Trapezium: "Multiple Star" or "Cluster"?
Posted 23 October 2012 - 05:08 PM
I suppose a better question might be, "does it matter?"
Posted 23 October 2012 - 08:04 PM
since you've taken solved orbits off the table, you have to derive the system potential from the separate stellar masses, trajectories and distances. i think the current outer bound for "fragile" binaries that are just waiting to be broken up is (depending on which source you look at) a comoving pair separated by 10,000 to 30,000 AU; i use 25,000 AU only because i have seen it cited more than once. that is 0.12 parsec, or about 0.4 light year. since a cluster radius is usually multiples of light years, the fundamental difference is perhaps simply dimensional.
you could also set an arbitrary number of components as the cluster cutoff. WDS includes a few systems with over a dozen components, and a multiple star such as BU 442 -- several very faint companions around a delicate triple star hidden in the cygnus milky way -- looks to me like cluster remnant, or perhaps a minicluster.
there is a similar vagueness about the difference between a cluster and an association. trumpler 10 and collinder 121 were both originally classified as clusters, but are now classified as OB associations. they are comoving, but so large and sparse that whether they are gravitationally bound or not is difficult to determine: the member motions are not orbitally around a center, but moving at equal speed away from a distant point.
Posted 27 October 2012 - 07:03 AM
Posted 29 October 2012 - 09:33 AM
Imagine the Trapezium if today it still contained these three stars.