Rebecca Oppenheimer, a curator and professor at the American Museum of Natural History (AMNH), delivered a talk on degenerate matter—which includes white dwarfs, neutron stars, and brown dwarfs—on Tuesday, April 23 for the annual Sturm Memorial Lecture. The lecture began at 7:30 p.m. in the Ring Family Performing Arts Hall, and was followed by a reception and observing at Van Vleck Observatory.
The Sturm Memorial Lecture—named after Kenneth Sturm ’40—invites an astrophysicist to campus to speak about their research. The public event is aimed at a general audience, and has been held annually since 1991, except for a three-year hiatus due to the COVID-19 pandemic. Funding was provided by the NASA Connecticut Space Consortium, the Albritton Center for Public Life, and the Astronomy Department, which selects speakers based on their accomplishments and communication skills.
“We look for someone who is both an outstanding, creative, accomplished astrophysicist, and who is an excellent communicator with students and the public,” Astronomy Department Chair Meredith Hughes wrote in an email to The Argus. “We have had so many wonderful Sturm lecturers over the year[s], including three Nobel laureates and an astronaut—that’s the level of awesomeness we work to bring to our campus!”
Dean of the Natural Sciences and Mathematics Martha Gilmore, a planetary scientist, introduced the event and described its history, while Professor of Astronomy Ed Moran introduced Oppenheimer. He noted that Oppenheimer was originally invited to speak in 2020, before the pandemic necessitated the cancellation of the Sturm Memorial Lecture that year.
“Dr. Oppenheimer has done as much as anyone to bring us closer to those goals, and she has made observational discoveries that have revolutionized our ideas about star and planet formation,” Moran said.
Oppenheimer began her lecture with a response to Moran, introducing herself and her topic.
“Ed, that was a really lovely introduction—actually, I’m a degenerate,” Oppenheimer said. “In physics, degenerates are something entirely different. In some ways, they’re supreme, and current theories and models show that the universe will eventually enter a degenerate state.”
Oppenheimer explained that degenerate matter behaves differently than the matter we encounter on Earth. When the mass of an object increases, we typically expect the size—or radius—of the object to increase proportionally. Degenerate matter, however, gets smaller when more mass is added.
“Think of two rocks,” Oppenheimer said. “You take them, put them together: it’s twice as big. Think of two pieces of this degenerate matter. You put them together and it shrinks—it gets smaller. And the more you put on it, the smaller it’ll get. There’s no such thing in any of our daily experience that behaves this way.”
Oppenheimer delved into some astronomical history to explain the significance of degenerate matter. Astronomer Friedrich Bessel detected a small companion orbiting Sirius, one of the brightest stars in the night sky, in 1844. He noticed that the companion is 4,000 times fainter than Sirius, yet it is massive enough to cause a large deviation—a wobble—in the motion of the star. A spectrum of Sirius taken in 1915 revealed that the companion, called Sirius B, contains the mass of the Sun within the radius of the Earth.
The immense density of Sirius B was not explained until the 1920s, when quantum mechanics described a new kind of pressure: degeneracy pressure. This pressure prevents extremely dense objects from completely collapsing under their own gravity. When an object is extremely compressed, electrons are forced into the lowest-energy states possible, creating degeneracy pressure. Displaying a slide filled with equations, Oppenheimer explained some of the mathematics behind degenerate matter, such as the Heisenberg Uncertainty Principle, the Pauli exclusion principle, and equations of state.
“I hope no one is terrified by this,” Oppenheimer said. “You should never be terrified by math. It’s fun, especially calculus.”
Although there is no degenerate matter on Earth, Oppenheimer provided three examples of degenerate objects: white dwarfs, neutron stars, and brown dwarfs. She showed a plot of mass-radius relations for these three objects, as well as Jupiter and M dwarfs, a type of star.
“Everything interesting in the Universe is on this plot,” Oppenheimer said.
Almost all stars eventually become white dwarfs, which contain half the mass of the Sun within the size of the Earth. These objects are composed entirely of hydrogen and frequently surrounded by planetary nebulae, the material ejected by a star when it becomes a white dwarf.
“They were called planetary nebulae because people at first thought they were planets,” Oppenheimer said. “These things are just gorgeous. They come in all kinds of shapes and sizes.”
As a part of her curatorial work at the AMNH, Oppenheimer created a three-dimensional model of the Helix Nebula, which has a white dwarf in its center. She showed this model as a part of her presentation.
“Part of the fun of working at the museum and being in the planetarium is occasionally I’ll get to make a new planetarium show,” Oppenheimer said.
Neutron stars are another example of degenerate matter, compressing 1.4 times the mass of the Sun into the size of Manhattan. These objects are the cores of certain stars—those that are 8 to 40 times as massive as the Sun—and remain after supernovae. Oppenheimer shared an image of the Crab Nebula, the remnant of a supernova.
“It’s just beautiful,” Oppenheimer said. “I mean, why wouldn’t people be interested in this? This kind of matter doesn’t exist here on Earth.”
A third kind of degenerate matter is a brown dwarf, an object with a mass between that of a large planet and a small star. Although brown dwarfs can vary widely in mass, their radii are fairly constant. While describing stellar masses, Oppenheimer took a moment to discuss a pet peeve.
“By the way, never tell anyone that the Sun is average,” Oppenheimer said. “It’s not. Ninety percent of stars are lower mass than it. The sun is also very quiet. Sort of a weird star. So, you don’t live around an average star. I hear that all the time, and it drives me up the wall.”
Oppenheimer spent some more time discussing the mass-radius relationships of celestial objects, noting that the classification of these objects into stars, planets, and brown dwarfs is arbitrary. She argued that it would make more sense to base a classification system on the mass-radius relationship of these objects, which tend to fall into four trends.
“Now if you look at this [mass-radius plot on the screen] and forget about these stupid labels, why wouldn’t you really characterize this as four different groups of objects?” Oppenheimer said. “One of the proposals a while ago was that we forget about these terms because nature doesn’t know anything about what we call planets.”
Having provided examples of degenerate objects, Oppenheimer switched focus to her technical work on imaging brown dwarfs. To emphasize the challenges of imaging these relatively faint objects, Oppenheimer shared a simulated image of what the Sun would look like if seen from 30 light years away. No planets were visible without suppressing the light of the Sun, and even without this light only Saturn and Jupiter could be easily seen.
Because stars are so bright, it is difficult to directly image orbiting planets. Jupiter, for example, is a trillion times fainter than the Sun. Oppenheimer works on coronagraphs, which block out the light of stars so that fainter objects—such as planets—can be seen.
“When you’re driving at night or standing on the side of the road and there’s a car coming towards you—the bright light in your eye—and you hold your hand up, you can actually see the car and maybe the side of the road,” Oppenheimer said. “That’s essentially what we’re doing.”
Oppenheimer described some of her specific projects building instruments for astronomical imaging, sharing photos of various telescopes, her lab, and her dog. Before Oppenheimer sends an instrument she has built to its new home, she brings the instrument to the dinosaur exhibits at AMNH after closing hours.
“I don’t think there are many astronomical instruments that have met a T-Rex before,” Oppenheimer said.
Oppenheimer noted that there are approximately 250 scientists at AMNH, which functions not only as a museum for the public but also as a research institution. The cases in the Collections Core at AMNH highlight different research departments, and two are dedicated to astronomy. Oppenheimer curated one of those cases.
“I can’t put a brown dwarf in there, so what I did is, one of the cases is about instrumentation,” Oppenheimer said. “It’s really wonderful to be able to put something like 30 years of your career together into a case to show the public.”
In addition to direct imaging, Oppenheimer explained that she also studies degenerate objects by measuring their movements via spectroscopy, similar to how Bessel detected Sirius B. If a star has companions, its spectrum will shift depending on the masses of those companions. One instrument Oppenheimer uses to detect Earth-sized planets is the Palomar Radial Velocity Instrument (PARVI).
“I’ve had so much of my time there because it’s such an effective observatory to do radical new types of instrumentation,” Oppenheimer said. “Most places don’t let you monkey around and try things out.”
Before the speaker and attendees left for the reception at the observatory, there was a question-and-answer session.
Professor of Astronomy Seth Redfield asked Oppenheimer what new developments in astronomy in the next 10 to 20 years she most looked forward to.
“I think one of the things we found with…all these thousands of worlds… is that they’re all different,” Oppenheimer said. “To me, it’s that range or diversity that I’m most interested in.”
Gilmore asked Oppenheimer whether a brown dwarf and a planet could be in the same system. Oppenheimer hinted that a paper she is working on will have more details about the answer.
“Actually, I will give you a little teaser,” Oppenheimer said. “I’m working on a paper on that brown dwarf that I found a long time ago…. This turns out to be a really interesting system, far more interesting than we knew in 1994 [when it was discovered]. There should be…a couple of papers coming out soon. There are three radial velocity–detected planets orbiting this thing within this 5 AU circle here…. They’re roughly about 1, 3, and 8 Jupiter masses…and there’s something very weird about this thing as well. And that’s all I can say at the moment.”
Although the Astronomy Department has wrapped up this semester’s public events, space nights and kids’ nights will resume in the fall.
Elias Mansell can be reached at emansell@wesleyan.edu.