March 28, 2008


Today, the faintest member of the zodiac, the constellation Cancer the Crab. According to Star Tales, Cancer is a minor character in the story of Hercules. The location of Cancer in the sky is such that in ancient times, the Sun was in Cancer on the date of the summer solstice. Hence, the line of latitude on Earth corresponding to the Sun's celestial latitude is known as the Tropic of Cancer. In modern times, the summer solstice location has moved into Taurus due to the Earth's precession.

Alpha Cancri is Acubens (from the Arabic for claw), but it isn't quite the brightest star in this constellation. Problem is, the handful of relatively bright stars in Cancer are all 4th magnitude or fainter, making this constellation invisible from even the outskirts of a lighted city. This is a hot binary star, actually a quadruple system, with another tight low mass pair orbiting at a great distance. Beta Cancri is Al Tarf, which translates to "the end" as it is at the end of one of the legs of the crab. This is an orange giant star, about 700 times the luminosity of our Sun and 50 times its diameter, but it is about 300 light years away and so very faint compared to the very similar star Aldebaran in Taurus which is only 60 light years away.

55 Cancri is a very faint sun-like star near the Northern end of the crab. What distinguishes it from the other stars is the presence of several planets in orbit, five in all, including one with a rather low (minimum) estimated mass of 11 Earth masses. That's a minimum because we don't know the inclination of the orbit so it could be a much more massive planet in a highly inclined orbit. Two other stars of mythological interest are found near the center of the Crab, bracketing "the manger" of stars known as the Beehive Cluster.

These stars are Asellus Borealis and Asellus Australis, which translate quite literally to the Northern and Southern Ass. They represent two donkeys that the Greek Gods Dionysius and Silenus rode into a battle between gods and giants. The giants were so startled by the sounds of the braying donkeys that they fled the battle, and so the gods honored the donkeys by putting them in an important place in the sky.

This brings us to the obvious place to start with deep sky objects in Cancer: Messier 44, an open galactic cluster known by many names, including the Beehive Cluster and Praesepe. The name Praesepe translates from the Latin for "manger" or "crib". This designation may come from the older term "stall" meant for the two donkey stars above and below it that I mentioned previously. Its true nature wasn't known until Galileo trained his telescope on this region of the sky and resolved this faint cloud into dozens of stars (we now know with deeper images and analysis that this cluster contains over 200 stars).

Praesepe is about 600 light years away, and a little less than a billion years old, and it shares both an age and proper motion with the Hyades cluster in the constellation Taurus. Though these clusters are widely separated (by about 500 light years), there is speculation that they are linked and perhaps formed from the same collapsing cloud at the same time.

About seven degrees due north of the Beehive is the remnant of a two or three galaxy collision known as NGC 2623. This faint object shows two clear nuclei (one elongated, so maybe it is two nuclei itself) and two sweeping tidal tails. It is about 70 million light years distant, so despite being a fairly bright starburst galaxy (as a result of the merger), it is extremely faint at 14th magnitude but quite a nice looking object for larger telescopes.

Also looking from the Beehive, if you follow a line about 10 degrees East of South for about eight degrees, you will run into the other Messier object in Cancer, M67. This is classified as an open galactic cluster, and as we've seen throughout the constellation series, most such clusters by an age of 1 billion years have been so disrupted by tidal forces within the disk of the galaxy that they are nearly unrecognizable as clusters. M67 sports an age of about 4 billion years! This cluster has roughly 500 stars (including nearly 200 white dwarf stellar remnants) and is located nearly 3000 light years away. How it has survived for so long remains a mystery.

Proceeding about 5 degrees Southeast from M67, we come to the final deep sky object I'll cover, the tightly wound spiral galaxy NGC 2775, about 20 million light years distant (at least, according to the redshift, though my linked source claims 60 million light years) and so much brighter than NGC 2623 by almost four magnitudes. For some reason, this galaxy has been something of a supernova factory, with five supernovae observed here within the past 30 years. In the image, you can also see a little cluster of galaxies much more distant. Such clusters literally blanket the sky at much fainter magnitudes, as the Hubble Deep Field showed.

Posted by Observer at March 28, 2008 11:50 PM

Comments on entries can only be made in pop-up windows while those entries are still on the main index page. Sorry for the inconvenience this causes, but this blocks about 99.99% of the spam the blog receives.

Now do the essay on M67 ... a cluster perhaps second only to the Hyades in terms of being a key sample for studies of stellar evolution. Not to mention the photometric calibration field of choice for visible-band imaging.

Posted by: Feff on April 9, 2008 11:50 AM

I couldn't find a good source to tell that story. Any recommendations?

Posted by: Observer on April 9, 2008 01:48 PM