by Chris Peterson » Fri Jul 05, 2013 2:46 pm
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.
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.
[quote="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.[/quote]
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.
[quote]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.[/quote]
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.
[quote]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?[/quote]
Actually, nobody has much idea how globulars form. Coming somehow from collisions is only one idea. They are not well understood phenomena.