Sunday, June 14, 2015

The Crab in the Sky

Cancer, the dimmest constellation in the Zodiac

The crab is one of the 12 constellations of the Zodiac. The zodiacal constellations are located along the ecliptic, a path in the sky where also the Sun, the moon and the planets travel around the year. By following the pattern proposed by H.A. Rey, this constellation certainly looks like a Crab once the 6 brightest stars are connected. The challenge lies in being able to see those stars, as they are not very bright in apparent magnitude. Asellus Australis or δCnc has an apparent magnitude of 3.90 and is the brightest star in the constellation, but even that is not enough to be seen with the naked eye under a Bortle 8-9 sky.

Even though the crab appears invisible under severely polluted skies, in its entrails there is one of the brightest DSOs of the entire Messier list: M44. At the beginning of my in amateur astronomy, M44 was one the first objects I observe by accident as I did not know it was a Messier object.

But how to find M44 if not even one of the Crab’s stars is visible? With a pair of Binoculars it is not a difficult task. The crab is located between Leo and the middle of Gemini heads –Castor and Pollux.  M44 is easily found by following a path from Pollux in a straight line to Regulus, and it is located at approximately half the distance between both stars.

When I processed this sketch, I had to re-size it because I realize my 2012 sketch did not match with the wide field of view in the binoculars. I performed this sketch in company of a setting waning moon, from a Bortle 9 sky in Bogotá, Colombia.

The most distinct group of stars I found in the Beehive cluster was the quadrilateral figure formed by the double star HIP 42549, ε Cnc, EP Cnc and HIP 42578; which was decorated by a small triangle in which the star HIP 42497A was part of the upper apex. 

The beehive is perhaps the best cluster known in this constellation but the crab has something else to offer for the followers of the Messier list: M67. It is located west of the star Acubens (αCnc). With an apparent magnitude of 6.90, this open cluster seemed not big enough to be resolved in the binoculars.

During this most recent observation, I counted with acceptable atmospheric conditions and no moon from a Bortle 5 sky. Transparency was 3/5, seeing 3/5 and the constellation was at its zenith. Jupiter was transiting the area and I had the perception to have seen M44 as a faint smudge.

From the about 500 components of this cluster, I could only see what I thought was HIP43465 with a closer star of apparent magnitude 9.80. This duplet seemed like a blurry star at one of the edges of the cluster and much dimmer than HIP 43519, on the opposite side of the cluster.  The group of stars seemed more like a fuzzy nebulous object, better seen with averted vision. I also saw a dim and fuzzy spot of light around the lowest star in the cluster.

This open cluster difficult to resolve with low power offered a similar view of other open clusters like M36, M37 and M38. But it is worth it to enjoy the view of what science estimates as one of the oldest known open clusters: 3.2 to 5 billion years old.

I hope you have enjoyed this short review of such a nice constellation.


Edited by Jennifer Steinberg (editor in chief)


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