APOD: Millions of Stars in Omega Centauri (2014 May 29)

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Expand view Topic review: APOD: Millions of Stars in Omega Centauri (2014 May 29)

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by Nitpicker » Fri May 30, 2014 1:11 am

This is one of the best amateur images of Omega Cen I've ever seen. (South is up, in case you're wondering.)

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by Chris Peterson » Thu May 29, 2014 9:37 pm

fastartcee wrote:That's very interesting. So you are suggesting that spherical clusters are dynamic enough to resist 'pancaking', but not so dynamic as to disperse?
You get fluid dynamic effects when there is a high density of dust and gas. Once a system evolves to the point that most of that material is tied up in stars, you just have a bunch of isolated masses interacting gravitationally. They aren't close enough to collide to transfer much momentum via multiple body interactions.
Your comment that 'it's pretty easy to perturb them into a sphere' is something I hadn't appreciated. Would you by any chance be able to post a link to a computer animation that shows such a perturbation? It would be fascinating to see this phenomenon in action.
Such simulations exist. But the best example is simply the evolution of a spiral galaxy into an elliptical galaxy, usually as the result of a collision with another galaxy. When the spiral formed, it was from a region rich in gas, and the material settled into a disc (just as accretion discs do). But today, galaxies don't contain much of their mass as free gas or dust. There is no force to pull them back into discs if they are disrupted.
Your explanation does not, however, include anything on how the pre-existing gas clouds (that evolved into spherical clusters) came to be in the first place. Any ideas on that?
The gas clouds that formed galaxies (and maybe globular clusters... nobody has worked that out with much certainty) were a natural product of the Big Bang. Natural density variations propagated from quantum level fluctuations very, very early in the evolution of the Universe. Once you had any density variations at all, gravity amplified them.

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by Chris Peterson » Thu May 29, 2014 9:30 pm

Qweenie wrote:The Omega Centauri cluster is 150 ly in diameter, giving a volume of about 1.7 million cubic ly. With 10 million stars crammed into that space, they must average about 0.2 ly apart, which would put them well inside the Solar System's Oort cloud but just outside the Kuiper belt.
The density varies strongly with distance from the center. A large globular cluster like this has a central density approaching 1000 stars per cubic light year, or an average spacing of 0.1 ly or so. At the outside, stars are over a light year apart. Actually defining the diameter is a bit tricky.
I'm really curious how stable any star could be in such a crowded environment. Why do we not see a whole slew of novas or supernovas in these clusters?
What do you mean by "stable" in this context? The stars themselves are not affected by proximity to other stars. Nothing makes them "unstable" in the sense of being prone to become supernovas. The density is nowhere near high enough to allow for a meaningful number of actual collisions. The orbits are quite unstable, which is responsible for the slow evaporation of globular clusters over tens of billions of years. And almost certainly, we would not find many planetary systems around stars in globulars.

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by Ann » Thu May 29, 2014 8:29 pm

fastartcee wrote:
But it's great fun to exchange views with knowledgeable people like yourself, Ann, even when they are trying to nudge me back on the right path!
Thanks! I enjoy having to explain to myself (and therefore to others) why your suggestions didn't seem plausible to me.

As for what happened soon after the Big Bang (and after the era of inflation?), well, I'm not the right person to have much of an opinion on that. I am after all an autodidact, with no formal training in astronomy or physics at all, and with little grasp of mathematics.
Galaxy M87 with globulars. Credit and copyright:
Canada-France-Hawaii Telescope, J.-C. Cuillandre (CFHT), Coelum

I nevertheless find it hard to believe that the disk and bulge of our galaxy would not originate from the same Bang as the globular clusters. (Bear in mind that probably all large galaxies have many globular clusters. If they all originate from alternative Bangs, then the Fourth of July fireworks would have nothing on the cosmic light show of the creation of trillions of trillions separate Little Bangs birthing globular clusters.)

Ann

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by fastartcee » Thu May 29, 2014 8:19 pm

Chris Peterson wrote:
fastartcee wrote:I am always struck by the nearly-perfect spherical shape of these 200-odd clusters. (Yeah, I know... they're called 'spherical clusters'!) I cannot understand how there could have been all these spherical hydrogen clouds at the outset, all with well-distributed denser regions ready to condense into stars.
That's probably not what was going on.

When you have high enough material densities for fluid dynamics to come into play, gravity tends to pull material into disc-like structures. If the conditions don't allow for momentum transfer via fluid dynamics, gravity tends to pull material into spherical structures. Once discs no longer support fluid dynamics (e.g. mature galaxies) it's pretty easy to perturb them into spheres. That may explain the formation of globular clusters from miniature galaxies, for instance (something perhaps supported by observational evidence for a net angular momentum or global rotation).
That's very interesting. So you are suggesting that spherical clusters are dynamic enough to resist 'pancaking', but not so dynamic as to disperse? Your comment that 'it's pretty easy to perturb them into a sphere' is something I hadn't appreciated. Would you by any chance be able to post a link to a computer animation that shows such a perturbation? It would be fascinating to see this phenomenon in action.

Your explanation does not, however, include anything on how the pre-existing gas clouds (that evolved into spherical clusters) came to be in the first place. Any ideas on that?

Thanks for your comment.

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by Ann » Thu May 29, 2014 8:13 pm

Qweenie wrote:The Omega Centauri cluster is 150 ly in diameter, giving a volume of about 1.7 million cubic ly. With 10 million stars crammed into that space, they must average about 0.2 ly apart, which would put them well inside the Solar System's Oort cloud but just outside the Kuiper belt. I'm really curious how stable any star could be in such a crowded environment. Why do we not see a whole slew of novas or supernovas in these clusters?
Undoubtedly the globular clusters originally contained some very high-mass stars. But these stars are gone now, and several of them almost certainly exploded as supernovas.

Now all the stars inside globular clusters are relatively low-mass stars. Even if two of the existing highest-mass stars in today's globular clusters were to merge, the star that would result from such a merger would not be massive enough to evolve into a core-collapse supernova. Of course, if two of the highest-mass stars were to merge, and the stellar product of that merger was to keep on merging, then eventually we would get a star that was massive enough to explode as a supernova Type II. But it is almost certainly not that easy to get stars to merge so efficiently.

Certainly a massive white dwarf in a globular cluster might explode as a supernova Type Ia, if it was part of a binary and was the recipient of matter from its companion. I would hazard a guess that the more massive white dwarfs with companions inside globular clusters have exploded already. Since all the surviving stars inside today's globular clusters are relatively low-mass stars, they will produce rather low-mass white dwarfs (unless some of them never turn into white dwarfs at all, but that's another question). And low-mass white dwarfs are not likely to ever go supernova.

I think it is pretty hard to get stars to collide and merge. Also globular clusters are not quite as crowded as we may think they are. We may marvel at the idea of millions of stars inside a volume of only about 1.7 million cubic light-years. But I think we still underestimate the small size of those stars compared with the (relatively speaking) huge volume of 1.7 million cubic light-years.

Ann

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by fastartcee » Thu May 29, 2014 8:08 pm

Ann wrote:
fastartcee wrote:
I've contemplated whether some kind of mega-nova could eventually produce a spherical cluster, but, as you've pointed out, unless the expanding debris of a nova 'bumps into' something (a molecular cloud, or another expanding debris field), star genesis will not occur. But what if the entire mass of a spherical cluster simply 'popped into' existence, just like the mass/matter/energy of the Big Bang popped into existence? (As Lawrence Krauss might say, "Something from nothing.") Could that eventually result in what we see as a spherical cluster? Has the math ever been done?
There is a problem here, because the globular clusters of the Milky Way are most certainly parts of our universe, let alone parts of our own home galaxy.

The way I understand it, physicists aren't too sure what the Big Bang really was. Was it an explosion? Maybe it was, but then again, maybe it wasn't.

In any case, the Big Bang gave rise to our entire universe. Nothing that exists in our universe was not caused by the Big Bang. And nothing that exists in our universe is also a part of another universe. That is the way I understand it, at least.

But now consider the possibility that globular clusters were formed by mini-mini bangs (Small Bangs?). Wouldn't even these Small Bangs cause incredible havoc if they suddenly intruded incredibly violently into our pre-existing universe? Shouldn't we see remnants of those unbelievable shocks of energy that suddenly burst into our universe? Shouldn't pre-existing galaxies, even proto-galaxies, be completely ripped apart by the titanic forces of the Small Bangs? And later, when the mass and energy reassembled itself into structures like galaxies, is it even conceivable that the disk and halo of the Milky Way would have been made of matter that originates from the original Big Bang, while the globular clusters would trace their origins back to matter cooked up by the Small Bangs?

That seems like a completely unrealistic scenario to me.

Ann
I value your comments; they've given me much to think about.

The posited Little Bangs (I like that) would have been absolutely nothing compared to the Big Bang (whatever it was). After all, they each resulted in only millions of stars, compared to billions of trillions. And I'm thinking of them as occurring within our Universe. If quantum mechanics permits virtual particles to pop into and out of existence, and it Krauss (in 'A Universe from Nothing') believes that our entire Universe popped into existence, then why is it not conceivable that under some circumstances--say, right after the Big Bang--the stuff of spherical clusters could pop into existence within our Universe? Certainly those events would have generated gravity waves, but the waves related to, say, a million stars would have been pretty puny, so maybe our technology cannot detect them. Also, I am speculating that these Little Bangs occurred before large galaxies formed... so there were no existing large structures to disrupt. Eventually, these mini-galaxies were swept up by the nearest mega-galaxies that formed, and settled into various orbits where they are found today, in the central bulges of large galaxies. (Okay, I freely admit it: this is all wild speculation! But it's great fun to exchange views with knowledgeable people like yourself, Ann, even when they are trying to nudge me back on the right path!)

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by neufer » Thu May 29, 2014 7:49 pm

Qweenie wrote:
The Omega Centauri cluster is 150 ly in diameter, giving a volume of about 1.7 million cubic ly. With 10 million stars crammed into that space, they must average about 0.2 ly apart, which would put them well inside the Solar System's Oort cloud but just outside the Kuiper belt.
The Kuiper belt lies within 0.001 ly.

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by Qweenie » Thu May 29, 2014 7:29 pm

The Omega Centauri cluster is 150 ly in diameter, giving a volume of about 1.7 million cubic ly. With 10 million stars crammed into that space, they must average about 0.2 ly apart, which would put them well inside the Solar System's Oort cloud but just outside the Kuiper belt. I'm really curious how stable any star could be in such a crowded environment. Why do we not see a whole slew of novas or supernovas in these clusters?

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by Chris Peterson » Thu May 29, 2014 7:06 pm

fastartcee wrote:I am always struck by the nearly-perfect spherical shape of these 200-odd clusters. (Yeah, I know... they're called 'spherical clusters'!) I cannot understand how there could have been all these spherical hydrogen clouds at the outset, all with well-distributed denser regions ready to condense into stars.
That's probably not what was going on.

When you have high enough material densities for fluid dynamics to come into play, gravity tends to pull material into disc-like structures. If the conditions don't allow for momentum transfer via fluid dynamics, gravity tends to pull material into spherical structures. Once discs no longer support fluid dynamics (e.g. mature galaxies) it's pretty easy to perturb them into spheres. That may explain the formation of globular clusters from miniature galaxies, for instance (something perhaps supported by observational evidence for a net angular momentum or global rotation).

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by Chris Peterson » Thu May 29, 2014 7:00 pm

BDanielMayfield wrote:I can agree with all of what you say here except for the "take them away and nothing would change" part. As a fun thought experiment, let's say that the Milky Way's 4.1 million solar massed Supermassive BH where to instantly vanish. Everything orbiting it would then fly outward in basically straight lines with the same rapid velocities they had at whatever points they where in their orbits when the BH vanishes. The core region of our galaxy would swell, and objects closest to the BH would have velocities fast enough to escape the Milky Way altogether.
Yes, obviously something would happen. That would be true removing even one star. But the results would be trivial. Very little is orbiting the central black hole. The central density exceeds a few million solar masses at a very small radius. Even close to the center, stars are primarily orbiting the center of mass created by millions of other stars. So a small number of stars would go flying off, and some others would see their orbits tweaked. I question whether somebody watching our galaxy from far away would be able to tell anything was happening. Our galaxy wouldn't look significantly different, either shortly afterward or many millions of years later.

A few million solar masses at the very center just isn't enough to matter much. Only a handful of galaxies have been identified where the central black hole is so massive it appears to be playing an important role in the structural evolution of those galaxies.

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by Ann » Thu May 29, 2014 6:50 pm

fastartcee wrote:
I've contemplated whether some kind of mega-nova could eventually produce a spherical cluster, but, as you've pointed out, unless the expanding debris of a nova 'bumps into' something (a molecular cloud, or another expanding debris field), star genesis will not occur. But what if the entire mass of a spherical cluster simply 'popped into' existence, just like the mass/matter/energy of the Big Bang popped into existence? (As Lawrence Krauss might say, "Something from nothing.") Could that eventually result in what we see as a spherical cluster? Has the math ever been done?
There is a problem here, because the globular clusters of the Milky Way are most certainly parts of our universe, let alone parts of our own home galaxy.

The way I understand it, physicists aren't too sure what the Big Bang really was. Was it an explosion? Maybe it was, but then again, maybe it wasn't.

In any case, the Big Bang gave rise to our entire universe. Nothing that exists in our universe was not caused by the Big Bang. And nothing that exists in our universe is also a part of another universe. That is the way I understand it, at least.

But now consider the possibility that globular clusters were formed by mini-mini bangs (Small Bangs?). Wouldn't even these Small Bangs cause incredible havoc if they suddenly intruded incredibly violently into our pre-existing universe? Shouldn't we see remnants of those unbelievable shocks of energy that suddenly burst into our universe? Shouldn't pre-existing galaxies, even proto-galaxies, be completely ripped apart by the titanic forces of the Small Bangs? And later, when the mass and energy reassembled itself into structures like galaxies, is it even conceivable that the disk and halo of the Milky Way would have been made of matter that originates from the original Big Bang, while the globular clusters would trace their origins back to matter cooked up by the Small Bangs?

That seems like a completely unrealistic scenario to me.

Ann

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by fastartcee » Thu May 29, 2014 5:57 pm

Ann wrote:
fastartcee wrote:I am always struck by the nearly-perfect spherical shape of these 200-odd clusters. (Yeah, I know... they're called 'spherical clusters'!) I cannot understand how there could have been all these spherical hydrogen clouds at the outset, all with well-distributed denser regions ready to condense into stars. Rather, spherical clusters look for all the world like the products of mega-explosions, with chaotic interaction of its denser portions giving rise to stars with various highly elliptical orbits around the center of combined mass.

A wilder idea I've had is that these 200 mega-explosions were, in fact, 200 Mini-Big-Bangs... some kind of 'echoes' of the original Big Bang. Would this not result in spherical mini-galaxies--nearly as old as the Universe--made up of stars having the observed radial distribution: densest in the middle, grading to vacuum at the periphery? I do not pretend to have even the vaguest idea as to how this could be; it is only wild speculation from a non-astrophysicist!
The only naturally occurring powerful explosions in space that astronomers definitely know about are the explosions of supernovas. (It is possible that, say, the merger of two supermassive black holes might cause the kind of titanic upheaval that can only be described as an explosion.)

While supernova explosions can indeed trigger star formation - by compressing nearby gas clouds so that they reach the critical density where they start forming stars - a single supernova explosion will not cause the kind of massive star formation that leads to the birth of a globular cluster.

Massive star formation in the nearby universe is often caused by galactic interaction and mergers. Take a look at this animation, showing the interaction and ongoing merger between the two galaxies NGC 4038 and 4039, which are also known as the Antennae Galaxies.

Ann
Nice animation!

I've contemplated whether some kind of mega-nova could eventually produce a spherical cluster, but, as you've pointed out, unless the expanding debris of a nova 'bumps into' something (a molecular cloud, or another expanding debris field), star genesis will not occur. But what if the entire mass of a spherical cluster simply 'popped into' existence, just like the mass/matter/energy of the Big Bang popped into existence? (As Lawrence Krauss might say, "Something from nothing.") Could that eventually result in what we see as a spherical cluster? Has the math ever been done?

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by Ann » Thu May 29, 2014 5:41 pm

BDanielMayfield wrote:
The core region of our galaxy would swell, and objects closest to the BH would have velocities fast enough to escape the Milky Way altogether.
Some stars do that already, even though the black hole has not evaporated yet. :wink:

Ann

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by BDanielMayfield » Thu May 29, 2014 5:31 pm

Ann wrote:They orbit their common center of mass. On this page, you can find an animation of how stars orbit the black hole at the center of our galaxy. Basically, the orbits of stars inside a globular cluster would be similar.
Thanks for providing us with that excellent link Ann.
Chris Peterson wrote:It isn't necessary to have a black hole at the center of a galaxy. Such black holes have little or no effect on the dynamics of galaxies. Take them away and nothing would change. They lack the mass to have anything other than very local effects.

Black holes are common at the center of galaxies because of something about the way galaxies form. They are a consequence of that process.

Some globular clusters have central black holes, some (probably most) do not. Again, it doesn't matter in terms of dynamics.
I can agree with all of what you say here except for the "take them away and nothing would change" part. As a fun thought experiment, let's say that the Milky Way's 4.1 million solar massed Supermassive BH where to instantly vanish. Everything orbiting it would then fly outward in basically straight lines with the same rapid velocities they had at whatever points they where in their orbits when the BH vanishes. The core region of our galaxy would swell, and objects closest to the BH would have velocities fast enough to escape the Milky Way altogether.

Bruce

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by Ann » Thu May 29, 2014 5:13 pm

fastartcee wrote:I am always struck by the nearly-perfect spherical shape of these 200-odd clusters. (Yeah, I know... they're called 'spherical clusters'!) I cannot understand how there could have been all these spherical hydrogen clouds at the outset, all with well-distributed denser regions ready to condense into stars. Rather, spherical clusters look for all the world like the products of mega-explosions, with chaotic interaction of its denser portions giving rise to stars with various highly elliptical orbits around the center of combined mass.

A wilder idea I've had is that these 200 mega-explosions were, in fact, 200 Mini-Big-Bangs... some kind of 'echoes' of the original Big Bang. Would this not result in spherical mini-galaxies--nearly as old as the Universe--made up of stars having the observed radial distribution: densest in the middle, grading to vacuum at the periphery? I do not pretend to have even the vaguest idea as to how this could be; it is only wild speculation from a non-astrophysicist!
The only naturally occurring powerful explosions in space that astronomers definitely know about are the explosions of supernovas. (It is possible that, say, the merger of two supermassive black holes might cause the kind of titanic upheaval that can only be described as an explosion.)

While supernova explosions can indeed trigger star formation - by compressing nearby gas clouds so that they reach the critical density where they start forming stars - a single supernova explosion will not cause the kind of massive star formation that leads to the birth of a globular cluster.

Massive star formation in the nearby universe is often caused by galactic interaction and mergers. Take a look at this animation, showing the interaction and ongoing merger between the two galaxies NGC 4038 and 4039, which are also known as the Antennae Galaxies.

Ann

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by jed » Thu May 29, 2014 4:34 pm

I estimate about 24 stars within a 1 light year radius?

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by jed » Thu May 29, 2014 4:12 pm

I'm thinking it's not very dark at night on a planet orbiting one of those stars!

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by fastartcee » Thu May 29, 2014 3:51 pm

I am always struck by the nearly-perfect spherical shape of these 200-odd clusters. (Yeah, I know... they're called 'spherical clusters'!) I cannot understand how there could have been all these spherical hydrogen clouds at the outset, all with well-distributed denser regions ready to condense into stars. Rather, spherical clusters look for all the world like the products of mega-explosions, with chaotic interaction of its denser portions giving rise to stars with various highly elliptical orbits around the center of combined mass.

A wilder idea I've had is that these 200 mega-explosions were, in fact, 200 Mini-Big-Bangs... some kind of 'echoes' of the original Big Bang. Would this not result in spherical mini-galaxies--nearly as old as the Universe--made up of stars having the observed radial distribution: densest in the middle, grading to vacuum at the periphery? I do not pretend to have even the vaguest idea as to how this could be; it is only wild speculation from a non-astrophysicist!

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by Chris Peterson » Thu May 29, 2014 2:58 pm

tjpstewart38 wrote:Given all the odd shapes of galaxies due to gravitational interactions (I assume), it seems strange to me that the globular clusters are able to maintain their spherical shape. Why is it that they don't get torn apart by tidal (is that the term?) forces from the Milky Way?
Because the tidal forces are too small to overcome the gravitational binding (most of the stars in a globular cluster have a high escape velocity with respect to their orbital velocity).

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by tjpstewart38 » Thu May 29, 2014 2:44 pm

Given all the odd shapes of galaxies due to gravitational interactions (I assume), it seems strange to me that the globular clusters are able to maintain their spherical shape. Why is it that they don't get torn apart by tidal (is that the term?) forces from the Milky Way?

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by Chris Peterson » Thu May 29, 2014 1:48 pm

zbvhs wrote:The problem with black holes is that they are apparently invisible. Apart from x-ray observations that suggest something strange at the center of the galaxy, nothing else is visible there. Observers see extreme motions of stars at the galactic center but nothing obvious that might be causing the motion. If globular clusters can exist without black holes, why is it necessary to have one at the center of the galaxy?
It isn't necessary to have a black hole at the center of a galaxy. Such black holes have little or no effect on the dynamics of galaxies. Take them away and nothing would change. They lack the mass to have anything other than very local effects.

Black holes are common at the center of galaxies because of something about the way galaxies form. They are a consequence of that process.

Some globular clusters have central black holes, some (probably most) do not. Again, it doesn't matter in terms of dynamics.

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by Chris Peterson » Thu May 29, 2014 1:44 pm

coyote@alum.mit.edu wrote:Many stars are blue or red, some of similar size and near one another. Is this true color and does the image display red- and blue shifts? Are they really this obvious?
Color is a physiological phenomenon, not a physical one (except that when astronomers talk about the color of a star, they're talking about its temperature, not its appearance). The colors you perceive in this image are not the same as you'd perceive looking at this object through a telescope. As is usually the case with astronomical images, the instrumentation and processing result in colors more saturated than the actual visual appearance.

The colors in the image convey accurate information about the stars, but you'd never see this object looking quite like it does here.

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by Observer » Thu May 29, 2014 1:16 pm

Why does it seem that there are so many small star arcs in and around the cluster center? It seems odd that there are so many liner "empty" zones between the arcs. Or is it that my monitor is old?
Thoughts?

Re: APOD: Millions of Stars in Omega Centauri (2014 May 29)

by BDanielMayfield » Thu May 29, 2014 12:42 pm

superwillbee wrote:
zbvhs wrote:So, if stars in a globular cluster orbit a common center of mass, i.e., a virtual mass, why is it necessary to have a black hole at the center of the galaxy? Why couldn't the whole galaxy be rotating about a virtual mass?
Well said!
No need for immense masses and 'black holes'. All that is needed is a vortex, a tornado or supercell if you like. I think that vortexes and/or tornados i.e rotation itself is the basic force of our universe, and not gravity. What we call gravity might be cohesion or some kind of 'stickyness', being a result of rotation. [Or attached to rotation]
I have to disagree. There is most certainly a need for an immense mass at the common focus of all the S-stars rapidly orbiting around something at the center of our galaxy. The nature of black holes can be debated, but their existence should be viewed as a well established fact.

Bruce

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