I've always been interested in M29. It's a simple, beautiful little cluster in a constellation (Cygnus) cluttered with interesting things.
I didn't realize that controversy existed over which stars belonged to the cluster. It turns out that some of the bright stars above---in what look like a cluster---do not! The problem is that the stars are too far away to measure their proper motion across the sky. Also, dust extinguishes light along our line of sight by several magnitudes. Without knowing the stars' intrinsic brightness, their distance is harder to infer.
But a study published in 2014 helps resolve these concerns. The study carefully compares the spectra of the stars with their brightness and the brightness of stars in the local vicinity to better estimate their distance and the effect of dust (another, larger 2015 study by many of the same authors examined extinction in the area of the cluster). The 2014 study estimates that at least 15 stars lie together at 2.2 kpc, about 7,100 light years.
The stars numbered in this image (linked here from the study) are probable cluster members:
Some of the brightest stars in the cluster fail the test. The white star in the top row (HD194378) and another bright star in the bottom row (HD 229238) are probably not cluster members. The study reminds us that space is a busy place full of unexpected surprises; sometimes line of sight alignment tricks us into thinking items come as a unit.
My image is 22x180" with the Atik460EXC through the 203mm Synta ONTC Newtonian at f/4.95 and an Astronomik CLS filter.
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