Comments on: Ep. 68: Globular Clusters

By: Val Parks

Val Parks — Thu, 29 Jan 2009 11:29:53 +0000

Although perhaps not answered in the show, these comments were very (Dan Gerhard) helpful. My observations/questions were:
1. GC's looks like a bunch a static stars, hovering around the core. So I guess it looks that way because no obvious clue is given in any photo of rotation, like for a spiral galaxy (which cries out, "I am rotating")

2. So if all the GC stars are not being drawing in to the supermasive Black Hole at the center, then they must be swarming around just like bees, all with different odd angle orbits. And always changing a bit. (How does this get started?)

3. With so many stars flying around every where in the GC, why don't we see occasional collisions? Is this really so remote a possibility. Are we looking out for these?

By: Taras

Taras — Fri, 15 Feb 2008 20:09:59 +0000

You guys didn't touch on some globulars having higher mass stars than expected given by the age of the cluster. It is believed these stars may have been caused by stellar mergers.

By: Jerry Fox

Jerry Fox — Thu, 03 Jan 2008 18:34:37 +0000

Thanks for the reply, Dan. I think I understand it now, though I would have intuitively expected many collisions to occur. Maybe they did, originally, and what we see now are the stars that got lucky!

To Linda – Wikipedia ("nearest star") gives Proxima Centauri at 4.3ly with all its co-ordinates, but that doesn't sound like Pamela's.

By: Linda Krouse

Linda Krouse — Thu, 03 Jan 2008 16:55:57 +0000

Pamela quoted: "Our nearest star of noticeable brightness is a little more than three light years away."

Can you tell me what star that is and its apparent and absolute magnitude. If you know, could you also give RA and dec?

Thank you. I'm asked this question so often by students.

Very nice production. Thanks. I so appreciate these shows and the continuous availability of each episode, so that I can go back again and again to listen or catch a point that has slipped away from me.

By: Dan Gerhards

Dan Gerhards — Thu, 03 Jan 2008 15:08:24 +0000

The cluster doesn't collapse because all the stars are orbiting each other like planets orbit the sun. The cluster won't collapse any more than a planet will suddenly change direction and plunge into the sun. Because there are so many stars, the orbits are hideously complex, and don't often repeat. Still, they operate under gravity as normal.

The mechanism for "floating to the top" (as they explain) comes out of the complex orbits. A star's path around the cluster could take it very close to several other stars at the same time, or very close to a massive star. This will speed it up as it "falls" towards the extra mass. Sometimes, a star gets sped up enough that it gets flung into the outer regions of the cluster. It will tend to stay there because of its higher orbital speed. A small, light star will be tossed around more easily than a massive one, so massive ones stay closer to the center after one of these interactions. You don't need a repulsive force to make this work.

It doesn't really make sense to talk about the rotation of the whole cluster as if it was a solid object. It's like talking about the rotation of a swarm of bees. Flattening can happen, but the reasons are more complicated than centrifugal force.

"There are always things _just_ too small to make out." Tell me about it! The larger your telescope, the larger you want it. I'm up to a 16-inch diameter scope myself. It's bigger than I am!