The Illusive Crab

If I were to hand the average person a star chart of the constellation Cancer the Crab and asked them to find it in the sky, I am sure they would be hard pressed in identifying it. Unlike bright celestial patterns such as Orion, the Big Dipper and so on, Cancer is not the easiest to recognize. However, to the seasoned astronomer who knows the sky like the back of their hand, Cancer is flanked with the Gemini Twins to its west and Leo (Major) the Lion to its east. Both of these bordering constellations possess bright suns.

So why bother to locate this less than impressive constellation? The only redeeming object the Crab has as a tenant is M44. Commonly known as the Beehive cluster, M44 is a beautiful wide open cluster that is best seen in binoculars. At about 570 light-years away, M44 is one of the closest open clusters to us. Astronomers estimate its age to the tune of 600 million years. Back in the day, Galileo reported seeing about 40 stars but with today’s telescopes, the number seen is now in the hundreds.



With Cancer situated on the ecliptic and a member of the astrological zodiac, planets from time to time slide through its boundaries as they orbit the Sun. Such is now the case with the Red Planet. On February 4th, Mars passed to the north of the Beehive by a little less than three degrees. Mars is still moving westward (retrograde motion) and will slow down and become stationary on March 10th. After that date, it then moves eastward and passes M44 by twice the width of the full moon on April 16th. Before it does that, Mars will scoot ten arc minutes or a third the width of the full moon, south of the 13th magnitude elongated galaxy NGC 2577 on March 31st.

In all constellations, stars with the brightest to faintest values or magnitudes, follow the Greek alphabet in descending order such as Alpha, Beta, Gamma, Delta etc. Such is not the case with magnitude 4.3 Acubens – deemed the alpha star. Is does not even rank second. Somehow Acubens stands behind magnitudes 3.8 Beta, 4.2 Iota and 4.2 Delta. But this class A star has strange absorption readings indicating a "metallic line" or making it an "Am" star.

Acubens is also a double star system with its companion is a mere 0.1 arc second from the primary star. The two suns are very close together – comparable to the distance of our Sun and Jupiter. When the Moon occults Acubens, the star does not vanish instantly but fades over a very short time frame.

Beta Cancri – aka Al Tarf is 290 light-years and shines 660 times that of our Sun. It is a class K giant star that would extend more than halfway to planet Mercury. Now that is an orange giant, astronomers are now sure at what stage it is at but for certain is coming to the end of its life. Al Tarf rotates very slowly, in the order of one revolution every two earth years. By comparison, our Sun spins once every twenty-five days at its equator and about thirty days at its polar regions.



Positioning your telescope two degrees to the west of Acubens to be rewarded with a superb star cluster catalogued M67. As large as the full moon and hovering around the naked limitation, this magnitude 6.1 swarm of about 500 stars is estimated to be around four billion years old. It is believed that about one hundred stars are like our Sun in its physical properties as well as a couple of hundred white dwarfs.

The Beehive and M67 are the only two open clusters residing within the borders of Cancer. But there are a few great looking galaxies to locate, such as NGC 2775 situated to the lower-left section of the constellation. Located some 60 million light-years from us, this galaxy has produced five supernovas in just the last 30 years. NGC 2775 has a delicate arm structure that shows a few areas of star formation.

Next, we will look at NGC 2750. This peculiar-looking galaxy is listed at 125 million light-years away and is a faint, face-on galaxy that is almost triangle shaped. It appears that one of its arms is distorted or bent semi-straight.

Located a little north and west of Mars is NGC 2535. This is another face-on galaxy whose two faint main arms are wide open with the southern one a bit brighter. Again, many star-forming regions can be glimpsed in this magnitude 12.7 object. NGC 2536 located south is not elongated.

The planet Venus is climbing up the western skies. Like tag team wrestlers, Venus and Jupiter tagged and Venus entered the ring as Jupiter is bowing out. On the night of March 3rd, Venus will slide south of Uranus by 41 arc minutes. This might pose a bit of a challenge as Uranus will only be only 12 arc minutes from the Sun.

Saturn is now rising about an hour after local sunset and on March 1st, it will be eight degrees for the north and east of the moon.

Time change occurs on March 14th as we spring ahead one hour in most time zones. Six days later on the 20th, the Spring Equinox officially begins at 1: 32 p.m. EDST.







Open cluster


RA:08h 40m  6.0s  Dec:+19d 59'


Open cluster


RA:08h 50m 24.0s  Dec:+11d 49

NGC 2535



RA:08h 11m 12.0s  Dec:+25d 12

NGC 2577



RA:08h 22m 42.0s  Dec:+22d 33'

NGC 2750



RA:09h 05m 42.0s  Dec:+25d 26

NGC 2775



RA:09h 10m 17.9s  Dec:+07d 02


This month’s article marks my 50th in the Sky This Month series.

Till next month, clear skies everyone.

Gary Boyle

eNews date: 
Sunday, February 28, 2010