Celestial hemisphere:  Southern  ·  Constellation: Scorpius (Sco)  ·  Contains:  M 4  ·  NGC 6121
M4, Willem Jan Drijfhout
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M4, Willem Jan Drijfhout
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Description

An LRGB image of Messier 4, or NGC6121. Interestingly, during processing some bright green stars popped up. At first this seemed like an error with the equipment or the processing. But the most likely scenario was mentioned in this thread. The green stars are RR Lyrae variable stars. These are stars with pretty significant amplitude swings with a period of 0.5-2 days. So when imaging over several nights and colour filters are applied at long enough intervals, each filter may catch these stars at a significantly different brightness, resulting in strange colours.

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M4, Willem Jan Drijfhout