by Ann » Fri Mar 11, 2022 8:06 am
nam888id wrote: ↑Thu Mar 10, 2022 1:18 pm
I find the structure of globular clusters kind of stunning and inspirationally incandescent.
I wanted to know more of what was going on.
https://apod.nasa.gov/apod/globular_clusters.html provides some interesting information, including a computer simulation, but I don't feel complete.
Let's look at a computer simulation again.
Click to play embedded YouTube video.
The cluster in question may be a rich open cluster or a globular cluster, or even two merging clusters. Never mind. Note the following aspects:
1) At first the cluster is dominated by bright blue-white stars.
2) Note how more and more of the bright blue-white stars go orange and then pop. The bright blue stars turn into red giants, and then they either explode as supernovas or shed their outer layers more gently and turn into tiny white dwarfs.
3) Note that stars disappear from the cluster by moving out of it. Some stars in the outskirts of the cluster just move quietly away, as they have slipped the gravitational grip of the cluster. A close encounter between two stars near the center of the cluster sends both stars flying out of the cluster in opposite directions.
4) Over time, the cluster therefore becomes fainter and poorer in stars. This means that today's globular clusters, which are impressive enough, were unimaginably magnificent when they first formed.
How and why did all those globular clusters form? Wikipedia doesn't have an answer for us:
Wikipedia wrote:
The formation of globular clusters is poorly understood.
Okay. But we may note a few things. Let's begin by looking at the Universe!
According to the NASA illustration above, the Universe is 13.77 billion years old. The first stars formed when the Universe was 400 million years old. According to
Wikipedia, 47 Tuc is estimated to be 13.06 billion years old, which means that it formed when the Universe was 700 million years old. What were things like in the Universe back then?
Well... First of all the Universe was full of gas! Imagine that the cars in these images all represent a chunk of gas. Compare the amount of gas in the Universe then and now:
Why was there so much more gas in the Universe back then than there is now? The answer is - little red dwarf stars. (And the expansion of the Universe, because the available gas is spread inside a larger volume as the Universe expands).
But let's talk about the little red dwarfs. Consider cluster NGC 6397. According to a rather old caption by
ESO, the age of NGC 6397 is 13,400 ± 800 million years. Well, if the Universe is 13.77 billion years old, and the first stars appeared when the Universe was 400 million years old, then an age of 13.4 billion years would make NGC 6397 precocious indeed. It is probably just a little younger.
Let's look at a closeup of NGC 6397. (And while we are at it, let's look at a massive and mass-losing young Wolf-Rayet star, too.)
Please note that the bright stars in NGC 6397 are not as blue as they look, because an orange and an infrared filter were used to create this image. But never mind. Let's focus on the small orange stars in NGC 6397.
Most stars formed in any starburst are going to be small red dwarf stars. These stars look tiny, but they are surprisingly massive. The average mass of a red dwarf star may be half the mass of the Sun. That is not so little. And because they are so incredibly numerous - a lot more numerous than the closeup of NGC 6397 suggests, because a lot of small red stars have undoubtedly drifted away from NGC 6397 since it first formed - the red dwarf stars contribute mightily to the combined mass of all the baryonic ("ordinary") mass of the Universe.
What happens to the gas that ends up inside a red dwarf star? Answer: It stays there, and for a much longer time than the current age of the Universe. Little red dwarf stars are gas-traps, sweeping up the gas of the Universe.
Massive stars, by contrast, give back much of their gas to the Universe. The picture of massive Wolf-Rayet star WR 124 dramatically demonstrates the violent mass loss during the late stage of their lives. Of course, any star that explodes as a supernova is going to give back most of its original gas to the Universe. And any star that sheds its outer layers to end up as a white dwarf is going to give back much of its gas to the Universe. I believe that the Sun, which is expected to end up as a white dwarf perhaps 5 billion years from now, is going to give back 50-60% of its original gas to the Universe.
But all those little red dwarfs are stingy hoarders that hang on to their gas for a much longer time than the Universe has existed. And when they finally release some of it, the Universe will have grown so incredibly large because of the acceleration caused by dark energy, that it will be very hard for massive gas clouds to form and to give birth to new stars.
So, to summarize. Most globular clusters are very old, 11-13 billion years old. Back then the Universe was quite literally smaller (because you must remember that it has been expanding ever since), and it was incredibly full of gas, because few stars had yet formed. Huge gas clouds must have collided and sparked incredible bursts of star formation. Think of it as an incredible fireworks show!
Okay. So what happened afterwards? When the fireworks show was over?
Take a look at the pictures of NGC 3603, a very massive and very young cluster in the Milky Way, and the double Cluster in Perseus. The age estimates of NGC 3603 vary from 300,000 years to 1.5 million years, and the age estimate of the Double Cluster in Perseus is ~12 million years.
You can see that there is still a lot of gas in the vicinity of NGC 3603, but the mighty cluster itself has cleared a cavity in the gas. As for the Double Cluster in Perseus, we see no sign of any gas cloud or any nebula whatsoever. The strong stellar winds must have blown away all the gas that remained after the Double Cluster themselves had formed.
Consider galaxy M82. Its gas, its starforming material, is rushing out of it because of a violent starburst in its center. M82 has already lost its ability to form stars in its disk. Just because it is forming stars (in its center) so violently, its starforming days will soon be over.
That's how globular cluster once formed, and that's probably why they generally stopped forming.
(And Chris will tell you that the gas that went into forming the globular clusters was quite different than the gas that is available in the Universe today, because the current gas is so heavily mixed with heavier elements forged in massive stars that not even a massive young metal-rich cluster like NGC 3603 can ever turn into a "true" globular cluster. I'll leave it to Chris to elaborate on that.)
I should mention, too, that some globulars are "stolen" cores of small galaxies. It looks something like this:
These stellar streams are the hapless remnants of dwarf galaxies captured by the Milky Way. The disks of the galaxies have spread out into long, long streamers, but you can see a "thickening", an almost spherical object, in one stellar stream. That would be the core of the captured dwarf galaxy. The largest globular cluster of the Milky Way, Omega Centauri, is believed to be the core of a captured dwarf galaxy.
Ann
[quote=nam888id post_id=321199 time=1646918281 user_id=144983]
I find the structure of globular clusters kind of stunning and inspirationally incandescent.
I wanted to know more of what was going on.
https://apod.nasa.gov/apod/globular_clusters.html provides some interesting information, including a computer simulation, but I don't feel complete.
[/quote]
Let's look at a computer simulation again.
[youtube]https://www.youtube.com/watch?v=E8nw2x6YV0A[/youtube]
[clear][/clear]
The cluster in question may be a rich open cluster or a globular cluster, or even two merging clusters. Never mind. Note the following aspects:
1) At first the cluster is dominated by bright blue-white stars.
2) Note how more and more of the bright blue-white stars go orange and then pop. The bright blue stars turn into red giants, and then they either explode as supernovas or shed their outer layers more gently and turn into tiny white dwarfs.
3) Note that stars disappear from the cluster by moving out of it. Some stars in the outskirts of the cluster just move quietly away, as they have slipped the gravitational grip of the cluster. A close encounter between two stars near the center of the cluster sends both stars flying out of the cluster in opposite directions.
4) Over time, the cluster therefore becomes fainter and poorer in stars. This means that today's globular clusters, which are impressive enough, were unimaginably magnificent when they first formed.
How and why did all those globular clusters form? Wikipedia doesn't have an answer for us:
[quote][url=https://en.wikipedia.org/wiki/Globular_cluster#Formation]Wikipedia[/url] wrote:
The formation of globular clusters is poorly understood.[/quote]
Okay. But we may note a few things. Let's begin by looking at the Universe!
[img3="Timeline of the expansion of the Universe. Illustration: NASA/WMAP Science Team - Original version: NASA; modified by Cherkash."]https://upload.wikimedia.org/wikipedia/commons/thumb/6/6f/CMB_Timeline300_no_WMAP.jpg/1280px-CMB_Timeline300_no_WMAP.jpg[/img3]
[clear][/clear]
According to the NASA illustration above, the Universe is 13.77 billion years old. The first stars formed when the Universe was 400 million years old. According to [url=https://en.wikipedia.org/wiki/47_Tucanae]Wikipedia[/url], 47 Tuc is estimated to be 13.06 billion years old, which means that it formed when the Universe was 700 million years old. What were things like in the Universe back then?
Well... First of all the Universe was full of gas! Imagine that the cars in these images all represent a chunk of gas. Compare the amount of gas in the Universe then and now:
[float=left][img3="Traffic jam in New Delhi."]https://innovationorigins.com/app/uploads/2018/10/traffic-jam-file-new-delhi-1004x674.jpg[/img3][/float][float=right][img3="Almost empty road in the capital of Myanmar."]https://img-9gag-fun.9cache.com/photo/aGzxe05_460s.jpg[/img3][/float]
[clear][/clear]
Why was there so much more gas in the Universe back then than there is now? The answer is - little red dwarf stars. (And the expansion of the Universe, because the available gas is spread inside a larger volume as the Universe expands).
But let's talk about the little red dwarfs. Consider cluster NGC 6397. According to a rather old caption by [url=https://www.eso.org/public/sweden/images/eso0425a/]ESO[/url], the age of NGC 6397 is 13,400 ± 800 million years. Well, if the Universe is 13.77 billion years old, and the first stars appeared when the Universe was 400 million years old, then an age of 13.4 billion years would make NGC 6397 precocious indeed. It is probably just a little younger.
Let's look at a closeup of NGC 6397. (And while we are at it, let's look at a massive and mass-losing young Wolf-Rayet star, too.)
[float=right][img3="Mass loss from Wolf-Rayet star WR 124. Image: ESA/Hubble"]https://upload.wikimedia.org/wikipedia/commons/thumb/7/7f/A_cosmic_couple.jpg/1024px-A_cosmic_couple.jpg[/img3][/float][img3="Closeup of a part of NGC 6397, containing only low-mass stars. Image: NASA, ESA and H. Richer (University of British Columbia)"]https://cdn.spacetelescope.org/archives/images/screen/heic0608b.jpg[/img3]
[clear][/clear]
Please note that the bright stars in NGC 6397 are not as blue as they look, because an orange and an infrared filter were used to create this image. But never mind. Let's focus on the small orange stars in NGC 6397.
Most stars formed in any starburst are going to be small red dwarf stars. These stars look tiny, but they are surprisingly massive. The average mass of a red dwarf star may be half the mass of the Sun. That is not so little. And because they are so incredibly numerous - a lot more numerous than the closeup of NGC 6397 suggests, because a lot of small red stars have undoubtedly drifted away from NGC 6397 since it first formed - the red dwarf stars contribute mightily to the combined mass of all the baryonic ("ordinary") mass of the Universe.
What happens to the gas that ends up inside a red dwarf star? Answer: It stays there, and for a much longer time than the current age of the Universe. Little red dwarf stars are gas-traps, sweeping up the gas of the Universe.
Massive stars, by contrast, give back much of their gas to the Universe. The picture of massive Wolf-Rayet star WR 124 dramatically demonstrates the violent mass loss during the late stage of their lives. Of course, any star that explodes as a supernova is going to give back most of its original gas to the Universe. And any star that sheds its outer layers to end up as a white dwarf is going to give back much of its gas to the Universe. I believe that the Sun, which is expected to end up as a white dwarf perhaps 5 billion years from now, is going to give back 50-60% of its original gas to the Universe.
But all those little red dwarfs are stingy hoarders that hang on to their gas for a much longer time than the Universe has existed. And when they finally release some of it, the Universe will have grown so incredibly large because of the acceleration caused by dark energy, that it will be very hard for massive gas clouds to form and to give birth to new stars.
So, to summarize. Most globular clusters are very old, 11-13 billion years old. Back then the Universe was quite literally smaller (because you must remember that it has been expanding ever since), and it was incredibly full of gas, because few stars had yet formed. Huge gas clouds must have collided and sparked incredible bursts of star formation. Think of it as an incredible fireworks show!
[float=left][img2]https://www.trafalgar.com/real-word/wp-content/uploads/sites/3/2021/10/fireworks-display-nye-1-750x400.jpeg[/img2][/float]
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Okay. So what happened afterwards? When the fireworks show was over?
[float=left][img3="NGC 3603, one of the most massive young clusters in the Milky Way. Image Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration"]https://upload.wikimedia.org/wikipedia/commons/thumb/8/80/NGC_3603_HST_ACS.jpg/808px-NGC_3603_HST_ACS.jpg?20071006160907[/img3][/float][float=right][img3="The Double Cluster in Perseus. Image: Stephen Rahn from Macon, GA, USA"]https://upload.wikimedia.org/wikipedia/commons/thumb/a/a8/Double_Cluster_in_Perseus_%2849571872261%29.jpg/800px-Double_Cluster_in_Perseus_%2849571872261%29.jpg?20201110214510[/img3][/float]
[clear][/clear]
Take a look at the pictures of NGC 3603, a very massive and very young cluster in the Milky Way, and the double Cluster in Perseus. The age estimates of NGC 3603 vary from 300,000 years to 1.5 million years, and the age estimate of the Double Cluster in Perseus is ~12 million years.
You can see that there is still a lot of gas in the vicinity of NGC 3603, but the mighty cluster itself has cleared a cavity in the gas. As for the Double Cluster in Perseus, we see no sign of any gas cloud or any nebula whatsoever. The strong stellar winds must have blown away all the gas that remained after the Double Cluster themselves had formed.
[img3=M82 and its outflow of gas due to its central starburst. NASA, ESA, and The Hubble Heritage Team (STScI/AURA)" ]https://upload.wikimedia.org/wikipedia/commons/thumb/c/ce/M82_HST_ACS_2006-14-a-large_web.jpg/1280px-M82_HST_ACS_2006-14-a-large_web.jpg[/img3]
[clear][/clear]
Consider galaxy M82. Its gas, its starforming material, is rushing out of it because of a violent starburst in its center. M82 has already lost its ability to form stars in its disk. Just because it is forming stars (in its center) so violently, its starforming days will soon be over.
That's how globular cluster once formed, and that's probably why they generally stopped forming.
(And Chris will tell you that the gas that went into forming the globular clusters was quite different than the gas that is available in the Universe today, because the current gas is so heavily mixed with heavier elements forged in massive stars that not even a massive young metal-rich cluster like NGC 3603 can ever turn into a "true" globular cluster. I'll leave it to Chris to elaborate on that.)
I should mention, too, that some globulars are "stolen" cores of small galaxies. It looks something like this:
[img3="Stellar streams in the Milky Way, discovered in 2007. NASA - http://www.spitzer.caltech.edu/images/2138-sig07-008-Rings-Around-the-Galaxy-Annotated-"]https://upload.wikimedia.org/wikipedia/commons/9/95/Sig07-008.jpg[/img3]
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These stellar streams are the hapless remnants of dwarf galaxies captured by the Milky Way. The disks of the galaxies have spread out into long, long streamers, but you can see a "thickening", an almost spherical object, in one stellar stream. That would be the core of the captured dwarf galaxy. The largest globular cluster of the Milky Way, Omega Centauri, is believed to be the core of a captured dwarf galaxy.
Ann