Starship Asterisk*

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

JohnD wrote: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.

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?

JohnD wrote: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.

nstahl wrote:

JohnD wrote: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.

JohnD wrote: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

Chris Peterson wrote: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.

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.

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.

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.

Chris Peterson wrote:
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.

http://en.wikipedia.org/wiki/Messier_19 wrote:
<<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.>>

http://en.wikipedia.org/wiki/NGC_4622 wrote:

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<<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.>>

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

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

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

JohnD wrote: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?

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

Chris Peterson wrote:

rstevenson wrote: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.

Ann wrote:
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.