and amplitudes,
are determined by what’s going on inside the star. This lets us backwards engineer those and figure out the properties of the star itself, e.g. its age. If I include a magnetic field in the model, this shifts the frequencies of the star’s pulsations and can greatly affect my prediction of the star’s properties, so the star’s magnetic field is really important in understanding all of the star’s properties.

What do you find so interesting about the study of stellar magnetic fields?

When you first think about the actual task of measuring how fast or how magnetic the core of a star is, that seems impossible. You can’t see through the star’s surface, and you can’t fly up to it, so how do we get that information? Asteroseismology is the only way to make measurements like that directly. And then magnetism specifically piqued my interest because it is a massive open problem in stellar physics. If you ask a stellar physicist to explain some surprising result, they’ll tell you that it’s probably magnetism-related.

Why is it important to study stellar magnetic fields in a broader sense?

Magnetic fields in stars change how they rotate. This, in turn, influences how chemical elements are moved around in the star, and can bring extra hydrogen fuel from the outer layers into the fusing core. This prolongs the lifetime of the so-called main sequence and significantly alters your predictions of the star’s age. Stellar ages are used in many fields of astronomy. For example, we use the ages of many stars to reconstruct the evolutionary history of the Milky Way, or when understand- ing the properties of exoplanets, we use the age of the host star as a clock to track the evolution of the planetary system. If all your estimates of the stellar ages are wrong, then all those timelines are wrong too.

What is a surprising result that you’ve discovered so far?

We know there are magnetic effects working in stars that impact them in some way. But we don’t know the degree to which this is the case and its significance. I expected to detect magnetic fields in the cores of most stars, and we know that there are theories to suggest that this should be the case. But in my recent project, which looked at 300 stars, I only found significant signatures of core magnetism in around 20 of them. Why strong magnetic fields occur in the cores of some stars and not others is a mystery, and shows there is a lot we still don’t understand about stellar magnetism.