Stars in globular star cluster NGC 6397, Hubble Space Telescope (HST) image. The stars in globular star clusters are densely packed. Here, the core of NGC 6397 has stars only a few light weeks apart compared to the 4 light years that separates the Sun from its nearest neighbour. The stars are so crowded that collisions occur every few million years. Direct collisions produce the hot bright blue straggler stars seen among the old yellow stars. NGC 6397 is around 8200 light years away in the constellation Ara.
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