By Daniel Acker
According to Egyptian records from about 2,000 BC, Cancer the Crab, as we know it today, was described as Scarabaeus, the sacred emblem of immortality. Much later, in Greek mythology, Cancer is associated with Hercules and Hydra, when Hercules was doing battle with the multi-headed Hydra and Hera, the disgruntled wife of Zeus, sent Cancer, the giant crab to help Hydra kill Hercules. As we all know, the mighty Hercules crushed the crab with his foot, and Hera, placed the crab in the night sky as a reward for its service.
Cancer, Latin for Crab, is a faint zodiacal constellation that lies between Gemini to the west and Leo to the east. The constellation is often lost in urban light pollution due to its lack of bright stars. Here we find that the brightest star in the constellation is not α (Alpha) Cancri, but the 3.5 magnitude star β (Beta) Cancri. α Cancri is a magnitude 4.2 class A (A5) star, meaning its spectrum displays ultra-strong absorptions of particular metals. During mid-March Cancer is crossing the local meridian around 02:00 UT and is well place for those of us at mid-northern latitudes. Cancer is poor in deep sky objects for small telescopes but it is home for two Messier clusters, M44 and M67 as well as a couple of interesting double stars.
ι Cancri
On some star atlases, ι (Iota) Cancri may be noted as S1268 (F. G. Wilhelm Struve) and the Washington Double Star Catalog (WDS) number is STF1268. ι Cancri is one of the showpiece double stars for telescopes in the 90mm and larger range. Iota is an easy target to locate simply by noting its position on your star atlas and pointing your telescope at it. The pair shines at magnitudes 4.1 and 6.6 separated by 31” (arc-seconds) in position angle (PA) 307°. 1The Cambridge Double Star Atlas states, “The Albireo of spring, Striking orange and clear-blue starry jewels!” *Here at the 90mm Observatory using the 60mm refractor at 35x (10mm eyepiece), ι Cancri is an easy split, according to our observing log of 24 January 2012 @ 03:52 UT, “The primary appears reddish, and the secondary is blue green in color.” We’re interested in learning what colors you see, so please take a moment to comment on this double star.
ζ Cancri
ζ (Zeta) Cancri, also know as S1196 is another fine double star for small telescopes and a 150mm (6 inch) reveals the third component making ζ Cancri a triple star. To locate ζ Cancri, center the star γ (Gamma) Cancri in your finder (you may notice the open cluster M44, we’ll come back to it later) and move west a little more than 8° and watch for a 6th magnitude star to appear. That should be ζ Cancri. Consult your atlas for exact position. ζ Cancri consists of a magnitude 5.3 and 6.3 stars separated by 5.9” in PA 75° and a third 6.3 magnitude component separated by 1.0” in PA 55°. All three stars appear yellow.
M44/NGC 2632
This is the famous Beehive cluster or known to some as Praesepe (Latin for “manger”). Although difficult here under the observatories light polluted skies, M44 is a naked eye object and it is easy to see from a dark sky location. Galileo reported resolving this object as “The nebula called Praesepe, which is not one star only, but a mass of more than 40 small stars.” On March 4, 1769 the French comet hunter Charles Messier add it to his catalog of objects to avoid as M44. To locate M44, center the star Asellus Australis (δ (Delta) Cancri) in your finder scope and you’ll notice M44 about 1.5° to the north. *Using the 90mm refractor at low power (34x) we listed this cluster as “Conspicuously seen cluster” in our observation log. Additional notes read, “Very sparse open cluster. Easy target with the 90mm refractor. At 34x the cluster fills the FOV. This cluster makes me think I’m looking at a group of wide double stars. About 32 stars, most of the same magnitude and blue in color.”
M67/NGC 2682
This is one of the oldest known open clusters being some 3.2 billion years old. It certainly is the oldest cluster cataloged by Charles Messier who recorded it on April 6, 1780. There are only a few open clusters found to be older then M67 and among them is NGC 188 in the constellation Cepheus listed as 5 billion years old and NGC 6791 in Lyra listed at around 7 billion years old. M67 is another easy target to locate with just a short star hop from Acubens (α (Alpha) Cancri). Center α Cancri in your finder and in the same field-of-view is M67, just a little over 1° to the north-west. *In the 152mm refractor at 54x the cluster seems to be concentrated toward one side forming a densely populated row of stars.
So, for the two double stars I mentioned, both are on the Astronomical League’s Double Star Program and the clusters are part of the Messier Program and the Urban Observing Program. These observing programs are offered by the Astronomical League and are a great way to enhance your observing skills and add to the enjoyment of backyard astronomy. If you’re not a member of the Astronomical League, I encourage you to join either through your local astronomy club or on your own as a member-at-large.
That’s going to do it for this month.
Source:
1The Cambridge Double Star Atlas.
*90mm Observatory observation logs.