In this series of posts, we sit down with a few of the keynote speakers of the 247th AAS meeting to learn more about them and their research. You can see a full schedule of their talks here, and read our other interviews here!
What’s smaller than a star but bigger than a planet? That would be a brown dwarf. While they’re made of the same material as “normal” main-sequence stars, brown dwarfs are too small to generate enough pressure in their cores to fuse hydrogen into helium. That fusion process is what makes stars shine, so brown dwarfs are sometimes called failed stars – but they offer plenty of opportunities for successful science. Dr. Burgasser, a professor of astrophysics and astronomy and director of the Cool Star Lab at UC San Diego, will be talking about his recent work on brown dwarfs in his Monday plenary talk, “The Brown Dwarf-Milky Way Connection: How Failed Stars Play a Unique Role in Galactic Archaeology.”
Right now is a great era for the study of brown dwarfs due to new instruments like JWST and the soon-to-launch Roman space telescope. Because they don’t shine like stars, brown dwarfs are cool and faint, and therefore hard to see. Until recently, almost all the known brown dwarfs were relatively near the sun, at distances of tens of parsecs. Thanks to the high sensitivity of JWST at the infrared wavelengths where brown dwarfs are brightest, dozens of brown dwarfs are now being identified at kiloparsec distances, extending the known population. That means we can see them not only in more of the galaxy, but out into older parts of the galaxy like the thick disk and halo.
“The vision I want to have for our field is to start to think about how we can view these objects not just as interesting sources in themselves but as unique probes of different galactic environments,” says Burgasser. Brown dwarfs slowly and steadily cool off over time, signatures of which can be seen in spectral data. They can therefore be used to infer the ages of the nearby stars they formed alongside. This technique has been used to date young open clusters for twenty-five years, but with new JWST data brown dwarfs can be seen in the much older globular clusters too. Spectra also reveal brown dwarfs’ metallicity, that is, their chemical composition, which reflects the composition of their environments. Since brown dwarfs are plentiful (some 20% of all stars) and long-lived (they can’t burn their fuel and can never burn out), they should be available everywhere as a tool to map the ages and composition of different parts of the Milky Way.
Besides serving as tracers of the galactic environment, brown dwarfs are a testbed for all sorts of interesting atmospheric chemistry; they’re similar in size to the largest planets, so the results have implications for exoplanets. One recent moment in his research that particularly excites Burgasser is the detection of phosphine in a brown dwarf. Phosphine is a molecule known since the 1970s to be plentiful in the atmospheres of Jupiter and Saturn. Models predicted that phosphine should be abundant in brown dwarf atmospheres…but astronomers looked carefully at brown dwarf spectra and it was nowhere to be found. Now Burgasser and colleagues have found one brown dwarf with a clear phosphine signature. “It’s interesting because it doesn’t solve a mystery, I think it actually makes a mystery more intriguing,” says Burgasser. What aren’t the atmospheric chemistry models capturing, and why should one brown dwarf have phosphine while the others don’t? Phosphine has been proposed as a sign of life on terrestrial planets, as on Earth it is only produced by organisms, but clearly we have a lot to learn before we can use it as a biosignature. “If we can’t get the chemistry of phosphine right in planets that have no life, then we should be very careful about making claims about this molecule in planets that do have life, or may have life.”
Besides researching brown dwarfs, exoplanets, and cool stars, Burgasser also conducts astronomy and physics education research and is active in equity and outreach efforts. Burgasser names the common thread to all of his professional activities as curiosity. “Every activity is a way to learn how to do better and to understand.”
Burgasser has been fascinated by astronomy since a young age, but he had career ambitions besides being a researcher along the way. As a physics undergraduate – at UCSD, where he is now a professor – “I had the very good fortune to work with Sally Ride, who was a professor here at UCSD at the time, and I was very inspired to become an astronaut. I was like, ‘Yes, this is definitely made for me, I want to do this!’ And one of the ways to do that is to get a graduate degree in a science or engineering discipline. So I went to grad school in physics, studying astrophysics, partly as a direction to become an astronaut, realized I was much better at astrophysics than being an astronaut, and went from there.”
When it comes to advice for students starting out in research, Burgasser’s is simple: Don’t overthink it! Specifically, don’t overthink the topic you want to research – the skills you gain will be useful even if you go in a different direction later. “Try to do a few different research projects, don’t worry if it’s not specifically around the direction you want to go in,” and if you find you don’t like a particular direction, that’s valuable information too. Burgasser himself tried a number of different research projects prior to coming to brown dwarfs in grad school, and “all these pieces really helped me, came in at different places in my career.” He sums up his own path as, “even though it was a very random walk process, it was the right process.”
Dr. Burgasser’s plenary talk, “The Brown Dwarf-Milky Way Connection: How Failed Stars Play a Unique Role in Galactic Archaeology,” will be on Monday, Jan. 5th, from 3:40-4:30 PM MT in West Building 301AB.
Read more on brown dwarfs on Astrobites
Edited by: Lindsey Gordon
Image Credits: AAS
