Sky and Telescope - August 2017 - 38

Vera Rubin: Her Life & Work

conﬁdant, he encouraged her to earn her PhD. Not long after
that decision, physicist George Gamow, then best known for
his theories on the origins of the chemical elements (and
who heard about Rubin through Robert's ofﬁce mate), called
Rubin to discuss her thesis. He became her PhD supervisor,
even though the only place to earn an astronomy PhD was at
Georgetown and he was at George Washington University.
After completing her PhD and joining the ranks at
Georgetown, Rubin returned to studying stars, starting in
the Milky Way. The working hypothesis at the time was that
stars in our galaxy, and all galaxies, should follow Newton's
law of gravitation: The tug two objects have on each other
is directly proportional to the product of their masses and
inversely proportional to the square of the distance between
them. Thus, the gravitational force two objects exert on each
other should drop precipitously as they move farther apart.
By extension, this law means that a star revolving around
a galaxy should experience an attractive force from all the
matter between it and the center of the galaxy, as though the
matter were concentrated right in the galaxy's center. The
more mass inside the orbit, the faster the star should move.
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A U G U S T 2 0 1 7 * SK Y & TELESCOPE

Visually, most of a galaxy's mass appears to be concentrated within a few tens of thousands of light-years of its
center. So outside that point, the effect of distance should
start to override the effect of mass: The gravitational force
should plummet with increasing distance from the galactic
center, meaning stars farther out should travel slower in
their orbits around the galaxy than stars closer in.
But when Rubin's team plotted the relationship of the
stars' distance from the galactic center against their velocities - a plot called a rotation curve - they found no drop-off
in velocity. The Milky Way's rotation curve seemed to be ﬂat.
Despite these paradoxical results, Rubin didn't immediately rush to the telescope to study the rotation curves of
other galaxies. It didn't immediately click that something
was off. Even if it had, female astronomers struggled to
obtain telescope time at the nation's leading observatories.
That, however, started to change for Rubin with a fortuitous move west. Robert took a fellowship at the University
of California, San Diego, in part so that his wife could work
with astrophysicists Geoffrey and Margaret Burbidge and
study the rotation curves of galaxies.

N ASA / ESA / HUBBLE HERITAG E TE A M (STSCI / AUR A)

p COMA CLUSTER In the 1930s Fritz Zwicky found that galaxies in the Coma Cluster (Abell 1656) move so quickly, they should fly out of the cluster
unless some invisible matter is holding them in place. This image is of a section about one-third of the way out from the cluster's center.

Sky and Telescope - August 2017

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