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Why do galaxies spin or rotate?
– Swara Dhat, age 13, Thane, India
Spinning is fun, and you can find many ways to do it. From carousels to an office chair, spinning is everywhere – even in outer space.
As astrophysicists, we know that spinning – or rotating – is extremely common in space. The Earth, the Moon, the Sun, the entire Milky Way, and many things inside and outside of it, rotate.
But why is that? Why does anything rotate at all in space? To answer this question, we first need to understand how objects spin.
Spinning has laws, too
The amount of spin that an object has is called its angular momentum. It depends on the mass of the object and how that mass is distributed around the object’s center of mass. The center of mass is the location at which an object could be balanced on a point. Have you tried to balance a ruler on your finger? You balance it in the middle, which is the center of mass.
If something is spinning, and its mass and shape stay the same, its spinning speed stays the same. If its mass or the shape changes, its spinning speed also changes. Outside forces can change an object’s total amount of spin.
Think about spinning on a chair. If you first try spinning with your legs splayed out, you’ll find that you can pull your legs in to spin faster. This phenomenon is explained by the law of conservation of angular momentum. Pulling your legs inward changes your shape, and brings mass – your legs – closer to your center of mass, making you spin faster. Later, the chair slows down and stops. Its spinning parts rub against each other and warm up due to friction, which converts moving energy into heat.
Another example is spinning figure skaters, who spin faster when they pull their arms in. The law of conservation of angular momentum applies to everything, which means it applies to galaxies.
How to build a galaxy
Now that we understand how objects spin, let’s look at how galaxies form and why they spin.
Galaxies are extremely massive. They’re made up of regular matter, including stars, planets, gas, dust and people. They also contain dark matter, which scientists know less about but can study through its gravitational pull on objects.
Both types of matter have mass: They can pull on each other through gravity, the same way Earth’s gravity keeps you anchored to the ground. When the universe first formed, dark matter helped pull together clouds of gas and dust to make galaxies. Imagine two clouds of gas and dust in space traveling toward each other from any direction. It would be unlikely for the clouds to collide perfectly head-on; more often, they approach off-center.
The clouds share a center of mass, and depending on their relative speed, direction and mass, gravity may cause the clouds to spin into each other and fuse. As such, if the clouds manage to pull on each other and spiral into one another, the initial spiraling motion is conserved into the final rotation of the merged cloud, under the law of conservation of angular momentum.
Once the clouds spiral into each other, the material inside them collides. This collision squishes the clouds, changing their overall motion and heating them up to the point of glowing. The clouds slow down as they pass through each other and move away, but they can no longer escape each other’s gravitational pull. They continue this spiral dance until they fuse into a larger rotating cloud. The process then repeats for bigger rotating clouds.
Depending on the angle, speed and mass of the clouds when they meet, these collisions can speed up, slow down or reverse the spinning. Eventually, the cloud becomes large enough that more collisions with smaller clouds do not affect the overall spinning motion by much.
Gas and dust also fall directly into the clouds, along with streams of matter called cosmic filaments. Together, clouds and filaments join to form a rotating galaxy. This galaxy will continue to spin, unless something happens to it, such as merging with another galaxy.
The big three
There are three main types of galaxies, each with different levels of organized spinning inside them.
Irregular galaxies are blobs of gas, dust and stars. They have a general direction of rotation, though some objects do not follow it.
Spiral galaxies are more organized. They have a flat disk containing spiral arms, and a central galactic bulge. Most of the matter in the disk follows the same rotation direction, while things get a little messier in the center. The spiral arms are like traffic jams where there are more stars, gas and dust, and it takes matter more time to leave the spiral arms and enter the space between them.
Elliptical galaxies have a lot of stars but very little gas and dust, and many are the result of galaxies merging together. The stars orbit the galactic center, but may or may not follow an organized rotation depending on its formation and merger history.
All galaxies spin, but not all galaxies spin the same way!
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The authors do not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.
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