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Hi there, this is a really interesting find.

What is so interesting about it is that the color is not typical of what you would expect for stars. However, when looking at the single exposures, this object is clearly there at many different points in time at the exact same location. This is in contrast to something transient like an asteroid, that might appear to have a weird color because it is only seen in one “band” (color) the one time it happened to be at that location, before whizzing along the sky to a different location.

I had a fun day tracking it down and it appears to be a failure mode of the first data processing pipeline that calibrates the data. This operates on the data before most astronomers (like me) start to play with it and analyze the data. Unfortunately, that pipeline seems to think that two stars, that are moderately faint, approximately equal brightness, and with a specific separation look like a cosmic ray. They don’t… but the algorithm gets confused and interpolates over them thinking that they are an artifact to be removed. Furthermore, this apparently happens most often in z-band (one of the color channels in the image) as opposed to the other bands, giving rise specific green looking star in the post-processed image, even though the star is totally normal.

So in summary, a real star(s) that are artificially being made to look like they have the wrong color. A super cool failure mode!

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