The count of stars in M13 is about 100,000, resulting in a core density about 100 times that in our sun's neighborhood. Since the stars in the cluster were formed together, and the cluster is over 11 billion years old, they should all be the same age--old. So why do we see blue stars, which have short lives? The blue stars we see here are Blue Horizontal Branch stars, which are about 1 magnitude brighter than blue stragglers, and far more numerous in a low-metallicity cluster like M13. They are the natural evolution of post-red giant branch single stars, after the "helium flash" occurs at the tip of the red giant branch. Essentially, they can be considered "helium main sequence" stars. For comparison, M3 in my previously posted
image is three times more populous in blue stragglers than M13 is.
Data
This represents 4.5 hrs of 9 x 600s SBIG RGB filtered subs using the RCOS 16" f/8.9 (3550mm focal length) Robotic Telescope at Sierra Remote Observatories, Auberry, California and a SBIG STX16803 16MP (4096 x 4096) CCD Camera.
Processing
I did calibration and stacking in DeepSkyStacker; L(synthetic)RGB composting, stretching, HDR, deconvolution, color calibration and NR in StarTools; and final touches in PhotoShop.
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