This fourth constellation of the zodiac is centred on the ecliptic is a faint constellation but easy to find in our north-western sky about April.
In ancient Greek mythology the friendly Crab was sent by Juno, Queen of the Heavens to pinch the feet of Hercules while he was slaying the seven-headed serpent Hydra. Hercules crushed the crab and the Queen of the Heavens placed it in the sky as the constellation Cancer the Crab.
On a dark night, locate Jupiter in the north-western sky with the naked eye, with some concentration the faint stars alpha, beta, delta and iota should be visible (see diagram). The brightest star of the constellation is Beta (β), a mag. 3.5 orange giant.
This is an excellent constellation to explore with binoculars or a small telescope…this is what I can see with my 7 x 50 binoculars and 78mm (3 inch) Takahashi refractor.
Asellus Australis (δ delta), named the Southern Donkey by the Romans, can be seen with binoculars as delta is a nice mag. 3.9 yellow star.
Between Delta Cancri (δ ) and Gamma Cancri (γ), the Northern Donkey (Asellus Borealis), is M44 (NGC 2632) The Beehive Cluster or Praesepe, visible with the naked eye. The Beehive has a “swarm” of approximately 200 stars between mag. 6 to 14, with 15 stars between magnitude 6.3 & 7.5 making it visible with the naked eye as a small cloudy region. This object is best viewed at low power as all the stars of the mag. 3.1 open cluster, covering approximately 1.5˚ of the sky will then fit into the field of view. With binoculars the cluster is brilliant lying in a dark portion of the sky off the main line of the Milky Way.
M44 is a prominent open cluster of stars. Nicknamed Praesepe and “The Beehive”, it is one of the few open clusters visible to the unaided eye. M44 was thought to be a nebula until Galileo used an early telescope to resolve the cluster’s bright blue stars. These stars are visible in the above image. M44, which is thought to have formed about 400 million years ago, is larger and older than most other open clusters. The Beehive Cluster lies about 580 light-years away, and spans about 10 light-years across. When viewed with a powerful telescope, hundreds of stars become visible.
My best view of M44 was with my 78mm telescope at 24x magnification with a 26mm Plossl eyepiece. Jupiter was visible with it’s 4 moons Io, Europa, Ganymede and Callisto at the edge of my field of view. Three bright triangles were visible within the cluster, with a multitude of stars surrounding them. Increasing the magnification to 37x with a 17mm Plossl eyepiece brought out 2 dark gas belts of cloud on Jupiter and a better view of the triangle star patterns of M44, but the complete cluster was no longer visible in the field. At even greater magnification 210x with a 6mm eyepiece and 2x Barlow more detail of the bands on Jupiter was evident and the Great Red Spot was visible as a pale greyish – white indent in the dark southern hemisphere belt. M44 was lost at this magnification.
I also had a look at M44 with our Celestron 8 (200mm) telescope at the lowest magnification available 51x with a 40mm eyepiece and the cluster doesn’t all fit into the field of view….so bigger telescopes are not always better!
Next I visited Alpha Cancri (α) – Acubens, the claw. It is a very faint double star at mag 4.2 it is just visible with the naked eye. I could not split the double as the companion star of mag 12 requires greater than 78mm to resolve…can you split it with your scope?
2˚ west of alpha you should see a very faint fuzzy spot, the faint mag 6.9 open cluster M67 (NGC 2682). The cluster has approximately 300 stars of mag 10 – 16 and with my binoculars and finder scope was a small nebulous spot. With the telescope at 105x with a 6mm eyepiece it appeared as a medium brightness scattered cluster and the stars were not well resolved.
Certainly not as impressive as M44 which is much closer at 515 light years, M67 is 2600 ly away.
One of the oldest known open star clusters. The stars of M67 are possibly around 4 billion years old, about the same age and with about the same elemental abundances as the Sun. Open clusters are almost always younger because they are dispersed over time as they encounter other stars, interstellar clouds, and experience gravitational tides while orbiting the center of our galaxy. Still, M67 contains over 500 stars or so and lies some 2,800 light-years away in the constellation Cancer. At that estimated distance, M67 would be about 12 light-years across.
My last stop for the night was with Zeta Cancri (ζ) a multiple star. I could just split the yellow binary pair of mag 5.1 and 6.2 at 37x magnification using a 17mm Plossl. They were nicely split at 63x with a 10mm eyepiece. The brighter of these stars can be split into a tight binary of mags. 5.6 & 6.0 with a telescope larger than 200mm (8 inch). I couldn’t split them with the Celestron 8. The stars are moving apart and a 150mm scope should split them by the year 2005.
Iota (ι) Cancri is also a double star mag 4.0 yellow giant with a blue – white companion mag 6.6, I didn’t try splitting these but they should be split with a small telescope.
I hope you enjoy exploring Cancer as even faint constellations hold interesting objects to observe.
I would like to thank William Simpson in advance as he will be writing up the constellation of the month on alternating months beginning next month.
References: Collins Pocket Guide Stars & Planets, Ian Ridpath, Ian Tirion; The Messier Objects, Stephen O’Meara; Hartung’s Astronomical Objects for Southern Telescopes, Malin & Frew