In message , Gavin Whittaker
writes
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.

In which case it seems to me that the interesting question is why some
objects are _not_ flat - the Oort Cloud (if it exists), globular
clusters, and elliptical galaxies, for instance.

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 impressive, but hardly likely to be stable...

I can't find my copy of Richard Baum's "The Planets: Some Myths and
Realities" but ISTR that William Herschel thought he had observed rings
around Uranus at right angles !
--
"It is written in mathematical language"
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