Pioneer : Anomaly Still Anonymous

Lester Zick replied to Jeff Root:

Globular clusters have little or no angular momentum,
but the individual stars have a great deal of angular
momentum. There are two ways they can lose some of it:
By throwing stars out of the cluster, and by collisions
between stars. Both are extremely rare events.

They're extremely rare events over the lifetime of the Milky Way?

Collisions are much rarer than expulsions, and expulsions
were more common in the past than they are now. They were
more common in the past because the globular clusters were
larger, with lower escape speed at the outer limits. Now
that the clusters are contracted, expulsions are rare.

I doubt it if globular clusters are the oldest things in
the galaxy.

They are the oldest things known. The only thing we can see
that is older is the cosmic background radiation. Otherwise,
globular clusters, and the stars which comprise them, are the
winners.

The discussion here seems to revolve around the age of globular
clusters in relation to the Milky Way. The only reason I can
see to argue that they should not have further contracted under
the influence of universal gravitation is that they're not older
than the Milky Way but younger. Gravitation got them going but
suddenly stopped while the Milky Way evolved. Doesn't sound
plausible on the face of it

The reason it doesn't sound plausible is that it is wrong.

since all we see are relatively light young stars with little
interstellar gas.

The stars are not young. They are the oldest stars we can
see in the entire sky. They are the oldest things we can see
anywhere in the Universe, except for the CMBR.

-- Jeff, in Minneapolis

Lester Zick replied to Jeff Root:

Globular clusters have no *net* angular momentum in any
direction, but all of the individual stars have lots of
angular momentum, and it is in all directions, so the
clusters are spherical.

Yes well that may be. But what I don't quite understand is how
individual stars acquired so much angular momentum if the gas
cloud they presumably condensed from had no net angular
momentum to begin with

The temperature of the cloud was more than absolute zero,
so the gas molecules were in random motion.

If the temperature of the original gas cloud had been zero,
the molecules would not have been moving, and gravity would
have collapsed them immediately into a single supermassive
black hole.

The cloud was intially hot. That's why it didn't collapse
immediately. But it cooled by radiation. So the molecules
moved less and less rapidly, and the cloud contracted. When
the temperature got low enough and the density high enough,
gravity made the cloud fragment into stellar masses and
collapse into individual stars.

Those stars attracted one another gravitationally. The
closer together two stars were, the stronger the attraction.
The attraction between two stars accelerated those stars in
opposite directions. So the newly-formed stars of the
cluster were accelerated in all directions, randomly.

Sort of a much larger-scale analogue of the original gas
molecules. Except that stars don't slow down when they
radiate heat.

The fastest-moving stars were ejected from the cluster.
That left the slowest stars behind, making the cluster
smaller, denser, and harder for the remaining stars to
escape.

and if the cloud of gas had non zero net angular momentum
how the globular cluster which emerged from it had none.

The original gas cloud had no net angular momentum. But
each gas molecule within the cloud had angular momentum.

And if you're positing some kind of gravitational stellar
capture

I don't know what you are referring to, so no, I'm not
positing it.

I consider the absence of net angular momentum even more
unlikely.

Nothing put a net angular momentum into the cloud, so it
had none.

Nor do I understand how processes of self gravitational
contraction which presumably produced the stars initially
suddenly stopped contraction while the Milky Way went on
its merry way evolving such that we see these same globular
clusters today which supposedly antedate the MIlky Way and
yet are filled with low density stars and little interstellar
gas. More than a little implausible as far as I'm concerned.

Contraction of globular clusters did not stop suddenly.
It has slowed over time, as more and more stars were lost,
making it harder and harder to contract any further.

When you say "low density stars", you mean "low mass stars".
Low-mass stars are the only stars in globular clusters which
have not yet collapsed to white dwarfs, neutron stars, or
black holes.

-- Jeff, in Minneapolis

Lester Zick writes:
....

A disc only has overall angular momentum in the plane of rotation. If
globular clusters have no angular momentum in any direction why
doesn't gravitation continue to collapse it?

Energy must still be conserved. A globular has a large store of
kinetic energy in its stellar motions, which cannot be easily
dissipated. Some globular clusters are have undergone "core collapse"
-- non-linear compaction of their cores. But this effect must be
balanced by the ejection (evaporation) of some stars from the cluster
at high velocity.

In any case, globular clusters do not have a perfectly zero angular
momentum, nor are they perfectly spherical. Example: in a sample of
100 globular clusters in the Milky Way, on average one axis was
flatter by about 7%. Globular clusters are also known to exhibit
rotation. Example: Omega Cen has significant rotation in its inner
regions. (Reijns et al 2006)

So please, no more straw men about "perfect" globular clusters.

CM


References
Reijns, R. A. et al. 2006, A&A, 445, 503
White, R. & Shawl, S. J. 1987, ApJ 317, 246

In article ,
"G. L. Bradford" wrote:

snip

"1.2*10^10 years" puts the supposed birth of the Milky Way pretty damn close
to the distant Big Bang / Planck / Speed of Light [constant] Horizon does it
not?

I believe the best current figure for the age of the CMBR is something
like 1.37E10 years, so one might say that the observable universe had
about one-eighth of its present age when the Milky Way's globulars were
formed. Their stars belong to what's called "Population II", along with
members of the galactic hub & halo (their spherical distribution giving
the Galaxy, were its spiral arms removed, the appearance of an
elliptical); most of them contain lower levels of metals than the later
"Population I" generations born in the spiral arms.

That said, we must beware over-generalizing about globular clusters;
some other galaxies whose haloes have been studied appear to have two or
more distinct populations of them, so we can expect these objects to
resist simplistic explanations.

--
Odysseus

"Lester Zick" wrote in message
...
On Sat, 29 Jul 2006 18:05:44 +0100, "George Dishman"
wrote:


"Lester Zick" wrote in message
. ..
On Sat, 29 Jul 2006 10:26:59 +0100, "George Dishman"
wrote:
"Lester Zick" wrote in message
news
...
And if globular clusters are old how is it they haven't progressively
collapsed to form disks?

As has already been pointed out, because a disc
has overall angular momentum while a globular
cluster has none.

If not why doesn't gravitation continue to collapse it to a point?

Because the stars have kinetic energy and that
is conserved as well as momentum.

Kinetic energy can be dissipated over time through radiation.

Not through radiation in the normal sense of EM. The
individual stars would cool but still move as fast.

What can and does happen that occasionally stars [1]
will pass very close and the 'gravitational slingshot'
effect we use for spacecraft will accelerate one star
while slowing the other relative to the group. The
result is that the faster star my pick up enough speed
to be ejected from the cluster. That reduces the total
kinetic energy and the cluster can collapse that way
but it is a very slow process.

Globular clusters with any net angular momentum have none in polar
directions and should have collapsed into discs long ago if they were
anything like older than the galaxy itself.

Not without violating either conservation of
momentum or of energy.

This is confusing. According to contemporary theory a large ball of
hydrogen gas without angular momentum contracted under the influence
of gravitation to produce stars.

Individual stars are probably spinning. AIUI there is
a minimum density for the gas above which it becomes
gravitationally unstable and tends to clump. As each
star forms the net momentum from the random motion of
the gas about the mean produces a net spin. The the
gas there is friction so differences in speed tend to
reduce rapidly producing some heating of the gas.

The same isn't true of the stars in the cluster since
there are negligibly few collisions and with too little
residual gas there is no reason for the stars to slow.

Somewhere along the way stars were
produced not only radiating but having non zero angular momentum
individually yet exactly zero net angular momentum in aggregate
whereupon the whole gravitational contraction process stopped while
the Milky Way was formed and evolved to its present state. Sorry I
just don't believe it.

Suit yourself, but that doesn't alter what we see.
By whatever method the present state arose, clusters
have random stellar orbits so no net angular momentum
but the stars still have kinetic energy so regardless
of how the state arose, it cannot now collapse into a
disc.

George

On 29 Jul 2006 22:57:47 -0400, Craig Markwardt
wrote:

Lester Zick writes:
...

A disc only has overall angular momentum in the plane of rotation. If
globular clusters have no angular momentum in any direction why
doesn't gravitation continue to collapse it?

Energy must still be conserved. A globular has a large store of
kinetic energy in its stellar motions, which cannot be easily
dissipated. Some globular clusters are have undergone "core collapse"
-- non-linear compaction of their cores. But this effect must be
balanced by the ejection (evaporation) of some stars from the cluster
at high velocity.

In any case, globular clusters do not have a perfectly zero angular
momentum, nor are they perfectly spherical. Example: in a sample of
100 globular clusters in the Milky Way, on average one axis was
flatter by about 7%. Globular clusters are also known to exhibit
rotation. Example: Omega Cen has significant rotation in its inner
regions. (Reijns et al 2006)

So please, no more straw men about "perfect" globular clusters.

Not my strawmen. I don't believe in zero net angular momentum for
globular clusters. Nor do I believe they don't flatten and contract.
Nor do I believe they're really all that old. Certainly not older than
the Milky Way galaxy.

Lester Zick
~v~~

On Sun, 30 Jul 2006 03:25:53 GMT, Odysseus
wrote:

In article ,
"G. L. Bradford" wrote:

snip

"1.2*10^10 years" puts the supposed birth of the Milky Way pretty damn close
to the distant Big Bang / Planck / Speed of Light [constant] Horizon does it
not?

I believe the best current figure for the age of the CMBR is something
like 1.37E10 years, so one might say that the observable universe had
about one-eighth of its present age when the Milky Way's globulars were
formed. Their stars belong to what's called "Population II", along with
members of the galactic hub & halo (their spherical distribution giving
the Galaxy, were its spiral arms removed, the appearance of an
elliptical); most of them contain lower levels of metals than the later
"Population I" generations born in the spiral arms.

That said, we must beware over-generalizing about globular clusters;
some other galaxies whose haloes have been studied appear to have two or
more distinct populations of them, so we can expect these objects to
resist simplistic explanations.

I don't disagree. As far as I can tell globulars look younger and act
younger than the Milky Way galaxy in general.

Lester Zick
~v~~

On 29 Jul 2006 17:49:45 -0700, "Jeff Root" wrote:


Lester Zick replied to Jeff Root:

Globular clusters have little or no angular momentum,
but the individual stars have a great deal of angular
momentum. There are two ways they can lose some of it:
By throwing stars out of the cluster, and by collisions
between stars. Both are extremely rare events.

They're extremely rare events over the lifetime of the Milky Way?

Collisions are much rarer than expulsions, and expulsions
were more common in the past than they are now. They were
more common in the past because the globular clusters were
larger, with lower escape speed at the outer limits. Now
that the clusters are contracted, expulsions are rare.

I doubt it if globular clusters are the oldest things in
the galaxy.

They are the oldest things known. The only thing we can see
that is older is the cosmic background radiation. Otherwise,
globular clusters, and the stars which comprise them, are the
winners.

This is curious. Obviously globular clusters of stars don't come with
a time and date stamp so we can only infer their age. Are you saying
stars in globular clusters are actually older than the Milky Way?

The discussion here seems to revolve around the age of globular
clusters in relation to the Milky Way. The only reason I can
see to argue that they should not have further contracted under
the influence of universal gravitation is that they're not older
than the Milky Way but younger. Gravitation got them going but
suddenly stopped while the Milky Way evolved. Doesn't sound
plausible on the face of it

The reason it doesn't sound plausible is that it is wrong.

since all we see are relatively light young stars with little
interstellar gas.

The stars are not young. They are the oldest stars we can
see in the entire sky. They are the oldest things we can see
anywhere in the Universe, except for the CMBR.

-- Jeff, in Minneapolis

Lester Zick
~v~~

On Sun, 30 Jul 2006 10:53:34 +0100, "George Dishman"
wrote:


"Lester Zick" wrote in message
.. .
On Sat, 29 Jul 2006 18:05:44 +0100, "George Dishman"
wrote:


"Lester Zick" wrote in message
...
On Sat, 29 Jul 2006 10:26:59 +0100, "George Dishman"
wrote:
"Lester Zick" wrote in message
news ...
And if globular clusters are old how is it they haven't progressively
collapsed to form disks?

As has already been pointed out, because a disc
has overall angular momentum while a globular
cluster has none.

If not why doesn't gravitation continue to collapse it to a point?

Because the stars have kinetic energy and that
is conserved as well as momentum.

Kinetic energy can be dissipated over time through radiation.

Not through radiation in the normal sense of EM. The
individual stars would cool but still move as fast.

What can and does happen that occasionally stars [1]
will pass very close and the 'gravitational slingshot'
effect we use for spacecraft will accelerate one star
while slowing the other relative to the group. The
result is that the faster star my pick up enough speed
to be ejected from the cluster. That reduces the total
kinetic energy and the cluster can collapse that way
but it is a very slow process.

Globular clusters with any net angular momentum have none in polar
directions and should have collapsed into discs long ago if they were
anything like older than the galaxy itself.

Not without violating either conservation of
momentum or of energy.

This is confusing. According to contemporary theory a large ball of
hydrogen gas without angular momentum contracted under the influence
of gravitation to produce stars.

Individual stars are probably spinning. AIUI there is
a minimum density for the gas above which it becomes
gravitationally unstable and tends to clump. As each
star forms the net momentum from the random motion of
the gas about the mean produces a net spin. The the
gas there is friction so differences in speed tend to
reduce rapidly producing some heating of the gas.

The same isn't true of the stars in the cluster since
there are negligibly few collisions and with too little
residual gas there is no reason for the stars to slow.

Somewhere along the way stars were
produced not only radiating but having non zero angular momentum
individually yet exactly zero net angular momentum in aggregate
whereupon the whole gravitational contraction process stopped while
the Milky Way was formed and evolved to its present state. Sorry I
just don't believe it.

Suit yourself, but that doesn't alter what we see.
By whatever method the present state arose, clusters
have random stellar orbits so no net angular momentum
but the stars still have kinetic energy so regardless
of how the state arose, it cannot now collapse into a
disc.

I don't see that stellar collapse is prevented by components of
collapsing stars having kinetic energy.

And I find it somewhat curious that people prefer to discuss this
particular subject so much more than gravitational doppler.

Lester Zick
~v~~

On 29 Jul 2006 17:57:43 -0700, "Jeff Root" wrote:


Lester Zick replied to Jeff Root:

Globular clusters have no *net* angular momentum in any
direction, but all of the individual stars have lots of
angular momentum, and it is in all directions, so the
clusters are spherical.

Yes well that may be. But what I don't quite understand is how
individual stars acquired so much angular momentum if the gas
cloud they presumably condensed from had no net angular
momentum to begin with

The temperature of the cloud was more than absolute zero,
so the gas molecules were in random motion.

If the temperature of the original gas cloud had been zero,
the molecules would not have been moving, and gravity would
have collapsed them immediately into a single supermassive
black hole.

The cloud was intially hot. That's why it didn't collapse
immediately. But it cooled by radiation. So the molecules
moved less and less rapidly, and the cloud contracted. When
the temperature got low enough and the density high enough,
gravity made the cloud fragment into stellar masses and
collapse into individual stars.

Except collapsing stars seem to be plenty hot and have plenty of
kinetic energy in their interiors and collapse despite all this. I
just don't see why analogous mechanical principles shouldn't apply to
globular clusters of stars as a whole.

Those stars attracted one another gravitationally. The
closer together two stars were, the stronger the attraction.
The attraction between two stars accelerated those stars in
opposite directions. So the newly-formed stars of the
cluster were accelerated in all directions, randomly.

Sort of a much larger-scale analogue of the original gas
molecules. Except that stars don't slow down when they
radiate heat.

The fastest-moving stars were ejected from the cluster.
That left the slowest stars behind, making the cluster
smaller, denser, and harder for the remaining stars to
escape.

and if the cloud of gas had non zero net angular momentum
how the globular cluster which emerged from it had none.

The original gas cloud had no net angular momentum. But
each gas molecule within the cloud had angular momentum.

And if you're positing some kind of gravitational stellar
capture

I don't know what you are referring to, so no, I'm not
positing it.

I consider the absence of net angular momentum even more
unlikely.

Nothing put a net angular momentum into the cloud, so it
had none.

Nor do I understand how processes of self gravitational
contraction which presumably produced the stars initially
suddenly stopped contraction while the Milky Way went on
its merry way evolving such that we see these same globular
clusters today which supposedly antedate the MIlky Way and
yet are filled with low density stars and little interstellar
gas. More than a little implausible as far as I'm concerned.

Contraction of globular clusters did not stop suddenly.
It has slowed over time, as more and more stars were lost,
making it harder and harder to contract any further.

When you say "low density stars", you mean "low mass stars".
Low-mass stars are the only stars in globular clusters which
have not yet collapsed to white dwarfs, neutron stars, or
black holes.

-- Jeff, in Minneapolis

Lester Zick
~v~~