Look across the Universe, and you’ll see that almost everything is rotating. The Earth rotates on its axis as it orbits the Sun. And the Sun itself is rotating. As you can probably guess, we even have galaxy rotation with our Milky Way galaxy.
Our galaxy is rotating incredibly slowly, however. It takes the Sun 220 million years to complete a single orbit around the galaxy. In the 4.6 billion years that the Sun and planets have been here, they’ve only rotated around the center of the galaxy about 20 times.
We know that galaxy rotation is happening because the Milky Way is a flattened disk, in the same way that the Solar System is a flattened disk. The centrifugal force from the rotation flattens out the galactic disk. All stars in the galactic disk follow roughly circular orbits around the center of the galaxy. Stars in the halo can have much different orbits and speeds.
The calculation of the high rotational speed of the galaxy led to the discovery of dark matter. If our galaxy contained just the matter we can see – planets, gas, etc – the galaxy rotation should cause it to spin apart. Instead, there’s much more mass holding the galaxy together. In fact, astronomers have calculated that the total mass of the galaxy is probably 10 times greater than the sum of all the stars in it. 90% of this is invisible dark matter, holding the galaxy rotation together. And only 10% is the regular matter that we can see. Our galaxy really has a mass of more than 1 trillion suns, and extends out more than 600,000 light-years; a third of the distance to the nearby Andromeda galaxy.
All the galaxies we can see are rotating. It’s this rotational force that counteracts the inward pull of gravity from all the galaxies. If galaxies didn’t rotate, they’d collapse inward and just join the supermassive black holes at the hearts of galaxies.
We have written many articles about galaxies for Universe Today. Here’s another article about the rotation of the Milky Way.
We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.