Why do galaxies form discs rather than spheres?

OK I know the answer here, let me see if I can explain it adaquately.

BIG BANG: Matter moves outwards, gravity attracts particles, the begin to cluster and form protogalaxies. The densest material sinks to the center of the cloud. The sheres we see on hubble are these protogalaxies, call them nebuli or what have you they are spherical because they havent condensed yet.

This matter becomes massive enough that it fires up like a giant reactor. A galaxy is created. Now the heat being released coupled with incoming debris creates friction which creates spin. Debris comes from different directions you get galaxies, and the same for solar system even, that spin.

OK A SPINNING BODY

Take a basketball on your finger spinning on an axis of rotation. Now imagine standing on a pole, i.e. where your finger is. Its easy enough to stay there, you get dizzy but the spin force is equal in all directions, so you dont fly off. Now imagine you are on the equator of the ball, you are at radius=x from the center of the ball and parallel to CENTRIFUGAL FORCE. This is the key, here you are being flung out from the center.

OK so a cloud of debris: gas, metal, alkalines, etc. Densest stuff goes to the center, you get a star or a back hole depending on your scale: solar system vs galaxy. Sound planets closest to the sun held there by the suns gravitational pull. Gas body form father out, gas being less dense is easier to hold onto, less gravity needed=farther from sun. The centrifugal force of the spinning galaxy or solar system flattens out the debris is the field from the original spin axis of the debris field, and whala - disk shaped body.

Old thread I know. but i need to add my two cents worth.

Ok all you folks have some good ideas. Stellar collisions aren't likely though. Planets are spread in a solar system some 10^5 times their diameter apart, stars are spread in a galaxy some 10^6 times their diameters apart. Coupled with the fact that they are co-rotating makes collisions so unlikely to be negligable.

Why do galaxies form discs.... its to do with interactions and conservation of angular momentum.

Our galaxy - the Milky Way probably had its spirals made by the LMC and SMC satellite galaxies. They create a non even tidal force on our galaxy. If the galaxy started off Elliptical or speroidal then it would have been elongated. However all the matter in our galaxy is gravitationally bound so cannot escape. This causes the elongated shape to rotate with the satellite galaxy around the centre of the elliptical one with matter colse to the edge being pulled round more thus winding the galaxy up into a spiral shape.

Other loose spiral structure in some galaxies is caused by cons of ang mom. In the earth moon system the moon drift slightly further out in its orbit to keep angular momentum the same. The earths rotation is slowed by tidal heating effects. With galaxies this needn't happen as they can shed matter. This matter (stars) which are flung off to conserve angular momentum during a collision form a loose spiral type shape.

Hope all this makes sense.

because the planets mass must also affect each other. they start orbiting the main star , and get strechted out into a line. in a spherical model they orbit the sun but also start pulling on each other. the suns pull is strongest, so the path of least resistance wins, which is a plate like shape.

Actually it's propably entirely other way.
Most of galaxies with lot of collisions/interaction with others are elliptical. (like galaxies in centers of bigger/denser galaxy groups/clusters)
Even our galaxy will become elliptical galaxy after few billion years after collision/merging with Andromeda.
Also that fits well to fact that observed elliptical galaxies have very little newer stars and they almost completely lack dust and gas clouds.
Shockwaves and gravity anomalies of collisions compress those clouds to stars. (whose radiation pressures scatter that what is left from those clouds)

And neither are Magellanic clouds massive enough to have much effect to Milky way.
In fact big galaxies like Milky way are shredding them to pieces/cannibalizing their smaller companion galaxies.


What is the origin of spiral structure in galaxies?
curious.astro.cornell.edu...


What you're describing is birth of star where radiation pressure stops gravity from collecting more matter/mass to star.
Stars form galaxies but galaxies aren't some bigger fusion reactors and fusion reaction doesn't have anything to do with shape of them.

I think the LMC/SMC will be doing to the Milky way something like this

bcs.whfreeman.com...|00110|&ns=450

I think E_T you are spot on about the evolution. In the early epochs of the universe right through to today dust clouds collapse and coallesce to form hot blue Irr galaxies. Which interact to form spirals or E/Sph. Which go on to form larger Ellipticals culminating in cD galaxies (very large diffuse ellipticals often with more than one core) which is very strong evidence for mergers. Also evidence for mergers is cD position at centre of clusters and high relative velocities of their cores ~ 1000kms compared to 100kms for their halo stars.

bah, i was gonna say it. beat me to it.


anyway, theres no such thing as centrifugal force. there is centripetal force, and that is pointed inward. centrifugal force is an imaginary force

If I am not mistaken, the flat component we see of, for example, a spiral galaxy is just the regular matter part, with a postulated spherical distribution of dark matter all around it...