The Naked Scientists

The Naked Scientists Forum

Author Topic: Why are solar systems and galaxies circular and not spherical?  (Read 3024 times)

Offline namaan

  • Sr. Member
  • ****
  • Posts: 195
    • View Profile
At least the physics that I know (101 physics mind you) dictates that solar systems and galaxies alike would be spherical.


 

Offline RD

  • Neilep Level Member
  • ******
  • Posts: 8134
  • Thanked: 53 times
    • View Profile
Have you ever seen pizza bases being made ? ...
NR=1
 

Offline namaan

  • Sr. Member
  • ****
  • Posts: 195
    • View Profile
I'm assuming you're joking lol. It's hardly an analogue...but yes, I have worked at a pizza shop in my time ;)
 

Offline graham.d

  • Neilep Level Member
  • ******
  • Posts: 2208
    • View Profile
This is a good question. I think it is fairly clear that gravitational attraction will tend to pull objects into larger conglomerations and that these, in turn, will mutually attract. The motion of these conglomerated bodies will not have zero momentum or zero angular momentum, in general, and so tend to be divided into parts that be in a mutual dance of complex motion under the influence of gravity. I think it is not at all obvious why this arrangement settles down into a disk with a large central core. This is true of solar systems or galaxies. I think it does so, and there are computer models that show this to be the case even with just Newtonian physics, but I don't think the maths is easy to solve analytically. Does anyone know of an analytical proof?

I guess this is the lowest energy state and I can imagine mechanisms that would gradually weed out the counter-rotating (or perpendicularly rotating) bodies but it does not seem obvious. I guess that the large central core is a likely outcome but stating this is far from being a convincing proof.
 

Offline Soul Surfer

  • Neilep Level Member
  • ******
  • Posts: 3345
  • keep banging the rocks together
    • View Profile
    • ian kimber's web workspace
Basically it is because of the law of the conservation of angular momentum.  The precise process is a statistical one and not clearly understood yet but as an approximately spherical lump of gas and dust contracts to form a star or galaxy it always possesses a net rotation along an axis and as the contraction takes place this rotation speeds up. now particles that are not travelling  in the disc that lies perpendicular to this axis through the centre of gravity are more likely to bump into other particles and get either knocked right out of the area or more into the line of the disc so eventually all the particles form a shearing rotating disc along the rotation axis while the ones knocked out are seen as bipolar flow along the line of the axis.
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums