Center of galaxies

Whenever there's a gravitationally "stable" system with objects
revolving around a center of mass and there's no wobble on the object
which sits close to the center of mass, this must mean (according to my
meager physics years ago) that this object must posses mass at least
equal to the sum of masses of all the revolving objects. (or that such
an object must have mass much larger roughly speaking than the mass of
the rest of the members).

So, speaking of a galaxy containing 2 billion solar masses in official
nomenclature, doesn't it follow that "something" sits at the center of
mass of that galaxy which contains _at least_ 2 billion solar masses in
order to sustain the system gravitationally?

I've heard of theories that claim that black holes lie in the center of
many galaxies, but it seems to me that one probably sits at the center
of _every_ galaxy. Correct me if I am wrong, but this is mind boggling.
I just saw some diagrams of some giant doubled lobed galaxies, like DA
240 and 3C 236 (Cygnus A?). Whatever the heck sits at the center of mass
of those giants?
--
I. N. Galidakis
http://users.forthnet.gr/ath/jgal/
------------------------------------------
Eventually, _everything_ is understandable

Ioannis wrote:
Whenever there's a gravitationally "stable" system with objects
revolving around a center of mass and there's no wobble on the object
which sits close to the center of mass, this must mean (according to my
meager physics years ago) that this object must posses mass at least
equal to the sum of masses of all the revolving objects. (or that such
an object must have mass much larger roughly speaking than the mass of
the rest of the members).

I don't see why that's necessarily so. First of all, there can be no
wobble only if the central object lies exactly at the center of mass of
the remaining objects. I doubt that such a system is dynamically stable
over cosmological periods of time.

Secondly, the wobble increases in magnitude, but slows in time, the
further out the perturbing object is. Move the "satellite" out by a
factor of 4, and the wobble increases by a factor of 4, but it slows
down by a factor of 8. All in all, it's probably more detectable the
closer in the object is.

If the central object is roughly spherical, then its effect on a nearby
"satellite" is to accelerate it as though all the mass of the central
object were concentrated in a point source at the very center. That's
what Newton found. It only holds true, though, if the satellite lies
entirely outside the central object. If it's plowing through some of
the outer layers of the central object (as in a galactic nucleus), then
to a first approximation, the outer layers don't accelerate the satellite.
Only the layers closer to the center than the satellite affect it.

What implicates a black hole, then, is not merely the presence of things
revolving around a center, or even merely the presence of things revolving
around a center with evidently great mass (such as a few million solar
masses), but the presence of things revolving in such a way *in great
proximity* to the center. They should be whipping around so quickly that
the central object must be both very massive and very small. This
mandates the high density that is the hallmark of the black hole.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Brian Tung wrote:

[snip]

What implicates a black hole, then, is not merely the presence of things
revolving around a center, or even merely the presence of things revolving
around a center with evidently great mass (such as a few million solar
masses), but the presence of things revolving in such a way *in great
proximity* to the center. They should be whipping around so quickly that
the central object must be both very massive and very small. This
mandates the high density that is the hallmark of the black hole.

So, does it make sense, for example, to have a central object of 10
solar masses and 2 billion stars revolving around it forming a galaxy?

How could a small object "hold" 2 billion stars in orbit? Sorry, I am
having trouble how this can be so. Please explain. Thanks,


Brian Tung
--
I. N. Galidakis
http://users.forthnet.gr/ath/jgal/
------------------------------------------
Eventually, _everything_ is understandable

On Wed, 15 Sep 2004 21:56:50 +0300, Ioannis wrote:

So, does it make sense, for example, to have a central object of 10
solar masses and 2 billion stars revolving around it forming a galaxy?

How could a small object "hold" 2 billion stars in orbit? Sorry, I am
having trouble how this can be so. Please explain. Thanks,

The central object isn't "holding" things in orbit. Objects in the galaxy are
orbiting around the center of mass of all the other objects that make up the
galaxy. You could take away the object at the very center and it wouldn't have
any significant effect on the motion of the galaxy.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Chris L Peterson wrote:

[snip]
The central object isn't "holding" things in orbit. Objects in the galaxy are
orbiting around the center of mass of all the other objects that make up the
galaxy. You could take away the object at the very center and it wouldn't have
any significant effect on the motion of the galaxy.

So, if I understand you well, you could take away the Sun, for example,
and still have all the planets in the solar system revolve around their
center of mass?

Thanks,

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
--
I. N. Galidakis
http://users.forthnet.gr/ath/jgal/
------------------------------------------
Eventually, _everything_ is understandable

Ioannis wrote:
So, if I understand you well, you could take away the Sun, for example,
and still have all the planets in the solar system revolve around their
center of mass?

Perhaps, but it's an empty conditional, since the antecedent isn't right:
you don't, in fact, understand him well.

The object at the center of the Milky Way actually has a small percentage
of the entire mass of the nucleus. So, if you were to take it away, the
motion of the rest of the galaxy would not be affected.

One cannot conclude from that, however, that such a rule applies to the
solar system, where the Sun has 99.9 percent of the mass in the entire
system, and if you were to remove it, there would be a dramatic displacement
of the center of mass. The planets are sparsely enough distributed that
they would probably fly off in different directions (with each planet
likely retaining their satellites, though).

Imagine a three-body system, with two large bodies of equal mass, revolving
in a common circular orbit, and a third smaller body in the center. Such
a system is in equilibrium (possibly unstable), and if you were to remove
the smaller body from the center, very little would happen, because it
isn't holding the other two bodies in place. The only detectable effect
on the two remaining bodies is that they would revolve somewhat slower,
and the system would become more stable since there is no perturbation on
the third body to worry about.

In fact, if anything, the galaxy's great mass is what *removes* the
necessity of having a discrete central mass, because the rest of the stuff
*is* self-gravitating. It is only if the rest of the galaxy were very
sparse, that having a central mass is necessary to holding the rest of it
together.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

So, if I understand you well, you could take away the Sun, for example,
and still have all the planets in the solar system revolve around their
center of mass?

Nope - they are going too-fast for this. But this is something
very different.

Now, look at this: Imagine a double star. Nothing else around.
The two components are with absolutely equal weight and size.
They are spinning around the common center of gravity - the
exact middle of the straight line from one to the other.
Around empty space.

Now, imagine you going to the center of the galaxy from the
previous message, getting the pebble that we left there, and
putting it smack in the middle between those two stars.

Do the stars revolve around it? Yes. Does it wobble? No.
Is it heavier than both the stars? No way! Is this pebble
holding the stars in place? No.

Now, just imagine the same thing, but with a bit more than
two stars - here is a galaxy that revolves around nothing.
(I'm not saying that there is nothing in the middle of each
galaxy, but that it's not necessary to have something heavy
there in order to keep the galaxy turning and turning and
turning...)

- Alex

In article 1095277448.173432@athnrd02, wrote:

Chris L Peterson wrote:

[snip]
The central object isn't "holding" things in orbit. Objects in the galaxy are
orbiting around the center of mass of all the other objects that make up the
galaxy. You could take away the object at the very center and it
wouldn't have
any significant effect on the motion of the galaxy.

So, if I understand you well, you could take away the Sun, for example,
and still have all the planets in the solar system revolve around their
center of mass?

Thanks,



Yes, certainly, if there were billions of planets. It's always mass vs speed.

Ioannis wrote in message news:1095274618.139766@athnrd02...
Brian Tung wrote:

[snip]

What implicates a black hole, then, is not merely the presence of things
revolving around a center, or even merely the presence of things revolving
around a center with evidently great mass (such as a few million solar
masses), but the presence of things revolving in such a way *in great
proximity* to the center. They should be whipping around so quickly that
the central object must be both very massive and very small. This
mandates the high density that is the hallmark of the black hole.

So, does it make sense, for example, to have a central object of 10
solar masses and 2 billion stars revolving around it forming a galaxy?

How could a small object "hold" 2 billion stars in orbit? Sorry, I am
having trouble how this can be so. Please explain. Thanks,

Galactic bungee cords. :-)

Seriously, there are some large gaps in our understanding how the Universe
works. Think back to the recent revelation that only 4% of the Universe's
mass comprises baryonic matter (i.e., electrons, protons, ... = atoms) and
that 96% comprises dark matter and dark energy which remain unknown and very
probably require extra dimensions to characterize. In those probable extra
dimensions gravity might not be the weak force it is in our "space".

Thad Floryan wrote:
Seriously, there are some large gaps in our understanding how the Universe
works. Think back to the recent revelation that only 4% of the Universe's
mass comprises baryonic matter (i.e., electrons, protons, ... = atoms) and
that 96% comprises dark matter and dark energy which remain unknown and very
probably require extra dimensions to characterize. In those probable extra
dimensions gravity might not be the weak force it is in our "space".

The problem with that approach is that if you say "Who knows what gravity
could do on galactic scales?" then you can make it responsible for any
effect at all. Thus, it explains relatively little, except to say that
we don't understand gravity perfectly. That's not an earthshattering
revelation.

I read the question to mean, "How *do we understand* gravity to permit
a smaller mass to be orbited by a larger mass?" Obviously, we cannot
answer with complete certainty how things really are. But at that level
of discussion, we're forced to throw our hands up in defeat. I don't
think Ioannis really wanted that.

It's true that dark matter and dark energy are baffling in the sense that
we really don't know what they are. But I don't trust them to do anything
arbitrary with space-time, and I don't think people in general should do
that, either. We should trust them insofar as the phenomena they reflect
can be fit into a consistent model, with as few underived parameters as
possible.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt