Postdoc Spotlight: Stella Ocker Explores the Space Between the Stars
Stella Ocker, a postdoctoral fellow at the Carnegie Observatories, studies the interstellar medium—the gas and dust between stars—and the diffuse ionized material that shapes galaxies like our own. Her research draws on radio signals from many sources, including data from Voyager 1’s Plasma Wave System, which she uses to infer the density of the interstellar gas the spacecraft is now crossing.
In this postdoc spotlight, Ocker talks about her path into science, her work on fast radio bursts, and what Voyager 1’s approach to the one light-day from Earth means for both her research and for humanity.
Stella’s Path Into Science
Q: Did you always think you would be a scientist?
I didn’t decide to study physics until high school. Before that, I didn’t have a particularly strong interest in science. Starting in late elementary school through early high school, I was much more interested in philosophy—particularly natural philosophy and philosophy of the mind. Also, as many adolescents are, I was interested in understanding why we’re here and why we exist. Ultimately, those questions led me to astronomy and physics, and those underlying questions still motivate me today.
Q: Was your love of science shaped by a particular experience or mentor?
I’ve had multiple mentors. But when I think back on it, there was one moment and one person in particular who played an important role in igniting my interest in physics and astronomy. During my sophomore year of high school, my science teacher, Mr. Hayden, off-handedly mentioned that time passes differently at the bottom of a mountain than at the top. He explained that this is because of a relationship between mass gravity and the passage of time. And I remember thinking, “What on earth are you saying? This makes no sense.”
When I approached him after class, he said, “If you want to learn more, read this book.” And he handed me Stephen Hawking’s The Universe in a Nutshell. That book was my first introduction to the concept of general relativity. My mind was blown by the idea that our perception of the world can be so skewed that time can pass differently for an astronaut in space than for a person on Earth. The fact that so much of our lives is shaped by our narrow field of perception still fascinates me. That curiosity nudged me to want to learn more about astronomy—physics in particular.
The fact that so much of our lives is shaped by our narrow field of perception still fascinates me.
Q: Who is your “Science Superhero”?
It’s crazy because I didn’t know she existed until last year when I read a book called Figuring by Maria Popova, which offers a broad overview of lesser-known historical figures, particularly from the 19th and 20th centuries. Her name is Mariah Mitchell, and she was a 19th-century astronomer who grew up in a Quaker family in Nantucket.
From a young age, she had a passionate interest in astronomy. Her father built an observatory on the rooftop of their family home, and every night she would go up there to observe the stars with her father using a two-inch telescope. During that time the King of Denmark, who was a patron of astronomy, offered gold medal prizes to anyone who discovered a comet using a telescope.
In 1847, Mariah was awarded one of these prizes for discovering a comet that would be named “Miss Mitchell’s Comet,” making her a bit of a celebrity. The process of astronomical discovery at the time was painstaking. She had to observe the sky every night for years and record the positions of known objects. And then, looking through a pretty rudimentary telescope, she had to clock any variations with her own eyes and note, “Oh, this object I see now wasn’t there before.” It’s a remarkable achievement, particularly for a woman at that time.
She went on to break a lot of barriers for women in science. She never got a college education, yet she was appointed as the first female professor of astronomy at Vassar College by its founder, Matthew Vassar. She also became the first woman elected to the American Academy of Arts and Sciences and the American Association for the Advancement of Science. In addition to her accomplishments in science, she was a social activist and early feminist, championing abolition and the women’s suffrage movement. She also helped found the Association for the Advancement of Women.
She lived this incredible life. It was mind-blowing to learn about this female astronomer who was a leader in the field, especially at a time when so few women were practicing science of any kind. I just thought, “This is the coolest lady I’ve heard of in a long time.”
She lived this incredible life. It was mind-blowing to learn about this female astronomer who was a leader in the field...
Research and Discovery
Q: What is your general field of study?
I study the interstellar medium—the gas and dust that exist in the space between stars within a galaxy—as well as the diffuse ionized gas that extends throughout the Milky Way and other galaxies. My research examines the distribution of that ionized gas using cosmic sources that emit strong radio waves, including neutron stars and fast radio bursts (FRBs), intense energy pulses emitted from celestial objects that have puzzled astronomers since their discovery.
I’m interested in understanding how ionized gas flows into and out of galaxies, and how that process shapes the galaxies we observe today, including our own.
Q: What is the coolest thing you’ve worked on so far in your career?
As an undergrad, I did summer research at McGill University, where I was first introduced to the field of FRBs. At the time, these fast radio bursts were a novel curiosity in radio astronomy. They had only been observed a handful of times, manifesting as these extremely energetic bursts of radio emission. They were unlike the radio transient sources we observed in our own galaxy, so we had an idea that they were coming from other galaxies. But we didn’t know exactly how they were produced, how common they were, or where they were coming from.
We were observing these bright radio flashes going off in the sky, but we didn’t know what they were—they were a complete and total mystery. My summer at McGill happened to coincide with their astronomy department building a major survey telescope called CHIME to search for more FRBs. I was so lucky I had the opportunity to work with the team as they were constructing this groundbreaking instrument.
As I progressed through undergrad, there were a couple of major advances in the study of FRBs. The first was that the location of an FRB was pinpointed to another galaxy, offering direct proof that these radio bursts are cosmological sources, meaning they come from galaxies other than our own. That was a huge breakthrough. Another pivotal observation was that FRBs were observed as repetitive phenomena, with multiple bursts detected from a single source.
That was a game-changer in our understanding of FRBs because it immediately ruled out a whole class of models that hypothesized the physical sources of these radio bursts. One of these models, the cataclysmic model, proposed that these bursts might be produced when something cataclysmic happens—like when two black holes collide. However, that specific model was predicated on one major event producing a single burst. But we were witnessing multiple bursts coming from the same source, so we knew those models had to be wrong. Instead, we deduced that there must be some physical source that could produce multiple bursts over a long period.
My undergrad experience and my summer at McGill marked my initiation into the world of scientific research. And I was extremely intrigued by the FRB puzzle. So, when I was applying for grad school and considering what I wanted to study, the field of FRBs was a natural choice.
During my first year of grad school at Cornell, I attended an American Astronomical Society (AAS) meeting, where professional astronomers from around the world gather to discuss the latest research. I couldn’t believe it when I saw a press conference on the agenda about the first results from CHIME. When I left McGill two years earlier CHIME was still under development.
Having been at McGill while it was under construction, I was deeply invested in its outcome and couldn’t wait for the session. As I sat in the audience, they announced they had discovered more repeating FRBs. Before this discovery, there was only one known repeating FRB source. My mind was blown, but I remember it being so quiet in the room. And there I was, screaming on the inside with excitement thinking, “Oh my God, this is such a big deal. Why aren’t people jumping out of their seats?” Those discoveries only served to deepen my interest in FRBs.
So, when you ask what is the coolest thing I’ve worked on so far in my career, it’s interesting because the field has changed so rapidly.
A couple of years into grad school, thanks to my advisor’s connections, I had the opportunity to work on a project with a substantial collaboration of scientists from around the globe. We were observing FRBs using a telescope in China called FAST. FAST is the most sensitive radio telescope in the world. It’s a massive radio dish—500 meters in diameter—that essentially fills up an entire basin in a mountain range. During this collaboration, we detected this one particular repeating FRB that was highly unusual. One of the main reasons it was an anomaly is because it appeared to have a lot of dense gas in its host galaxy, making it wholly distinct from all of the other FRBs we had observed up to that point. A substantial portion of my dissertation ended up centering on this specific FRB. It was one FRB of only about 20 that had been localized to their host galaxies. With this very small sample, we knew the exact location of their host galaxies, the types of galaxies, and all of the host galaxy properties. These are all really critical pieces of information that help us better understand the physical mechanism producing these energetic radio bursts.
So, when you ask what is the coolest thing I’ve worked on so far in my career, it’s interesting because the field has changed so rapidly. And, when I look back, major discoveries and collaborations were happening during these pivotal moments in my academic career—from undergrad and my summer at McGill, when I first learned about how scientific research works, through to grad school as I embarked on my dissertation journey. I have had the great fortune to be at the forefront as the study of fast radio bursts has evolved.
Q: What do you think is the most exciting research direction happening in your field right now?
The most exciting thing in my field right now is the advent of these sensitive surveys that can pinpoint an FRBs’ exact location in the sky at the same time it is detected. This is a significant technological advance that only became the standard in the field a year or two ago and is largely thanks to a unique radio telescope called DSA-110. In fact, one of the reasons I came to the Carnegie Observatories in Pasadena was to work with the research team at Caltech that was building DSA-110.
While telescopes like CHIME and FAST are very sensitive, they aren’t capable of the high-precision localization that DSA-110 offers. Before DSA-110, localizing an FRB required much more telescope time because they are totally unpredictable events, so we never knew when to expect them. That put us at the mercy of telescope committees, and us having to persuade them to give us more time with their instruments. You may get 10 hours of observation time and not see anything. DSA-110 does everything at once.
Q: Looking 10 years ahead, what future development do you think will have the greatest impact on your field?
There is another major FRB survey called DSA-2000, which is currently being designed for a site in Nevada. It is the more powerful successor to DSA-110. The numbers in the names of the telescopes refer to the number of individual radio antennas that are used in tandem to make a whole array that forms the radio telescope. The more individual antennas a telescope has, the more sensitive its combined antennas are collectively. Like DSA-110, DSA-2000 will be capable of detecting and localizing FRBs simultaneously. The big advancement in DSA-2000 is that it will be much more sensitive. It will have a sensitivity comparable to telescopes like the Arecibo Telescope in Puerto Rico, which was the world’s second most sensitive radio telescope in the world—after FAST—until it collapsed in 2020. DSA-2000’s increased sensitivity will allow it to detect many more FRBs, and it will be a long-term survey that is expected to start in roughly four years and last into the 2030s.
The more individual antennas a telescope has, the more sensitive its combined antennas are collectively.
We anticipate it will allow us to detect thousands of FRBs and localize them to their host galaxies. This represents a major sea change in the field of FRBs because instead of having under 100 FRBs localized to their host galaxies, we will have hundreds and eventually thousands in several years. And that will dramatically change the type of science we can do using FRBs.
Q: Do you have a favorite FRB and why?
If I had to name one favorite, it would be FRB 190520, the one I studied in grad school where I had the opportunity to work with the team that discovered it. I wrote a couple of papers about it. I’m partial to it because it shows a lot of unusual properties—it’s highly variable and seems to live in a very dynamic environment. At the time of its discovery, it defied many of our expectations about FRBs, making it especially interesting. Often in science, we’re chasing those things that go against our expectations or that we don’t immediately understand—things that motivate us to solve the puzzle and, hopefully, make new discoveries. And this FRB definitely challenged us and exposed a gap in our knowledge.
Often in science, we’re chasing the things that go against our expectations—those mysteries motivate us to solve the puzzle.
Voyager 1 Nears One Light-Day
Q: Voyager 1 will soon become the first spacecraft so distant that a radio signal from Earth takes a full day to reach it. What does this milestone mean to you?
Voyager continues to provide invaluable and extremely unique scientific information about the environment around our Sun. This mission is the only source we have for direct measurements of interstellar gas and cosmic rays in our solar neighborhood. This new record it has set in light travel-time from the Earth reinforces how special this mission is. I count each day that Voyager operates as a new and exciting milestone for humanity’s journey to the stars.
Q: Does Voyager play a role in your research?
I use voltage measurements [data] from the Voyager Plasma Wave System to infer the density of gas that Voyager travels through. These data help us infer what our Solar System neighborhood looks like, and what our Solar System will encounter as it continues its journey through the Galaxy.
[Voyager] is the only source we have for direct measurements of interstellar gas and cosmic rays in our solar neighborhood.
Career Growth and Advice
Q: What are some of the challenges you have encountered so far in your career and how did you address them?
Transitioning to the postdoc experience came with an interesting set of challenges. As a postdoctoral fellow, I am funded to do my own research, which means I’ve been working completely independently. That’s a big change from grad school, where I had an advisor I could regularly consult when I encountered problems or roadblocks in my work. In my current situation, it’s just me. The experience has forced me to confront where I get motivation.
I didn’t realize until I left grad school how much external motivation I received from the people around me, particularly my advisors. In some ways, this new setup has been liberating because it allows me to take the time to think about my interests and what I want to pursue. At the same time, it has been a struggle because I don’t have a single, obvious person to consult when I get stuck, especially when a specific problem is more complicated than I originally thought. I’m learning that proactively connecting with other scientists is important in this environment. But, as an introvert, that can be difficult sometimes.
That said, since starting my postdoc I’ve had some illuminating conversations with people from both Carnegie and Caltech who are doing highly interesting research. And while they work on completely different things than me, they still have valuable insights because our research techniques are generally the same.
When I was in college and applying for grad school, I had one particular experience that really threw me off the rails. I got appendicitis the week before the physics GRE. It was also within a few weeks of when my grad school applications were due. The whole process of juggling the surgery, the recovery, and the residual anxiety from that experience, coupled with writing applications and taking tests, made that an exceedingly difficult period. It forced me to question my motivations—it was hard for me to remember why I wanted to pursue science in the first place. There were so many different things at play simultaneously and my mental bandwidth was significantly impaired. But I’m also the sort of person who has a hard time deviating from “the plan” once I set my mind on something and decide to do it. If I were a different person, I might have taken a step back and put off applying for grad school. But I couldn’t allow myself to change course, so I put myself in a tough spot.
What helped in my first year of grad school, when I was still recovering from all this, was that I learned to devote more time and energy to taking better care of myself and setting distinct boundaries between school and my time outside of school. I became really regimented about creating more balance and setting aside dedicated time for things like exercise, meditation, and cooking. That helped me heal in many ways. It also taught me critical time management skills that helped me later in grad school and beyond.
I learned to devote more time and energy to taking better care of myself and setting distinct boundaries between school and my time outside of school.
Part of what made it possible for me to create that space for myself was that I knew grad school was a time of exploration. When I started my grad program it became clear that no one knew exactly what they would be doing several years down the road. Research is a very creative process. It can take you in directions you don’t expect and that openness helped release me from having a rigid set of goals to work toward. I knew it was a time to explore a subject in deeper detail and discover where the interesting areas of inquiry were. So having that sense of freedom in the academic space helped me relax in the personal space. It taught me I could take care of myself on a personal level, while also pursuing the things I wanted to pursue in science.
Q: What advice would you give graduate students and others just starting their scientific careers?
My advice would be to learn about subjects outside of your chosen discipline. When I was in college, I spent a decent amount of time taking classes in things like literature and history. I strongly believe that exploring other subjects shaped me into the researcher I am today. For example, studying literature taught me how to parse text—regardless of whether it’s fiction, nonfiction, or scientific articles—with a level of attention to detail that I would not have had otherwise. Also, I learned how to search for general themes and patterns in writings, whether historical or contemporary. As you advance in your career and go deeper into your discipline, it becomes increasingly important that you know how to zoom out to identify those big problems. Learning that critical skill gave me a unique perspective on my field that I would not have had otherwise.
Studying literature taught me how to parse text with a level of attention to detail that I would not have had otherwise.
Q: Why did you decide to do a postdoctoral position at Carnegie Science?
My training is in radio astronomy, and as an institution specializing in optical astronomy, Carnegie has these powerful optical telescopes available to its researchers. I was drawn to Carnegie because I knew it was a place where I could expand my horizons and learn from people using tools and telescopes I had never used before.
Q: What has your experience at Carnegie meant to you so far?
It’s been a very stimulating experience. Carnegie has allowed me to take my research in directions I wouldn’t have expected. For example, I’m using the Magellan Telescopes, which I couldn’t have imagined two years ago when I was in grad school, using radio telescopes. Also, Carnegie is a special community because it consists mostly of research staff and postdocs. That’s very different from being a postdoc in most university departments where the predominant constituency is students and faculty, with postdocs representing a minority. At Carnegie, I believe postdocs are the majority and it’s very nice to be surrounded by scientists at the same career stage as me and going through similar transitions.
Carnegie is a special community because it consists mostly of research staff and postdocs. That’s very different from being a postdoc in most university departments where the predominant constituency is students and faculty, with postdocs representing a minority.
In addition, I’ve had the opportunity to travel a lot to various conferences where I’ve met other scientists and set up new collaborations. Being granted the freedom to travel has been huge for me because it has helped me expand my professional network exponentially in the span of just several months. Carnegie also offers its postdocs so much flexibility in their research.At OBS, we are each funded individually to do our own independent research, making it a uniquely interesting environment because everyone is working on completely different projects. There is such a diverse range of scientific inquiry here, which I find really exciting.
