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Why are most galaxies and solar systems 'flat'?



 
 
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  #1  
Old October 23rd 03, 08:13 AM
Richard Dickison
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Default Why are most galaxies and solar systems 'flat'?

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.



  #2  
Old October 23rd 03, 10:06 AM
Gavin Whittaker
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Default Why are most galaxies and solar systems 'flat'?

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
  #3  
Old October 23rd 03, 11:18 AM
Will Trash-Spam
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Default Why are most galaxies and solar systems 'flat'?


"Gavin Whittaker" wrote in message
...
Richard Dickison writted:

[snip]
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


Treasure Planet per chance? Disney always took/take a liberty with the laws
of science....


  #4  
Old October 23rd 03, 05:21 PM
Martin
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Default Why are most galaxies and solar systems 'flat'?


"Richard Dickison" wrote in message
...
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.


I also think that Centripatal force would play a part here. Any Galaxy
(like ours) that spins around a central point will tend to spread it's mass
out along the plane of the spin motion. I understand that calculating the
rate a Galaxy turns is one method used to calculate it's mass?

Martin


  #5  
Old October 23rd 03, 05:39 PM
Pete Lawrence
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Posts: n/a
Default Why are most galaxies and solar systems 'flat'?

On Thu, 23 Oct 2003 07:13:03 GMT, "Richard Dickison"
wrote:

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.


In terms of a Solar System, most of the material that comprises the
final Star/Planet system will be hoovered up by the star in the centre
of the system.

As the cloud shrinks, any rotation in the original cloud will quicken
(the skater bringing their arms in trick) and the remaining material
will tend to flatten out into a disk around the main body (the star).
As the star turns on, any residual material will be vapourised or
blown away.

Heavier material is harder to blow away than lighter gases which is
why the planets nearer the Sun are rocky and the planets further away
are gaseous.

--
Pete
Homepage at http://www.pbl33.co.uk
CCD/digicam astronomy
  #6  
Old October 23rd 03, 07:10 PM
Richard Dickison
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Posts: n/a
Default Why are most galaxies and solar systems 'flat'?


"Martin" wrote in message
...

"Richard Dickison" wrote in message
...
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.


I also think that Centripatal force would play a part here. Any Galaxy
(like ours) that spins around a central point will tend to spread it's

mass
out along the plane of the spin motion. I understand that calculating the
rate a Galaxy turns is one method used to calculate it's mass?

Martin


Centripetal force would seem to play a part if the debris were being thrown
out of the nucleus, but since the nucleus is pulling the debris in, I'm not
sure if the intuitive 'water sprinkler' galaxy applies.


  #7  
Old October 23rd 03, 07:42 PM
Martin
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Posts: n/a
Default Why are most galaxies and solar systems 'flat'?


"Richard Dickison" wrote in message
...

"Martin" wrote in message
...

"Richard Dickison" wrote in message
...
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.


I also think that Centripatal force would play a part here. Any Galaxy
(like ours) that spins around a central point will tend to spread it's

mass
out along the plane of the spin motion. I understand that calculating

the
rate a Galaxy turns is one method used to calculate it's mass?

Martin


Centripetal force would seem to play a part if the debris were being

thrown
out of the nucleus, but since the nucleus is pulling the debris in, I'm

not
sure if the intuitive 'water sprinkler' galaxy applies.


Is it not a matter of balancing of forces? What the gravitational effect of
the nucleus of the galaxy V the centripatal force? Some galaxies spin very
slowly (I assume that most spin to a degree?) hence the galaxy will tend to
be much more compressed and look less like ours or say Andromeda.

Martin


  #8  
Old October 23rd 03, 07:47 PM
Martin
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Posts: n/a
Default Why are most galaxies and solar systems 'flat'?


The link below might be of interest to those reading this thread.

Martin

http://helios.astro.lsa.umich.edu/Co.../tf_intro.html


  #9  
Old October 23rd 03, 09:06 PM
Jonathan Silverlight
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Posts: n/a
Default Why are most galaxies and solar systems 'flat'?

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"
Remove spam and invalid from address to reply.
  #10  
Old October 23rd 03, 10:33 PM
Martin
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Posts: n/a
Default Why are most galaxies and solar systems 'flat'?


"Jonathan Silverlight" wrote
in message ...



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.


Good point, I guess the interaction between Galaxies, black holes and
whatever else we have yet to discover may have big influences on how exactly
Galaxies end up the way they do.

Martin


 




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