APOD: Globular Star Cluster NGC 6752 (2013 Jul 05)

[APOD Robot]

Globular Star Cluster NGC 6752

Explanation: Some 13,000 light-years away toward the southern constellation Pavo, the globular star cluster NGC 6752 roams the halo of our Milky Way galaxy. Over 10 billion years old, NGC 6752 follows clusters Omega Centauri and 47 Tucanae as the third brightest globular in planet Earth's night sky. It holds over 100 thousand stars in a sphere about 100 light-years in diameter. Telescopic explorations of the NGC 6752 have found that a remarkable fraction of the stars near the cluster's core, are multiple star systems. They also reveal the presence of blue straggle stars, stars which appear to be too young and massive to exist in a cluster whose stars are all expected to be at least twice as old as the Sun. The blue stragglers are thought to be formed by star mergers and collisions in the dense stellar environment at the cluster's core. This sharp color composite also features the cluster's ancient red giant stars in yellowish hues.

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[saturno2]

A old cluster with some blue stars

[JohnD]

I'm still puzzled by the structure of globular clusters, or rather the apparent lack of it.

Spiral galaxies 'obviously' rotate, the velocity of individual stars keeping them in orbit around the common CoG, and mutual attraction organising them into spiral arms. It seems as if the laws of physics act like traffic laws, keeping everyone on the correct side of the road, and in lanes. The space apart of stars makes collision unlikely anyway, but this 'streaming' makes it even more improbable.

In contrast, a globular cluster looks like traffic mayhem, or market day in a third world country. The stars are orbiting, but their orbits are random, yet the cluster looks calmer than any spiral galaxy, with its dust, gas and new star forming regions, where collision and mayhem are the rule. In this respect globular clusters are as regimented as soldiers on parade.

So do I misunderstand the structure of a globular cluster? Does it rotate 'in one piece' rather than as a swarm of stellar bees? They are said to be the product of ancient galactic collisions. How have they achieved this karmic stasis from such apocalyptic events?

John

[Boomer12k]

Wonderful Cluster shot as usual...a real....PEACH. ...here are some cluster videos...
http://www.spacetelescope.org/videos/ar ... /clusters/

The first one on the evolution of how BSS's migrate over time to the central area is interesting....it would then appear they form on the outer edges of the cluster. There was an APOD last year, or at least a comment on an APOD, I think on a simulation of incoming "globs" of gasses to a galaxy...I am wondering if that would contribute to the formation of new stars in an old cluster. Bringing new "life" to the cluster, and hence its continuation. Collision Theory seems off...that would mean crashes...explosions...scattering of gasses, dust, etc, there would be evidence of Nebula, I would think...I imagine it is not a homogenous union. There does not seem to be evidence of that. And the migration of BSS's into a cluster would seem to negate that theory. However....where is the Stellar Cloud? The Nursery at the end of the cluster? Where are the "NEW" Blue Stars forming???? There has to be a mechanism...They are very odd indeed...

:---[===] *

[nstahl]

Does it rotate 'in one piece' rather than as a swarm of stellar bees?
John

The stars orbit the center of mass, with no doubt a lot of perturbations. Apparently there's still enough room between stars (a light week is still a long way) that they don't collide very often. So from a distance it may seem stately. But just try keeping a planet in orbit around a star in there; I seem to recall one or two such have been found, to great surprise, but it's got to be quite rare.

[zbvhs]

Could Blue Stragglers grow so large through accretion that fusion at the core couldn't sustain them against gravity? Would they develop some sort of solid core with fusion on the surface of the core? Would a super-super-massive star have to go through a super-nova process in order to reach the black hole state? Could such a star grow large enough to have a surface escape velocity equal to the speed of light? Could something like this be found at the core of a globular cluster? Where does the theory go?

[Chris Peterson]

I'm still puzzled by the structure of globular clusters, or rather the apparent lack of it.

Spiral galaxies 'obviously' rotate, the velocity of individual stars keeping them in orbit around the common CoG, and mutual attraction organising them into spiral arms. It seems as if the laws of physics act like traffic laws, keeping everyone on the correct side of the road, and in lanes. The space apart of stars makes collision unlikely anyway, but this 'streaming' makes it even more improbable.

Spiral galaxies don't rotate; many of the stars in them orbit on a plane, and in the same direction. Of course, this only applies to the disc. The stars in the bulge behave just like those in a cluster, orbiting at all different inclinations. And galaxies other than spirals also have their stars orbiting over a much wider range of inclinations. If anything, it is spiral galaxies that are the exception, with their stars still orbiting within the parameters of the formation structure.

In contrast, a globular cluster looks like traffic mayhem, or market day in a third world country. The stars are orbiting, but their orbits are random, yet the cluster looks calmer than any spiral galaxy, with its dust, gas and new star forming regions, where collision and mayhem are the rule.

It only looks calm because we see it over such a short time span. If we could observe for a few million years, a globular cluster would look like a swarm of bees.

There are virtually no stellar collisions in clusters, nor in galaxies, nor anywhere. There is no mechanism other than extreme (bad?) luck that can cause two stars to collide. Even at the densest stellar concentrations in the center of galaxies or globular clusters, the distance between stars is many, many orders of magnitude greater than their sizes. The activity in galaxies occurs because there is (often) a large amount of gas and dust. That's what collides, and that what results in star formation. Clusters are old, stable, and cleared of gas and dust. There's simply no material to create significant activity in most cases.

So do I misunderstand the structure of a globular cluster? Does it rotate 'in one piece' rather than as a swarm of stellar bees? They are said to be the product of ancient galactic collisions. How have they achieved this karmic stasis from such apocalyptic events?

Actually, nobody has much idea how globulars form. Coming somehow from collisions is only one idea. They are not well understood phenomena.

[emc]

Happy aphelion day!
This is as far as most of us get from the sun!

[JohnD]

Thnaks, Chris!
I need a certain amount of mental readjustment, especially on "Spiral galaxies don't rotate", yet the individual stars are in orbit.
I'll be grateful for a bigger explanation of that, please.
John

[Chris Peterson]

I need a certain amount of mental readjustment, especially on "Spiral galaxies don't rotate", yet the individual stars are in orbit.
I'll be grateful for a bigger explanation of that, please.

I guess it just depends on what you mean by "rotate" when you're talking about a body made up of mainly uninteracting components. Certainly, the disc part of a spiral galaxy has a net angular momentum, in that all of the stars are orbiting on a similar plane and in the same direction. That's a kind of rotation. But in many spiral galaxies, most of the stars, and most of the mass (ordinary matter) is found in the bulge, and there you don't necessarily have a large value for angular momentum.

Mainly, I meant that in many respects the rotation of a spiral galaxy is a bit of an illusion, created by the apparent rotation of the arms, which does not represent the actual rotation of physical structures (as the arms are continually being formed and destroyed, not actually rotating).

[geckzilla]

There's a Wikipedia article and the external link leads to some animations on the matter.

http://en.wikipedia.org/wiki/Gravitatio ... ned_orbits

Animations: http://rqgravity.net/SpiralStructure

[Galaxian]

Does it rotate 'in one piece' rather than as a swarm of stellar bees?
John

The stars orbit the center of mass, with no doubt a lot of perturbations. Apparently there's still enough room between stars (a light week is still a long way) that they don't collide very often. So from a distance it may seem stately. But just try keeping a planet in orbit around a star in there; I seem to recall one or two such have been found, to great surprise, but it's got to be quite rare.

"Rare" on our scale or "rare" on cosmic scales? From Kepler and other robot missions it seems that there are milliards of planets around but most are hot Jupiters or SuperEarths or some other combination of not quite Hawaii. That makes Terran objects with Hawaii-style beaches "rare", but rare on our scale. Cosmically, there are still probably *lots* of nice Earth-like worlds, enough for an Asimovian or "Star Wars" Galactic Empire.
Which leads to the question, are we talking about thousands of worlds per Galactic Cluster, with maybe hundreds being nice and NewNew Englandish? Which would mean tens of thousands of habitable worlds scattered among the many hundreds of GC's in the Local Group, or are we thinking of really rare with maybe two or three solid worlds within millions of light years? Is "Nightfall" very unlikely, or spectacularly unlikely?
That aside, it's a gorgeous image and I would *love* to see this sort of thing up close with my own eyes.
And pareidolia strikes again, in the very centre of the standard image on the APoD page I see both a glowing, lovely bird light years long and a strikingly beautiful witch riding a broom correctly, brush leading. So now I know where the wife is...

[Galaxian]

I'm still puzzled by the structure of globular clusters, or rather the apparent lack of it.

Spiral galaxies 'obviously' rotate, the velocity of individual stars keeping them in orbit around the common CoG, and mutual attraction organising them into spiral arms. It seems as if the laws of physics act like traffic laws, keeping everyone on the correct side of the road, and in lanes. The space apart of stars makes collision unlikely anyway, but this 'streaming' makes it even more improbable.

In contrast, a globular cluster looks like traffic mayhem, or market day in a third world country. The stars are orbiting, but their orbits are random, yet the cluster looks calmer than any spiral galaxy, with its dust, gas and new star forming regions, where collision and mayhem are the rule. In this respect globular clusters are as regimented as soldiers on parade.

So do I misunderstand the structure of a globular cluster? Does it rotate 'in one piece' rather than as a swarm of stellar bees? They are said to be the product of ancient galactic collisions. How have they achieved this karmic stasis from such apocalyptic events?

John

Does it help to see them as a swarm of bees with gravity and inertia taking the place of legs holding the furball together?
Start by imagining a binary star system. Add a third star of nearly the same size as the others. Slowly add into you mental picture five or six others. Watch the sparks zoom around the centres of mass within semi-chaotic trajectories that don't even approximate stable orbits. Add more and more sparks, with millions of millions of moving, changing, overlapping and transient centres of mass. Some stars orbit "up-down" for a while, some in the "equatorial" plane for a time. They all interfere with all of the others.
The model isn't a rotating object, it's a dustball with every single mote of luminous, sparkling dust falling through space in twisty, curly, non-repetitive trajectories.
Stars are tiny, tiny things falling serenely through vast volumes but their gravity is infinitely extensive in theory. Every star in a Galactic Cluster, or a Galactic Centre or a spherical galaxy influences all of the others a small bit as it falls through its life. Even a great swarming hugeness like a cluster will have few, if any collisions during the deep ages of its existence.
Though the occasional ingestion of a star by the supermassive central singularity might provide some excitement.
Does any of the above help?

[rstevenson]

There are virtually no stellar collisions in clusters, nor in galaxies, nor anywhere. There is no mechanism other than extreme (bad?) luck that can cause two stars to collide.

And yet, following links in the APOD text, I find...

From http://hubblesite.org/newscenter/archiv ... es/1997/35

For the first time, astronomers have confirmed that a blue straggler in the core of a globular cluster (a very dense community of stars) is a massive, rapidly rotating star that is spinning 75 times faster than the Sun. This finding provides proof that blue stragglers are created by collisions or other intimate encounters in an overcrowded cluster core.

From http://heritage.stsci.edu/2003/21/caption.html

The stars in NGC 6397 are in constant motion, like a swarm of angry bees. The ancient stars are so crowded together that a few of them inevitably collide with each other once in a while. Near misses are even more common. Even so, collisions only occur every few million years or so. That's thousands of collisions in the 14-billion-year lifetime of the cluster.

From http://en.wikipedia.org/wiki/Blue_straggler

Overall, there is evidence in favor of both collisions and mass transfer between binary stars. In M3, 47 Tucanae and NGC 6752, both mechanisms seem to be operating, with collisional blue stragglers occupying the cluster cores and mass transfer blue stragglers at the outskirts.

Seems like there is some reason to believe that collisions do occur, at least in the crowded centers of globular clusters.

Rob

[neufer]

[b][color=#0000FF][size=150]M19: The most oblate globular cluster?[/size][/color][/b]

---

Mainly, I meant that in many respects the rotation of a spiral galaxy is a bit of an illusion, created by the apparent rotation of the arms, which does not represent the actual rotation of physical structures (as the arms are continually being formed and destroyed, not actually rotating).

• However, unlike some globular clusters, the flattening of a spiral galactic disk is no illusion.

<<Messier 19 or M19 (also designated NGC 6273) is a globular cluster in the constellation Ophiuchus. M19 is one of the most oblate of the known globular clusters. This flattening may not accurately reflect the physical shape of the cluster because the emitted light is being strongly absorbed along the eastern edge. This is the result of extinction caused by intervening gas and dust. When viewed in the infrared, the cluster shows almost no flattening. It lies at a distance of about 8.8 kpc from the Solar System, and is quite near to the Galactic Center at only about 2.0 kpc away.>>

---

<<The spiral galaxy, NGC4622 (also called backward galaxy), lies 111 million light years away in the constellation Centaurus. NGC 4622 is an example of a galaxy with leading spiral arms. In spiral galaxies, spiral arms were thought to trail; the tips of the spiral arms winding away from the center of the galaxy in the direction of the disks orbital rotation. In NGC4622, however, the outer arms are leading spiral arms; the tips of the spiral arms point towards the direction of disk rotation. This may be the result of a gravitational interaction between NGC 4622 and another galaxy or the result of a merger between NGC 4622 and a smaller object.

NGC 4622 also has a single inner trailing spiral arm. Although it was originally suspected that the inner spiral arm was a leading arm, the observations that established that the outer arms were leading also established that the inner arm was trailing.

These results were met with skepticism in part because they contradicted conventional wisdom with one quote being “so you’re the backward astronomers who found the backward galaxy.” The fact that a pair of arms could lead was not easy to accept. Astronomical objections centered on the fact that dust reddening and cloud silhouettes were used to determine that the outer arms led. The galaxy disk is tilted only 19 degrees from face-on making near to far-side effects of dust hard to discern and because clumpy dust clouds might be concentrated on one side of the disk, creating misleading results.

In response, the “backward astronomers” determined NGC4622’s spiral arm sense with a method independent of the previous work. The new Fourier component method is actually assisted by the small tilt, and dust reddening and cloud silhouettes are not used in the latest analysis. The Fourier component method reveals two new weak arms in the inner disk winding opposite the outer strong clockwise pair. Thus the galaxy must have a pair of arms winding in the opposite direction from most galaxies. Analysis of a color-age star formation angle sequence of the Fourier components establishes that the strong outer pair is the leading pair.

While the presence of backward arms in a galaxy may seem like an inconvenient truth to many, two independent methods now indicate that NGC4622’s arms do indeed behave in a very unusual fashion, with the outer arms winding outward in the same direction the disk turns.>>

[Galaxian]

Actually, nobody has much idea how globulars form. Coming somehow from collisions is only one idea. They are not well understood phenomena.

Sir, thank you, your post was far clearer than mine. I guess that comes of you being a real, practising Scientist while I'm mostly an autodidactic dilettante.
I don't have the mathematics to describe it but I picture a gigantic cloud of almost pure primordial hydrogen condensing slowly into milliards of nodes. These begin falling past each other in complex trajectories in every plane and inclination. The condensing micro-clouds light up and we have a Galactic Cluster or a Centre or an elliptical galaxy.
That sort of picture makes intuitive sense to me. There is a slight issue with it, though. It has no way of falling into the flat fried egg of a spiral galaxy. That takes a bit of doing. Ragged irregulars are easy, they are just spherical swarms with less sphericity and more randomnity but spirals are odd.
I know the *universe* did it. Lots of times. Lots and lots and really *lots* of times but short of the hand-waving "there are tiny effects which, over deep time in huge spaces will dominate when they are the only residual effects" forming spiral galaxies is not easy to visualise.
Lots of times, I wish I had more maths.
There is a corollary to my picture, the nodes of condensing gases form stars but there should also be old micro-stars, red and brown dwarves and even things that could be described as planets, huge Jupiters. Forming as they do at the beginning of time, Galactic Clusters won't have had much "metals" so terrestrial planets are unlikely until new stars are born from the old but that still means there could now be many of them.
"Many" on a human scale.
We desperately *need* an FTL drive.

[Chris Peterson]

Seems like there is some reason to believe that collisions do occur, at least in the crowded centers of globular clusters.

That's why I said "virtually no collisions". They are literally one-in-a-million events. Extremely rare, and not important for understanding the dynamics of globular clusters.

[Chris Peterson]

However, unlike some globular clusters, the flattening of a spiral galactic disk is no illusion.

I hope I didn't say anything that suggested otherwise!

[JohnD]

Thanks to all!
Esp. geckzilla for the link to the animation page.

OK;
A/ I should have read my Hitchhikers Guide, "“Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.” So, no star collisions, even in a chaotic system, like a globular (?).

B/ The Spiral doesn't turn, but instead is a standing wave (?) as the result of the precession of the orbits of all the stars in a galaxy. But I still don't see why that precession should produce this structure in the galaxy, when as Chris says, interaction between stars even at a distance is minimal. Or is that minimal interaction enough?

Thanks for the continuing tutorial!
John

[Chris Peterson]

But I still don't see why that precession should produce this structure in the galaxy, when as Chris says, interaction between stars even at a distance is minimal. Or is that minimal interaction enough?

Empirically, the interaction must be enough. There really are no mechanisms other than gravity that can explain spiral arms (even if that particular explanation is still being clarified).

To be clear, when I talked about a minimal interaction, I was referring to that between individual stars. In the disc of a spiral galaxy, any individual star is largely interacting with the net gravitational field of all the other stars. In elliptical galaxies, the bulge of spiral galaxies, or globular clusters that is similarly the case, but because the individual orbits are much more random, and the stellar density may be higher, you also have individual stellar interactions. These don't typically result in collisions, but they do create large perturbations, with the result that stellar orbits are frequently changed (sometimes even ejecting the stars from the body).

[rstevenson]

... So, no star collisions, even in a chaotic system, like a globular (?). ...

Yes, there are collisions -- in particular in the center of globular clusters. See above.

Rob

[Anthony Barreiro]

Seems like there is some reason to believe that collisions do occur, at least in the crowded centers of globular clusters.

That's why I said "virtually no collisions". They are literally one-in-a-million events. Extremely rare, and not important for understanding the dynamics of globular clusters.

But very important in understanding how there can be big, bright, blue stars in a globular cluster!

Lovely image, by the way. I've got to get to the southern hemisphere one of these years and see these big globular clusters for myself!

[Ann]

Extremely few of the blue stars that we see in this globular are likely to be products of collisions. Virtually all the blue stars that can be seen here are undoubtedly blue horizontal branch stars. They represent a late evolutionary phase of extremely metal-poor stars.

The blue horizontal branch stars are less massive than the blue stragglers, but they are considerably brighter. The blue stragglers are main sequence stars, typically of spectral classes A and F, whereas the blue horizontal branch stars have used up their core hydrogen and shine by other means. They are either the same temperature as the blue stragglers but considerably larger, or else they are the same size as the blue stragglers but considerably hotter.

I'm certainly not trying to suggest that there are no blue stragglers in this cluster. However, the straggler stars are far less conspicuous than the blue horizontal branch stars, which means that most or all the blue stars that we see so clearly in this picture are likely to be horizontal branch stars.

Ann

[neufer]

I'm certainly not trying to suggest that there are no blue stragglers in this cluster. However, the straggler stars are far less conspicuous than the blue horizontal branch stars, which means that most or all the blue stars that we see so clearly in this picture are likely to be horizontal branch stars.

But the blue stars near the center of NGC 6752 aren't very conspicuous (except for their color)

http://outreach.atnf.csiro.au/education ... sters.html

[Beyond]

Hmm... i don't see any blue ones in your linked picture, Art. But then i may be getting feeble and color blind.