View Full Version : Some questions re: Open clusters

2009-Dec-24, 05:46 AM
Ok, sort of related to from my earlier question about type O stars, I have a few questions about open star clusters.

When looking for info on them, I often find references to their "tidal radius". e.g. on wikipedia the Pleiades is described as follows:

The cluster core radius is about 8 light years and tidal radius is about 43 light years.

I guess that means that most of the stars are packed into a volume of radius 8 lightyears, but what exactly does "tidal radius" mean in this context? There's a link to "Roche limit" on wiki there, but that doesn't sound like the right thing - I can't imagine it means that nearby stars get torn to bits if they get closer than 43 lightyears to the cluster. I'm guessing it is more likely to mean that the orbits around the galaxy of other stars that are within 43 ly of the core can be affected by the cluster?

Does anyone have any more info on the Double Cluster in Perseus? The wiki page is somewhat short about those - do we know how large they are, how many B stars they contain, and how many stars are contained within their "core radii"? I'd imagine that they're smaller than the Pleiades since they're a lot younger (and probably contain more B and O stars too).

And how common are open clusters? Do we have a "number density" for these star clusters? Would we expect to see for example, one per 100 cubic lightyears or something?

2009-Dec-24, 08:35 AM
On the first question, apparently the tidal radius is the radius within which an object will remain attracted to that object rather than being pulled out of its range into something else. The moon is within the earth's tidal radius, but if were much further out it would end up being stolen by the sun away from the earth.

2009-Dec-24, 12:13 PM
So tidal radius = radius of Hill sphere?

2009-Dec-24, 04:46 PM
That sounds about right.

Note though, that orbits would become unstable well inside the strict limits of the tidal radius...