08 March 2007

Clustering on a really large scale

Star clusters open a pandora’s box of interesting little paradoxes. They have sets and groups which are used to track a cluster’s age and the progress of indivisual stars within the cluster.

However, a 14Ga-old cluster will have orbited our galaxy more than 100 times, passing through the galactic disk twice each time, and somehow remained together & compact through all of this.

On top of this, there are... oddities in how stars’ ages are tracked. Once a star has chewed through all of its hydrogen, it expands & burns redder. At this point, it no longer fits the main stellar sequence, so the point is called “the main sequence turn-off point”.

The age of the cluster can be calculated by measuring stars at the turn-off point and plotting them in a sequence based on our Sun. It’s a nice theory, but there’s no way of testing it. There are also a few problems, like most of the turned-off stars have a far lower mass than they should.

The ages typically come out in the 13-15Ga range, which is uncomfortably close to the (apparently variable) age of the Universe, & is consistent enough to point more towards regularities in the theories than in the stars themselves.

There are also (welcome to Real Life™) problem children known as “blue stragglers” which are too blue, hot & bright to fit any of these sequences. Where did these come from? How do we measure them?

People reckoned that they were intruders, but they fit too well with the other core stars for that to be reasonable. Then there was the idea that the cluster’s “horizontal branch” crossed the extended main sequence, but blue stragglers are too different from branch stars for this to be reasonable. Were they recently formed, then? No, there is a dearth of star-forming materials in the clusters.

Then we come to more interesting theories, such as collided, collapsed & exhausted stars dumping their mass into the newly blue stars.

Speaking of one of Milky Way’s 160 clusters, Ferraro’s (astronomy) team suggests that “M80 is in the middle of an evolutionary phase in which stellar collisions are generating the huge population [305] of blue stragglers that are retarding the overall collapse of the cluster’s core”. All very adventuresome.

We can see no new clusters forming today, so it seems reasonable that the processes which formed them are no longer active. If the actual processes are much more catastrophic than those proposed, explaining blue stragglers becomes easier: blue stragglers were (are, generally) heavier stars.

However, adding new unobserved processes to the existing collection of unobserved processes doesn’t exactly simplify the pandora’s box which clusters represent, does it? (-:

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