Here's M13 taken under an almost full moon on a crisp clear night.
I took a number of exposures of various durations from 30 seconds up to 4 minutes.
In the end I just averaged them all up and did a bit of stretching and colour balancing.
I was pleasantly surprised with the final result but was somewhat alarmed to see a lot of 'blue' stars dotted across the cluster.
Was this some sort of artifact due to the moonlight? Was my camera somehow responsible for the blue stars?
Surely, I thought, globular clusters are incredibly old and should therefore contain only old red stars.
A bit of research on the web soon came up with the answer. The stars within M13 are predominantly old red giants that have expanded well beyond their original diameter. These stars can apparently orbit a common point in the centre of the cluster's mass and occasionally, as the centre of M13 is so densely packed, the old stars can stray too close, collide, and creating a new type of star known as a "blue straggler". This collision mechanism is one possible explanation to the appearance of "young", hot stars inside globular clusters like M13.
There is, however, another more recent theory that assumes that most blue stragglers are members of binary star pairs that gradually pull matter from their partners and this cosmic cannibalism allows smaller, aging stars to swell to sunlike proportions, extending their lives by hundreds of millions of years.
www.solstation.com/x-objects/bluestrag.htmgives a definitive account of what's going on.
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