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The constellation of Cancer is small, dim and really doesn’t resemble a crab (or a lobster for that matter) – but it is one of the twelve constellations of the zodiac. Positioned on the ecliptic plane, Cancer is positioned between Gemini to the west and Leo to the east, Lynx to the north and Canis Minor and Hydra to the south. It consists of five main stars and has 76 Bayer/Flamsteed designated stars.
In mythology, Cancer was part of the Twelve Labors of Hercules. While Hercules was busy fighting the multi-headed monster, Hydra, the goddess Hera, who did not like Hercules, sent the Crab to distract him. Cancer grabbed onto the hero’s toe with its claws, but barely breaking the rhythm of his great battle with Hydra, Hercules crushed the crab with his foot. Hera, grateful for the little crustacean’s heroic but pitiful effort, gave it a place in the sky. The crab did not win, so the gods didn’t give the crab bright stars.
For small telescopes, start off with Alpha Cancri – Acubens. Positioned right on the ecliptic, this white A-type main sequence dwarf with an apparent magnitude of +4.26 can often be occulted by the Moon, but right now keep an eye out for its companion, ? Cancri B, is an eleventh magnitude star located 11 arc seconds away. From studying its light curve during occultation, it is thought that ? Cancri A may itself be a close binary, consisting of two stars with similar brightness and a separation of 0.1 arc seconds.
Now, grab your binoculars and head over to Beta Cancri. Al Tarf is actually the brightest star in Cancer and about 660 times brighter than our Sun. This K-class orange giant star is about 290 light years away from Earth, and if you take a look through a really big telescope you might find out that 29 arc seconds away there’s a 14th magnitude star that joins it. It is so far away – about 65 times the distance of Pluto from the Sun – that their orbital period is at least 76,000 years!
Now, shift your binoculars north towards Delta Cancri – orange giant star approximately 136 light-years away. Its name is Asellus Australis which in Latin means “southern donkey colt”. If you ever need a laugh during an outreach program, you may simply refer to it as the “southern ass”. A bit further north brings you to Gamma Cancri – a 158 light year distant white A-type subgiant named Asellus Borealis or… you got it… the “northern ass”! What’s so important about these two stars? If you didn’t notice before, look between them. Here is where you will find spectacular open cluster, M44 – also known as the “Beehive Cluster”.
Messier 44 is the nearest open cluster of its type to our Solar System, and it contains a larger star population than most other nearby clusters. Under dark skies the Beehive Cluster looks like a nebulous object to the unaided eye; thus it has been known since ancient times. The classical astronomer Ptolemy called it “the nebulous mass in the heart of Cancer,” and it was among the first objects that Galileo studied with his telescope. The cluster’s age and proper motion coincide with those of the Hyades stellar association, suggesting that both share a similar origin. Both clusters also contain red giants and white dwarfs, which represent later stages of stellar evolution, along with main sequence stars of spectral classes A, F, G, K, and M. So far, eleven white dwarfs have been identified, representing the final evolutionary phase of the cluster’s most massive stars, which originally belonged to spectral type B. Brown dwarfs, however, are extremely rare in this cluster, probably because they have been lost by tidal stripping from the halo.
But don’t put away your binoculars just yet, go back to Alpha and move slowly west until you spy the stellar cloud of open cluster M67. Put a telescope on this one and be blown away! M67 is not the oldest known galactic cluster, but there are very few in the Milky Way known to be older. M67 is an important laboratory for studying stellar evolution, since all its stars are at the same distance and age, except for approximately 30 anomalous blue stragglers, whose origins are not fully understood. M67 has more than 100 stars similar to the Sun, and many red giants. The total star count has been estimated at over 500. The cluster contains no main sequence stars bluer than spectral type F, since the brighter stars of that age have already left the main sequence. In fact, when the stars of the cluster are plotted on the Hertzsprung-Russell diagram, there is a distinct “turn-off” representing the stars which are just about to leave the main sequence and become red giants. As the cluster ages, the turn-off will move down the main sequence. It appears that M67 does not contain an unbiased sample of stars. One cause of this is mass segregation, the process by which lighter stars (actually, systems) gain speed at the expense of more massive stars during close encounters, which causes the lighter stars to be at a greater average distance from the center of the cluster or to escape altogether.
For telescopes, try your hand at magnitude 11 spiral galaxy NGC 2775. Positioned some 60 million light years away, NGC 2775 is a peculiar blend of spiral galaxy with a smooth bulge in the center. According to NOAO data, the spiral pattern starts very abruptly outside of this region showing a mind-boggling complexity (especially at higher resolutions). The star formation is confined to this ring of tightly wound arms. Indeed this galaxy has been host to 5 supernovae explosions in the past 30 years!
For larger GoTo telescopes, take a look at DX Cancri (RA 8 29 49 Dec +26 46 37 begin_of_the_skype_highlighting +26 46 37 end_of_the_skype_highlighting). While it is quite faint at magnitude 14, it sure isn’t without pizazz. DX Cancri is a cool red dwarf star that is less than 9% of the mass of our Sun. It is a flare star that has intermittent changes in brightness by up to a five-fold increase. This star is far too faint to be seen with the naked eye, even though it is the 18th closest star system to the Sun at a distance of 11.82 light years, and is the closest star in the constellation Cancer.
Now set your mark on 55 Cancri (RA 8 52 35 Dec +28 19 59). Also known as Rho1 Cancri, here we have a binary star approximately 41 light-years away from our own solar system… and a whole solar system of its own! The system consists of a yellow dwarf star and a smaller red dwarf star, separated by over 1,000 times the distance from the Earth to the Sun. As of 2007, five extrasolar planets have been confirmed to be orbiting the primary, 55 Cancri A (the yellow dwarf). The innermost planet is thought to be a terrestrial “super-Earth” planet, with a mass similar to Neptune, while the outermost planets are thought to be Jovian planets with masses similar to Jupiter. The 55 Cancri system is as of September 2008 the only planetary system known to have five planets, and may possibly have more. 55 Cancri A is ranked 63rd in the list of top 100 target stars for the NASA Terrestrial Planet Finder mission.
There is only one meteor shower associated with the constellation of Cancer. The peak date for the Delta Cancrids is on or about January 16th. The radiant, or point of origin is just west of Beehive. It is a minor shower and the fall rate averages only about 4 per hour and the meteors are very swift.