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Vera Rubin graphed the rotation curves of galaxies, helping astronomers better understand the accelerated orbits of stars on the outskirts of galaxies. Her life's work generated some of the first solid evidence for dark matter in the universe.

Thumbnail Credit: Vera Rubin - Courtesy, Carnegie Institution of Washington
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Sources:

http://science.sciencemag.org/content/sci/295/5557/960.full.pdf?ck=nck
https://www.aps.org/programs/outreach/history/historicsites/carnegie.cfm
http://www.marciabartusiak.com/uploads/8/5/8/9/8589314/spins_the_stars.pdf
https://blogs.scientificamerican.com/guest-blog/vera-rubin-didnt-discover-dark-matter/
http://www.astronomy.com/news/2016/10/vera-rubin

Images:

https://carnegiescience.edu/sites/carnegiescience.edu/files/RubinAtLowellObs1965.jpg
https://commons.wikimedia.org/wiki/File:GOODS_South_field.jpg
http://hq-filemaker1.carnegiescience.edu/images_archives_search.php
It’s the 1970s.

Sean Connery is still James Bond, fashion is blindingly ugly, and astronomers are working tirelessly to solve the galactic rotation problem. Which is actually a very misleading name, because the problem was that galaxies rotated too well.

The stars on the outskirts of galaxies were orbiting much faster than astronomers expected, and they were starting to question what they thought they knew about the laws of physics. Eventually, they realized that they were looking at the first direct evidence for dark matter. And it was an astronomer named Vera Rubin who found it.

Rubin was born in 1928 in Philadelphia, Pennsylvania, and she loved to stargaze as a kid, she was especially fascinated by how the stars would move through the sky overnight. Her father, who was an electrical engineer, helped her make a telescope when she was 14, and she started going to meetings for amateur astronomers. In interviews, she used to claim that she faked her way through high school, since she turned every assignment into an excuse to write about astronomy.

That passion for the stars carried her to Vassar College in New York, which she chose because one of her awesome astronomer forebears, Maria Mitchell, had taught there. From there, she got a master’s degree in astronomy from Cornell, and a PhD from Georgetown. Her master’s thesis involved analyzing the movements of more than a hundred galaxies, and her PhD looked into how galaxies are distributed through the universe.

Maybe you're starting to sense a theme here: she was really interested in galaxies. After bouncing around between a few different professorships, Rubin landed at the Carnegie Institution in Washington, DC in 1965. It was there that she and fellow astronomer, Kent Ford, turned their attention to how stars orbited the centers of galaxies.

Ford had invented a more sensitive kind of spectrometer, a tool that splits up the light detected by a telescope based on its wavelength. Rubin and Ford used it to calculate how fast different parts of galaxies were moving. When they plotted the stars’ orbital velocities across a galaxy, they expected to see that the stars close to the center orbited really fast, with orbits getting slower and slower the farther the stars were from the center.

Like how in our solar system, Mercury moves much more quickly in its orbit around the Sun than Neptune does. But that’s not what they saw. They found that stars on the edges of galaxies were orbiting just as quickly as the stars closer in.

It made no sense. The stars on the outskirts of these galaxies were orbiting so fast that the galaxies should have basically flown apart, the mass of all the matter that they could see in each galaxy shouldn't have been enough to hold them together. Now, Rubin and Ford weren’t the first people to notice some odd galactic motions.

There had been a few isolated observations earlier in the century. For example, in the 1930s, American astronomer Horace Babcock observed that the nearby Andromeda galaxy was spinning way too fast. Jan Oort, the Dutch astronomer who the Oort Cloud is named after, saw something similar with the Spindle Galaxy in the constellation Sextans, and so did a Swiss astronomer named Fritz Zwicky, in some of the galaxies in the Coma galaxy cluster.

They came up with a few different explanations for this behavior. Babcock thought it might have to do with light absorption, or maybe that objects on the outskirts of galaxies had some different dynamics that we didn’t have the math yet to describe. Oort and Zwicky both independently suggested that there were halos of non-luminous matter around the galaxies, aka dark matter.

What Rubin and Ford discovered was that this problem didn't just exist for one or two galaxies at a time. It showed up all across the sky. Babcock, Oort, and Zwicky hadn't found some weird anomalies; they saw specific examples of a widespread phenomenon.

Rubin graphed the motions of these galaxies in rotation curves, plotting the velocities of objects from their centers out to their edges. In the 1970s, she gathered and published a huge amount of data, showing clearly and incontrovertibly that the galactic rotation problem was typical galactic behavior, and that there was some kind of unexplained physics at work. She also realized that the dark matter hypothesis was consistent with her observations for all these galaxies.

If there was a bunch of matter in them that we couldn't detect, that would explain why the galaxies were rotating so fast. After that, astronomers started finding more and more evidence for dark matter, and these days, most astronomers think that 84% of the matter in the universe is dark matter. Rubin died in December 2016 at the age of 88.

Through her decades of work on galaxies and dark matter, she laid the foundation for what’s now a huge field of research. Astronomers still have no idea what dark matter is, and there are thousands of researchers all over the world trying to figure it out. So, in a lot of ways, Vera Rubin is still contributing to our knowledge of the universe.

Thanks for watching this episode of SciShow Space. And for more on dark matter, check out our video “What we don’t know about dark matter,” which explores some of the possibilities astronomers have considered over the years.