DSO of the Month
Caldwell 13
AKA: Owl Cluster, ET Cluster; NGC 457.
Position: 01 hr 19.5 min 58 deg 17 min 27 sec
Due south at 21:39 (BST) on 15 November. It is practically overhead.
Image of Caldwell 13 by HAS Member Martin Gill. Used with permission.
Having looked at several planetary nebulae and globular clusters recently, we now return to open clusters and specifically Caldwell 13. Officially known as NGC 457, this cluster in Cassiopeia has been given several informal names, such as the Owl Cluster, the ET Cluster (after the character the film “E.T. the Extra-Terrestrial”), the Kachina Doll Cluster, and the Skiing Cluster. These names are based on the fact that the cluster appears to have two “eyes”, which in fact are the stars Phi1 and Phi2 Cassiopeiae (at the bottom left in the above image). None of these names are at all old, although the cluster itself was discovered by William Herschel in 1787. It appears that David Eicher was the first to at least popularise the name “Owl Cluster” in about 1987, although this name could lead to confusion with the Owl Nebula, M97 in Ursa Major. The name “Kachina Doll Cluster” was invented by the Saguaro Astronomy Club, probably in the 1990s. I cannot find the origins of ET Cluster, but it cannot date before the film which came out in 1982 and it seems to have been first used in the 1990s. The Skiing Cluster appears to be of very recent origin. Other more formal names for the cluster include Collinder 12 and Melotte 7. Phi1 is almost certainly not part of the cluster, ironically, as it is only 2,000 light years away, whereas the cluster is almost 8,000 light years distant. This relatively young cluster, only 21 million years old, does however contain about 40 stars, many of which can be seen in a small telescope. The cluster is roughly half the size of the Moon and is just below naked eye brightness so you will definitely need binoculars or a telescope to see it in Havering. It is directly below Gamma Cassiopeiae and just below the line between Epsilon and Delta Cassiopeiae (Ruchbah). It is also on the same level as another star cluster M103. M103 is to the west of Ruchbah and Caldwell 13 is to the east of it, but further away. Although M103 has the cachet of being a Messier object, Caldwell 13 is generally considered to be more impressive visually. The red supergiant variable V466 Cassiopeiae with a magnitude of 8.7 can be seen near the centre of Caldwell 13, but once again at just under 1,700 light years, it cannot be part of the cluster.
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