APOD: M13: The Great Globular Cluster in... (2021 May 20)

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APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by APOD Robot » Thu May 20, 2021 4:06 am

Image M13: The Great Globular Cluster in Hercules

Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent." Of course, M13 is now less modestly recognized as the Great Globular Cluster in Hercules, one of the brightest globular star clusters in the northern sky. Sharp telescopic views like this one reveal the spectacular cluster's hundreds of thousands of stars. At a distance of 25,000 light-years, the cluster stars crowd into a region 150 light-years in diameter. Approaching the cluster core upwards of 100 stars could be contained in a cube just 3 light-years on a side. For comparison, the closest star to the Sun is over 4 light-years away. The remarkable range of brightness recorded in this image follows stars into the dense cluster core. Distant background galaxies in the medium-wide field of view include NGC 6207 at the lower right.

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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by DAVESWAY69@OUTLOOK.COM » Thu May 20, 2021 5:09 am

Is there any indication of black hole presence and or stellar collisions in any of the clusters.

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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by Ann » Thu May 20, 2021 10:26 am

DAVESWAY69@OUTLOOK.COM wrote:
Thu May 20, 2021 5:09 am
Is there any indication of black hole presence and or stellar collisions in any of the clusters.

Stellar collisions, you bet your boots! Take a look at the two diagrams above. They are color-magnitude diagrams, and they plot the brightnesses of stars versus their luminosities. In the diagram at right, you can see the true color of the stars that are located in different parts of it. And in the diagram at left, you can see a number of abbreviations, which give you the names of the different parts of the stellar populations.

"MS" means main sequence. Here we find the stars that still fuse hydrogen to helium in their cores (like the Sun does). "TO" means turnoff point. Here we find the stars that have just used up the hydrogen in their cores. How fast a star uses up its core hydrogen depends on how much mass it was born with. The more massive the star, the quicker it reaches its turnoff point. Therefore, as time goes by, more and more star use up their core hydrogen, and the turnoff point of the entire cluster keeps moving "downward".

Yes but now look at "BS". When it comes to globular clusters, "BS" means blue stragglers. Blue stragglers are stars that remain on the main sequence, even though other stars of the same mass have left it. It is believed that the blue straggler stars are stars that have gained extra mass through stellar interactions and sometimes collisions, and therefore they have received extra helpings of hydrogen to fuse in their cores.

What about black holes in globular clusters?


I suspect you are talking about intermediate-mass black holes in the center of globulars, and apparently one has been found in one of the densest globular clusters of the Milky Way, 47 Tucanane (47 Tuc).
Bülent Kızıltan, Holger Baumgardt & Abraham Loeb wrote:

Here we show there is evidence for a central black hole in 47 Tucanae with a mass of 2,300 (+1,500, -850) solar masses when the dynamical state of the globular cluster is probed with pulsars.
NASA recently claimed that a number of stellar-mass black holes had been found near the center of globular cluster NGC 6397.
Mike Wall of Space.com wrote:

The globular cluster NGC 6397, a conglomeration of stars about 7,800 light-years from Earth, likely harbors a clump of small black holes at its heart, a new study reports.

Researchers studied the movement of stars in NGC 6397 using NASA's Hubble Space Telescope and the European Space Agency's Gaia spacecraft. These motions revealed the existence of a hidden mass at the cluster's center — a "central dark component" that makes up 0.8 to 2% of NGC 6397's total mass.
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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by orin stepanek » Thu May 20, 2021 11:32 am

M13_FA12STXL11002_LRGB_2021-05_1024.jpg

Star clusters just amaze me! they are like a tiny galaxy by their own
right! funny there isn't a lot of push and shove among the resident
stars! :mrgreen:
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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by Chris Peterson » Thu May 20, 2021 1:13 pm

DAVESWAY69@OUTLOOK.COM wrote:
Thu May 20, 2021 5:09 am
Is there any indication of black hole presence and or stellar collisions in any of the clusters.
Stellar collisions will be very rare in a cluster. They can only occur when the orbits of two stars come within a stellar diameter of each other. Typical distances between stars is about seven orders of magnitude (10 million) times greater than the size of stars. Of course, you have hundreds of thousands of stars and billions of years to interact, so there have been collisions. The vast majority of close interactions result in a transfer of orbital angular momentum, with the consequence of one star dropping into a lower orbit and the other being ejected from the cluster, which is why such clusters dissipate over billions of years.

Most globular clusters do not appear to have massive black holes at their center. But at least one or two appear to.
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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by E Fish » Thu May 20, 2021 1:15 pm

This is the first non-planet that I found in a telescope on my own. It remains one of my favorites. Globular clusters are so beautiful and so interesting. Imagine how bright the night sky would be if we lived in one of them. :)

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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by dunce » Thu May 20, 2021 1:39 pm

Are all the stars in the pic moving inward to the center of the cluster or are they kinda static or orbiting like planets?

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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by Chris Peterson » Thu May 20, 2021 1:48 pm

dunce wrote:
Thu May 20, 2021 1:39 pm
Are all the stars in the pic moving inward to the center of the cluster or are they kinda static or orbiting like planets?
They are all in Keplerian (elliptical) orbits around the common center of mass, and at random inclinations. Were they not in orbit the cluster would collapse into a supermassive black hole.
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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by Ann » Thu May 20, 2021 4:29 pm

dunce wrote:
Thu May 20, 2021 1:39 pm
Are all the stars in the pic moving inward to the center of the cluster or are they kinda static or orbiting like planets?
Click to play embedded YouTube video.

This video gives you a pretty good idea of how stars move in a rich and dense cluster.

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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by neufer » Thu May 20, 2021 6:27 pm

Ann wrote:
Thu May 20, 2021 10:26 am
DAVESWAY69@OUTLOOK.COM wrote:
Thu May 20, 2021 5:09 am

Is there any indication of black hole presence and or stellar collisions in any of the clusters.
I suspect you are talking about intermediate-mass black holes in the center of globulars, and apparently one has been found in one of the densest globular clusters of the Milky Way, 47 Tucanane (47 Tuc).
Bülent Kızıltan, Holger Baumgardt & Abraham Loeb wrote:

Here we show there is evidence for a central black hole in 47 Tucanae with a mass of 2,300 (+1,500, -850) solar masses when the dynamical state of the globular cluster is probed with pulsars.
NASA recently claimed that a number of stellar-mass black holes had been found near the center of globular cluster NGC 6397.
Mike Wall of Space.com wrote:

The globular cluster NGC 6397, a conglomeration of stars about 7,800 light-years from Earth, likely harbors a clump of small black holes at its heart, a new study reports.

Researchers studied the movement of stars in NGC 6397 using NASA's Hubble Space Telescope and the European Space Agency's Gaia spacecraft. These motions revealed the existence of a hidden mass at the cluster's center — a "central dark component" that makes up 0.8 to 2% of NGC 6397's total mass.
https://www.syfy.com/syfywire/wait-do-b ... r-ngc-6397
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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by johnnydeep » Thu May 20, 2021 8:03 pm

As others have already expressed, I have come to find globular clusters almost as interesting - and indeed beautiful - as galaxies. Their spherical symmetry, and oftentimes great age (12 Gy in this case), make me think of them fondly as wanna-be ellipsoidal galaxies, with eccentricity=0.

One observation: given the age and size of this GC, it seems inevitable that there would be a much-greater-than-stellar mass black hole in the center!

Now for the questions:

- Is there any evidence for a dark matter cloud around these clusters, meaning a concentration of dark matter greater than in the non-cluster areas of the Milky Way? [ Why does it seems almost every post these days asks about dark matter? :) ]
- With a cluster so old, yet with stars in chaotic orbits that often eject members, how is this reconciled? Did the cluster have many more member stars in the past, but the ejection rate - though larger than in true galaxies (if it happens at all there) - is still not large enough to dissipate the cluster even after 12 Gys, yet still leave such a large cluster even now?
- And how is the cluster age determined? By finding some very old stars?
- What are the black dots close to the core and best observable in the full image? They seem not to just be gaps in the population of stars, though perhaps that's just an optical illusion caused by the greater contrast with the more numerous stars toward the core. [Image below.]
- Other than the large background galaxy at lower left, how many others do people see? I have only found two very small ones on the far left middle. [Image below.]

Dots at Center of M13.JPG
Background Galaxies in image of M13.JPG

EDIT: oops! Just found two more small galaxies toward the middle bottom. I won't spoil the surprise with another pic.
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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by Sa Ji Tario » Thu May 20, 2021 11:46 pm

Johnny Depp, I see 20 galaxies in the left half and 9 in the right half.

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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by Ann » Fri May 21, 2021 7:14 am

johnnydeep wrote:
Thu May 20, 2021 8:03 pm
As others have already expressed, I have come to find globular clusters almost as interesting - and indeed beautiful - as galaxies. Their spherical symmetry, and oftentimes great age (12 Gy in this case), make me think of them fondly as wanna-be ellipsoidal galaxies, with eccentricity=0.

One observation: given the age and size of this GC, it seems inevitable that there would be a much-greater-than-stellar mass black hole in the center!

Now for the questions:

- Is there any evidence for a dark matter cloud around these clusters, meaning a concentration of dark matter greater than in the non-cluster areas of the Milky Way? [ Why does it seems almost every post these days asks about dark matter? :) ]
- With a cluster so old, yet with stars in chaotic orbits that often eject members, how is this reconciled? Did the cluster have many more member stars in the past, but the ejection rate - though larger than in true galaxies (if it happens at all there) - is still not large enough to dissipate the cluster even after 12 Gys, yet still leave such a large cluster even now?
- And how is the cluster age determined? By finding some very old stars?
- What are the black dots close to the core and best observable in the full image? They seem not to just be gaps in the population of stars, though perhaps that's just an optical illusion caused by the greater contrast with the more numerous stars toward the core. [Image below.]
- Other than the large background galaxy at lower left, how many others do people see? I have only found two very small ones on the far left middle. [Image below.]
Johnny, you asked:

1) - Is there any evidence for a dark matter cloud around these clusters, meaning a concentration of dark matter greater than in the non-cluster areas of the Milky Way?

Answer: I'm too lazy to google, and I'm not hugely interested in dark matter. (Because there are no lovely colors in dark matter, are there?) But I'm pretty sure I've heard that, no, globular clusters don't seem to contain dark matter. But why not google it for yourself?

2) - With a cluster so old, yet with stars in chaotic orbits that often eject members, how is this reconciled? Did the cluster have many more member stars in the past, but the ejection rate - though larger than in true galaxies (if it happens at all there) - is still not large enough to dissipate the cluster even after 12 Gys, yet still leave such a large cluster even now?

M53 NGC 5053 Bob Franke.png
Sparse globular cluster NGC 5053 and rich globular cluster M53.
Photo: Bob Franke.

Answer: The globular clusters that we see today have most certainly lost a lot of stars since they were born! So yes, indeed, these clusters were even richer in stars than they are today when they were born some ~12 billion years ago! Some globular clusters have lost more stars than others, or, alternatively, they were born with fewer stars than others.

And that's also why a rich open cluster like R136a in the Large Magellanic Cloud will not be as rich as, say, M13 is today in twelve billion years' time. (Even though R136a is almost certainly going to exist as a cluster 12 billion years from now.) Actually, it wouldn't totally surprise me if M13 is still going to be richer in stars than R136a will be in twelve billion years.


3) - And how is the cluster age determined? By finding some very old stars?


Color-magnitude diagrams are the principal tools for astronomers to determine the age of a cluster. In a young cluster like the Pleiades, nearly all stars fall along the main sequence line, where all the stars fuse hydrogen to helium in their cores, and the stars are brighter and bluer the more massive they are. (Similarly, the stars are fainter and redder the less massive they are.) In the Pleiades cluster, ~100 million years old, almost all the stars belong to the main sequence. But the most massive stars in the Pleiades have begun to evolve off the main sequence. They have grown larger and a bit brighter and also a little less blue. That fact is not very clear in the diagram at left.

In globular clusters, only low mass stars are still on the main sequence (labeled as MS in the diagram). Medium-mass and high-mass stars have evolved off the main sequence long ago. The really massive stars that once belonged to the globular clusters have died billions of years ago, and few stars in today's globulars are more massive than the Sun. Even the 12 billion year old red giants - and you can see the red giant branch, the RGB - are relatively low-mass stars in a globular cluster.

Rich globular cluster M53 and sparse globular cluster NGC 5053 both have color-magnitude diagrams similar to the color-magnitude diagram of globular cluster M3. That's how we know that NGC 5053 is a "real" globular cluster and not a much younger open cluster .

The more stars have evolved off the main sequence, the older the cluster is. The lower mass of the stars that have evolved off the main sequence, the older the cluster is.

But the ages of stars are also determined by their metal content, which is seen in their spectra. "Metals" in astro-speak mean elements heavier than hydrogen and helium, i.e., elements that were created by earlier generations of stars which died as either supernovas or red giants and in either case produced a lot of heavier elements during their death throes. These heavier elements then got mixed into the gas reservoir of galaxies, and later generations of stars, formed from gas richer in heavier elements than previous generations, will show their metal content in their spectra.

Globular clusters are so old that most of them are extremely metal-poor indeed. Very young clusters, like R136a in the Large Magellanic Cloud, are much more metal-rich (even though I believe that R136a is much more metal-poor than the Sun). For that reason, R136a will never evolve into a globular cluster similar to great globular cluster M13. For one thing, 12-billion-year-old R136a will lack the blue horizontal branch stars that are so prominent in M13. These blue stars represent a late evolutionary phase of very metal-poor stars, but more metal-rich stars never "turn blue" when they have evolved off the main sequence (except a very small number of extremely massive stars which may "turn blue" after they have "turned red" during their later evolution. But such exceptional stars don't affect the later evolution of clusters like R136a.).

What about your other questions? Sorry, you'll have to ask someone else!

Ann
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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by johnnydeep » Fri May 21, 2021 11:50 am

Sa Ji Tario wrote:
Thu May 20, 2021 11:46 pm
Johnny Depp, I see 20 galaxies in the left half and 9 in the right half.
Wow. You convinced me to try again, and I'm now up to 14 total. Your eyes must be better than mine! Or else I need to consider more "blurry smears" to be galaxies :)

EDIT: here's my final attempt. I might have been a little too charitable with my promotion of some of these to galaxy status. I've got about 35 of them circled in red:

Background Galaxies Near M13.jpg
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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by johnnydeep » Fri May 21, 2021 12:34 pm

Thanks Ann! The way you feel about math is the way I currently feel about color magnitude-diagrams and what they mean. But I'm working on that. How the metallicity of stars relates to their age I do think I understand. Now, back to globular clusters. My googling got me this - https://astrosociety.org/news-publicati ... r-clusters:
In fact, it’s common for UFD [Ultra Faint Dwarf galaxy] candidates to need extra-detailed observations just to determine their galaxy-hood! See, UFDs are thought to be the most dark matter-dominated objects in the Universe, meaning they should have much more dark matter than visible matter (like stars). Globulars, on the other hand, were thought to contain almost no dark matter with a mass-to-light ratio of about 1-to-4. (A mass-to-light ratio is the relationship between how much mass is inferred in a system, from the velocities of its stars, say, to how much light the system puts out.) A lower mass-to-light ratio equals less dark matter, and vice versa.

This vast difference in mass-to-light ratios between UFDs and globular clusters is the main way astronomers designate one from the other. Despite these differences, when other astronomical traits are compared, globulars and UFDs appear more similar than not.

For 58 of these nearby tiny systems, researchers from Universidad de Chile looked at the relationship between the effective radius (the radius out to where half of the total light of that system is emitted) and Sérsic index (that is a relationship between intensity of emitted light and distance from the center of the system, from Marchi-Lasch et al. 2019). In this view, globular clusters with lower surface brightness and UFDs are very similar! The nature of effective radii and Sérsic index lend to the idea that these two types of celestial systems may actually form in similar ways—[spoiler alert!]—in their own dark matter halos.
There's more at the link, but it's somewhat foggy to me. I don't understand why, if a galaxy is embedded in a vast halo of dark matter, that same dark matter wouldn't also permeate anything in the galaxy and nearby, like its attendant globular clusters. And if dark matter is affected by gravity - which it definitely is - why that dark matter wouldn't be concentrated exactly where the matter is concentrated, meaning there should be more of it toward the galaxy center, as well as in any clusters of stars.

PS - those googleusercontent.com links to pics you sometimes use are still broken for me, though I know from my past posts about this that others see them just fine.
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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by Ann » Fri May 21, 2021 1:58 pm

johnnydeep wrote:
Fri May 21, 2021 12:34 pm
Thanks Ann! The way you feel about math is the way I currently feel about color magnitude-diagrams and what they mean. But I'm working on that.

I think it is easier to understand the color-magnitude diagram if the diagram in question is in fact in color.

Look at the picture at left. The higher up in the diagram a star is, the brighter it is. And the farther to the left a star is, the bluer it is, and the farther to the right it is, the redder it is.

Blue stars are hot, and red stars are cool.

Look at the thick stellar "stream" starting at lower right and moving up towards the upper left in the picture at left. This stellar stream is cut off about halfway. The stream moving diagonally upwards from lower right is the main sequence. All stars on the main sequence shine by fusing hydrogen to helium in their cores. As you can see, the bottom of the main sequence is red, because the stars there are red dwarfs. The stars at the top of the main sequence, before it is cut off in a typical globular cluster, are typically yellow to yellow-white. These are Sunlike stars, or even stars that are a little less massive then the Sun.

At the turnoff point the stars have used up the hydrogen in their cores. Other energy-generating processes now take over inside the stars. The stars grow larger, brighter and redder. They are now red giants.

To the left of the main sequence is the horizontal branch. Unique to extremely metal-poor populations of stars, they undergo a "late blue evolutionary phase" after they have left the red giant branch. These stars, which used to be red giants, swollen, cool and red, now shrink and become compact, hot and (more or less) blue - they become bluer the smaller they get. The stars on the horizontal branch are brighter than the Sun - some 30-50 times brighter than the Sun, I think (I'm too lazy to google), but they are typically less massive than the Sun, at least the stars on the horizontal branch in a circa 12-billion-year-old globular cluster. Fancy that.

Even metal-rich stars leave the red giant branch, shrink a bit and become a bit paler in color than they used to be. But they are still swollen and reddish, although a bit less so. But whether the (low- and medium-mass) stars are metal-rich or metal-poor, they end their lives by first rising on the Asymtotic Giant Branch, when they become very red, swollen and unstable, and then by shedding their outer atmospheres and turning into white dwarfs, which at first are surrounded by planetary nebulas.

Hopefully these diagrams make a bit more sense to you, Johnny.

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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by johnnydeep » Fri May 21, 2021 3:57 pm

Ann wrote:
Fri May 21, 2021 1:58 pm
johnnydeep wrote:
Fri May 21, 2021 12:34 pm
Thanks Ann! The way you feel about math is the way I currently feel about color magnitude-diagrams and what they mean. But I'm working on that.

I think it is easier to understand the color-magnitude diagram if the diagram in question is in fact in color.

Look at the picture at left. The higher up in the diagram a star is, the brighter it is. And the farther to the left a star is, the bluer it is, and the farther to the right it is, the redder it is.

Blue stars are hot, and red stars are cool.

Look at the thick stellar "stream" starting at lower right and moving up towards the upper left in the picture at left. This stellar stream is cut off about halfway. The stream moving diagonally upwards from lower right is the main sequence. All stars on the main sequence shine by fusing hydrogen to helium in their cores. As you can see, the bottom of the main sequence is red, because the stars there are red dwarfs. The stars at the top of the main sequence, before it is cut off in a typical globular cluster, are typically yellow to yellow-white. These are Sunlike stars, or even stars that are a little less massive then the Sun.

At the turnoff point the stars have used up the hydrogen in their cores. Other energy-generating processes now take over inside the stars. The stars grow larger, brighter and redder. They are now red giants.

To the left of the main sequence is the horizontal branch. Unique to extremely metal-poor populations of stars, they undergo a "late blue evolutionary phase" after they have left the red giant branch. These stars, which used to be red giants, swollen, cool and red, now shrink and become compact, hot and (more or less) blue - they become bluer the smaller they get. The stars on the horizontal branch are brighter than the Sun - some 30-50 times brighter than the Sun, I think (I'm too lazy to google), but they are typically less massive than the Sun, at least the stars on the horizontal branch in a circa 12-billion-year-old globular cluster. Fancy that.

Even metal-rich stars leave the red giant branch, shrink a bit and become a bit paler in color than they used to be. But they are still swollen and reddish, although a bit less so. But whether the (low- and medium-mass) stars are metal-rich or metal-poor, they end their lives by first rising on the Asymtotic Giant Branch, when they become very red, swollen and unstable, and then by shedding their outer atmospheres and turning into white dwarfs, which at first are surrounded by planetary nebulas.

Hopefully these diagrams make a bit more sense to you, Johnny.

Ann
Getting there! Still digesting... Thanks again for the patient explanation.
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Re: APOD: M13: The Great Globular Cluster in... (2021 May 20)

Post by Ricardo Sala » Wed Jun 02, 2021 4:25 pm

This image inspires me to want to see an APOD as a stereoscopic GIF, also called a wigglegram.
This could be achieved artificially (with a computer simulation, inputing calculated distances of each stellar object)...
But most spectacular would be to achieve it naturally by means of the parallax effect provided by our planet's orbit around the Sun. Of course, the stellar objects would have to be near enough to appreciate the depth of field.

Does somebody here know about such a certified naturally-made astronomical wigglegram?