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NET Angular Momentum of Globular cluster of stars



 
 
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  #11  
Old March 27th 17, 05:28 AM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Default NET Angular Momentum of Globular cluster of stars

[[Mod. note -- I apologise to readers for the delay in processing this
article, which the author submitted on 2017-03-23. Unfortunately there
were some garbled characters in one of the article headers that took a
bit of time to straighten out.
-- jt]]

In article ,
writes:

On Tuesday, March 21, 2017 at 1:18:55 PM UTC-7, Steve Willner wrote:
In article ,
writes:
Is the NET angular momentum of a globular cluster of stars, zero?


OK, it can be difficult to communicate in words rather than pictures and
sketches.............trying again:

I'm just trying to get us on board for a qualitative exploration.


While a qualitative understanding is necessary, it is not sufficient. As
Lord Kelvin said, "I often say that when you can measure what you are
speaking about, and express it in numbers, you know something about it;
but when you cannot measure it, when you cannot express it in numbers,
your knowledge is of a meagre and unsatisfactory kind."

Sure,
with ellipticals, as with globulars, there is a range of oblatness to
the spherical geometry. A flattened structure may (and may not) have
net angular momentum, it depends on whether there are stars rotating in
a preferred (or in counter rotational
e.g. the spiral ngc4138 is counter rotating, so it's possible for stars
in ellipticals to also have this feature
https://en.wikipedia.org/wiki/NGC_4138)
directions about the flattened plane's normal axis. ie, we have E0
through E5 ellipticals.


Note that this refers only to the appearance. So if we look at
something cigar-shaped along the major access, or something shaped like
a disk along the minor axis, we see E0 in both cases.

BUT,

COMPARED TO.... a simple spiral disk, with no central bulge, with all
stars moving around a center in the SAME direction..........like
Saturn's rings. ........... an elliptical and or a globular have

ESSENTIALLY, ZERO angular momentum .


As has been noted here, this doesn't seem to be the case, based on
current observations. There seems to be an appreciable number of
"fast-rotating" ellipticals.

In other words, can we agree that if I have the same number of stars,
and the same orbital radii, ellipticity etc. for every star, with the
sole exception that the globular has stars that appear as spherical
morphology, where as a flat spiral has purely single angular momentum
axis.............

The globular will be nearly zero compared to the flat spiral.

Agreed? If so then I can move on.

rt

  #12  
Old April 4th 17, 05:29 AM posted to sci.astro.research
Mike Dworetsky
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Posts: 715
Default NET Angular Momentum of Globular cluster of stars

Steve Willner wrote:
In article ,
writes:
Is the NET angular momentum of a globular cluster of stars, zero?


I think that's the case for most clusters, but I vaguely remember
that there are a tiny number that show rotation. I might be mistaken
on either part of this.


As I recall dimly from either reading or hearing talks about this, Omega Cen
appears slightly oblate, although I do not know what if any radial velocity
observations confirm that this is due to rotation. And it may be the core
of a small galaxy captured by the MWG long ago, rather than being a
"classical" globular.

Mike Dworetsky


Is the NET angular momentum of an elliptical galaxy and or the
central bulge of a spiral, zero or close to zero


For elliptical galaxies, a quick web search turned up
https://academic-oup-com/mnras/artic...6.2011.18496.x

The authors claim 86% of early type galaxies are "fast rotators." I
haven't studied the paper to find out what that means or what sample
they defined.

I expect there are many more works on this subject; as I say, it was
a very quick search.

There are also lots of theory papers simulating major mergers, from
which elliptical galaxies are supposed to form. The simulation
results must include a final angular momentum.


--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)
  #13  
Old April 4th 17, 10:10 PM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default NET Angular Momentum of Globular cluster of stars

In article ,
(Eric Flesch) writes:
If dark matter resides throughout an elliptical galaxy or
globular cluster (or the Galactic halo) then it could raise the
ambient background gravitational level to where a resident star no
longer feels the gravitational effect from its neighbours or from
the system centre. Instead, the star would follow thre contours
(potentials) of the dark matter structure. So stars would just
mingle throughout without following orbits as such.


What distribution of dark matter are you postulating? If the
distribution is uniform, then the dark matter has no effect
regardless of how much of it there is. If the distribution is
spherically symmetric, then we have the familiar case where each
object responds to the amount of matter interior to that object's
position relative to the center of the dark matter distribution. If
the distribution is more complicated, then so are the effects, but in
any case, stars are affected in the usual way by other stars. I
suppose if you have clumps of dark matter (say tens or hundreds of
solar masses each) whizzing around randomly, the overall effects
could look random, but it's hard to see how such dark matter clumps
could form or be held together or why they should inhabit elliptical
but not spiral galaxies.

A nice calculator for galactic rotation curves is at
http://burro.astr.cwru.edu/JavaLab/R...eWeb/main.html
but it didn't work when I tried it just now. (I suspect some kind of
Java compatibility problem.)

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #14  
Old April 7th 17, 04:50 PM posted to sci.astro.research
Martin Brown[_3_]
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Posts: 189
Default NET Angular Momentum of Globular cluster of stars

On 04/04/2017 05:29, Mike Dworetsky wrote:
Steve Willner wrote:
In article ,
writes:
Is the NET angular momentum of a globular cluster of stars, zero?


I think that's the case for most clusters, but I vaguely remember
that there are a tiny number that show rotation. I might be mistaken
on either part of this.


As I recall dimly from either reading or hearing talks about this, Omega Cen
appears slightly oblate, although I do not know what if any radial velocity
observations confirm that this is due to rotation. And it may be the core
of a small galaxy captured by the MWG long ago, rather than being a
"classical" globular.


There was something in ApJ early stellar dynamics measurements mid
1990's showing that the luminosity oblateness varies with radius more or
less spherical near the middle and far out but oblate in between.

https://arxiv.org/pdf/astro-ph/9612184.pdf

They credit someone else in 1983 as having first measured the isophotes
in 1983 and claim mean 0.121 minimum 0 up to 2', maximum 0.25 at 10' and
then becoming rounder as you go further out. (see p6)

--
Regards,
Martin Brown
  #16  
Old April 10th 17, 12:31 PM posted to sci.astro.research
Eric Flesch
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Posts: 321
Default NET Angular Momentum of Globular cluster of stars

On Fri, 07 Apr 2017 11:52:34 EDT, Eric Flesch wrote:
My point is that "dark matter" could be gravitationally opaque even as
it gravitates. We know it has different qualities than baryonic ...


[[Mod. note -- The notion of "gravitationally opaque" doesn't exist
in general relativity, so if you want this then you need to come up
with a new theory of gravity.
-- jt]]


I'm pretty sure that general relativity is not a theory of gravity.

[Moderator's note: I've kept this bit in because the rest of the post
might prompt some interesting discussion. I don't think that this is
the place to debate whether GR is a theory of gravity. -P.H.]

I

can however rephrase my point, Think of dark matter as a medium.
Compare stars in a dark matter environment to fish in a lake. Fish
don't fall because they are buoyant in the water. In a dark matter
lake, stars would similarly have some gravitational buoyancy. The
gravitational medium would modify or nullify the inverse square law.
How could it not? And that's pretty much all of my point.

  #18  
Old April 16th 17, 02:29 AM posted to sci.astro.research
Eric Flesch
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Posts: 321
Default NET Angular Momentum of Globular cluster of stars

On Tue, 11 Apr 2017, Phillip Helbig wrote:
... (Eric Flesch) writes:
gravitational medium would modify or nullify the inverse square law.
How could it not? And that's pretty much all of my point.


... But the kind of dark matter we are talking about has little if
any interaction with baryonic matter.


Um, sorry Phil, but dark matter is supposed to account for (1) spiral
galaxy rotational profiles, and (2) "great attractors". Given the
existence of dark matter (for this argument), it definitely interacts
with baryonic matter. And if it does so, it must have a major effect
on the dynamics of galactic haloes, elliptical galaxies and globular
clusters. Maybe fish-in-water isn't the right analogy, but the
presence of a gravitational scalar diminishes (or nullifies) the
gravitational influence of near neighbours. So the boundary of our
solar system may simply be where our Sun's gravitational influence is
supplanted by that of the "dark matter" medium, with neighbouring
stars not in the equation. That's the idea, anyway.

I also don't think that it would work quantitatively. Can you show that
this idea (even neglecting the point I mentioned above) results in the
very simple MOND law?


I'm not up with MOND. Is it simple? Simple is good but not
sufficient. I expect that MOND is a "top down" system designed to
best account for observed behaviour, as opposed to a "bottom up"
system which builds on known physical law. A top-down system must, of
human necessity, be simple, else nobody will pay attention.

thanks, Eric


[[Mod. note --
1. I think Phillip Helbig was referring to dark matter having little if
any *non-gravitational* interaction with baryonic matter.

2. There's an extensive history of people studying what the consequences
of a gravitational scalar field might be. See, for example, section
5.3 of Clifford M Will, "Theory and Experiment in Gravitational Physics"
(Cambridge University Press, 1981, 1985), or section 3.3.2 of Will's
review paper "The Confrontation between General Relativity and
Experiment" (open-access at
http://www.livingreviews.org/lrr-2014-4 ).
To summarize, compatability with various solar-system experiments
constrains the free parameters in these theories so as to give
results almost identical to general relativity.
-- jt]]
  #20  
Old April 16th 17, 09:43 PM posted to sci.astro.research
Eric Flesch
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Posts: 321
Default NET Angular Momentum of Globular cluster of stars

On Sat, 15 Apr 2017 21:29:20 EDT, Eric Flesch wrote:
[[Mod. note --
2. There's an extensive history of people studying what the consequences
of a gravitational scalar field might be. See, for example, section
5.3 of Clifford M Will, "Theory and Experiment in Gravitational Physics"
(Cambridge University Press, 1981, 1985), or section 3.3.2 of Will's
review paper "The Confrontation between General Relativity and
Experiment" (open-access at http://www.livingreviews.org/lrr-2014-4 ).


Thank you for this outstanding reference which I'll be reading for
days. I wasn't aware of Clifford Will's work.

[[Mod. note -- Will is also the author of the excellent popular book
"Was Einstein Right?" (Basic Books, paperback 2nd Edition 1993) on
experimental tests of general relativity.
-- jt]]
 




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