47 tucanae

[orin stepanek]

What keeps the stars apart? Are they by any chance in orbit around the clusters core? Seems at though they wouled converge in toward the cores center.
Orin

[ta152h0]

Is there a repulsive force of greater intensity than gravity ????

[makc]

What keeps the stars apart?

My guess is "time".

[Kim]

I thought at first that they must all orbit the common centre of gravity, but that would not account for the stars near the axis of rotation. Cosidering that (I seem to remember reading somewhere) the average distance between the stars in a GC is about 1 LY, I wonder how much gravitational influence each has on its neighbors, maybe they all pull in different directions fairly equaly, maintaining some kind of gravitational status quo?

[makc]

...maybe they all pull in different directions fairly equaly, maintaining some kind of gravitational status quo?

This is what Kid (aka N00b) asked not that long ago, so I thought someone had to point out that this is NOT possible for purely mathematical reasons.

Consider this:

Star .............. Planet .............. Star
8) .................... .................... 8)

As long as planet is right in the middle, it will not move neither to the left, nor to the right. Now, imagine some meteorite hit it from the left, moving our Planet 1cm right.From now on, right star pulls it stronger, and left star pulls it weaker, so planet starts to fall towards right star, and there is nothing now to revert disbalance.

[orin stepanek]

If the stars are all moving together along the same direction. would this tend to keep them apart? Kind of like the shot out of a shotgun? After all; we are moving through space.
Orin

[craterchains]

But, if the shot pellets are the size of planets their gravity would attract each other into one mass, eventually.

Norval

[orin stepanek]

So; Makc! How does time keep the stars apart???
Orin

[Odegard]

Stars in globular clusters orbit around a common center of gravitaty. It's like a mini-galaxy in the real galaxy.

They are kept apart due to individual directions of velocity, i.e. they orbit. It's the same with earth-moon system. They orbit a common center of gravity.

The earth-sun also orbit a common center of gravity. If you want to calculate the common center of gravity between moon-earth-sun it will save a a lot of work if you consider the two centers instead three bodies, e.g. where is the center of gravity for the moon-earth system, and the (moon-earth)-sun system.

If you want to calculate the individual tug of gravity felt by a single star in a globular cluster you would need all the computing power in the world and a lot of time. (Simulations use models to describe the field of gravity, they don't take each star into account.)

So it's nothing weird about globular cluster except that they orbit real close to each other, but still, they are several light-hours if not light-days apart from each other, also in the center. (Pluto is a few light-hours away from the Sun).

Many think that a galaxy, or a globular cluster, is brimfull of stars, standing shoulder to shoulder. The fact is that stars are so small compared to the distance between them. When e.g. to galaxies crash into eachother there is almost a negible chance that two stars would collide. (what causes the fun fun action is the gas between the stars which heats up and make new stars.)

[makc]

I was saying that stars (mostly) have no enough time to hit each other.

[Kid]

makc, is there a need to emphasize my older name such as

(aka N00b)

?

[Dog-dog]

The gravitational dynamics of local globular clusters, within themselves and as "children" of the Milky Way, just doesn't seem to make sense. We have these wonderful, nearly perfectly spherical balls of stars, wonderfully orbiting a much huger gallactic plate. Since when did nature begin permitting such discrete fields of operation? Assuming the shape of the Milky Way is a result of gravity, then why didn't globular clusters get sucked into the malestrom centuries ago, like all of the water in my sink after I am done the dishes?

Re internal dynamics of globular clusters, has anyone ever compared the Doppler shift of stars in the near side of a local globular cluster, relative to us, to the Doppler shift of stars in the far side of the same cluster? I am especially interested in stars on near and far sides close to the center.

[craterchains]

A most interesting line of quest Dog-dog.

[Odegard]

Since when did nature begin permitting such discrete fields of operation? Assuming the shape of the Milky Way is a result of gravity, then why didn't globular clusters get sucked into the malestrom centuries ago, like all of the water in my sink after I am done the dishes?

This is not an anomaly of nature. You are infact using the same arguments as the church did in the 16th century "all orbits are circular, nature cannot permit any other imperfect orbit."

Globular cluster are indeed being sucked into the "malestroem", albeit slowly. While the water in a sink can be drained in seconds, the Milkyway rotates once every 300 million years or so. And also while water in the sink rotates pretty much horizontally before going down the drain, globular clusters tend have orbits out of and through the galaxy plane, adding a new dimension(!) to the question. Things just take time.

Re internal dynamics of globular clusters, has anyone ever compared the Doppler shift of stars in the near side of a local globular cluster, relative to us, to the Doppler shift of stars in the far side of the same cluster? I am especially interested in stars on near and far sides close to the center.

The relative difference between the near side and the far side is zero, since doppler shifts are measured radially from or towards us and not parallell to us. I tried searching the NASA ADS and arXiv for keywords like globular,cluster,dynamics,origin,evolution but didn't find anything in particular relating to your question, I guess this one is the closest.

Globular cluster may form from immensely huge molecular clusters. One scenario is that the first stars form in the outskirts and when they ignite, the stars send shockwaves inwards, triggering new star formations. This results in a local starpopulation with stars with about the same age. Globular clusters have been used to measure distances in the universe up to around 50.000 light years using HR-diagrams. It's fairly accurate.

[Empeda]

There are also motion theories that put intermediate blacks holes at the centre of Globular Clusters - they could also provide a mechanism to build up the supermassive blacks holes at the centre of galaxies....

[Dog-dog]

Thank you for the answers. They're good.

Where I am coming from is this: I have always been suspicious of assumptions that spiral galaxies net-out to gravitational agglomerations. Years ago, as a child, I thought, "They sure look like they are coming out of the center, sort of like a rotating quasar in space, with twin jets firing into space matter pulled to our frame of reference from another point in space and time." I wondered if the hub of the Milky Way, on our side of the center, is blue shifting toward us. My child's logic was that if spiral galaxies were emerging from their own centers, then matter closer to the edge, having been emitted from the center earlier than material closer tom the center, would have had much more time to decelerate due to the gravitational attraction of other already-emitted matter in the galaxy, slowing us down relative to the hub, generating blue shift relative to the hub, on our side of the hub.

Lo and behold, matter near the hub on our side of the hub is blue-shifting toward us.

I was curious if globular clusters were characterized by the same relative blue-shift-near-the-center phenomenon.

[Empeda]

I see the reasoning - but that wouldn't explain why the centre of the galaxy has the highest abundance of older stars - in fact hardly any young ones (relatively speaking).

Since globular clusters appear to orbit the galactic centre, and are randomly distributed, some are blue-shifted, some are red-shifted. There are curious bodies though since they contain some of the oldest stars in the Galaxy, some nearly 12/13 billion years old.

[craterchains]

Is the methodology of dating a star's age theoretical, or factual?

If theoretical, then that may have to be changed based on sciences oldest tool, "observation".

Norval

[Empeda]

As with anything it's theorectical, but stars do follow stict rules within an HR diagram which will allow calculation of their age - globular clusters are particularly good for this, as there is a 'drop off' on the diagram, showing us that the stars are older - and also allowing us to predict a pretty precise eage for the cluster.

Where does it say that this side of the centre is blue shifting?

There is a proper motion of the sun when compared to other points - we aren't on a static disk -for example, the sun bobs up and down within the disk as well as orbiting the galactic centre.

The other thing to consider off course is the lack of gas clouds at the centre of the galaxy that are required to produced new stars - the process described above would also not explain the existance of elliptical, barred-spiral or starburst galaxies.