APOD: Millions of Stars in Omega Centauri (2024 Mar 28)

[APOD Robot]

Millions of Stars in Omega Centauri

Explanation: Globular star cluster Omega Centauri, also known as NGC 5139, is 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. It's 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. With a yellowish hue, Omega Centauri's red giant stars are easy to pick out in this sharp, color telescopic view.

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[Ann]

So... globular clusters are not my favorites because they are not so colorful. Anyway...

Let's compare the two largest globular clusters of the Milky Way!

Millions of Stars in Omega Centauri
Image Credit & Copyright: Massimo Di Fusco and Mirco Turra

47 Tucanae. Credit: Don Goldman.

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Let's look at some differences. First, however, bear in mind that the pictures may not be directly comparable. The scale and "degree of closeup" may be different, and the sizes of the images are definitely different - the Omega Centauri image is almost 2 MB, whereas the 47 Tuc image is some 300 KB.

According to an old catalog of mine, Sky Catalogue 2000.0 Vol.2, globular clusters are classified according to their degree of concentration. The most highly concentrated globulars are class 1, while the loosest ones are class 12. According to Sky Catalogue 2000.0 Vol.2, Omega Centauri is class 8, so it's fairly loose. 47 Tuc, on the other hand, is class 3, so it is much more concentrated. And you can see in Don Goldman's picture how concentrated its center is.

Another aspect of globular clusters is their metallicity, typically measured as how much iron they contain relative to hydrogen compared with the Sun. If the metallicity, [Fe/H], is -2, it means that each member of the globular contains, on average, 1/100 as much iron relative to hydrogen as the Sun. Well, the metallicity of Omega Centauri is -1.59, and the metallicity of 47 Tuc is 0.79. So both Omega Centauri and 47 Tuc are metal-poor compared with the Sun, but Omega Centauri is more metal-poor than 47 Tuc.

And this has consequences. What it means, in particular, is that Omega Centauri contains a class of evolved blue stars, blue horizontal branch stars, but 47 Tuc lacks them.

Color magnitude diagram of a metal-poor globular cluster, M55. The stars are classified according to their colors and magnitudes. The blue horzontal branch stars are at upper left. Only very metal-poor stars ever become blue horizontal branch stars. B.J. Mochejska, J. Kaluzny (CAMK), 1m Swope Telescope

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Color magnitude diagram of 47 Tuc. Note the very short horizontal branch at upper left. The stars here are not blue. Credit: Chegg.com

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Color magnitude diagram of Omega Centauri. The 'arc' at left is the blue horizontal branch. The stars here are blue. Credit: Michael Hilker

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When you compare the color-magnitude diagrams of Omega Centauri and 47 Tuc, your first question is probably, Why are the "undersides" of the color magnitude diagrams so different for Omega Centauri and 47 Tuc? Well, I guess that the lower part of the Omega Centauri diagram has just been cut off, so that it doesn't show us the faintest, reddest stars. Note that the reddest, faintest stars of the Omega Centauri diagram have a B-V index of about 0.5, whereas the reddest, faintest stars of 47 Tuc have a B-V index of about 1.8. We can be sure that the there are small red stars in Omega Centauri that are as red as the faintest red stars of 47 Tuc, but we don't see them in this diagram.


There is a Hubble closeup of stars inside Omega Centauri. If you ask me, the colors are pretty horribly saturated and "too much", but you can certainly see that there are blue stars inside Omega Cen. There is also a picture of 47 Tuc that shows you that there are no blue stars here. (Yes, there are blue stragglers in 47 Tuc, stars that have accreted gas from a companion star and grown hotter and bluer than they "should" be, but the blue stragglers are so few and far between - and fairly faint, too - that they don't affect the overall color of the cluster.)

Stars of Omega Centauri. There are both blue and red stars here, but the real colors are not so exaggerated. Credit: Hubble

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Stars of 47 Tuc. There are no blue stars here. Credit: NASA and Ron Gilliland (Space Telescope Science Institute)

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As a consequence, because there are blue stars in Omega Centauri but not in 47 Tuc due to its higher metallicity, Omega Centauri is whiter in overall color than 47 Tuc. The B-V of Omega Centauri is 0.68, which is just a hair yellower than the B-V index of the Sun, 0.656, (or the B-V index of Polaris, 0.636). The B-V index of 47 Tuc, by contrast, is 0.89, which is a little yellower than Capella (0.795).

Very pale yellowish white star Polaris, similar in color to the Sun, and similar in color to Omega Centauri. Credit: Credit: NASA/ESA/HST, G. Bacon (STScI)

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Pale yellow star Capella, relatively similar in color to 47 Tuc. Credit: I don't know.

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Omega Centauri is really different from most other Milky Way globulars, and it is certainly different from 47 Tuc, in that it is believed to be the core of a dwarf galaxy cannibalized by our own big bully of a galaxy.


Small spiral galaxy NGC 300 may just possibly be similar to the dwarf galaxy that was captured by the Milky Way, and whose core became Omega Centauri. The best picture I could find of the core of NGC 300 is printed on a zip pouch!

Small galaxy NGC 300. It might possibly be similar to the galaxy that was captured by the Milky Way billions of years ago, and whose core became Omega Centauri. Credit: ESO.

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The center of dwarf galaxy NGC 300, printed on a zip pouch. With the exception of the small very bright core and the signs of youth, such as the blue-white open clusters and the dark dust lanes, the central part of NGC 300 could be similar to Omega Centauri. Or perhaps it is the very bright tiny core of NGC 300 that is similar to Omega Centauri?

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The Milky Way has cannibalized many small galaxies over its many billion-year life time. The hapless small galaxies have left their marks on our galaxy in the form of stellar streams.

Illustration of a few of the stellar streams of the Milky Way. Note the small galaxy in the large stream at top. Credit: NASA

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Is there a stellar stream connected to Omega Centauri, too? Yes, apparently, a small one! It's called the Fimbulthul stream, and it was discovered only in 2019. Its mass is just 318 solar masses, so it's a lightweight stream for sure!

The left-hand panel shows stars belonging to Omega Cetauri, shown in blue, and stars belonging to the Fimbulthul stream, shown in red. They mostly overlap. The right-hand panel shows background stars that don't belong to either Omega Centauri or the Fimbulthul stream. Credit: Rodrigo A Ibata et al.

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And that's all for me! I'm off!

Ann

[smitty]

Is gravitational attraction causing the stars to move toward the center of the cluster? If not, why not?

[Christian G.]

In his unique style Richard Feynman wrote that globular clusters are one of the most beautiful things in the sky, adding "If one cannot see gravitation acting here, he has no soul."

[Chris Peterson]

Is gravitational attraction causing the stars to move toward the center of the cluster? If not, why not?

For the same reason that the gravitational attraction of the Sun isn't causing the Earth and other planets to move towards the center of the Solar System. They're in orbit. (Of course, we can understand orbits as "falling" in towards the dominant mass, but never reaching it because of the tangential motion component.)

[Roy]

Globular clusters are leftover galaxy cores? The 2010 count of them was 157. We see large numbers of them in the Andromeda galaxy, and other galaxies. So, based on that number crunching, clusters are two or three orders of magnitude more common than galaxies.

[pclark@bigpond.net.au]

Omega Centauri is reported to contain a black hole. A study in 2008 provided evidence for an intermediate-mass black hole at the center of Omega Centauri, based on observations made by the Hubble Space Telescope and the Gemini Observatory. The black hole is estimated to be roughly 40,000 times the mass of the Sun. This finding is significant because it suggests a continuous range of black hole masses, from small stellar types to supermassive. Is this information correct?

[Chris Peterson]

Globular clusters are leftover galaxy cores? The 2010 count of them was 157. We see large numbers of them in the Andromeda galaxy, and other galaxies. So, based on that number crunching, clusters are two or three orders of magnitude more common than galaxies.

There is no evidence that globular clusters are "leftover galaxy cores".

[zendae]

I've asked this before re the Hercules cluster: do astronomers know the approximate distance between the closest stars to each other here? They seem to know the approximate number of stars, and the size and distance of the structure. I'm assuming things are not homogeneous tho: some stars are relatively closer to each other than others.
It certainly is a good representation of how close stars appear to be to each other from our visual perspective, yet how far they actually are from each other, and both how great and miniscule the span of distance a light year is.

If we were viewing from the center of this, I do wonder what the "sky" would look like compared to ours.

[Chris Peterson]

I've asked this before re the Hercules cluster: do astronomers know the approximate distance between the closest stars to each other here? They seem to know the approximate number of stars, and the size and distance of the structure. I'm assuming things are not homogeneous tho: some stars are relatively closer to each other than others.
It certainly is a good representation of how close stars appear to be to each other from our visual perspective, yet how far they actually are from each other, and both how great and miniscule the span of distance a light year is.

If we were viewing from the center of this, I do wonder what the "sky" would look like compared to ours.

Globular clusters are not static structures. All of the stars are moving with respect to each other. So the best way to understand this is in terms of density, not distance. There's a density gradient, with more stars per unit volume near the center than at the outside. In general, these gradients are easy to measure, and the total number of stars is easy to estimate with pretty good accuracy. And, the orbital dynamics are well understood and readily modeled. So usually the structure of globular clusters is pretty well described, including the average range of distances between nearby stars in different parts of a cluster.

[Ann]

I've asked this before re the Hercules cluster: do astronomers know the approximate distance between the closest stars to each other here? They seem to know the approximate number of stars, and the size and distance of the structure. I'm assuming things are not homogeneous tho: some stars are relatively closer to each other than others.
It certainly is a good representation of how close stars appear to be to each other from our visual perspective, yet how far they actually are from each other, and both how great and miniscule the span of distance a light year is.

If we were viewing from the center of this, I do wonder what the "sky" would look like compared to ours.

Wikipedia wrote:
The stars in the core of Omega Centauri are so crowded that they are estimated to average only 0.1 light-year away from each other.

Here is a picture of what it might look like to to be inside a globular cluster (in this case, 47 Tuc):

Inside globular cluster 47 Tuc. The bright white spot at lower right is the brilliant core of 47 Tuc. The yellowish spot at upper left is another globular cluster near 47 Tuc, NGC 362. Credit: William Harris and Jeremy Webb

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Ann

[Ann]

Globular clusters are leftover galaxy cores? The 2010 count of them was 157. We see large numbers of them in the Andromeda galaxy, and other galaxies. So, based on that number crunching, clusters are two or three orders of magnitude more common than galaxies.

There is no evidence that globular clusters are "leftover galaxy cores".

I take it that there is some evidence that Omega Centauri is?

Ann

[Chris Peterson]

Globular clusters are leftover galaxy cores? The 2010 count of them was 157. We see large numbers of them in the Andromeda galaxy, and other galaxies. So, based on that number crunching, clusters are two or three orders of magnitude more common than galaxies.

There is no evidence that globular clusters are "leftover galaxy cores".

I take it that there is some evidence that Omega Centauri is?

Ann

I would not characterize the remnant of a collision as a "leftover galaxy core"! In any case, the hypothesis that Omega Centauri isn't a "true" globular cluster, but rather something left from a collision, is based on the observation that its stellar population looks very different from that of other globular clusters... the overwhelming majority of which present stellar populations that look nothing like galaxy cores.

[johnnydeep]

There is no evidence that globular clusters are "leftover galaxy cores".

I take it that there is some evidence that Omega Centauri is?

Ann

I would not characterize the remnant of a collision as a "leftover galaxy core"! In any case, the hypothesis that Omega Centauri isn't a "true" globular cluster, but rather something left from a collision, is based on the observation that its stellar population looks very different from that of other globular clusters... the overwhelming majority of which present stellar populations that look nothing like galaxy cores.

What of this paper then - https://www.nature.com/articles/s41550-019-0751-x

Abstract
Omega Centauri (ω Cen) is the Milky Way’s most massive globular cluster, and has long been suspected of being the remnant core of an accreted dwarf galaxy. If this scenario is correct, ω Cen should be tidally limited and tidal debris should be spread along its orbit. Here we use N-body simulations to show that the recently discovered ‘Fimbulthul’ structure is the long-sought-for tidal stream of ω Cen, extending up to 28° from the cluster. Follow-up high-resolution spectroscopy of five stream stars shows that they are closely grouped in velocity, and have metallicities consistent with having originated in that cluster. Informed by our N-body simulations, we devise a selection filter that we apply to Gaia mission data to also uncover the stream in the highly contaminated and crowded field within 10° of ω Cen. Further modelling of the stream may help to constrain the dynamical history of the dwarf galaxy progenitor of this disrupting system and guide future searches for its remnant stars in the Milky Way.

[Chris Peterson]

I take it that there is some evidence that Omega Centauri is?

Ann

I would not characterize the remnant of a collision as a "leftover galaxy core"! In any case, the hypothesis that Omega Centauri isn't a "true" globular cluster, but rather something left from a collision, is based on the observation that its stellar population looks very different from that of other globular clusters... the overwhelming majority of which present stellar populations that look nothing like galaxy cores.

What of this paper then - https://www.nature.com/articles/s41550-019-0751-x

Abstract
Omega Centauri (ω Cen) is the Milky Way’s most massive globular cluster, and has long been suspected of being the remnant core of an accreted dwarf galaxy. If this scenario is correct, ω Cen should be tidally limited and tidal debris should be spread along its orbit. Here we use N-body simulations to show that the recently discovered ‘Fimbulthul’ structure is the long-sought-for tidal stream of ω Cen, extending up to 28° from the cluster. Follow-up high-resolution spectroscopy of five stream stars shows that they are closely grouped in velocity, and have metallicities consistent with having originated in that cluster. Informed by our N-body simulations, we devise a selection filter that we apply to Gaia mission data to also uncover the stream in the highly contaminated and crowded field within 10° of ω Cen. Further modelling of the stream may help to constrain the dynamical history of the dwarf galaxy progenitor of this disrupting system and guide future searches for its remnant stars in the Milky Way.

What of it?

[johnnydeep]

I would not characterize the remnant of a collision as a "leftover galaxy core"! In any case, the hypothesis that Omega Centauri isn't a "true" globular cluster, but rather something left from a collision, is based on the observation that its stellar population looks very different from that of other globular clusters... the overwhelming majority of which present stellar populations that look nothing like galaxy cores.

What of this paper then - https://www.nature.com/articles/s41550-019-0751-x

Abstract
Omega Centauri (ω Cen) is the Milky Way’s most massive globular cluster, and has long been suspected of being the remnant core of an accreted dwarf galaxy. If this scenario is correct, ω Cen should be tidally limited and tidal debris should be spread along its orbit. Here we use N-body simulations to show that the recently discovered ‘Fimbulthul’ structure is the long-sought-for tidal stream of ω Cen, extending up to 28° from the cluster. Follow-up high-resolution spectroscopy of five stream stars shows that they are closely grouped in velocity, and have metallicities consistent with having originated in that cluster. Informed by our N-body simulations, we devise a selection filter that we apply to Gaia mission data to also uncover the stream in the highly contaminated and crowded field within 10° of ω Cen. Further modelling of the stream may help to constrain the dynamical history of the dwarf galaxy progenitor of this disrupting system and guide future searches for its remnant stars in the Milky Way.

What of it?

Well, doesn't it say that Omega Centauri is - or very well could be - the remnant core of a cannibalized galaxy? And aren't you arguing that it's not?

[Chris Peterson]

What of this paper then - https://www.nature.com/articles/s41550-019-0751-x

What of it?

Well, doesn't it say that Omega Centauri is - or very well could be - the remnant core of a cannibalized galaxy? And aren't you arguing that it's not?

I am not. I'm suggesting that "leftover galaxy core" is a lousy term for it, and that it isn't even a globular cluster, except visually. My point is that there's no evidence that globular clusters (plural, in general) are "leftover galaxy cores".

[johnnydeep]

What of it?

Well, doesn't it say that Omega Centauri is - or very well could be - the remnant core of a cannibalized galaxy? And aren't you arguing that it's not?

I am not. I'm suggesting that "leftover galaxy core" is a lousy term for it, and that it isn't even a globular cluster, except visually. My point is that there's no evidence that globular clusters (plural, in general) are "leftover galaxy cores".

Ok, but it seems the paper is arguing (or might be - I don't have full access) that it is indeed a "remnant galaxy core", which I take to be the same as a "left over galaxy core".

[Chris Peterson]

Well, doesn't it say that Omega Centauri is - or very well could be - the remnant core of a cannibalized galaxy? And aren't you arguing that it's not?

I am not. I'm suggesting that "leftover galaxy core" is a lousy term for it, and that it isn't even a globular cluster, except visually. My point is that there's no evidence that globular clusters (plural, in general) are "leftover galaxy cores".

Ok, but it seems the paper is arguing (or might be - I don't have full access) that it is indeed a "remnant galaxy core", which I take to be the same as a "left over galaxy core".

I don't think the two terms are really synonymous. But in any case, the important point is that this singular object is very different from other bodies we call "globular clusters".

[Roy]

Some astronomers think there is a black hole in omega Centauri. Five others think that if that is so, it could be the center of a small galaxy stripped of its peripheral stars. They look in that area of the sky, and find 309 stars that seem to be of the same age as omega Centauri, in an arc pointed at the cluster. They name it "Fimbulthul". Lots of "what ifs" and conjecture and data selection in that logic sequence. Can it be falsified?
My original remark about "leftover" galaxy cores referred to a galaxy being stripped of all except the central core stars after it had passed through the large galaxy, on the way to becoming an orbiting globular cluster - that there were too many globular clusters for that process to be the source of them.
In this case remnant and leftover are substantially the same semantics.

[Chris Peterson]

Some astronomers think there is a black hole in omega Centauri. Five others think that if that is so, it could be the center of a small galaxy stripped of its peripheral stars. They look in that area of the sky, and find 309 stars that seem to be of the same age as omega Centauri, in an arc pointed at the cluster. They name it "Fimbulthul". Lots of "what ifs" and conjecture and data selection in that logic sequence. Can it be falsified?
My original remark about "leftover" galaxy cores referred to a galaxy being stripped of all except the central core stars after it had passed through the large galaxy, on the way to becoming an orbiting globular cluster - that there were too many globular clusters for that process to be the source of them.
In this case remnant and leftover are substantially the same semantics.

As long as you are restricting the comment to this object, and only this object (not other globular clusters), fine.

[johnnydeep]

I am not. I'm suggesting that "leftover galaxy core" is a lousy term for it, and that it isn't even a globular cluster, except visually. My point is that there's no evidence that globular clusters (plural, in general) are "leftover galaxy cores".

Ok, but it seems the paper is arguing (or might be - I don't have full access) that it is indeed a "remnant galaxy core", which I take to be the same as a "left over galaxy core".

I don't think the two terms are really synonymous. But in any case, the important point is that this singular object is very different from other bodies we call "globular clusters".

Ok.

[Roy]

M31, Andromeda, has over 500 Global Clusters, GCs. M87, according to a paper by John Huchra of Harvard, has approximately 20,000 GCs. No suggestion of them being leftover cores. I'll stick by my statement that there are too many GCs for them to originate by the stripping method imputed in the original description.

[Chris Peterson]

M31, Andromeda, has over 500 Global Clusters, GCs. M87, according to a paper by John Huchra of Harvard, has approximately 20,000 GCs. No suggestion of them being leftover cores. I'll stick by my statement that there are too many GCs for them to originate by the stripping method imputed in the original description.

I find no such suggestion in today's caption.

[zendae]

TY Chris and Ann.
Ann, that is too many stars to have in our sky for me lol..
I very much like night time, and the night sky.
It must be quite an energetic place for so many stars to be so near each other. Talk about turbulence - and the insane math that goes with it...
And the next wonderment: so if the Proxima Centauri family were 600 billion miles away instead if ~4 light years, would Earth care?