Why are galaxies round as a pizza instead of round like a sphere?
In: Physics
Same reason pizzas are disks: a galaxy is a big amount of mass rotating in space, angular momentum is conserved but kinetic energy is dissipated as heat. Wait long enough and we have a disk.
Ever watched one of the many videos that the astronauts on the ISS send back to earth? They’re fascinating! Many videos involve water (or other liquids) and how it behaves in free fall. Put a bunch of water together, just floating, and it will stay a sphere.
If you spin it, however, it flattens out like a pizza! Centrifugal force from the spin causes the molecules to try to escape, but water’s tendency to stick to itself holds it together at low speed and makes the water go oval. If you did the same thing with a more sticky substance (like pizza dough) it could go faster and flatten out even more before breaking apart.
You could have a spinning sphere if the water was able to spin in every direction at once, but since the molecules bump into each other, they all cancel each other’s movement out until they’re all moving in the same direction at the same speed. That’s why galaxies mostly all spin like pizza dough.
Some galaxies are round, known as globular clusters. These take on a roughly globe shape. Some are irregular and are hard to define into any shape like the Magellanic Clouds that neighbor our galaxy. Some galaxies are elliptical or are so tightly wrapped, they look like flying saucers more than pinwheels, like the Sombrero Galaxy. Galactic scale formations are bound by gravity just like the contents of our solar system. Edwin Hubble was the first to characterize galaxy types.
Being that all galaxies form as a result to gravity, that can explain why a globular cluster galaxy makes sense. However all the stars and planets in a galaxy would drop in toward the center. What’s to keep them from dropping into the center? Spin or more properly orbits. The original gas and dust would slowly orbit around a common center. From there, suns and planets and whatnot would form. But all of them are orbiting a common center. This will be the center of any future galaxy. How fast the stuff orbiting around the center will partly determine its shape. The total galactic mass and what happens at the center will also contribute to what it looks like. There is also a sort of evolutionary period that can radically change galaxies as a result of mergers. For instance, our Milky Way galaxy and the Andromeda galaxy will merge one day far off.
Irregular galaxies have very low spin. So their affiliation stays loose. These galaxies tend to be the smallest. Some may change into Globular Clusters as they age and gain mass and spin. The Large Magellanic Cloud is one of those. It’s very close to our Milky Way galaxy.
Globular Cluster galaxies have not so low a spin. They are a larger in mass over that of Irregular galaxies (seeing a theme here?) A Globular Cluster is normally much smaller than the Elliptical or Spiral galaxy. This type of galaxy is relatively uniform and doesn’t have any shenanigans going in in the center.
Elliptical galaxies are are dead boring. They are the oldest types and are usually made from old red dwarf stars. They are also characterized as being somewhat uniform in feature, like a Globular Cluster. Unlike Globular Clusters, which are small, Elliptical galaxies can be any size. They are most commonly thought of as “not much going on”. Just very old stars living in a very clean neighborhood where everything is in order. Yup, there’s Bob Redstart 417964 mowing his lawn again, like he dies every day.
Spiral galaxies spin the fastest. Because of their total mass and rapid spin, arms appear. One may observe this when looking at whirlpools. These are called density waves and they are more visible because a large proportion of the galactic ‘disk’, is in those arms. However there is more. It is believed that galaxies that are spiral shaped or have large bulges in their centers, contain a clue as to why these types, and ours, are spiral. There is a lot you g on in the galactic center. It’s believed that there is a large or a series of large black holes at their cores. The center’s of spiral galaxies are so stuffed with stars, the entire region is bathed in extreme radiation of all kinds. This brightness contributes to a easily visible bulge. The mass of a typical Spiral galaxy is much larger than a Irregular or Globular Cluster Galaxy.
Boy Scout Astronomy badge comes in handy. My only qualification.
Pardon typos. I’m on a mobile device.
Galaxies (and solar systems) start out as large balls of gas (mostly hydrogen). If you get enough gas together, gravity takes over and starts to pull the gas towards the center. If none of the gas was moving, it would all pull towards the center and converge on one point at the center of the ball of gas. But due to the random motion of those particles, inevitably there will be one direction of motion, or rather one direction of rotation, that has more motion than all the other directions. So the ball of gas begins to converge, however since it’s spinning/rotating in that one direction, it ends up flattening like a pizza, instead of compressing to a point.
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They start out as a bunch of things revolving around a central point, and it looks like a sphere. However, it does not move like a spinning sphere; if you spin a sphere, only an infinitely-thin circular piece of it revolves around the center.
Over time, all these random intersecting orbits make the objects crash onto each other or pull gravitationally on each other, and all that’s left are the circular paths which don’t intersect, which is pizza-shaped.
Because spinning.
The same thing is happening within our solar system. All of the planets are spinning around the Sun in “pizza” formation, not as a sphere.
Fun fact: It’s the same reason that the Earth technically isn’t a sphere. It’s an oblate sphereoid. That means it’s round… but a little bit wider in the middle than top-to-bottom.
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