Globular clusters

[Outbackmanyep]

I hope you had time to take a breath in all that Les! I'll have a read of the links!
Cheers!

[Paddy (Patrick)]

From Les' post,

'Paddy wrote:

"I have one question arising from this. If the Milky Way's GCs are so old and were formed with the galaxy itself, what has prevented the gravity of the disc pulling them into the disc?"

The simple answer is "the conservation of angular momentum". Plus, gas is "sticky", stars (or balls of them) aren't.'

Thanks Les for your detailed and informative entries - fantastic! Please be patient if I pick your brains a tad more. My understanding is quite limited, but I would have thought that if conservation of angular momentum was to prevent GCs moving into the disc, their orbits would have to be roughly orthogonal (now I haven't used that word in some decades to the plane of the disc). If the orbit was in a plane parallel to the disc, I imagine the plane of orbit of the GC would gradually merge with that of the disc.So do they orbit in a plane "perpendicular" to the plane of the disc or have I misunderstood? Could you also say a little more about gases being sticky whilst GCs are not and how this influences their positions/orbits?

I looked at the entries in wikipedia and they are very,very helpful, but you are adding a lot more information with your entries here and I really appreciate your contribution.

[Brian W (Brian)]

Originally Posted by xelasnave

I feel we are very privelged Les to have your input ...just think how many folk are now better informed... great stuff..we now know what you know
alex

Yes we are privileged but for myself I now have read what he wrote but I sure don't know what he knows!

[ngcles]

Hi Paddy & all,

Perhaps I have expressed myself in a slightly clumsy fashion.


_So far as I understand_ (and again, I'm no astrophysist or dynamycst) the orbits of the GCs are random about the galactic centre at all angles to the plane of the disc. Some may be at right-angles, most probably not. Some have shorter orbits than others. Some, the outer halo and extreme outer halo clusters (like NGC 2419) have such huge orbits, they orbit the Milky Way but never move through the disc at all.

Once a GC (as a whole) is in an orbit around the centre of mass of the Milky Way, it will remain in that orbit unless an outside force acts to alter its orbit, and its orbital angular momentum as a whole will remain constant. The majority of the galaxy is fairly empty -- a GC can pass through the disc and stars won't crash into one another unless they are very, very, very unlucky. The disc therefore will not do much to brake the cluster's motion and change its orbit.

However, the repeated passages through the disc and core will have an effect on individual cluster stars and tidal forces will eventually rob the cluster of individual members and therefore mass making it easier in turn for the next member to escape, and the next, and the next and so on.

Clusters that make repeated (frequent) dives through the disc or more importantly through the core of the Milky Way loose members that will straggle both behind and in front of the cluster in its orbit but generally remaining in the galactic halo. This is what is happening to Pal 5 at present (and several other clusters) and eventualy they will completely evaporate, loosing all their stars into the halo. The tidal tails of Pal 5 are in total about 13,000 ly long. The cluster will eventually become a "stream" of stars within the halo. I seem to remember reading somewhere that if the Milky Way does not add to its globulars, then in 10gyr time less than 1/2 what are there now will remain then. It is easier for small GCs to evaporate than
big ones.

Gas behaves diferrently to stars (that are point-like in the scheme of things). It is easy for a two clusters of stars to pass right through each other and exit the other side completely unscathed. Gas has volume and is viscous and when gas-cluds ram into each other they can easily loose momentum (angular or otherwise) and become stuck together (the likely result is star formation). Think of the stars as "needle-like" bullets and the gas like a block of wood. Which one will pass through a human body most easily at high-speed? We see this all the time in merging and interacting galaxies the stars won't hit each other -- the gas clouds do and violently. Interacting/merging galaxies are alive with star-forming activity for just this reason.

Paddy wrote:

"If the orbit was in a plane parallel to the disc, I imagine the plane of orbit of the GC would gradually merge with that of the disc."

But they can't orbit parallel to the disc, they have to orbit the centre of mass of the galaxy, but their orbits are generally all well tilted with respect to the plane of the disc. Some outer halo clusters never sully themselves with the disc. The inner halo clusters spend half their time above, half below. Think of them as being a bit like the comets that are not necessarily confined to the plane of the solar-system, but come and go at all angles -- but all still orbit the Sun!

That's how I understand it works anyway. Please, if there is a physicist out there and I've got it wrong -- let us all know!

Best,

Les D

[OneOfOne (Trevor)]

Another factor that reduces the impact suffered by any GC going through the plane of the galaxy is that it is so thin. So for the millions of years it may take a cluster to go around the galaxy once, only a very tiny amount of this time will be spent under the potential influence of stars in the parent galaxy. Sort of like the rings of Saturn, they are very thin and consist mostly of nothing. Any effects caused by stars interacting with the galaxy would be fleeting at best, gravitationally or otherwise.

It would be interesting to know if any GCs have been found actually inside the galaxy and just passing by? Statistically I would expect this would be very unlikely for the reasons above.

[Paddy (Patrick)]

Many thanks Les, that's a very helpful discourse! I now feel a lot clearer. You keep saying you're no astrophysicist, but the amount of fascinating material that you can impart is truly impressive. So much to learn, so little time!