APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

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APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by APOD Robot » Fri Sep 01, 2023 4:06 am

Image The Great Globular Cluster in Hercules

Explanation: In 1716, English astronomer Edmond Halley noted, "This is but a little Patch, but it shows 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.

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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by emc » Fri Sep 01, 2023 5:22 am

Wot Gorilla :mrgreen:

I thought this Cluster would make a good spot to point out that there’s a new post for Friends to cluster around in Open Space: Discuss Anything

This is kinda like a pop up ad :mrgreen:
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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by Ann » Fri Sep 01, 2023 8:25 am

emc wrote: Fri Sep 01, 2023 5:22 am Wot Gorilla :mrgreen:

I thought this Cluster would make a good spot to point out that there’s a new post for Friends to cluster around in Open Space: Discuss Anything

This is kinda like a pop up ad :mrgreen:
I agree with emc, it would be nice if more people would chime in and say something in the Friends thread in the Open Space forum.

Unfortunately people have left and even died since you were here last, emc. But those of us who are here can still be friends. 🧑‍🤝‍🧑👫👬

As for the APOD: It's nice!

M13-totale-en-cours-crop8_1024[1].jpg
M13, the Great Globular Cluster in Hercules.
Credit: Serge Brunier, Jean-François Bax, David Vernet OCA/C2PU

Me being me, what I particularly like about the picture is that it gives us a good idea of the large number of blue horizontal branch stars in M13. My impression is that these blue stars are more plentiful in M13 than in most globulars.


I'm going to use up my three attachments and show you the typical populations of a globular cluster:

M55 blue stragglers and turnoff point.png
Stellar populations in globular cluster M55.
Credit: B.J. Mochejska, J. Kaluzny (CAMK), 1m Swope Telescope

What I particularly like about the picture I have annotated is that the star colors are correct, if saturated. The blue stars of M13 are members of the blue part of the horizontal branch (upper left). Only stars that contain particularly low amounts of elements more massive than hydrogen and helium (so called metal-poor stars) can ever be part of the blue horizontal branch found in globular clusters.

The broad yellow-red feature moving diagonally upwards from lower right in the color-magnitude diagram is the main sequence, where the stars sustain themselves by fusing hydrogen to helium in their cores (like the Sun does). In old globular clusters, all the stars that are still on the main sequence are relatively low or very low in mass, and they shine relatively feebly with a yellowish or a reddish glow.

At the main sequence turnoff point, the stars have just used up the hydrogen in their cores. From now on, they will sustain themselves by other means than core hydrogen fusion, until they die. First they rise on the red giant branch. Here their cores shrink, releasing heat that in itself makes the stars grow big, bright and reddish. Finally their cores get hot enough to sustain helium fusion, as helium fuses into carbon and oxygen. At this point the stars shrink quite dramatically, but their "surfaces", their photospheres, also heat up.

It is when the stars reach the top of the red giant branch that their cores have become hot enough to sustain core helium fusion. The stars now "back down and move left" and enter the horizontal branch, which is, as I said, characterized by stars fusing helium to carbon and oxygen in their cores.

But not all globular clusters ever get a blue horizontal branch. Look at the puny little horizontal branch of bright globular cluster NGC 104 (also known as 47 Tucanae or 47 Tuc):


Look at that puny little black clump at magnitude 14 on the Y axis! That's the horizontal branch of NGC 104 or 47 Tuc, and it's a red horizontal branch. Actually our familiar constellations in the night time sky contain large numbers of modestly yellow "metal-rich" stars that are on the red horizontal branch. Their spectral class is typically K0 or K1. Well-known examples of these K0-type stars are Pollux in Gemini and Dubhe of the Big Dipper:


But in age-old, metal-poor stars like the ones we find in many globulars, the horizontal branch stars don't turn into pale yellow "Polluxes and Dubhes". Instead they turn into blue stars the color of, well, maybe the Pleiades. But the blue horizontal branch stars are typically a bit fainter (though not a lot fainter) than the bright stars of the Pleiades, but they are a lot less massive.



The color and brightness of the old evolved low-mass horizontal branch stars of globular cluster M13 is about the same as the color and brightness of the sprightly young, mostly unevolved and relatively massive bright stars of the Pleaides.

Who would have thought?

Ann
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Last edited by Ann on Fri Sep 01, 2023 10:13 am, edited 1 time in total.
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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by jeffbax » Fri Sep 01, 2023 9:25 am

Thank you Ann for this very interesting comment. Glad you like this image.

JF

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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by emc » Fri Sep 01, 2023 9:58 am

Ann wrote: Fri Sep 01, 2023 8:25 am
Who would have thought?

Ann
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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by Christian G. » Fri Sep 01, 2023 12:25 pm

Great image, great object! To say that there is more than meets the eye in the universe is the ultimate understatement, and in the case of a globular cluster, the "more" in question that stuns me each time is that however densely packed with stars it seems, it contains far more empty space than stars… Good grief is it VAST out there!

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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by Ann » Fri Sep 01, 2023 5:28 pm

Chris Alex wrote: Fri Sep 01, 2023 12:25 pm Great image, great object! To say that there is more than meets the eye in the universe is the ultimate understatement, and in the case of a globular cluster, the "more" in question that stuns me each time is that however densely packed with stars it seems, it contains far more empty space than stars… Good grief is it VAST out there!
Absolutely! Even globular clusters are mostly emptiness!


A larger version of the picture of a dwarf galaxy behind a globular cluster is here.


Even the core of our galaxy's largest globular cluster, Omega Centauri, doesn't seem impossibly crowded:


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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by orin stepanek » Fri Sep 01, 2023 6:15 pm

M13-totale-en-cours-crop8_1024.jpg
Oh me; Whenever I see a globular cluster; I have to wonder how
these stars can remain so near each other! I would expect them to
drift apart or bump into each other! :evil:
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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by Chris Peterson » Fri Sep 01, 2023 7:10 pm

orin stepanek wrote: Fri Sep 01, 2023 6:15 pm M13-totale-en-cours-crop8_1024.jpg
Oh me; Whenever I see a globular cluster; I have to wonder how
these stars can remain so near each other! I would expect them to
drift apart or bump into each other! :evil:
They don't bump into each other because they are immensely far apart compared with their diameters. Like houseflies in a football stadium. Individually, they don't stay close to each other, but are all independently orbiting the center of mass of the cluster (the total mass of which is what keeps the stars grouped). Over a long time- billions of years- globular clusters do evaporate away as stars are steadily lost after near misses slingshot them away.
Chris

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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by johnnydeep » Fri Sep 01, 2023 8:29 pm

It's interesting to me that the astrobin link shows a gray-scale mouse-over image with the center stars lost in a solid mass of black. Are the much more discernible central stars in the color image the result of software processing?
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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by orin stepanek » Fri Sep 01, 2023 9:25 pm

Chris Peterson wrote: Fri Sep 01, 2023 7:10 pm
orin stepanek wrote: Fri Sep 01, 2023 6:15 pm M13-totale-en-cours-crop8_1024.jpg
Oh me; Whenever I see a globular cluster; I have to wonder how
these stars can remain so near each other! I would expect them to
drift apart or bump into each other! :evil:
They don't bump into each other because they are immensely far apart compared with their diameters. Like houseflies in a football stadium. Individually, they don't stay close to each other, but are all independently orbiting the center of mass of the cluster (the total mass of which is what keeps the stars grouped). Over a long time- billions of years- globular clusters do evaporate away as stars are steadily lost after near misses slingshot them away.
Thanks Chris :)
Orin

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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by C0ppert0p » Fri Sep 01, 2023 11:21 pm

Is there a good explanation of how a globular cluster forms?
I can visualize a single or a binary star resulting from a contracting/rotating gas and dust cloud.
I find it harder to visualize a giant cloud collapsing and forming individual concentrations of gas/disk that rotate independently to form a giant cluster of stars.

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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by Chris Peterson » Sat Sep 02, 2023 4:18 am

C0ppert0p wrote: Fri Sep 01, 2023 11:21 pm Is there a good explanation of how a globular cluster forms?
I can visualize a single or a binary star resulting from a contracting/rotating gas and dust cloud.
I find it harder to visualize a giant cloud collapsing and forming individual concentrations of gas/disk that rotate independently to form a giant cluster of stars.
It remains a poorly understood area.
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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by Ann » Sat Sep 02, 2023 4:59 am

C0ppert0p wrote: Fri Sep 01, 2023 11:21 pm Is there a good explanation of how a globular cluster forms?
I can visualize a single or a binary star resulting from a contracting/rotating gas and dust cloud.
I find it harder to visualize a giant cloud collapsing and forming individual concentrations of gas/disk that rotate independently to form a giant cluster of stars.
Is there a good explanation of how a globular cluster forms? There isn't.

I recommend that you read what Wikipedia has to say on the matter. Here are some excerpts:
Wikipedia wrote:

The formation of globular clusters is poorly understood.
Observations of globular clusters show that their stars primarily come from regions of more efficient star formation, and from where the interstellar medium is at a higher density, as compared to normal star-forming regions.
Globular cluster formation is prevalent in starburst regions and in interacting galaxies.
Some globular clusters likely formed in dwarf galaxies and were removed by tidal forces to join the Milky Way.
In elliptical and lenticular galaxies there is a correlation between the mass of the supermassive black holes (SMBHs) at their centers and the extent of their globular cluster systems. The mass of the SMBH in such a galaxy is often close to the combined mass of the galaxy's globular clusters.
No known globular clusters display active star formation, consistent with the hypothesis that globular clusters are typically the oldest objects in their galaxy and were among the first collections of stars to form.
Very large regions of star formation known as super star clusters, such as Westerlund 1 in the Milky Way, may be the precursors of globular clusters.
Many of the Milky Way's globular clusters have a retrograde orbit (meaning that they revolve around the galaxy in the reverse of the direction the galaxy is rotating), including the most massive, Omega Centauri. Its retrograde orbit suggests it may be a remnant of a dwarf galaxy captured by the Milky Way.

Wikipedia wrote that the stars of globular clusters primarily come from regions of more efficient star formation and where the interstellar medium (i.e., the concentration of gas and dust) is at a particularly high density. They have also come from starburst regions and interacting galaxies.

Let's look at the famous interacting (colliding) Antennae galaxies, NGC 4038 and NGC 4039:


In the picture, yellow means old stars, blue means young stars, pink means regions of recent or ongoing star formation, and brown means high concentrations of gas and dust, ripe for new star formation. You can see that there are many pink regions mixed inside the largest region of brown dust. Lots of star formation is taking place here.

As you can see, most of the pink and blue regions are in the galaxy at top, NGC 4038. The largest and most massive brown region is found between the two galaxies, where they actually ram into one another. Some very massive young star clusters are born in the region between the two galaxies, but also in NGC 4038. However, few young stars are born in NGC 4039. There is not enough gas and dust for efficient star formation there.

A galaxy that is not interacting with another galaxy, but which is still experiencing high levels of star formation, is NGC 6946. This galaxy contains a super star cluster.

NGC 6946 closeup.png
The super star cluster of NGC 6946.

Another super star cluster is R136a in the Tarantula Nebula in the Large Magellanic Cloud. Note that in both R136a and in the super star cluster in NGC 6946, there is a central concentration of stars surrounded by a "halo" of young stars.


The Large Magellanic Cloud, where R136a is found, is itself a galaxy rich in star formation.


We can say, at least, that the formation of globular clusters required high concentrations of gas, and the gas was made to contract and form huge numbers of stars.

Ann
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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by jeffbax » Sat Sep 02, 2023 6:44 am

johnnydeep wrote: Fri Sep 01, 2023 8:29 pm It's interesting to me that the astrobin link shows a gray-scale mouse-over image with the center stars lost in a solid mass of black. Are the much more discernible central stars in the color image the result of software processing?
Hi Johnny. This inverted version is automatically generated by Astrobin and has nothing to do with the original luminance. I set it like this because it shows the large diameter of the globular.

Cheers.

JF

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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by johnnydeep » Sat Sep 02, 2023 11:48 am

jeffbax wrote: Sat Sep 02, 2023 6:44 am
johnnydeep wrote: Fri Sep 01, 2023 8:29 pm It's interesting to me that the astrobin link shows a gray-scale mouse-over image with the center stars lost in a solid mass of black. Are the much more discernible central stars in the color image the result of software processing?
Hi Johnny. This inverted version is automatically generated by Astrobin and has nothing to do with the original luminance. I set it like this because it shows the large diameter of the globular.

Cheers.

JF
Hmm. I don't think I've ever seen that before on Astrobin. What I'm used to is a mouse-over that shows celestial sphere coordinates and annotations that call out various objects in the FOV.
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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by jeffbax » Sun Sep 03, 2023 10:17 am

johnnydeep wrote: Sat Sep 02, 2023 11:48 am
jeffbax wrote: Sat Sep 02, 2023 6:44 am
johnnydeep wrote: Fri Sep 01, 2023 8:29 pm It's interesting to me that the astrobin link shows a gray-scale mouse-over image with the center stars lost in a solid mass of black. Are the much more discernible central stars in the color image the result of software processing?
Hi Johnny. This inverted version is automatically generated by Astrobin and has nothing to do with the original luminance. I set it like this because it shows the large diameter of the globular.

Cheers.

JF
Hmm. I don't think I've ever seen that before on Astrobin. What I'm used to is a mouse-over that shows celestial sphere coordinates and annotations that call out various objects in the FOV.
When you publish your image, you can make the choice of this option instead of the celestial sphere coordinates. You can even set a different version of the image.

JF

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Re: APOD: The Great Globular Cluster in Hercules (2023 Sep 01)

Post by johnnydeep » Sun Sep 03, 2023 1:01 pm

jeffbax wrote: Sun Sep 03, 2023 10:17 am
johnnydeep wrote: Sat Sep 02, 2023 11:48 am
jeffbax wrote: Sat Sep 02, 2023 6:44 am

Hi Johnny. This inverted version is automatically generated by Astrobin and has nothing to do with the original luminance. I set it like this because it shows the large diameter of the globular.

Cheers.

JF
Hmm. I don't think I've ever seen that before on Astrobin. What I'm used to is a mouse-over that shows celestial sphere coordinates and annotations that call out various objects in the FOV.
When you publish your image, you can make the choice of this option instead of the celestial sphere coordinates. You can even set a different version of the image.

JF
Ah, thanks.
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