M38 is an open cluster located 3800 light years away in the constellation of Auriga at a declination of 36 degrees. It is a magnitude 6.4 cluster which spans 20 arc-minutes in our apparent view. This corresponds to a diameter of 22 light years. It is one of the oldest galactic open clusters.
Comprised of over 100 stars, this cluster has a mixture of bright blue stars and orange giants. It is somehow nicknamed the Starfish cluster, but I don't see the resemblance to a Starfish in its shape. In fact, that nickname is #14 on my top 25 Worst DSO Nickname list:
Worst DSO Nicknames Others have suggested that this cluster resembles shapes such as the Greek letter Pi, an irregular arrow and an oblique cross. The brightest star in this cluster is the yellow giant, shown towards the top of the image just to the right of center, with a magnitude almost 1000 times brighter than our Sun.
A nice band of HII emission to the lower right provides additional interest to the image. Also seen in the image, below M38, is the denser open cluster NGC 1907, located about twice as far away.
The image presents a good eye test. The faint PN Abell 7 (PK172+00.1), identified in the mouseover, is barely visible in this RGB image as a faint gray angular shape. It is much easier seen in my longer focal length narrowband image of it:
Abell 9 This faint elliptical planetary nebula is 13,000 light years away, much further away than M38. The PN spans 30 arc-seconds in our apparent view. This corresponds to the standard PN diameter of 2 light years. Like many PN, the outer rim is stronger in HII while the inner section has some OIII. A curious dimming of the outer ring occurs at the top.
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