Why do galaxies form discs rather than spheres?

I was think about the formation of the galaxy and solar system last night when a strange question occured to me. Why do galaxies form discs rather than spheres.
From what I understand in my own amatuerish way of cosmology is that the galaxyies formed after the big bang had spread matter and time throughut the universe. As the galaxy expanded, gravity caused great clouds of dust gas and plasma to condense. Between the momentum imparted by the big bang and the force of the gravity these clouds began to spin. However what I don't understand is why they began to spin in an single direction. The image used to llustrate this is of water flowing down a drain, the hole representing the center of gravity pulling the matter in. But shouldn't gravity have imparted a gyroscopic motion to the young galaxy? If gravity pulls in all directions equally shouldnt matter have moved into the center from all directions equally? And shouldn't that have imparted a gyroscopic spin?
Why did the galaxy flatten out to produce a disc? does gravity have an up and down? (as viewed from outside)

I know I did not explain my question very well but I hope those of you with the knowledge and education to awnser my question can understand it enough to tell me what I am missing.

First off, there are galaxies that look like spheres. Just look at some of the deep space pictures taken by Hubble. However, most of them are more disc like and this could be because many galaxies have supermassive Black Holes at their centers so the galaxy itself is like a giant Event Horizon of stars and gas.

It's because if galaxies would be spheres matter's (dust, gas, planets,stars...) orbits around galaxy's center would cross each others which would cause collisions between them. These collisions would eventually change orbits so that they would settle closer to same level. (it's same like if everybody would drive at freeway without any rules and continuously changing lane or from driving from roads side to other it would cause collisions and would eventually block whole traffic)

This same applies to forming of solar systems.

Great question.

I think the galaxies that appear to be spheres,Jazz,are just seen from a different view.

And great answer E-T.

I'm just wondering also if a strong primary axis of revolution at the center might play a part.I don't know if moons are known to orbit at an angle less than a certain degree from the axis of a planet?

John Bull....so you think that there are no spherical galaxies? So, would they be sort of "turned upside down" from the Earth's perspective...not literally, but you know what I mean?

PS- Am I correct in saying that most galaxies centers are supermassive Black Holes, and that the stars and space dust, gas, etc. are like the Event Horizon orbiting the singularity? If this is incorrect then please inform me otherwise.

Thanks E.T. I knew I was missing something.

Not quite sure of the event horizon idea, I thought that was more of a theoretical model rather than something we can see with out eyes, but I may be wrong.

E_T's suggestion seems to make a lot of sense. Also once a disc like structure does start to develop, wouldn't the gravitational pull from the mass of matter already in the disc tend to pull other matter in to join it?

just wondering.

Good question. I asked similar questions about black holes which are probably actually spheres not holes. Just like the sun spins on an axis the black "sphere" does also. Just like solar system planets, and moons orbiting planets tend to do so around the mid point of the spin object. Look at Saturns rings to see the effect. Why are those rings at the axis? Why are they not spread over the entire planet like a cloud?

Our planets are generaly lined up around the axis of our sun.

If you look at black holes you can see as you get closer in, that the dust clouds do tend to form a sphere around the center and suns tend to move toward the disc or axis. So the heavier the objects orbiting appear to move toward the axis to orbit there, causing the disc shape. I bet smaller younger galaxies are the more spherical ones as they have not moved out and settled about the axis. Imagine if our planet in our solar system all had odd orbits...

I also bet just outside the event horizons that there are very dense rings like you see at Saturn but on a much larger scale.

Most of this is speculation based on things Ive read. I am not a cosmologist, but I stayed in a Holiday Inn Express last night

[edit on 3-8-2004 by Xeven]

[edit on 3-8-2004 by Xeven]

It's same with Saturn's rings.
curious.astro.cornell.edu...


And matter orbiting/falling to black hole forms disc called "accretion disc".

Yes,I would say so except for immature galaxies that have not had time to form the disc shape.

I'd also suggest that the rotating axis at the center becomes defined slowly alongside the disc formation,each feeding and defining the other.

Another thing that is quite interesting is that the same thing has already happened on a smaller scale in our solar system.Debris rotating around the sun made of different elements (heavier elements nearer the sun gases further away) colliding and creating planets.

I'm just wondering if the final model of this galaxy or even the universe is an object of huge mass with a series of giant planets representing the different group of mass elements orbiting on just a larger scale to our solar system? As slowly the chaos becomes conformity?

So what would the Roche limit of a galaxy be?

Is this even a valid question?

I don't think it really matters because once it's torn apart it the satellite becomes debris perhaps reforming to it's original size or,more likely forming a smaller satellite/s the rest becoming part of the primary body.

Or that's how I see it.It would definately be part of the process but one eventually leading to a stable and predictable conformity.


[edit on 3-8-2004 by John bull 1]

Well the Ort Cloud and Kupier belts are yet to be maped or seen or in case of Ort cloud even proven to exisit! Yes they probably do. The further away from the suns gravity you get the less effect it has on objects orbiting it so further out some things may orbit outside fo the axis. I bet though, that the ort cloud, if it is out there, is more concentrated (thicker), near the axis of our sun than throughout the entire span of it all.

In other words the Ort cloud may extend in a spherical nature around our sun, but there would be more mass (dust,rocks roids, comets) near the axis. Ort cloud is were they think comets come from right? Most of the comets probably come from the axis area of the ort, and that is why comets orbit near axis as well (not sure if this is true)?

Why do you assume that the oort cloud is thicker at some part than the other?

The spherical oort cloud surrounds the sun at a very large distance (radius), way farther out than the orbit of pluto. If its affected by sun's gravity, it would be uniform throughout the sphere. I think planets' gravity would be negligible since its so far away.

Talking about comets, they do come from the oort cloud according to the findings. Comets are found to have huge eccentricities and they don't really follow the sun's axis of rotation or that of other planets'.

One of my central questions had been why do objects in the universe rotate on an axis rather that spin in a gyroscopic motion. Last night I was reading "The science of discworld" (a truly great popular science book if you get a chance to read it) When my question was awnsered in terms of solar systems. Apaarently when the gas clouds which form into solar systems are young they are spherical hwever the side which is closest to the galatic center moves faster than the opposite side. this is what cases the cloud to flatten into a disc shape.

I don't think the answer is necessarily because the further from the center the slower the rotation...

I think it is because of the "plane of rotation" (as I call it) which is perpendicular to the axis of rotation and bisects the diameter of the rotating object along the axis.

The plane of rotation is where the rotation is the fastest, the poles of an object rotate very slowly, the equator very quickly.

Therefore objects towards the "equator" are more likely to be "flung" outwards.

Because of the nature of the genesis of galaxies, some are discs and others are spheroidal.

The discs were created around a supermassive black hole, which created an "accretion" disc because of this plane of rotation effect, and as stars were created, they moved out into this plane as well. Hence the oldest stars are in the disc.

So as the black hole rotates, the galaxy rotates around it, and the stars migrate to where there is the most force which happens to be where there is the most angular momentum.

I don't think it is because the galactic center moves faster, the center is a point and has no axis or plane of rotation.

It is because the rotating object (in this case a black hole) has an axis and a plane of rotation and the most angular momentum is at the equatorial region and thus a disc forms along that region as matter in the "bulge" migrates outwards.

After all you can only be thrown off when the force is great enough, so objects towards the poles of the bulge will not be thrown out along the disc, they must first migrate towards the equator and then they are pushed outwards.

I may not have explained this well.
The reason the side of the solar system closest to the galatic center spins fastest is that the gravitic influence of the galactic center is greater. As a result the sde closest to the center is accelerated. Imagine a bike wheel whch you spin by slapping one side. I will try to remember to bring the book in tommorow so I can quote it properly.

wouldnt the black holes cause the galaxies to spin, thus causing the flat shape from the spinning. thats my guess.

Black holes aren't those which cause rotation of galaxies... and that matter was there even before those black holes were born.

curious.astro.cornell.edu...