As you learned in the first article on rotating galaxies, astronomers in the early-20th century measured light from hydrogen to measure the speed at which spiral galaxies spun around on their axis like great celestial hurricanes. Astronomers expected galaxies to rotate according to the same laws that govern the motion of our solar system, the laws discovered by Kepler and Newton. What they found, eventually, was something quite astonishing…
Here’s what astronomers expected to see in rotating spiral galaxies…
A star near the center of the galaxy’s disk feels the gravity of all the other stars in the galaxy pulling it equally in all different directions, so it feels a net gravitational force close to zero and barely rotates at all around the center. A star orbiting a little further from the center feels the pull of gravity of the stars closer in, so it rotates a little faster.
Further out, stars feel even more gravity from the closer-in stars and so move even faster. Stars near the visible edge of the galaxy feel the influence of nearly all the stars in the galaxy and move fastest of all (see the red and white curves in the image below).
Beyond that, the outlying stars, astronomers reasoned, should move around the galactic center at a slower rate because they are farther away from the combined mass of all the other stars. You’ve seen this effect before. Kepler’s Third Law says that bodies orbiting farther away from a central object, whether the Sun or an entire galaxy, should rotate at a speed which decreases with distance. So this is what astronomers expected to find when they measured the rotation of stars across the visible face of edge-on spiral galaxies (see the red curve in the image below). This plot of rotation speed vs. distance from the galactic center is called a “rotation curve”.
Because galaxies are so faint, measuring the rotation curve is a difficult business. But by the early 1970’s, instruments advanced far enough to allow very precise measurements of galaxy rotation from the center out to the visible edge of the disk. American astronomer Vera Rubin made a painstaking study of galaxy rotation and discovered the rotation rate does not fall off with distance from the center. It stays constant (see the white curve in the image above). This implied more mass, much more, must extend beyond the visible realm of a spiral galaxy and make the stars at the edge of the galaxy rotate faster than expected.
How did astronomers react to Rubin’s discovery?
Many did not believe her. Rubin was no stranger to adversity: she began her training in the 1950’s when women were a rarity in most astronomy departments. She was not even allowed to apply to Princeton University because they refused female applicants until 1975. But she graduated with a Ph.D. from Georgetown University and earned the respect of many colleagues, finally landing a position at the prestigious Carnegie Institution in Washington.
But Rubin was confident in her measurements, and by the early 1980’s other astronomers verified her work. So they had a real puzzle on their hands. Radio observations of cold hydrogen gas beyond the visible disk revealed insufficient mass to explain Rubin’s results. So this missing matter is not simply cold hydrogen or anything else visible with optical or radio telescopes.
There were hints of this unseen matter before. In 1933, the Swiss/American astronomer Fritz Zwicky noticed the visible mass of galaxy clusters could not account for their motion. He suggested there must be hundreds of times more invisible matter than visible matter in these galaxies. He called this material dunkle Materie — dark matter– because it had mass and therefore gravity but emitted no light.
What is this dark matter? Is it a form of everyday matter that simply emits no light? Or is it something else?
We leave that until the next article…