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

[Ann]

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

[Chris Peterson]

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.

[Chris Peterson]

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).

[Qweenie]

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?

[neufer]

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.

[fastartcee]

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!)

[Ann]

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

[fastartcee]

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.

[Ann]

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.

Click to view full size image

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

[Chris Peterson]

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.

[Chris Peterson]

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.

[Nitpicker]

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