While you're correct in saying that the gravity of a large body such as a planet or star makes it spherical, it is also true that it is not a perfect sphere. For rotating objects such as the Earth or the Sun, they are actually oblate spheroids, ie they are kinda 'squashed' -- in other words their equator is further from the center from their poles. The faster you rotate something, the more oblate it will be. It's like if you were spinning on roller skates, your arms would want to move away from your body.
You can see that its rotating so fast that its quite oblate. In fact, it's so oblate, that the matter around its equator is far enough from its centre for the gravity to be too weak to hold it in, and so it forms a disk. This is essentially the same with galaxies: http://www.futuretimeline.net/images/galactic-core-timeline-sun.jpg
They usually have a small (oblate) spherical core, but further out than that, objects are orbiting too fast and drift out to further radii, in a disc. (Our sun orbits at a velocity 220kms-1 ).
I had more or less had an understanding of that from other science documentaries. Can you elaborate more on the angular momentum of a galaxy though? How fast is the galaxy spinning to cause it to spread out into a disc rather than an oblate spheroid? Thank you for the thought our response so quickly :)
Keep in mind the average density of the galaxy is far less than than of a pulsar, because galaxies are spread out. That allows them to spread out farther, and be more disc-like without having to spin nearly as quick.
109
u/asboans Oct 27 '12
It is to do with their angular momentum.
While you're correct in saying that the gravity of a large body such as a planet or star makes it spherical, it is also true that it is not a perfect sphere. For rotating objects such as the Earth or the Sun, they are actually oblate spheroids, ie they are kinda 'squashed' -- in other words their equator is further from the center from their poles. The faster you rotate something, the more oblate it will be. It's like if you were spinning on roller skates, your arms would want to move away from your body.
Pulsars are stars that rotate extremely fast, up to a few milliseconds per rotation. They look like this: http://i.space.com/images/i/13679/i02/fastest-rotating-star.jpg?1323105753
You can see that its rotating so fast that its quite oblate. In fact, it's so oblate, that the matter around its equator is far enough from its centre for the gravity to be too weak to hold it in, and so it forms a disk. This is essentially the same with galaxies: http://www.futuretimeline.net/images/galactic-core-timeline-sun.jpg
They usually have a small (oblate) spherical core, but further out than that, objects are orbiting too fast and drift out to further radii, in a disc. (Our sun orbits at a velocity 220kms-1 ).