Rounding out our trio of influential women in science is astronomer Vera Rubin, whose observations and calculations provided the first empirical evidence of a mysterious unseen entity lurking throughout the universe. In the 1970s, while observing the nearby Andromeda Galaxy, Rubin and her collaborator Kent Ford noticed that the stars in this spiral galaxy appeared to defy the fundamental laws of physics. The entire galaxy, it seemed, was spinning at the same speed—despite most of the mass being concentrated in the center. In fact, the outer stars were moving so fast that they should have been hurled out of the galaxy completely as their inertia exceeded the stabilizing hold of gravity. But the stars seemed stable enough in their rapid orbits. Rubin calculated that, for Andromeda to spiral in this way, there must be at least ten times as much mass in the galaxy than what was expected based on visible stars. But where is that mass, and why can’t we observe it? Physicists have come to call this invisible material dark matter. Dark matter may just hold our entire universe together, and the mystery of its identity has haunted physicists for the past several decades.
Born in Philadelphia in 1928, Vera Rubin was fascinated with astronomy from a young age. By twelve, she routinely spent her nights watching the movement of the stars through the sky using a cardboard telescope her father helped her build. During the day, she would go to the library and read books about astronomy—after gaining written permission from her mother to check out books from the adult readers section. And every night, she came back to those curiously moving stars. “What fascinated me was that if I opened my eyes during the night, they had all rotated around the pole,” Rubin said in a 1995 interview for the American Institute of Physics. “And I found it inconceivable. I just was captured.”
While her parents were supportive of her interest in astronomy, many of Rubin’s teachers were less than encouraging. At the time, there was very little precedent for women interested in the field. The only female astronomer Rubin knew about at the time was Maria Mitchell—the first woman to study astronomy professionally in the United States. Mitchell taught at Vassar College from 1865–1888 and was a director of the observatory there. Inspired by Mitchell’s scientific journey, Rubin decided to apply to Vassar for her undergraduate degree and was accepted with a scholarship. She was the only astronomy major in her class, and she graduated after three years in 1948. The same year, Rubin married a physical chemist and joined him at Cornell University, where she worked on completing a master’s in astronomy and had her first child. Rubin’s master thesis on the velocity distribution of galaxies was met with backlash and skepticism at the time, but she would ultimately return to build upon it years later.
After getting her master’s, Rubin took a break to raise her son, but she found herself incessantly drawn back to Astronomy. “After a few months of being out of school, I wanted to learn more,” Rubin wrote in her 2011 autobiography. “I would push David to the playground, sit him in the sandbox and read the Astrophysical Journal.” In the end, it was her husband who encouraged her to pursue a PhD from Georgetown University after their family moved to DC. At Georgetown, Rubin studied under George Gamow—a nuclear physicist and cosmologist who was an early proponent of the “Big Bang” theory and the originator of the “liquid drop” model of atomic structure (which was crucial to Lise Meitner’s explanation of nuclear fission).
After earning her PhD in 1954, Rubin taught astronomy at Georgetown for ten more years. In 1965, she was offered the chance to make observations using the Carnegie telescope at Palomar, a facility that wasn’t normally available to women. In fact, Rubin notes in her autobiography that the building only had a men’s restroom—she recalls covering up the door sign with her own crude drawing of a “skirted woman.” The experience at Palomar reminded her of the love she had for observing the stars. Shortly after, she left her teaching position at Georgetown for a research position at the Carnegie Institute where she met Kent Ford. Ford had just developed a uniquely powerful spectrometer that could pick apart the light from individual stars in distant galaxies.
When looking at stars in other galaxies, resolution is a big issue. Any telescope has a diffraction limit—the absolute smallest distance between two objects that it can discern from a given distance. Eyes have diffraction limits too (check out this physics question on how big Legolas’ eyes must be to see as well as he can in LOTR), which is why you can’t read very small text from far away. You can think of diffraction limit as the pixel size of the world you, or your telescope, can see. If you look up at the night sky, what you might call “stars” is often really a cluster of stars, but at that distance, all those stars melt together into a single “pixel” of light. The Andromeda galaxy, one of the closest galaxies to Earth, can actually be seen by the naked eye (from the Northern hemisphere on a clear night). It appears as a fuzzy patch of light. Even with a fairly powerful telescope, it is difficult to pick apart the individual stars in that galaxy. That’s where Ford’s spectrometer comes in.
A spectrometer splits apart light into different wavelengths, making it easier to separate them out and categorize them. Ford’s spectrometer used special photomultipliers— devices that sensitively convert photons of light into electrical signals to create photographic plates. This special spectrometer made it possible for Rubin and Ford to dissect distant galaxies and determine how they work. They turned their gaze to the Andromeda galaxy to look more closely at how its individual stars rotated. Galaxies, much like solar systems, have the most mass concentrated in the center, so it is reasonable to assume that the objects in the center would rotate the fastest. In our solar system, for example, Mercury orbits the sun over eight times faster than Neptune. This property is all about the center of gravity and inertia, and all rotating systems behave in this way. Think about how an ice skater spins for example. Going into the spin, they pull their arms into their body as tightly as possible. This allocates more of their mass to the center, speeding them up. When they want to slow back down, they may stretch out their arms again to disperse their mass and reduce their speed.
According to the basic physics of rotating objects, Rubin and Ford expected that the velocity of stars in the Andromeda galaxy would decrease in proportion to their distance from the center, but that’s not what they found. Instead, all the stars seemed to be traveling at the same speed. Even at the fringes of the galaxy, where the gravity holding the stars together should be weak, the stars were still rotating around the center at the same breakneck speed. Instead of being hurled out of the galaxy, like unseatbelted children flung out of a cheap carnival ride, the stars at the periphery of the Andromeda galaxy seemed to be held in place by some invisible force.
Rubin and Ford quickly started taking observations of other nearby galaxies and kept getting the same results: the stars were all moving at the same speed, pinned in place by a gravitational force much stronger than predicted. The idea of “dark matter” was theoretically proposed in 1933 by Fritz Zwicky, but it was mostly disregarded as conjecture until Rubin and Ford’s startling results came to light in the 60s and 70s. Since that initial demonstration, no one has been able to identify a culprit for dark matter. The suspects range from inconceivably small subatomic particles—smaller than even an electron—to massive, slow-moving particles or even solar-sized black holes left behind from the dawn of time. But thanks to Rubin’s extensive characterization of galaxy rotation, few scientists can dispute that dark matter exists—and it may even be more prevalent than “light” matter. “In a spiral galaxy, the ratio of dark-to-light matter is about a factor of 10,” Rubin said in an interview in 2000. “That’s probably a good number for the ratio of our ignorance-to-knowledge. We’re out of kindergarten, but only in about third grade.”
Despite never winning a Nobel prize, Rubin did gain recognition for her discovery. She was elected to the National Academy of Science in 1981, and in 1993, she was awarded a National Medal of Science by President Bill Clinton. All four of her children have also pursued doctorates in science. She died in 2016 at the age of 88 on Christmas Day.
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