Ottawa Centre Monthly Observing Challenges

Beginning in 2007, Ottawa Centre has introduced a new item to its monthly meetings: an observing challenge object. Each month the centre will post a description of an interesting object. Members are invited to observe, sketch, photograph, or otherwise learn about this object. At the subsequent monthly meeting, an agenda item will provide an opportunity to share your thoughts, photos, sketches, or comments.

January 2007:

Deep Sky:

1. Melotte 20: Alpha Persei Association open cluster;
2. IC 2169: reflection nebula in Monoceros

Thanks go to Glenn LeDrew for suggesting our two deep-sky challenge targets for this month. Glenn also created the excellent finder charts, and offers the accompanying text to share with us his own impressions of Mel 20 and IC 2169.

If you would like to recommend a deep-sky object for our Observing Challenge series, please contact Meeting Chairman Brian McCullough at (613) 831-4932, or by email.

Melotte 20, The Alpha Persei Cluster

Lying relatively nearby at a distance of 570 light-years, this cluster is visible to the unaided eye as a dim stream of stellar sparks trailing away from Mirfak for about three degrees in the direction of Auriga. Any pair of binoculars will really show Mel 20 in its full glory. Yellowish Mirfak is an evolved F-type supergiant, having once been a blue main-sequence B-type star up until a few short millions of years ago. In the not-to-distant future it will quite likely explode as a blazing supernova. Given the cluster's age of 50 million years, it's not surprising that at least one other more massive member has already blown up, as current evidence indicates. The dozen or so of the other brightest cluster members are all blue-white jewels of spectral type B, either on or near the main sequence. They are arranged in a curious, elongated S-shape, with alpha at the NW end. The chrome orange star sigma Per is not a member, but it does add a nice touch of additional colour. Cluster members lie inside a 5 degree diameter circle, but visually the main mass of stars occupies a 2 by 3 degree ellipse. The ideal instrument with which to appreciate Mel 20 is a binocular having a field of view of 5-7 degrees. For my money, a wide-angle 10X50 offers the most pleasing view.

IC 2169, a Reflection Nebula in Monoceros.

IC 2169 is the largest of the nebulae in the Monoceros R1 complex of reflection nebulae. Mon R1 lies at the southern edge of the dark cloud Barnard 37, and is involved with the Mon OB1 association. The brightest object in Mon OB1 is the Christmas Tree cluster, NGC 2264, which is enmeshed in a large but exceedingly dim emission nebula. Mon OB1 lies about 3,000 light-years distant, which is twice as far away as the Orion Nebula.

To find IC 2169, locate S Mon, the brightest star (magnitude 4.6) in the Christmas Tree, and slew your 'scope just a tad less than 2.5 degrees due west. The star illuminating IC 2169 is eighth magnitude V727 Mon, a variable with a small amplitude of about 0.1 magnitude, located at 06h 31m 9s, + 09* 56.4'. Be prepared to see a fairly large, more-or-less evenly illuminated haze surrounding V727, but offset a little toward the south. The full extent of the nebula is 26x18 arc minutes -- close to the diameter of the Moon! However, in typical rural skies you'll likely see half that size or so.

Desktop Universe Finder chart & image below by Glenn LeDrew. Click for larger version.

For those using rich field telescopes or giant binoculars, Barnard 37 should be quite obvious as a very star-poor void some 2.5 degrees long, oriented north-south and wider at the south end. IC 2169 lies at the very southernmost extension of B37. As another aid to locating the nebula, look just south of the midpoint between two 6th magnitude yellowish stars which are separated by 1.75 degrees and oriented east-west. I found this nebula with 25X100 binoculars under less than ideal circumstances. It had not yet culminated in the south, it was seen through a small amount of light pollution, and there was some very thin cirrus cloud present. Moreover, having never thought to seek it out before, I was only roughly aware of its location. Yet with only a little scanning the soft haze was seen reasonably distinctly and could be held continuously with averted vision. IC 2169 will be more of a challenge for the Cyclopian set, i.e., those who must squint with one eye through a common telescope. Remember, two-eyed viewing affords a gain in signal-to-noise by a factor of 1.41, which translates to 0.37 magnitudes. Having seen the nebula only once through the 100mm binos, I feel a that 70-80mm bino could reveal it, while a somewhat larger one-eye 'scope of about 90-100mm should be considered a minimum aperture (but do try smaller apertures anyway!) Because it's a reflection nebula, IC 2169 will absolutely not benefit from the use of a narrow-band nebula filter, and perhaps even a broad-band light pollution filter may not improve the view. Your best bet is to seek out a very dark and transparent sky.

DSO challenge descriptions by member Glenn LeDrew.

Previous Challenge Objects

(none yet)

Header Image Credit

The icon leading to this page and image in the header are images of globular cluster M3 produced by Ottawa Centre member Albert Saikaley. This was chosen as the icon of the "Challenge Objects" section because the globulars are within the range of beginners with modest telescopes, yet advanced amateurs are still challenged by teasing out the spectacular colours and details such as those in Albert's image.

February 11, 200707