Vera Rubin, the Rotation Curve, and the First-ever Evidence of Dark Matter

From the Lecture Series: Great Heroes and Discoveries of Astronomy

By Emily LevesqueUniversity of Washington

Vera Rubin was born in Philadelphia in 1928. As a child, she would lie in bed and close one eye, then the other, noticing how the pictures that hung on her bedroom wall seemed to jump back and forth. Her questions about how things move and the rotation curve of the galaxies would eventually prove invaluable for the entire field of astrophysics.

A group of five women posing for a picture in front of a building.
Astrophysics owes a lot to Vera Rubin (second from the left) and her studies. (Image: NASA/Public domain)

Vera Rubin

Rubin received a scholarship from Vassar College and finished her degree in 1948 after only three years, taking astronomy coursework during the school year and carrying out research during the summers. She then earned a master’s degree from Cornell and a Ph.D. from Georgetown, studying the motion and clustering of distant galaxies, and went on to work at Georgetown for the next decade. 

While teaching a graduate class at Georgetown in 1962, Rubin posed a unique challenge to her students, asking whether they could use data from cutting-edge star catalogs to measure the rotation curve of the Milky Way.

Understanding the Rotation Curve

To understand what a rotation curve is, let’s imagine a simple scientific plot. On the horizontal axis, we can measure how far away a star or clump of gas is from the center of a galaxy. On the vertical axis, we can measure velocity, or how fast that star or clump of gas is moving. The shape that this plot makes can tell us how an object’s location in a galaxy is related to its speed, and that in turn can tell us a lot about what a galaxy is made of and how it works.

Let’s imagine a galaxy that’s built like a solid disk, spinning like a turntable or a merry-go-round. If we measure the speed of several points on that spinning disk, we can see that points closer to the center of the galaxy move more slowly, while points closer to the edge move faster. In a solid disk galaxy, this would mean that stars near the edge of the galaxy have higher velocities, and so the rotation curve of that galaxy would look like this—a straight line.

The rotation curve of a spiral galaxy
An object’s speed and location in the galaxy can help scientists to learn more about galaxies. (Image: Mario De Leo/Public domain)

However, we know that galaxies aren’t actually solid disks. In this way, the motion of objects in a galaxy might bear more resemblance to the motion of something like our solar system, with small masses orbiting a large center. 

In the 17th century, Johannes Kepler calculated physical laws for the motion of planets orbiting our Sun. Kepler’s third law stated that planets farther away from the Sun would move more slowly, with a speed proportional to one over the square root of the distance. According to Kepler the rotation curve of a galaxy would look like this, a decreasing curve.

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Rubin’s Study of the Motion of the Stars

When Rubin first started studying the motion of stars in the Milky Way with her class in 1962, most astronomers expected that a real galaxy’s rotation curve would look like a combination of the two curves: an increasing line near the center of the galaxy where the densely-packed core would act as a solid body, and a decreasing curve throughout the rest of the galaxy following Kepler’s third law. 

The rotation curve of the Milky Way
The actual flat rotation curve of the Milky Way was astonishing as astronomers expected different results. (Image: Soonclaim/Public domain)

However, Rubin and her students found something else. As they measured the motions of stars further and further from the galactic center, the rotation curve was strangely flat; it did not decrease as would be expected for stars obeying Kepler’s laws of motion.

The Carnegie Image Tube

Over the next few years, Rubin continued studying the velocities of stars in and beyond the Milky Way. In 1965, she moved to a new job at the Carnegie Institution of Washington and began working with another astronomer, Kent Ford. 

When Ford began working with Rubin, he had also recently developed a groundbreaking new tool for capturing astronomical data, the Carnegie Image Tube. Ford’s new image tube took advantage of something called the photoelectric effect. When photons with a specific amount of energy hit a material, it will emit electrons. 

Finally, Ford’s image tube took advantage of something called a phosphor screen, which would emit photons when electrons hit it. Putting it all together, the net effect was that a few photons could enter one end of the tube and, by magnifying the photoelectric effect, a ton of corresponding photons would come out of the other end through the phosphor screen. By photographing from the far end of the tube, you could take brilliant and detailed images that would have been far too dim to capture directly.

Measuring the Rotation Curve of Other Galaxies

Together, in 1970, Rubin and Ford were able to measure the rotation curve of the Andromeda galaxy, the Milky Way’s nearest neighbor. Again, the rotation curve came out flat. No matter how many galaxies Rubin and Ford studied, the results stubbornly remained the same. After puzzling over the results for months, Rubin realized that the data could be explained perfectly if we moved beyond what we saw. 

If these galaxies had invisible haloes of matter—around five to 10 times as much as the stars, gas, and dust combined suggested—they would have the flat rotation curves she had been observing since that 1962 class at Georgetown. She had identified the first indirect observational evidence of dark matter.

Common Questions about Vera Rubin, the Rotation Curve, and the First-ever Evidence of Dark Matter

Q: What was the challenge that Vera Rubin posed to her students?

During her time at Georgetown University, Vera Rubin asked her students to measure the rotation curve of the Milky Way using advanced stellar catalog data.

Q: What does Kepler’s third law state?

In the 17th century, Johannes Kepler calculated the physical laws for the motion of planets orbiting the Sun in the solar system. Kepler’s third law states that the farther away the planets are from the Sun, the slower their motion is. According to Kepler’s law, the galaxy’s rotation curve would look like a decreasing curve.

Q: What was the result of the study on the rotation curve by Vera Rubin?

When Vera Rubin began studying the motion of Milky Way stars with her students, most astronomers expected a galaxy’s rotation curve to be a combination of a straight line and a decreasing curve. But the studies showed otherwise, as Rubin found out that the rotation curve was strangely flat.

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