Interviewed by Professor Jess Werk
On a lovely Tuesday afternoon in March, I sat down with Tom Wagg over steaming cups of vanilla rooibos tea from Tea Republik. The British astrophysicist, currently completing his Ph.D. at UW, has quickly established himself as a rising star in the field of stellar evolution, particularly through his pioneering work on binary star systems and their complex interactions.
Tom’s research focuses on massive stars—rare cosmic giants that, despite their scarcity, dramatically shape the galaxies they inhabit. As our conversation unfolds, it becomes clear that Wagg's fascination with these celestial behemoths goes beyond academic interest; it's a genuine passion that lights up his entire demeanor.
Q: What draws you to massive stars specifically?
"Stars are wonderful because they kind of interact with everything. Massive stars in particular are rare in terms of the initial mass function. You're not going to get that many of them, but they're extremely impactful in what they do on galaxies. They give you supernovae, which not only creates huge amounts of feedback that changes the way your galaxies evolve, but also throws things across the galaxy making runaway stars and hyper-velocity stars. Stars turn into black holes and neutron stars, they merge and you get gravitational waves. Stars and their by-products touch everything in the Universe."
What fascinates him most is the connective nature of stellar research: "It's this fun thing where you can study a specific object and then apply it to a population, and that can have an effect on an entire galaxy. So, you start on one scale, and it really expands into many scales. I think that's why I like stars."
Q: Do you have a favorite type of star?
"Any star that's got a companion," he replies without hesitation. "Binary stars for sure."
This preference makes sense given that most massive stars exist in binary systems. "For a solar-type star, it's like a 50-50 chance. And then for an O-type star, it's like 100%, and of those, 70% will still be interacting, so they'll have some form of accretion, spin up, common envelope or merger. That's hugely affecting their evolution."
Changing Perspectives
Wagg's research has significantly shifted his understanding of how binary interactions reshape stellar evolution. "I think I've really started to properly comprehend quite how much the binary interactions can change things," he reflects. "I've been focusing on that and, for example, seeing how if you take on a bunch of mass from a star, even if it only lands on the surface of the star, because of the increased mass and therefore pressure and density and temperature, you're going to change the size of your convective core."
The implications are far-reaching: "You're going to change how the star evolves for the rest of its main sequence. Even though the mass transfer only happened for half a million years, the next 160 million years, it changes everything vastly."
Q: Why do you think this wasn't appreciated in previous research?
"It's a mixture of two things," Wagg explains. "It was hard—it’s much easier to evolve a single star. So if you're going to evolve one thing first in your simulations, that's where you're going to start. At the same time, the data weren’t necessarily there to prove that all of these stars are in binaries."
He points to a pivotal moment in the field: "There was some work from Dany Vanbeveren back in the day, but it was really Hugues Sana and collaborators in 2012 who published a paper in Science basically showing that all O-type stars are binaries. They're all interacting. Everyone suddenly was like, 'Oh…we should be caring about this.'"
Mentorship Mosaic
Wagg's academic journey has been shaped by a constellation of mentors across different institutions. When I ask about influential advisors, he describes assembling his own network of expertise.
"It's a whole conglomerate of people," he says. "I've had a sort of strange path going through the Ph.D., wanting to try a couple of different things and eventually realizing I do really love massive stars. A lot of what I want to do requires expertise that's not around the University of Washington, so I spent time learning from a lot of people who aren't actually here."
At UW, his advisor, Professor Eric Bellm, has been a consistent source of support. "Eric and I chat once every two weeks to check in," Wagg explains. "Eric has been great at really supporting me and what I want to do. He's there ready to give me the support I need when I want to write a proposal, and he's happy to co-write it with me and provide all the admin things that go with it and teach me what goes into it. At the same time, while maybe not being as enthusiastic about all the science that I'm doing as the stuff that he loves most, he's happy to come to my meetings and give opinions and see what the plots are showing. He gives me the freedom to do what I want while supporting me at every moment that I need."
Beyond UW, one key connection began with a mathematical disagreement. "There's Katie Breivik, who is a professor at CMU. I was doing a paper on LISA detections of gravitational waves, trying to re-derive the equations in one of her papers to calculate signal-to-noise ratios. I was following her paper, and the equations I was working out weren't matching."
Rather than a setback, this became an opportunity: "I emailed her to ask for help. Long story short, what was written in the papers turned out to be wrong, and Katie was lovely and very nice about it. We ended up writing a whole Python package together that repackaged all the equations. Now, everyone uses that package as a safe way of making the calculations. Katie gives such lovely advice and is very supportive of not just the science, but of the person."
He speaks with similar admiration about Mathieu Renzo, a professor at Arizona who “has an amount of literature knowledge in his head of everything massive stars and binaries-related that is truly insane. It's far more effective to ask him a specific question than to read a paper seeking the answer."
Finding Balance
When not peering into the cosmos, Wagg finds refuge in Seattle's natural spaces. "In terms of clearing my mind and refreshing, you can often find me wandering around a park somewhere. I live right next to Ravenna Park right now, just taking a wander through there in the mornings is always very relaxing."
His athletic pursuits include skiing—he notes he's skied faster than he's ever driven a car,
reaching 73 mph down an Olympic downhill run—and field hockey, playing on both a men's team and a mixed team in Seattle. "I'll also do climbing and play some pickleball with Andy," he adds.
What outside of astronomy sparks joy for you?
"Mostly people," he reflects. "Kind of talking with people about whatever they're getting up to, whether it's family or friends, and just hearing about life in other circles other than astronomy is always fun. I like interacting with people who aren't in academia quite a lot."
Community Constellation
As he prepares to move on to a postdoctoral position at the CCA (Center for Computational Astrophysics), Wagg looks back fondly on his time at UW. "What I’m going to miss most is all of my friends and fellow grad students, which is really the same group of people. It's a 100% overlap, because they are wonderful."
He particularly values the collaborative environment: "I love having a corridor that I can walk down and I'm like, 'Oh, I have a question about these observations of stars,' and I can go and ask Andy, or 'I have this question about clusters,' and I can go and ask Tobin, or 'I'm thinking about jets,' and I can go and talk to V. There's someone in the grad hallway that knows just the thing I’m looking to learn more about, and we're happy to chat with each other."
As our teacups empty and the light begins to fade, Wagg's enthusiasm for his research remains undimmed. His friends from college are flying in for his defense, and his mother is coming from England for his graduation—an event he's particularly excited about "because we don't do it in England, and I didn't have an undergrad one because of COVID."
Tom Wagg represents a new generation of astrophysicists: collaborative, interdisciplinary, and deeply engaged with both the technical intricacies and the grand cosmic questions that spur our entire field forward. As he ventures toward his next career chapter, one thing is certain—the stars that have fascinated him since childhood will continue to guide his path forward.