Richard Dickison writted:
: Why are most galaxies and solar systems 'flat'? If there is a cloud of
: dust/gas that starts to coalesce around a localized density in the middle of
: the dust/gas, it seems it would attract from all 3 dimensions equally. But
: galaxies and solar systems attract primarily in a single plane. What
: happened to all the debris above/below the plane?

: I understand there will probably be an unequal distribution of matter
: surrounding the central object and there will consequently be a resultant
: angular momentum after a while. But that doesn't seem to explain why there
: still isn't debris spiraling in from above/below the primary plane.

The trick is to think about what would happen if there were a second
plane rotating at an angle to the primary plane. If there were (for the sake
of simplicity) two discs, you get interactions between the components of
both - collisions, gravitational interactions, etc - until some
stable distribution and movement evolved. This stable form would end up
as a (more or less) single plane. If you add more discs, you start to
get a more realistic picture, but it makes the mathematics and
conceptualisation more complicated, and doesn't change the basic idea.

You get a similar phenomenon in planets with ring systems - Saturn,
Jupiter, Uranus all have multiple rings, but in a single plane. I was
watching a film on TV the other night which had a rather nice gas planet
with two ring systems intersecting at an angle of around 30 degrees. Very
impresive, but hardly likely to be stable...

HTH, Gavin