I recently did a spot of observing with my trusty 20x60mm binoculars. It was also an opportunity to reacquaint myself with some of the familiar celestial sights after a winter of little observational activity.
Just past the meridian, as soon as darkness falls during April evenings, is the zodiacal constellation of Cancer, the Crab. Though dim, and literally a footnote in the legend of Hercules (the mythological strongman crushed the hapless crab underfoot during his battle with the multi-headed Hydra), Cancer nevertheless holds a number of delightful objects for the binocular user.
The Crab’s pattern is not readily apparent at first but it is usefully flanked by the prominent constellations of Gemini and Leo – the former presently hosting the planet Saturn, the latter containing Jupiter. Only from a dark site though, will you begin to trace the Crab’s outline as just two stars are above fourth magnitude.
The brightest is b Cancri (+3.59), or Al Tarf (“the End”), an orange giant that is estimated to be fifty times the diameter of our Sun. It is also a double, although the fourteenth magnitude companion is well beyond the range of binoculars. The physical separation between the two is quite large – on the order of 2,600 astronomical units – while their orbital period is reckoned to be about 76,000 years!
a Cancri, (+4.25), or Acubens (“the Claw”), is ranked only the fourth brightest star in the Crab and is a pure white sun of spectral class A. Interestingly enough, it is a tight spectroscopic double with both stars of equal luminosity separated by just 5 astronomical units. The presence of a nearby faint twelfth magnitude red-dwarf companion that is itself a double makes for a rather interesting quadruple system.
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Acubens is also the jumping off point for our venture into deeper space to find the ancient galactic cluster, M67. Keeping the star towards the left (eastern) edge of the field of view (3°) in the 20x60s, the duster is seen as an elongated mottled haze about two degrees to the west. On my second evening out observing the area, I was able to see a sprinkling of the brighter 9th magnitude members of this association while an unrelated 8th magnitude sun lay just outside the northern edge of the group.
M67 is considered to be one of the oldest galactic dusters known, with an estimated age of four billion years. This poses the question as to how its members have remained gravitationally-bound over such a long period of time. Most open star dusters are disrupted after a few hundred million years following encounters with other clusters or by successive passages through giant interstellar gas clouds during their circuits around the galaxy. The key to the survival of M67, however, lies in its own orbit that brings it to a great distance (1,500 light years) high above the plane of the Milky Way.
The vast majority of open star clusters are spawned amongst the great clouds of gas and dust that are concentrated within a few hundred light years of the galactic plane. M67, on the other hand, is well removed from its original nursery as its orbit takes it high above the disk leading to less frequent adverse encounters with its kin. The age of the cluster’s stars is similar to that of our Sun and, indeed, its members happen to have a similar chemical composition as the Sun.
One of the most celebrated objects in the sky is Praesepe, or M44, the Beehive star duster. It is visible to the naked-eye as a spot of light framed by d, g, h, and q Cancri, makng up the “body” of the Crab. The proper names for g and d Cancri – Asellus Borealis and Asellus Australis – come to us from Greek myth where they represented the donkeys on which the gods Dionysos and Silenus rode into battle against the Titans, who were so frightened by the animals’ braying that the gods won. As a reward, the asses were put in sky together with the manger – presumably to ensure a perpetual supply of hay!
But before having a peek at the group, stop briefly at X Cancri, a dun-hued semi-regular variable that lies 2.75° east-southeast of d Cancri. The period is 195 days and the brightness fluctuates between magnitude +5.6 and +7.5. A handful of sixth and seventh magnitude stars are sprinkled across the field.
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Praesepe, or Phatne (the Manger), has been known since ancient times and the first recorded mention of it is in the great weather and sky lore poem Phaenomena by the Greek author Aratus who wrote in 260 BC:
“And when with deep-charged clouds the air’s opprest,
Phatne, the spot that shines on Cancers breast,
Attentive mark: if bright the spot appear, Soon Phoebus smiles with face serene and dear,
Nor the returning rain and tempest fear.”
Indeed, the visibility of the cluster with the naked eye has long been regarded in folklore as an indication of weather conditions. Although the sky transparency was not that good on the two nights I ventured out, Praesepe was clearly visible without optical aid but I have seen it more distinctly on occasions. It was not until Galileo turned his telescope skyward though, that the true nature of the Beehive became apparent; “The nebula called Praesepe, which is not one star only, but a mass of more than 40 small stars.”
Binoculars are the ideal instrument to examine this sprawling cluster and to resolve the stars swarming around an imaginary celestial honey pot. The brightest member of the group is e Cancri, a +6.3 magnitude star that is 70 times as luminous as the Sun. If we could place our Sun in the midst of M44, it would appear as a feeble magnitude +10.5 spark of light as seen from Earth.
I made no effort to count the number of visible members, as I was more intent on finding two doubles that reside within its boundaries – ADS 6915 (Burnham 584) and ADS 6921 (see chart of M44). Both lie towards the southern edge of the cluster as part of a sort of tipped-over “house”-shaped asterism that also reminded me of a miniature Cepheus.
A chart of M44, Praesepe
ADS 6915 is a triplet of suns of around seventh magnitude at the “roof” of the “house” with the star right at the apex of the triangular arrangement being brightest. ADS 6921 is a quadruple system but only the two more luminous members will be seen in binoculars. A fifth-magnitude satellite that sped through the field as I was observing was a nice bonus.
Recent measurements by Hipparcos place M44 at a distance of 577 light years with the cluster’s age estimated to be about 400 million years. This fixes it at the same age and distance as the equally well-known Hyades in Taurus and has led astronomers to believe that both groups were born in the same area of space long ago. A third member of the so-called Hyades superduster is NGC 1901 in the southern constellation of Dorado. The whole subject of galactic superdusters (as distinct from galaxy superclusters) offers a fascinating insight into stellar evolution and associations.
Taking our leave from M44, we now head northwards to the beautiful double star, i Cancri. The separation is 31″ and the magnitude +4.2 primary appears yellowish with the magnitude +6.6 companion glowing bluish-white. The star is set in a lovely field that shows two nice curving chains of stars tapering to a point. Again, I teased out a miniature constellation shape; this time the pattern partly reminded me of a diminutive Perseus. Another nice star field is centred on the line of a stars, tagged 1 through 4, from west to east. We finish our tour, though, with the rather unassuming 55 Cancri – a sixth-magnitude sun that is also labeled r1 on charts. The star is a member of the exclusive club of those stars recently found to posses a planetary system. Only one attendant planet is known at present though a second more massive body is suspected in the data.
The confirmed extra-solar planet orbits its parent in only 14.6 days at a distance of 16.5 million kilometres – or 28% the distance of Mercury’s orbit from the Sun. The mass is slightly less than that of Jupiter. 55 Cancri is a G-type dwarf slightly cooler than the Sun and is 41 light years away.
Before I packed up on the second night to head home I took a moment to reflect on how the tour of Cancer ranged from doubles to clusters, stellar evolution to the cutting edge of research on extra-solar planets, and finally, to the human connection with the skies above. We may have gazed at the night sky countless times, but when we become aware of the stories behind the sights, then we ourselves – and our hobby – are that bit richer.
The constellation of Cancer
Filed under: Astronomy Articles
