Constellation of the Month - Cancer and Canis Minor
Albert Brakel
Cancer
Cancer the Crab is a faint constellation between Gemini and Leo, but it is not hard to find because at its centre is the
Beehive Cluster (or Praesepe), which is so obvious as an area of haze that it has been known from ancient times. In mythology,
Cancer pinched the toes of Hercules as he was dealing with the multi-headed Hydra (last April's COTM), and ended up being
crushed. Canis Minor, the Little Dog, lies to the SW of Cancer, and is easily recognizable by its bright luminary Procyon. The
constellation is usually considered to represent one of Orion's hunting dogs, but it has also been regarded as the dog of Diana
or Actaeon, among others.
Let's start at the most obvious deep sky object, M44, the Beehive Cluster (RA 8h 40.1m, Dec +19 59'). Here is
something too big for a telescope, and more suited to binoculars or a finderscope. Even at a low 50x it did not fit into the
field of view of a C8. In 7x50 binoculars it was resolved, but rather small. The brighter stars occupying the central region
form patterns of trapesiums and triangles. The brightest member is 6th mag. Epsilon Cancri, a white star; other bright stars
are yellowish or yellow-orange.
Eight degrees to the SSE of M44 lies M67 (8h 50.4m, +11 49'), Cancer's "other" open cluster, that was just detectable
with a 6x30 finder or 7x50 binoculars. The main group occupied about half the diameter of the field of view at 77x. It appeared
as a rich group of completely resolved faint stars, the brightest (on the N side) being golden yellow.
In the other direction, 9 degrees N of M44, Iota Cancri (08h 46.7m, +28 46') is an orange-yellow and pale blue binary
that rivals Albireo in Cygnus. It is a joy to behold, and has to be in any list of the top 10 binaries. At a wide separation of
30", it is very easy even with low powers and small apertures. The magnitudes are bright at 4.2 and 6.6.
1.3 deg ESE of Iota Cnc is Rho-1 Cnc (also known as 55 Cnc) (8h 52.6m, 32 28.5'), a yellow undistinguished star, but
this is what the Sun would look like from 40 light years away. It is so Sun-like that it even has a Jupiter-sized planet (0.8
Mj) orbiting it, at a distance of only 0.1 AU.
About 2 deg. NE of Iota, a 6.0 and 6.5 mag. pair of orange stars form 57 Cnc (8h 54.2m, +30 35'), with a separation
of 1.5". As with all doubles closer than 2", the seeing is critical. I could only detect an elongation to the NW with 235x.
The same distance farther NE brings us to 66 Cnc (9h 01.4m, +32 15'), a pair of white mag. 6.0 and 8.1 components at
a less difficult 4.5" apart. By accident I stumbled across another double, STF 1294, about 4' E of nearby Sigma-2 Cnc,
and comprising a yellow primary and a colorless secondary slightly NW and about 15" apart (mags 9.9 and 10.9). Averted vision
helps to see the secondary.
Almost 5 deg. ESE of Iota, Phi-2 Cnc (8h 26.8m, +26 56') is an equal white pair (mag. 6.3), 5.2" apart and easily
resolved. A further 5 deg. E, on the border with Gemini, Struve 1177 (8h 05.6m, +27 32') is also not too difficult (mags. 6.6
and 7.5, 3.5").
Zeta Cnc (8h 12.2m, +17 39'), 7 deg. ESE of M44, appears as a beautiful yellow pair 6.4" apart. The brighter member
is actually double itself, but is only just past minimum 0.6" separation, and can only be resolved with 20 cm under excellent
seeing. Smaller scopes can't split it yet, but it will continue to widen until 2020. There is also an unseen fourth component,
probably a white dwarf.
On the border with Hydra, near Delta Hydrae, Struve 1245 (8h 35.9m, +6 37') is easily spilt with small apertures
(10.3" separation). It is an unequal yellow binary, of mags. 6.0 and 7.2.
For something different, try the mag. 10.3 Sa galaxy NGC2775 (9h 10.3m, +7 02'), also near the border with Hydra. It
appears with a fairly bright, tight core, and the fact that it could be seen even in the suburban skies of Downer is testament
to its brightness. (Let's hope it stays visible, and that the Outdoor Lighting Reform Action Group has success against light
pollution).
Canis Minor
Canis Minor, I'm afraid, doesn't have much to offer. Procyon's mag. 11.3 white dwarf companion is impossible to see with
amateur instruments in the overpowering glare of the mag. 0.4 primary, and I didn't have any luck with the other two doubles
there despite trying for about a dozen nights. Struve 1126 (7h 40.1m, +5 14'), 12' E of Procyon, comprises mag. 6.4 and
6.7 stars of spectral type A, but at 0.9" they need a steady night for resolution.
Eta Canis Minoris (7h 28.1, +6 57'), although with components 4.0" apart, suffers from a huge magnitude difference
(5.3 and 11.1), making this one for the eagle-eyed. Just to its NE, however, is the open cluster NGC 2394, which is such
a loose grouping that Hartung did not believe they were physically associated. An even wider scattering of stars continues to
its NE.
Well, that's it for this month. Galaxies have not featured prominently in this column so far this year, but that will be
remedied next time, when Peter Ogilvie boldly takes us deep into the heart of their megaparsec habitat.
|
