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.

August 2007

Deep Sky: 61 Cygni, Sh2-132, Sh2-119

By Glenn LeDrew

During late summer, and all through autumn, the northern milky way of Cygnus and Cepheus is well placed for observation from mid-northern latitudes. Three objects are featured in this installment; one is a very easy double star, and the other two are faint emission nebulae, one being moderately difficult and the other a real challenge. The nebulae were first catalogued by Sharpless in 1959, presumably after scrutinizing the newly released Polomar Sky Survey photographic plates.

61 Cygni

This is a fascinating object, both observationally and historically. It's a nearby, high proper motion binary whose members are both variables. And it features prominently in Man's first steps to determine the scale of the Universe.

Popularly known as Piazzi's Flying Star, this star's large proper motion of 5.28 arcseconds per year was first pointed out by Piazzi in 1804. Among the stars in the Hipparcos Catalogue (ESA, 1997), 61 Cyg has the 6th largest proper motion, and ranks as the fastest of the naked eye stars of 6th magnitude and brighter. It will drift across the stellar background by an amount equal to the Moon's apparent diameter in a mere 340 years.

In the 1830s, 61 Cyg had the fastest proper motion of any star known. This suggested nearness to Earth, and naturally led to its being the first target with which to test the method of trigonometric parallax. Bessel succeeded, in 1838, in determining a distance close to the modern value of 11.4 light-years.

Less than a decade earlier, Struve realized that the pair was indeed a true binary, whose components are designated 61 Cygni A and B, the former being the brighter. Individually, the stars shine at apparent magnitude 5.2 and 6.0, but their combined light is a brighter 4.8 magnitude. Both are main-sequence stars of spectral types K5 and K7, having 70% and 63% of the Sun's mass (and roughly corresponding diameters), respectively. While they are not very much smaller than our Sun, their luminosities are only 8.5% and 3.9% of the Solar value. 61 Cyg A is a BY Draconis variable, and B is a flare star. Each star has exhibited an activity cycle with periods of about 7.5 and 11.7 years--compare this with the Sun's 11 year cycle.

Both stars orbit the common centre of mass over a period of 659 years, the separation between them varying from 44 AU to 124 AU (for comparison, Pluto is some 30 AU from the Sun). The current angular separation of the stars is 28 arcseconds, which makes for easy resolution in a 10X binocular. Being K-type stars, their colour is a prominent golden orange.

The large proper motion is the result not just of nearness, but more importantly, the very non-circular orbit about the Galaxy. This is evidenced by the rather high space velocity of 120 km/sec with respect to the Sun. It is estimated that 61 Cyg is some 10 billion years old, or fully twice as old as our solar system. Over such a long lifetime, it's to be expected that a star will have had its orbit significantly perturbed into non-circularity due to encounters with massive molecular clouds. Some two dozen stars widely scattered across the sky share 61 Cyg's space motion and thus form an extended stream, almost certainly the remains of an ancient but relatively recently disrupted star cluster.

Another reason 61 Cyg is interesting is that it's one of the very few main-sequence K-type stars visible without optical aid. Even though K and M stars on the main-sequence are the most numerous, their intrinsic faintness renders them visible only at rather close distances. Virtually every K and M star you see with eye alone is either an evolved giant or supergiant near the end of its life.

As you observe with your binocular or telescope, contemplate the fact that you are easily resolving a separation equivalent to the distance between our Sun and a Trans-Neptunian Object, or TNO. Furthermore, if 61 Cyg has an Oort cloud of comets like our solar system, its apparent diameter on the sky would be about 15 degrees!

Finally, there is a possibility that 61 Cyg A has a companion of about 10 Jupiter masses orbiting approximately 2 AU distant.

Sh2-132

Emission nebula Sh2-132 is featured in few if any observing lists and programs. This led me to conclude that it must be a target suitable only for imaging. Until a couple of months ago, that is. I'd captured it in many of my own film photos over the years, but it was Desktop Universe's image mosaic, with its uniform presentation of the entire sky, that made me realize that Sh2-132 might indeed be observable, after I compared its appearance against other faint nebulae in DTU that I'd already glimpsed.

It's easy to locate because of its proximity to the distinctive triangle of stars formed by delta, epsilon and zeta Cephei (delta Cep is the prototypical Cepheid variable). From northwest to southeast, zeta, epsilon and Sh2-132 form an almost straight line. The distance between zeta and epsilon is 1.25 degrees, and between epsilon and the nebula is 1.0 degree. Helping to locate the nebula is a very wide optical double about 1/4 degree due west of it.

The full extent of the nebula is 90 by 40 arcminutes, but only the brightest part to the northeast, of about 20 by 15 arcminutes extent, will likely be visible. My first attempt to see it was with a 22x100 binocular equipped with Orion Ultrablock filters (equivalent to the Lumicon Ultra High Contrast, designed to pass both the OIII and H-beta wavelengths). Even without the filters in place there was a very faint milkiness superposed upon the rich star field. But the filters made it stand out quite definitely.

Sh2-132 lies in the field of the Cepheus OB1 association. If it is related to Cep OB1, whose distance is given as 10,000 to 12,000 light-years, then it would be located well out in the Perseus arm of our Galaxy, and be more than 250 light-years in extent.

Sh2-119

Emission nebula Sh2-119 is certainly a not widely observed object! The only print atlas I'm aware of which plots it is the Millennium Star Atlas. Other atlases often plot nebulae not observable by eye, but not Sh2-119. I find this surprising because it's both large and photographically bright. Perhaps leading to its neglect is the show stealer, the North America nebula, right next door.

Like Sh2-132, I'd not thought to try to observe it until a couple of months ago. And again, it was Desktop Universe which prompted me to give it a go. I've learned to use colour as a guide to the visibility of an emission nebula. More specifically, I try to ascertain the relative degree of reddening by interstellar dust. An unobscured emission nebula will have a pink, purplish or even bluish hue, while a reddened nebula will appear a more monochromatic, deep red. Because the dark adapted eye is rather insensitive to red, an otherwise bright nebula which is significantly reddened can be rendered invisible because the all-important OIII and H-beta emission (in the blue-green part of the spectrum) will be dimmed rather more strongly than the red (primarily H-alpha).

And large it is, at 2.5 by 2.0 degrees. In form it is a pronounced elliptical shell or ring, oriented roughly northeast-southwest, centered on 68 Cygni. The brightest portion of the ring is 1 degree long and 0.6 degree east of 68 Cyg. This is the part I observed in the 22x100 binoculars, but with the filters only. If you're already familiar with the North America/Pelican nebulae field, use the Pelican as your guide to the visibility of Sh2-119. If you can see the former, you have a good chance to see the latter.

That 68 Cyg lies right at the heart of Sh2-119 is no coincidence. Actually, is IS a coincidence! Whereas the illuminating stars of most nebulae were born of those clouds, 68 Cyg is merely an interloper--a runaway star--that happens to be passing through an unrelated interstellar cloud. With a spectral type of O8, it emits enough UV radiation to ionize the Hydrogen gas in its vicinity. But its vigorous stellar winds continually sculpt out a low density cavity surrounding the star in fairly short order, hence the shell-like structure.

The distance of this object hasn't been determined very accurately, but 2,000 light-years would seem reasonable, suggesting a diameter of about 85 light-years.

Previous Challenge Objects

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.

Modified: September 16, 2007