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

[APOD Robot]

Millions of Stars in Omega Centauri

Explanation: Globular star cluster Omega Centauri, also known as NGC 5139, is some 15,000 light-years away. The cluster is packed with about 10 million stars much older than the Sun within a volume about 150 light-years in diameter, the largest and brightest of 200 or so known globular clusters that roam the halo of our Milky Way galaxy. Though most star clusters consist of stars with the same age and composition, the enigmatic Omega Cen exhibits the presence of different stellar populations with a spread of ages and chemical abundances. In fact, Omega Cen may be the remnant core of a small galaxy merging with the Milky Way. This astronomically sharp color image of the classic globular cluster was recorded in March under Chilean skies from Hacienda Los Andes.

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[coyote@alum.mit.edu]

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? Adjacent red and blue stars would suggest a binary, right?

(Is there a way to embed an image in this field?)

Joe Horton

[geckzilla]

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? Adjacent red and blue stars would suggest a binary, right?

(Is there a way to embed an image in this field?)

Joe Horton

That's just the color of the stars, Joe.

[C0ppert0p]

I'm interested in knowing how a globular cluster would form. Were they formed before the galaxy formed? Can they exist inside the galaxy or do they only exist in the galactic halo?
How do the mechanics of star formation differ in globular clusters than from say stars forming in the gas and dust clouds of the spiral arms?

Thanks!

[Ann]

There is nothing preventing globular clusters from existing inside galaxies. Check out this page to read about a newfound globular cluster among the rich deep starfields in constellation Aquila.

However, globular clusters typically follow orbits that take them out into the halo again after passing through the disk of the Milky Way. That is one reason we astronomers believe that globular clusters formed before the Milky Way galaxy even had a disk. Another reason to believe that globular clusters are ancient is that the stars in them are composed of quite pristine gas, mixed with extremely small amounts of elements heavier than hydrogen and helium.

Ann

[grump]

I'm interested in knowing how a globular cluster would form...

And I'm interested in how the stars in a cluster would orbit around the centre?

[Ann]

I'm interested in knowing how a globular cluster would form...

And I'm interested in how the stars in a cluster would orbit around the centre?

I'm not absolutely sure what you mean here. Are you asking about the orbits of the stars inside a globular cluster? 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. However, it was recently found that stars inside at least some globular clusters orbit around a common axis, thereby (as I understand it) forming a precursor of a disk or a bar.

This picture shows the general type of orbits in the Milky Way. In the old yellow bulge, stars orbit "every which way", up and down and round and round, creating the rounded shape of the bulge. In the disk, the stars generally orbit in the plane of the disk, creating the flat shape of the disk. The globular clusters orbit the center of the Milky Way "up and down and round and round", creating a spherical distribution of the globulars.

Ann

[zbvhs]

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?

[starsurfer]

I think Omega Centauri is also suspected to have a black hole at its centre? Quite a few globular clusters are now known to possess multiple stellar populations. I think some of the globular clusters in the Milky Way come from dwarf galaxies that might have been absorbed by our galaxy.

[Ann]

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]

Hmmm. I note that you have solved a problem that our very best physicists and astronomers have not managed to understand.

Another way of putting this is that anybody's opinion is as good as anyone else's. Why should we listen to scientists, just because they have spent years carrying out experiments and observations, verifying data, constructing models, running computer tests, calculating uncertainties and margins of errors, taking alternative explanations into very careful consideration, and finally arriving at a conclusion? Why should they get any special credit for doing that, and why should we think their opinions carry more weight than others, when ordinary people have gut feelings that often disagree with the opinions of scientists?

Ann

[zbvhs]

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?

[grump]

I'm interested in knowing how a globular cluster would form...

And I'm interested in how the stars in a cluster would orbit around the centre?

I'm not absolutely sure what you mean here. Are you asking about the orbits of the stars inside a globular cluster?.....

Ann

Yes I was. Thank you Ann for pointing me at some explanations.

[BDanielMayfield]

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

[Observer]

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?

[Chris Peterson]

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.

[Chris Peterson]

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.

[tjpstewart38]

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?

[Chris Peterson]

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).

[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. 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!

[jed]

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

[jed]

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

[Ann]

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

[BDanielMayfield]

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.

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

[Ann]

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.

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. 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?