Re: Center of galaxies
From: Alexander Avtanski (avtanski_at_ispwest.com)
Date: 09/16/04
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Date: Wed, 15 Sep 2004 17:11:01 -0700
Garmachi wrote:
> [ ... ]
>
> Yes, but... in the example of the bicycle wheel, it is the spokes that
> bear the tension radially, thus keeping the mass at the edges (the
> rubber tube) from flying off tangentally. (Not to mention that the
> continuity of the rubber tube holds itself together, as opposed to being
> composed of billions of independent particles, free to bounce off of
> each other or to fly off tangentally as well...)
>
> On the other hand, the mass at the center (of a galaxy now, I'm done
> talking about wheels) still doesn't necessarily have to be greater than
> the sum of the remaining mass for the disk to be stable. If you pick a
> random star, say one that is neither at the center nor the exteme edges,
> chances are that it is in gravitational equilibrium because it is being
> "pulled" equally and in opposite directions by the stars surrounding it.
>
> I don't have the math to back this up, rather I'm talking off the cuff
> here. Can anyone substantiate or refute me? I don't claim to be right,
> and I'm curious to know too!
>
> Thanks...
Yeah, the "bicycle wheel" analogy was probably not very good.
Probably you can look at the other part of this thread - Brian
and Chris had some pretty good examples that I think clear the
matter.
I just want to add one small thing - you say that a star
somewhere in the disk (say 1/2 distance from the center) is
in roughly gravitational equilibrium. Yes, the stars nearby
really pull in all directions approximately (statistically
speaking) with the same force. However, one should not forget
that even the stars on the _other_ side of the galaxy have
their influence, albeit quite small. Each of them may not pull
with great force, but together they have their influence too.
The "imbalance" that causes the stars to revolve around the
center comes from those larger distances, where the structure
of the galaxy comes in play.
It is theoretically possible to have a ring of stars that will
turn like a merry-go-round around empty space. (This would be
highly unstable configuration I realize, but that's not the
issue.)
Regards,
- Alex
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