'The Sky is for Everyone': Talking with Princeton women in astrophysics

Neta Bahcall and Gillian Knapp

As the James Webb Space Telescope (JWST) delights stargazers with breathtaking images and groundbreaking discoveries — like the recent revelation that a planet 700 light-years away has a carbon dioxide atmosphere — we connected with some of Princeton’s astrophysics luminaries: Gillian “Jill” Knapp and Neta Bahcall, who have been at the University for a combined 87 years.

“It’s just astonishing how we can sit here, on Planet Earth, and figure out how the universe started,” said Bahcall. “Astronomy is an exciting field, and it’s an exciting time to be in this field.”

Bahcall and Knapp were the first two women on Princeton’s astrophysics faculty, and they have spent the past half-century shaping the department into a welcoming environment for astronomers of all backgrounds.

Book cover of The Sky is for everyone: Women Astronomers in their own Words, Edited by Virginia Trimble and David A. weintraub

“Neta and Jill are both considered to be among the most distinguished astrophysicists in the world,” said Tea Temim, one of the newest members of the astrophysics faculty at Princeton and a scientist on one of the space telescope’s instrument teams. “Having these women on the faculty made a huge difference to me. The climate in a department — how welcoming it is to women and underrepresented groups — is always something I consider when choosing where I’m going to spend my time and do research.

“Not only Neta and Jill but also Eve Ostriker, Jo Dunkley and Jenny Greene have all been extremely supportive, and have served as mentors to me in making the transition here in this challenging time,” said Temim, who joined the astrophysics research faculty at the height of the pandemic.

We sat down with Bahcall, Princeton’s Eugene Higgins Professor of Astronomy, and Knapp, an emerita professor of astrophysics, shortly before their essays appeared in the new book, “The Sky is for Everyone: Women Astronomers in Their Own Words,” published by the Princeton University Press and edited by David Weintraub and Virginia Trimble.

When did you first fall in love with the stars?

Jill Knapp: I was a sickly child, and my bedroom had a sloping ceiling. When I couldn’t sleep, I would watch the stars go by and see the moon and its craters. My mother used to bring me books from a secondhand bookshop in Edinburgh, and one day, she brought home “Splendour of the Heavens.” The covers were gone, and the first 50 pages were gone, but it hardly mattered.

And I was a nerdy kid who did physics experiments in the kitchen. There was the disaster of some rockets that I couldn’t wait to set off, and it was raining, so I set them off in the house. That kind of thing.

Neta Bahcall: My way came very differently. As I wrote in the book, I grew up in Israel, and I wanted to go to medical school. But at that time, and this was many years ago, the medical school only accepted children of medical doctors, so I couldn’t go. I greatly enjoyed science, physics, math. I loved the beauty of it, the cleanliness, the clear results, solving things in a logical way.

I majored in physics and math, then I met John, my husband [John Bahcall, a renowned astrophysicist who later joined the faculty of Princeton and the Institute for Advanced Study], when he visited Israel. We married and I moved to Caltech, where John was junior faculty. Caltech was and is a powerhouse in astronomy. I was fascinated by the fundamental questions addressed by astronomy, and I began working on some of these with astronomers there. I thought, “Wow, that is just so exciting.”

People can come to astronomy from many different directions — either falling in love with looking at the stars at a very young age, or later in life when you start understanding better what it all means.

Also, as I said in my article for the book, I had a wonderful science teacher who turned me on to science. Teachers are so very important at any stage, from kindergarten to college. Mentors and teachers can turn students on — and they can turn them off.

Jill Knapp: In my undergraduate physics lab, there were four girls in our class of 70, and the fellow who ran it was very strongly of the opinion that women had no business anywhere near any of this. So he would come up to one of the girls and harangue you as you’re trying to do your experiment, and he kept at it until you burst into tears. Then he would go away happy and go pick on the next girl. So we all learned to burst into tears in 10 seconds flat — it’s something I could do today if I had to — and that got him out of our faces right away. [Laughs]

A lot of it is quite absurd.

I tell my students: Don’t accept the idea that somebody can tell you that you can’t do this. They have no right.

Neta Bahcall: So much has changed. Jill and I have the long perspective. We can see the change, and it’s dramatic. It’s still not 50-50, but it’s getting close. You can see the trends vividly when you go to conferences. There used to be maybe five women out of hundreds of people. Now there are so many women, it has become common. Typical. In departments, colloquia, conferences and everywhere. This was not the case 20, 30 years ago.

Jill Knapp: I think that the glass ceiling is completely broken in astrophysics. I don’t think things are going back. Our department chair, Jerry Ostriker, was really committed to changing the department, and that was great.

Q: How exactly does one determine the “first” woman astrophysics professor at Princeton? Neta, you were here first, but Jill, you were a professor first.

Jill Knapp: When David called me to be in this book [“The Sky is For Everyone”], I said, “Technically, yes, I was the first professor, but Neta was already here when I got here, so I think you need to ask us both.” I was the first woman faculty member on our side of Washington Road — in all of the vast engineering complex, all of physics, all of math, plasma physics, I was the first woman on the faculty. But Neta had been a research scientist here for some years before that, so it’s a bit loose as to the “first” woman astrophysicist here.

Neta Bahcall: I had been a research scientist at Princeton from 1971 until I left to become head of the science program at the [Hubble] Space Telescope in ’83. I was there six years, and then I felt, “OK, everything is set up, we’re all ready, it’s operable.” John and I were then both offered various positions in excellent places; but I finally accepted Jerry's offer of faculty position here in Princeton, and we came back.

Jill Knapp: I remember they threw this very nice party: You were coming back as professor, and I’d been promoted to full professor — the two first women professors.

Jerry wanted me to be Director of Graduate Studies, something the department cared about. It was an opportunity to expand and diversify our program. There was one woman graduate student when I came, and we had never had any members of any minority group. So that was something I could change — and did — and I stayed DGS until I retired. And the National Academy decided about 10 years ago that this is the best astronomy graduate program in the country.

Neta Bahcall: When I started at Princeton as a postdoc, there were no women. I may have been the first woman postdoc, I’ve never checked.

Jill Knapp: You were.

Neta Bahcall: There were certainly no women on the astrophysics faculty; the entire faculty was only five men. But today — 50 years later — it’s a major change that occurred in one generation. One lifetime. We now have five women professors, and more as research faculty, as well as many women at all levels: researchers, postdocs, graduate students, undergraduates. I’m the director of the undergraduate program, and about half of our students are women. It’s just not recognizable, compared to what it used to be. The department has been wonderful and supportive all along. I don’t think there is any better place to be an astrophysicist.

Q: You’ve also opened up the department to outside audiences in many ways.

Jill Knapp: Yes. People give public talks about astronomy, and of course we have this fabulous subject. Astronomy is the most glamorous science, there’s no doubt about it.

Neta Bahcall: The public always wants to know about new planets and dark matter and black holes, all exciting topics. We’re very fortunate to work in a very exciting field. We also attract many new students because the topic is so fascinating.

It’s simply astonishing how we can sit here on Planet Earth and figure out how and when the universe began. How did it start? How is the universe expanding and evolving, and how did stars and galaxies form? Astronomers discovered that there are unexpected and mysterious components in the universe like dark matter and dark energy. And we do all of these by observing the universe and then modeling it using the laws of physics. It’s astonishing to think how much we have learned about the mysteries of our universe.

Jill Knapp: Now we know the age of the universe to a percent or something. I mean, honest to goodness!  What we now have as astronomers, it blows your mind! The universe is an open book — we can just read the whole history of the universe in an image.

One of my favorite things is the empty beer bottle sitting on my desk, called Super Jupiter Beer. We found, by direct imaging, a “super Jupiter,” a planet more massive than Jupiter around a nearby star. There was a lot of fuss about it in the press, and a brewery made Super Jupiter Beer and sent us a case of it. They said the good things in life should be celebrated.

Neta Bahcall: The advancements in the field in recent years have been astounding. Right now we are trying to figure out, How did exoplanets form? It’s not exactly clear. We thought we knew it, more or less, but now we have to revise some of our understanding based on the new observational results. That’s the beauty of science.

Q: The popular love of astronomy is almost primal. Since we first emerged from caves and looked up, there’s been something just magic about the heavens.

Neta Bahcall: There’s “something magic” about understanding our cosmos; this is not something that most people think about day to day: planets, galaxies, dark matter, the beginning of the universe. But many people are curious and interested in both learning about these topics and trying to figure these out. We all receive emails from people offering their ideas about planets or dark matter. We hear from engineers, from experts in other sciences or mathematics. Because it is fascinating to think about something that is much bigger than daily life.

Jill Knapp: And yet it is daily life, because we’re all made of the same stuff. The carbon you can see in stars is the same carbon in us.

Q: Jill, you were one of the founders of the Princeton Prison Teaching Initiative (PTI). How does one teach astronomy to individuals who are incarcerated? How do you open the wonder of the heavens to those who might not see the night sky?

Jill Knapp: Astronomy’s about measuring the sky, and indeed our prison students can’t go outside at night. We do do one sky-observing lab, taking various measurements of the sun. There isn’t time enough to tell you what it took to get this to happen, and I don’t mind saying that when I walked into the prison with a wooden SunSpotter telescope for the first time, I felt like God Herself.

Some of our labs are paper labs, i.e. “Analyze these data we give you.” Some are experimental, some observational. And of course, the Space Telescope Science Institute — the science operations center for Hubble and JWST — has lots of pictures and posters for schools. So even in these difficult circumstances, teaching a good solid course is pretty straightforward.

We’ve taught several science courses in prison by now: astronomy, physics, biology, chemistry, environmental science, most with labs. The students do a great job.

Q: What gives you the most hope for where the field is going?

Jill Knapp: I’m optimistic because I’ve seen the effects of community.

When you say, “I’m not going to live like this,” things change. And what’s changed in astronomy? Sure, the instrumentation is just fantastic, but also the modus operandi for how science gets done. And I think that really changed with the Sloan Digital Sky Survey. For so long, people would keep their data secret. A scientist should never, never not for a second, do that. It’s not your sky, for pity’s sake, it’s not my planet. Data needs to be open.

It's true that the national observatories, both ground- and space-based, open their data archives after a short proprietary period, but these are raw data, which require extensive processing to get any useful science out. This requires a lot of training and access to powerful computer resources, and therefore in practice you need to be a professional astronomer. But the SDSS data are fully processed and ready for scientific analysis, and you need only modest resources — a laptop and access to the internet — and you're on an equal footing with any professional astronomer.

Sloan was my husband Jim’s big concept [Jim Gunn, Princeton’s Eugene Higgins Professor of Astronomy, Emeritus], but we both worked to make the data public. I’m very proud of that. It took a lot of work, but we were right: The science is just pouring out.

It’s brought in a lot of scientists all over the world who get access to state-of-the-art data and find something in it themselves.

Astronomy is blossoming all over the developing world. I wouldn’t say we take credit for that, but SDSS was a very big piece of changing the mores of the field into something that was much more collaborative.

Neta Bahcall: In space-based astronomy, the data is almost all public right away. It started with the [Hubble] Space Telescope. Previously, with the big optical telescopes on the ground, people kept their data close, but for more than 30 years now, Hubble data has been public. The massive Hubble data archive is being utilized by astronomers from all over the world.

And when you have data, a piece of the sky, you have many objects there. One person can analyze the stars, and another can study the galaxies, and somebody else can look for planets.

Jill Knapp: The democratization of data — that’s the big picture that I’m optimistic about. I’m also very optimistic on small levels. At Reunions, quite a lot of our students came back. Five of our Ph.D. students came because their children are graduating. That shows your age, doesn’t it? But all of them are working quite hard with whatever groups are marginalized in their part of the world. It was just lovely to see that.

Neta Bahcall: I’m very pleased to see the growth in the department, the change, the improvement. I see it at all levels. I work with students day in and day out in the undergraduate program. We went from about five students to 20 now. We have many women and students of color and students who are first in their family to go to college among our astro majors. Our students receive excellent support and mentoring, and they are very happy here. This is where one can see how important it is to provide good support and mentoring to students, and how much these benefit them.

As I mentioned earlier, it was my science teacher in high school that influenced my love for science. When I speak with my students, it’s not just about the love of science but the encouragement to stay with it. Especially in a demanding place like Princeton, I urge them not to drop a course quickly if they fail an exam, or if they think they don’t understand as much as others. Persistence is important, I tell them, important to keep doing what you love.

I hope our book inspires young girls, young kids, young adults, students, and young professionals to do what they love, whatever it is — whether the exploration of our cosmos or something else.


Neta Bahcall, Princeton’s Eugene Higgins Professor of Astronomy, has served as the director of undergraduate studies for the department since 1993. Born in Israel in 1942, she completed her B.S. in physics at Hebrew University in 1963, her M.S. in Physics at the Weizmann Institute of Science in 1965, and her Ph.D. in Astrophysics from Tel Aviv University in 1970. She came to Princeton in 1971 as a postdoctoral researcher and stayed on as a research astronomer until 1983, when she left to lead the Hubble Space Telescope’s science team. She returned to Princeton in 1989 as a professor.

Gillian Knapp, an emerita professor of astrophysical sciences, served as the department’s director of graduate studies until she retired in 2014. Born in the United Kingdom in 1944, Knapp received her B.Sc. in Physics from the University of Edinburgh in 1966 and her Ph.D. in Astronomy from the University of Maryland in 1972. She came to Princeton in 1980 as a research astronomer, then joined the astrophysics faculty as an associate professor in 1984. In 2005, she (with then-postdoctoral researchers Mark Krumholz of the Class of 1998 and Jenny Greene, now a professor at Princeton) founded the department’s program to teach college courses in the state’s prison system, which grew into the University-wide Prison Teaching Initiative.