Because their sizes compared to their typical distances from one another of a light year or slightly less, make collisions very rare.
A typical core has perhaps 100,000 stars inside a region about 10 parsecs in radius...as an estimate. This works out to a typical density of 250 stars per cubic parsec and a distance between stars of about (1/250)^.333 = 0.16 parsecs or 0.5 light years.
The time between collisions is the reciprocal product of the density of objects, their cross section and their speed. For cluster stars, V is about 10 kilometers/sec, the density is 250 stars/cubic parsec, and their cross sections equal about ( one million kilometers)^2 so:
1/(n x sigma x V) = 1/(250pc^3 x 10km/s x (10^6 km)^2 )
= 3 x 10^17 years
for a pair of stars to collide under these conditions. Note, the pictures you see of stars inside clusters, like the image of Messier 13 above, make it look like the stars are nearly touching. This is because the light from each star is spread out into a fuzzy blob by the atmosphere or by the telescope optics..or both. No telescope can resolve the true disks of distant stars in pictures like this.
Copyright 1997 Dr. Sten Odenwald
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