Why Are There Galaxies?

"G=EMC^2 Glazier" wrote in message
...
My post says inside the hub. It should read outside the hub,and inside
the hub ithe stars are not in lock step Bert

It's kind of hart to tell what you mean when you use terms like "in lock
step", but it sounds like you had it right (sort of) the first time.

The bulge exhibits characteristics of solid-body rotation, the disk exhibits
characteristics of differential rotation.

Zinni to Bert:

It's kind of hart to tell what you mean
when you use terms like "in lock step",
but it sounds like you had it right (sort of) the first time.

Got a question for the gurus on non-Keplerian rotation of galaxies- In
our solar system, well over 99% of the mass of the system is contained
in the nucleus (the sun). And the rest of the system naturally obeys the
Keplerian laws of orbital mechanics, orbiting about the massive nucleus.
But in contrast to a solar system, a galaxy's nucleus
(presumably a BH) does not contain the overwhelming mass of the system;
rather it contains a miniscule percentage of the total mass. So the
question is Why should a galaxy be expected to display Keplerian
rotation? Would not a more unitary (or 'frisbee-like') rotation be
expected, just as is observed?

Or, is the mass of the nucleus sufficiently high that Keplerian rotation
*should* be expected? oc

"Bill Sheppard" wrote in message
...

Why should a galaxy be expected to display Keplerian
rotation? Would not a more unitary (or 'frisbee-like') rotation be
expected, just as is observed?

Hey Bill

In fact, Keplerian rotation is not expected IN THE DISK.

Let me back-track for a second. There is a well known result attributable to
Newton that says ...

- Given a spherical shell of uniform density, its net gravitational effect
on a test particle outside of the spherical shell will be exactly the same
as if all of its mass were concentrated in a single point at its centre.

and ...

- Given a spherical shell of uniform density, its net gravitational effect
on a test particle (anywhere) inside of the spherical shell will be exactly
ZERO.

This result also holds true for rings of uniform density and a test particle
in the plane of the ring either inside or outside of it.

So, if we view the galaxy as approximating a series of rings of uniform
density (a good approximation for most of the disk) only the mass internal
to the orbit of a test particle need be considered. As we move out from the
centre of the galaxy there is more and more mass internal to the orbit to be
considered. The rotation curve IN THE DISK is part way between solid-body
and Keplerian. It is only once we get to the outer reaches of the galaxy,
were the visible matter thins to such an extent that we would expect no
appreciable change in the mass internal to the orbit of our test particle as
we move out, that we expect a Keplerian rotation curve to kick in. We do not
however get what we expect, the orbital velocities do not start to drop
according to Kepler, thus leading to the theory of Dark Matter.

Hi oc The sun has 99% of all the mass of the solar system. and gravity
is a mutual attraction. Still it is hard for me to see Neptune and
Pluton so far away from the sun,and staying in orbit. Then we have
Mercury so close to the sun. It has to be revolving very fast as not to
fall into the sun. The planets are not in lock step nor is the solar
system like a solid frisbee. With the huge area of the Milky
Way,and gravity getting weaker the inverse square of distance. There is
no way the blackhole at the center of the hub even with a mass of2.6
million suns can keep our sun that is 26million LY away in orbit,and all
the other millions,and millions of stars from leaving the galaxy and
escaping into space. That is why when they add up the mass of the Milky
Way astronomers tell us 95% of the mass(gravity) is missing. This holds
true for all spiral galaxies(I don't know about elliptical) The term in
lock step means the stars are not gaining on each other. To me it means
the Big,and Little dippers will keep their shape. I hope that stays true
for they are the only constellations I can find Well oc even if they
find a lot of dark matter I can see a problem,for it would have to be
spread out between the stars very evenly,and with the right force of
gravity to accomplish what we observe. Bert