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. Telescopic views 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. Along with the cluster's dense core, the outer reaches of M13 are highlighted in this sharp color image. The cluster's evolved red and blue giant stars show up in yellowish and blue tints.
RocketRon wrote:The burning question, so to speak, is as to why these clusters aren't/haven't formed into rotating disks ?
Click to play embedded YouTube video.
Take a look at the simulation of a the evolution of a star cluster at left. We see no sign of the stars settling into a rotating disk.
Planets, however, from out of rotating disks around their own suns. Don't star clusters form in the same way? No, because there is rarely a single, huge, flattened disk where one enormous star is formed in the center, and smaller stars are formed like planets, orbiting the huge star.
I think that a gaseous, dusty disk is the key to a flattened shape. Most disk galaxies have a central dust lane which, in my opinion, is what makes the galaxy rotate and settle into the shape of a flattened disk. See here and here. However, there are also disk galaxies with no central dust lane, like NGC 3115. I believe that NGC 3115 used to have a dust lane, but the dust lane has been lost. We actually see several examples that the dust disk of a galaxy seems to be shrinking. See here and here.
Star clusters never seem to have dust lanes. I think that they deplete or at least wildly scatter their dust as they form, and then they have no inner "spine" to stabilize them.
Small galaxies also seem to mostly lack central dust lanes. An interesting example is NGC 2976, whose relatively small dust disk is chaotic. According to the paper Rotation and mass for NGC 2976, the mass of NGC 2976 is only 2 billion solar masses. By contrast, the mass of the Milky Way might be 8.5×1011 M☉, which would be some 200 times more than the mass of NGC 2976.
Perhaps starforming small galaxies upset their own dust too much for it to settle into a disk (because the chaos created by the birth and death of massive stars can't be counteracted and stabilized by the galaxy's own small gravity). And perhaps too much of their dust is lost into space as their massive stars go supernova, and again they have lost the chance to form a stabilizing central dust lane. Later, when they settle into premature old age and become dwarf spheroidal galaxies, they mostly lack dust.
Wonder how it would be to sit inside that cluster? The sky would be crowded with suns of different sizes. Is it hot? By the way, could there be any planets to land on, or do I have to stay on board my vessel?
The whole globular cluster has to rotate in some way to prevent the stars from collapsing into a common center that keeps them in this obviously very old configuration?
With the Universe it is important to fit it into its proper context
RocketRon wrote:The burning question, so to speak, is as to why these clusters aren't/haven't formed into rotating disks ?
From what I (poorly) understand, Globular Clusters, Elliptical Galaxies and the yellowed-reddish cores of spiral galaxies don't really rotate like a spinning football (a spherical one, not one of those deformed ones), they beehive. The individual stars have orbits, albeit orbits heavily influenced by everything else in the ball but the thing itself isn't coherent enough to behave like a unit. It's more a swarm of stars than a swirling cloud.
Is this an even vaguely accurate picture, Professor Chris, or am I colossally wrong?
I would like to know if Clusters, Cores and Ellipticals are all the same thing, just coming in various sizes or if there are intrinsic differences between those three sets of objects? I know about the collapsed objects, supermassive black holes in the cores of galaxies but I'm wondering whether those, too, are an epiphenomenon, a function of sheer size?
Do we actually know whether those classes of things are related or don't we have enough information, yet?
gorade wrote:Wonder how it would be to sit inside that cluster? The sky would be crowded with suns of different sizes. Is it hot? By the way, could there be any planets to land on, or do I have to stay on board my vessel?
The whole globular cluster has to rotate in some way to prevent the stars from collapsing into a common center that keeps them in this obviously very old configuration?
"Nightfall", a short SF story by Isaac Asimov. This is sometimes called the best SF short story ever written, more often the best ever written by that Author.
Not to spoil the ending, but it is set on a world of many suns.
Full disclosure, I never thought "Nightfall" was very good, but almost every one else likes it.
In the field of real physics, even in the core of the Cluster, as in the Core of our very own galaxy, stars are light years apart on average. They are certainly not close enough to each other to show as Suns in the skies of any planets.
Again, Prof. Chris will probably be around soon to tell me I'm wrong.
This striking configuration was one of the first marvels of the deep night sky I was shown as a child > as expect may well be true for many here.
Oh, and the usage "In 1716," - eschewing specific calendric reference - is refreshing. Further, it is perhaps even less divisive than the use of "CE" > Wisi enim ad usus profanos notatio
Life inside a globular cluster?
Image copyright: William Harris and Jeremy Webb.
Well, I googled "Life inside a globular cluster" and got this picture. What I particularly like about it is that it shows the brilliant core of the globular, which means that the globular in question is probably 47 Tucanae or M15! I don't think M13 has such a brilliant core.
The sky, as you can see, is jam packed with stars, although the night sky would still be dark.
The image seems to show many tendrils of dust inside the globular, some of them glowing red from Ha. I don't believe in that at all. Most globulars are completely devoid of gas, certainly the ones we know well in the Milky Way.
And by the way... there seems to be a second globular in the sky (at 11 o'clock). That settles it, the globular is 47 Tucanae! I think you might be able to see 47 Tuc's "neighboring globular", NGC 362, from inside the great globular cluster of Tucana. And maybe those faint red tendrils and slightly blue background belong to the Small Magellanic Cloud? Or, more likely, to the Milky Way?
I've always loved deep pictures of globulars. Properly exposed bracketed images that show the wispy extents as well as resolved stars all the way to the core. Congrats on such a superb image Adam and as always, kudos to the APOD folks for keeping these gems coming.
gorade wrote:Wonder how it would be to sit inside that cluster? The sky would be crowded with suns of different sizes. Is it hot? By the way, could there be any planets to land on, or do I have to stay on board my vessel?
The whole globular cluster has to rotate in some way to prevent the stars from collapsing into a common center that keeps them in this obviously very old configuration?
I imagine it's both hot & bright. When the centre does coalesce & collapse into itself a black hole might well be created followed by the development of a cluster wide vortex giving rise to a 'normal' spiral galaxy.
Can someone please tell me why globular clusters don't have concentrations of dark matter in them? All galaxies do, even (or indeed especially) small ones. What gives?
RocketRon wrote:The burning question, so to speak, is as to why these clusters aren't/haven't formed into rotating disks ?
When you have a collection of particles loosely bound together in some way, they tend to end up in a disk only if there's an efficient mechanism for transferring angular momentum. Multiple body gravitational interactions are not an efficient way to do that. What is required is a dense enough environment that fluid dynamic interactions come into play. That happens in dense gaseous regions. Thus we have protoplanetary disks, accretion disks, and presumably the early galaxy formation environments. But not gravitationally bound clumps of stars. (Which also explains why disrupted disk galaxies, which no longer have much gas and dust, are typically disrupted into elliptical galaxies over time- ellipticals behave a lot more like globular clusters, with their stars orbiting in random inclinations).
Chris
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Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
gorade wrote:Wonder how it would be to sit inside that cluster? The sky would be crowded with suns of different sizes. Is it hot? By the way, could there be any planets to land on, or do I have to stay on board my vessel?
The whole globular cluster has to rotate in some way to prevent the stars from collapsing into a common center that keeps them in this obviously very old configuration?
I imagine it's both hot & bright. When the centre does coalesce & collapse into itself a black hole might well be created followed by the development of a cluster wide vortex giving rise to a 'normal' spiral galaxy.
Globular clusters don't collapse. Just the opposite- they evaporate, gradually losing stars until nothing is left.
Chris
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Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
gorade wrote:
Wonder how it would be to sit inside that cluster? The sky would be crowded with suns of different sizes. Is it hot? By the way, could there be any planets to land on, or do I have to stay on board my vessel?
The whole globular cluster has to rotate in some way to prevent the stars from collapsing into a common center that keeps them in this obviously very old configuration?
I imagine it's both hot & bright.
Let's just take the case of sitting at the edge of M13:
M13 is a 6,800 parsecs distant ball of stars ~10 parsecs in radius of apparent magnitude +5.8. If you were situated 680 times closer (at the edge of that ball) it would fill the sky (in one direction) and be of magnitude -8.4 [= 5.8 - 5 log(680)]. This is 28 times dimmer than a (magnitude −12.7) full moon but 25 times brighter than a magnitude −4.9 Venus and 575 times brighter than a magnitude −1.5 Sirius. The sky would be dazzling with perhaps a dozen 'bright planet' stars and up to a thousand first magnitude stars. However, you would NOT be hot and would probably find it difficult to read a newspaper by the light.
heehaw wrote:Can someone please tell me why globular clusters don't have concentrations of dark matter in them? All galaxies do, even (or indeed especially) small ones. What gives?
Well, some globular clusters do appear to have a lot of dark matter in or around them.
But the real problem is that nobody really understands the mechanism behind globular cluster creation. With galaxies, the usual presumption is that they begin as dark matter concentrations. That explains why we find dark matter and galaxies together. But if globular clusters do not require dark matter to seed them, there's no reason that they should have significant dark matter halos today.
Chris
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Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
gorade wrote:Wonder how it would be to sit inside that cluster? The sky would be crowded with suns of different sizes. Is it hot? By the way, could there be any planets to land on, or do I have to stay on board my vessel?
The whole globular cluster has to rotate in some way to prevent the stars from collapsing into a common center that keeps them in this obviously very old configuration?
I imagine it's both hot & bright. When the centre does coalesce & collapse into itself a black hole might well be created followed by the development of a cluster wide vortex giving rise to a 'normal' spiral galaxy.
Globular clusters don't collapse. Just the opposite- they evaporate, gradually losing stars until nothing is left.
Then, what forces them apart? Rotation? Radiation?
With the Universe it is important to fit it into its proper context
Chris Peterson wrote:Globular clusters don't collapse. Just the opposite- they evaporate, gradually losing stars until nothing is left.
Then, what forces them apart? Rotation? Radiation?
They aren't forced apart. The stars are in chaotic orbits (all real-world orbital systems with more than two bodies are inherently chaotic). Gravitational perturbations transfer orbital angular momentum between stars, and that sometimes results in stars with hyperbolic orbits (that is, they are moving at greater than the cluster's escape velocity). Eventually the cluster evaporates away, over a time scale of a few tens of billions of years.
Chris
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Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
DomeLord wrote:
I imagine it's both hot & bright. When the centre does coalesce & collapse into itself a black hole might well be created followed by the development of a cluster wide vortex giving rise to a 'normal' spiral galaxy.
Globular clusters don't collapse. Just the opposite- they evaporate, gradually losing stars until nothing is left.
Then, what forces them apart? Rotation? Radiation?
Click to play embedded YouTube video.
This is a good video, better than the one I posted before. You can see how fast the stars move inside a dense cluster, and during their interactions they sometimes get accelerated to high speeds. Then they sometimes just zip out of the cluster.
MDB wrote:How does M-13's proximity to the rest of our galaxcy effect its ability to form a disk and does being in the halo make any difference?
Well, there's no mechanism to form a disk. Indeed, were it already a disk it would tend to be disrupted into a spherical shape over time by tidal interactions.
Chris
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Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
Ann wrote: Well, I googled "Life inside a globular cluster" and got this picture. What I particularly like about it is that it shows the brilliant core of the globular, which means that the globular in question is probably 47 Tucanae or M15! I don't think M13 has such a brilliant core.
You can download the pdf of the article "Life Inside a Globular Cluster" by William Harris & Jeremy Webb, from the July 2014 issue of Astronomy magazine here.
BTW: it is definitely 47 Tucanae.
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk. — Garrison Keillor
My very simplistic understanding of the formation of globular clusters and disc galaxies is that the globulars formed in the spherical halo, before the galaxy's gas had sufficient time to settle into a disc (assuming enough gas was present for a disc to form). The younger stars in the disc formed from the gas in the disc, which already had an angular momentum with a predominant direction. The presence of sufficient gas and time leads to the disc shaped formations.
I thought spirals formed from Mergers... they spiral material into the disc...it flattens from momentum.
Star Clusters act like COMETS coming from different angles around a central point, where most of the mass would be centered... or some such...it is almost as if there is a black hole there.
