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Southern Cross - November 2000

• Constellation of the Month - Aries & Triangulum
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Constellation of the Month - Aries & Triangulum

Albert Brakel

Aries the Ram is a small zodiac constellation, whose appearance in our evening sky marks the onset of summer. The only conspicuous thing about its appearance is the trio of its three brightest stars, which are magnitudes 2, 3 and 4 respectively. It appears to be of very ancient origin, and one theory has it that it was adopted into the zodiac by the Babylonians when its stars began to mark the vernal equinox. To insert it between Taurus and Pegasus, they had to cut off part of each; if correct, this may explain why both Taurus and Pegasus now represent only the front parts of the bull and winged horse respectively.

In Greek and Roman mythology, the ram was sent by the mother of Phrixus and Helle so that the children could flee on its back from the wrath of their wicked stepmother, Queen Ino of Boeotia. On the way, Helle fell off into the sea, which became the Hellespont. At journey's end in the land of Colchis, Phrixus sacrificed the creature to Zeus, and hung its fleece in the sacred grove of Ares, where it shone like the Sun and was guarded by a dragon. It became the object of the quest by Jason and the Argonauts, because an oracle had prophesised that Jason would reclaim his rightful throne if he brought back the fleece. This eventually happened, but afterwards the fleece lost its lustre and was placed in the sky by Zeus, to be the inconspicuous constellation we see today.

So what objects does Aries hold for the telescope? Essentially galaxies and double stars. A good starting point is Gamma Arietis (RA 1h 53.5m, Dec +19 18'), one of the finest examples of an equal white binary, with components of mag. 4.6 and 4.7 separated by 7.7". It was one of the first double stars known, found in 1664 by Robert Hooke while searching for a comet.

Not far away, forming a right angle with Gamma and Iota Arietis, is the brightest galaxy in Aries, NGC 772 (1h 59.3m, +19 01). It is a mag. 10.3 spiral, measuring 7' x 4.5', but how large you see it will depend on your aperture size and the conditions. With the 14" in the CAS dome and a quarter Moon in the sky, only the nucleus could be seen and even that was barely detectable. It is important for this and all the galaxies that follow that you have a dark sky free of haze. With everything in your favor, you can see the galaxy's brighter northwestern arm with a 10" scope, and the soft glowing blanket tucked around the central hub. About 5' to the south is the 1.3'-diameter elliptical galaxy NGC 770 (1h 59.2m, +18 57'), but with a magnitude of only 14.1 this is big scope territory.

NGC 821 (2h 08.4m, +11 00'), near the Cetus border, is the next-brightest galaxy (mag. 10.8). It's another elliptical, and shows up as a grey oval 3' long.

NGC 877 (2h 18.0m, +14 33'), an almost face-on spiral, is revealed as a 1.5' patch of dim luminosity rising slightly to the centre, with a star on its SE side. The challenge for large instruments is to find the mottling across the galaxy's face.

In the NE of Aries, near the border with Triangulum, lies NGC 972 (2h 34.2m, +29 19'), an obliquely-inclined spiral almost twice as long (3.6') as it is wide. It glows at magnitude 11.3, and rises in brightness towards its core. A faint double star lies to its south.

Extend a line from NGC 972 to 41 Arietis and beyond to 2h 59.7m, +25 14', and you can locate the irregular galaxy NGC 1156, lying in a field well-sprinkled with stars. The mag. 11.7 galaxy consists of a central oval of light with a fainter irregular halo that evinces a grainy to mottled texture if the atmospherics are in your favor.

While in the neighbourhood of 41 Ari, have a look 1.3 deg. ESE of it to find the double star Struve 326 (2h 55.6m, +26 52'). The two components, a yellow mag. 7.6 and a colorless mag. 9.8, lie an easy 5.9" apart.

There are some other doubles in Aries that will reward the effort to check them out. Struve 174 (1h 50.2m, +22 17') is yellow and white, mag. 6.2 and 7.4, with 2.7" separation.

Pi Ari (2h 49.3m, +17 28') shows three whitish stars almost in line. The mag. 5.2 and 8.7 A and B stars lie at a distance of 3.3", while faint mag. 10.8 C is 25.2" away. The brightest member is itself a spectroscopic pair, with a further invisible component discovered during a lunar occultation.

Epsilon Ari (2h 59.2m, +21 20') is an almost equal pair of mag. 5.2 and 5.5 whitish stars (Hartung describes them as pale yellow). At 1.5" separation, they need steadier seeing to resolve.

The constellation of Triangulum is located on the NW side of Aries, and is also fairly small, consisting of a narrow triangle of 3rd and 4th mag. stars. There is no mythology associated with it, although in antiquity it was apparently more equilateral in shape, and attempts were made to liken it to the Nile delta and the island of Sicily. There was also once a smaller Triangulum Minor situated immediately to the south, that fell into disuse long ago.

The big feature of Triangulum is of course the large face-on spiral galaxy M33 (1h 33.9m, +30 39'), the third-largest galaxy in the local group. Although about 80' across, more than twice the diameter of the Moon, it has a low surface brightness, and can be hard to find. The most eagle-eyed observers claim to have seen it with the unaided eye, something I have failed to achieve even in the perfectly dark and clear skies of Arnhem Land and central Australia.

With optical aid under those very favorable conditions, I have easily seen it as very bright with 7x50 binoculars, yet on another occasion, again in a very dark sky, I could barely find it with 10x80 binocs. It seems to be very sensitive to atmospheric conditions. Telescopically, you can on a good night make out the ghostly spiral pattern, the tiny nucleus, knots of dark dust clouds and bright star-forming regions, especially the star cloud that has its own designation of NGC 604. NGC 604 lies 11' northeast of the galactic core, and contains enough material to make a small galaxy by itself. Because it is also an emission region, a filter allowing through the hydrogen or oxygen III spectral lines will enhance it.

NGC 672 (1h 47.8m, +27 26'), about 2.5 deg. south of Alpha Trianguli, is often overlooked, e.g. by Hartung. It forms a mag. 11.6 oval of light 4.5' x 1.7' in size with a brighter centre, and is in fact a barred spiral. Large apertures may be able to detect some central mottling.

Finally, 6 Trianguli (2h 12.4m, +30 18') is a lovely, bright and easy, golden yellow binary. The mag. 5.3 primary has a mag. 6.9 companion located 4.0" at PA 070 deg.

I hope that this selection motivates you to have a go at Aries and Triangulum under a dark sky, and that you have better weather than I had recently.