November 2025 - Picture of the Month
The Lobster Claw (Sh2-157) emission nebula in Cassiopeia
This month's image is about 2.5 degrees top to bottom and the feature object, there are so many, straddles the border between Cassiopeia and Cepheus which cuts vertically through the centre of the Lobster's claw.
There are many other objects in this image, which can be seen even more clearly in the large version on David's website. We have open clusters including NGC 7654 (M52), which is obvious to the upper left of the image, and compact NGC 7510 to the right of the claw. Then there are some additional emission nebulae, the most obvious of which are NGC 7538 which that dense bright nebula directly to the right of the M52 on the opposite side of the main region of nebulosity, and the Bubble Nebula (NGC 7635) which is much closer to M52 to its lower right.
Sh2-157 itself broadly comprises two H-II regions: the northern part (Sh2-157a, SG 13 or Sim 13) is a ring nebula and the southern part is a compact H-II region which is probably quite separate and ionised by a different source.
Sim 13 surrounds the Markarian 50 (Basel 3) open cluster which contains many young and massive stars including Wolf Rayet star WR 157 (WN5), the stellar wind of which is primarily responsible for this nebula's structure. The neutral gas around the edge of this expanding ring nebula appears to be home to stellar formation, as indicated by the density of Young Stellar Object (YSO) candidates in this region.
Markarian 50 and Sim 13 are located in the Perseus arm of the Milky Way about 3.4 kpc (11,000 light-years) distant. The cluster is about 7.5 million years old and its stars are part of Cas OB2 association. Sim 13 is about 30 pc (100 light-years) in diameter and approximately 1.4 million years old.
Many of these objects are visible in relatively modest telescopes, but Sh2-157 is unlikely to be amongst them, it's an excellent imaging target as you can see. I enjoy images like David's that contain many targets within a single frame. There's such a lot to identify and research.
Reference
J. Vasquez, C. E. Cappa, S. Pineault, Multifrequency study of the ring nebula SG 13, Monthly Notices of the Royal Astronomical Society, Volume 395, Issue 4, June 2009, Pages 2045–2055, https://doi.org/10.1111/j.1365-2966.2008.14188.x https://academic.oup.com/mnras/article/395/4/2045/971449 (accessed 24 October 2025)
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