Constellation of the Month - Cygnus

Michael Nelmes

Cygnus the swan has been conspicuously absent from COTM since at least 1985. As with Hercules last month, you need a fairly low north horizon, and in fact part of the constellation is hidden from Canberra. The distinctive cross shape is however fully visible. As the Milky Way passes through it there are some nice dense star fields, and it's worthwhile just to scan Cygnus with binoculars. Lots of binaries too. Here's my selection; some of the magnitudes quoted will appear dimmer through atmospheric extinction.

Beta Cygni (Albireo). A classic binary for small scopes with a 34 arcsecond separation, the colour contrast in this magnitude 3 and 5 pair is very nice. Star A is deep yellow, star B (reportedly blue) appears green to me by contrast. At 36x in a 4.5 inch scope the surrounding star field is fairly rich, but this is the sort of object that would benefit from a wide field scope with a wide-angle eyepiece.

61 Cygni. A nice pair of almost identical (mags. 5.3, 5.9) orange K-type dwarf stars separated by an easy 28 arc seconds, 61 Cygni is famous for being the first star apart from the Sun to have its distance measured (Bessel in 1838, by parallax); distance is 11 light years, the fourth closest naked-eye star. With a huge proper motion of 5 arc seconds per year, here's a rare opportunity to actually detect the movement of a star over just a few years. Burnham (1978) mentioned a third member of the system, detected by its gravitational influence on the stars, weighing only 8 Jupiters - ie, a planet. As it doesn't feature in a recent listing of extrasolar planets (Sky & Telescope, March 1998), I presume it's now been discounted.

M29 (NGC 6913). A fairly compact trapezoid-shaped open cluster. In a 4.5 inch scope only 8 stars are visible; a few more pop into view in the C8. Distance is about 7200 light years.

M39 (NGC 7092). A loosely scattered open cluster best seen through binoculars or a very wide field small scope. Diameter is about half a degree, and distance around 800 light years. At 48 degrees north declination, it rises only 7 degrees from Canberra.

NGC 6826. A small (25 arc seconds), bright (mag. 9) but poorly defined planetary nebula, hard to distinguish from the fuzzy stars at a maximum elevation of 5 degrees. Called "the blinking planetary" - i.e., it disappears when you look at it.

Omicron 1 Cygni (31 Cygni) and 30 Cygni. Two stars, orange and bluish, mag. 4 and 5 separated by 6 arc minutes, with a third of mag. 7 near the latter. All three can be seen nicely in binoculars if you hold them steady. The primary star is an eclipsing binary involving a K-type giant of some 200 solar diameters - as, coincidentally, is the nearby 32 (Omicron 2) Cygni.

Mu Cygni. A close binary - a bit of a test at 1 arc second separation, I couldn't split it in the C8. Magnitudes 4.7 and 6.1. Wait until it's at maximum altitude (27 degrees). The third star (48 arc seconds) won't be easy either at mag. 11.

Chi Cygni. A Mira - type variable star with a magnitude range of about 14 to 4, which it will reach in October. A good one to watch with binoculars.

Using the 4.5-inch scope, 7 x 50 binoculars and C8 with deep sky filter, I couldn't find either of the brightest emission nebulae in Cygnus - NGC 7000 (the North America nebula) and NGC 6992 (brightest part of the Veil nebula). Don't let that stop you looking for them though. Just get the chainsaw onto that tree covering your north sky.

Find a black hole!

Cygnus also holds your best chance to spot a black hole - well, actually the mag.9 blue supergiant star HDE 226868 that's being devoured by it; the black hole is of course invisible. Cygnus X-1 was the first example of these weird objects to be identified, in 1971. It's marked on reasonably detailed sky atlases but it's easier to use the field photograph in Burnham (p.795 - I can provide photocopies). Distance is about 8000 light years.

What are we seeing here? The star is 33 solar masses (and diminishing), the unseen companion 16 (and growing). The orbital period of the two is 5.6 days. How do we know a black hole is involved? The best argument is that the X-rays, in order to show the variations they do over short time periods, must come from a source less that 300 km across - and at 16 solar masses, that pretty well rules out anything else.

For a listing of ten black hole candidates, see Sky & Telescope May 1996, p. 39. The next brightest example is magnitude 14, and they get rapidly dimmer from there!

Building a Backyard Observatory

Steve Johnston

So when my good friend Steve Crouch was considering a "C 14" his main worry was a house for it. He asked my advice and I persuaded him to build rather than buy an observatory. Taking my sales pitch one step further I said, "even I could do it" - before I knew it I was announcing to my better half my plans for the summer.

Now when you build for yourself you accept the fact that if it needs "maintenance" (a major rebuild because it fell down) you will get around to fixing it, but working for someone else means it has got to work and stay up! It was going to be a busy summer.

In planning the construction Steve had some definite ideas concerning size, orientation and roof type. This is the best part of building for yourself; you get what you want. A "C14" is a very large telescope and it's nice to have room to move in an observatory. But I must admit I kept hearing this booming voice talking of "cubits" and "two of everything". I felt a little worried. The basic layout was this, a square measuring 3x3 meters. The split roof had to open north-south, (this means the opening slit ran from east to west) and give maximum sky access, for the size and height of the observatory. It must have a big solid pier offset so it placed the scope's "dec" axis in the center of the observatory. Having given me these modest guidelines Steve then left me to my own devices.

I started gathering material at the same time doing the design layout in my mind. First the pier, this was supplied buy AGL, a length of gas pipe 28cm in diameter with a wall thickness of 12mm. We set this in concrete and blue metal to a depth of 80cm and filled it with sand, a plate was then welded on top for the wedge. Work then started in earnest. I used wood from the society stockpile for the wall frames; having measured the "C14" wedge I worked out the offset of the pier and set the footings to suit. The floor was tongue and groove pine set atop hardwood bearers; these were supported on concrete footings that I formed in-situ. We chose wood for the floor because it is a good insulator and because of the land slope, the floor was raised rather than dug in, also if you drop an eyepiece on wood it doesn't make the same crunch sound as on concrete.

The walls were made from fibro sheeting cut into 30cm strips glued and nailed like weatherboards this proved a very economical and visually neat cladding. The roof was built on the ribbed design of an aircraft wing very rigid and light, the two halves being covered with white "laserlite". These two sections needed to roll open on "U" channel steel rails, but commercial rollers were very expensive! ($15 each). "Toys r' us" came to the rescue, for $25 I brought a pair of roller blades and used their eight wheels, the result is a very smooth, quiet rolling roof. Because of the size of the roof sections ease of opening was a must, this was achieved by using a twin rope pulley system running over pushbike axles. The result was better than expected. One person could open the roof from a central position; this method eliminates flexure, which can bind the wheels in the "U" channels.

Power was supplied from an external source via a caravan socket and 15-amp cable thus eliminating the expense of electrical contractors, inside are plenty of outlets for computers etc. Drop down chart tables helped to free up floor space and corner shelving was utilized for storage. I fitted one white and one red interior light, these are "bulk head " light fittings very sturdy and safe.

While on the subject of safety great care was used in fitting storm clips to the roof sections; these eliminate the possibility of wind lifting the sections out of the rails (Steve suggested we print a return address on the underside in case they ever blew away). The steps are extra wide and edged in white; the entire steel rail frame was welded, with gussets added for strength. The roof on a shed is an integral part of the structural strength: a roll of roof observatory lacks this strength, so great care is needed to make up for this deficiency. Internal paneling aiding wall strength is vital.

Problems were encountered with water! Initially the roof joint leaked like a sieve! Capillary action was the main culprit but after several modifications this has almost been eliminated (recent heavy weather has caused a minor leak) but I am not beaten yet, one other problem has been condensation, while this is not too serious it must also be eliminated.

Steve's observatory is only the second such structure I have built and though it differed from mine in both size and roof style the principal construction method was the same. The result was a good home for a telescope that eliminates a long nightly set up. Steve's took much longer to build than I had expected but I am pleased with the results.

In conclusion I think that any permanent observatory needs careful thought - "do it once and do it right". Roll off roof designs are by far the easiest to construct but do offer restricted access to the horizon when compared to a dome. This is not generally a drama in the average urban back yard. A single roof section is sure to be watertight; this is a little more difficult to achieve with a split roof. On the plus side the cool down time is almost nil, and you don't have to keep moving the dome. All up we built a very well appointed good-sized observatory for well under $2,000. One other consideration about this type of observatory is that they don't advertise like a dome that there are expensive optical items inside, my neighbor still thinks we have a sauna in our backyard.

The New Tirion Sky Atlas 2000

Ross Gould

Although we have entered an era when computer generated charts are readily available, old style paper star atlases continue to multiply or re-generate, and I suspect it will continue to be the case that they complement each other and therefore co-exist. Each has advantages. The current (October '98) issue of Sky & Telescope carries news of Wil Tirion's newest offering, a revised edition of Sky Atlas 2000 which will be published soon. Some information is included in David Levy's Star Trails column, which features a biography of Tirion.

There have been rumours for some time of a revised SA2000. Although full details of the new atlas are not given, it is possible to do a preliminary evaluation from the information - and part of a chart - given by David Levy.

Levy's brief description mentions that nearly twice as many stars are shown, compared to the old edition. That had 43,000 stars to mag 8.0 - so the new atlas, going to mag 8.5, would have c. 80,000. The old edition showed about 2500 deep sky objects - the new version has about 2700, only a small increase. However all galaxies are now shown in correct orientation. This is useful, without being a giant step forward. And having more stars can make star hopping to deep sky objects easier. What then matters is having a fairly consistent boundary for which deep sky objects are shown, and which are excluded - so you don't show objects requiring 40cm scopes while missing others visible with 20cm, for example.

There is no indication yet of what databases have been used - always a vital point for mapping. A wrong choice of databases can seriously undermine the usefulness of an atlas.

The Old Edition

The previous version of Sky Atlas 2000, although a useful atlas, did not use the best options of its time for databases. For example, its stars were chosen from the SAO Catalog - good for positions, very erratic for photometry. Consequently some stars within its magnitude limits were excluded, others fainter than the limit were included. So star patterns didn't reflect the real sky. The SAO had used whatever photometry came to hand, and some stars were more than 1 magnitude wrong. It included some data from as far back as Argelander's BD, photometry by eye estimates with a small refractor. For the purposes of the SAO catalog, that hadn't been important.

As a result, some stars shown correctly on Becvar's Atlas Coeli - which was done in the 1950s - were wrongly included or excluded in the earlier Sky Atlas 2000. The SAO has magnitudes that can be wrong by large amounts. Some years ago I checked a region of sky where the difference between the SAO magnitude and the modern photoelectric measure were quite often of the order of 0.5 to 1.4 magnitudes.

Double stars have been a problem area for atlases - more so than clusters, nebulae and galaxies. A look at the Reference list in SA 2000 will show no use of a comprehensive double star source - presumably they relied on the double star flags in SAO, with some checking against Burnham's Celestial Handbook. This was not an adequate approach. A similar problem afflicts Uranometria, which again fails to list a comprehensive ds source.

One can only hope the new Sky Atlas 2000 will have chosen its star database for photometric accuracy; and have used a dedicated ds list.

Aspects of Showing Double Stars

Not using an authoritative douuble star list creates erratic coverage. To look briefly at Uranometria (another Tirion project), I have found some WDS pairs - including those visible with modest (15-20 cm) apertures - are not shown as double on Uranometria. Conversely, some stars marked double on Uranometria are not listed in the WDS. And some of the doubles marked in Uranometria are beyond amateur telescopes. These problems also occurred - though less often - with the old SA 2000.

SA 2000 (new version) marks doubles with a single short line each side of the star. This is readily seen when observing, even though no other information of the ease or difficulty of the pair is given. I have commented before that in using the Herald-Bobroff AstroAtlas, I find the more informative markings for doubles used there too difficult to make out easily under a dim red light when observing; the same is likely true for Millenium, which copies HB's style of marking doubles. SA 2000 like Uranometria retains the less informative but easier to read style of the two short lines.

Looking at a Chart

I have spent a little time comparing the part of a chart reproduced in S&T, with Uranometria and the earlier SA2000. I have also checked double stars against the current (1996) incarnation of the Washington Double Star Catalog (WDS). Some points of interest arise.

A number of stars in this area, marked double on Uranometria, were not found in the WDS. These were not shown double on the new SA 2000, an improvement. However some stars listed as double in WDS were not shown as double on the new SA 2000. While some of the doubles not marked as such are difficult, others are not, and would be within easy reach of a 20cm telescope.

A variable star, NO Aurigae, marked on Uranometria is not shown as variable on the new SA 2000 - Sky Catalog 2000 gives it a magnitude range of 6.1-6.3. Another variable, AE Aur, is marked as a variable on the old SA 2000 - it is not marked as variable on the new SA 2000. Sky Cat 2000 lists it as having a range of 5.78-6.08 mag.

The star close NW of AE Aur is marked double on the old SA 2000 - the new version shows it single. The WDS lists it as Es 170, a pair of mags 7.9 and 10.0 at 13.4" - which should be easy in any telescope. Nearby 16 Aurigae is a double, marked as such on Uranometria and the old SA 2000, not marked double on the new version of SA 2000. Other examples could be given from my brief sampling and comparisons.

The form of the nebulosity around AE Aur (IC 405) is shown quite differently on the old and new versions of SA 2000 - the new one makes it much larger in extent, no doubt following photographic versions of its size. This can mislead the visual observer, especially if they don't use a nebular filter, but I would agree with it as useful because many observers use filters, and some use atlases to choose objects for photography, whether on film or CCD.

The star Beta Tauri shows labeling differences - the old SA 2000 told us it was also called Gamma Aurigae (being on the constellations' common boundary line). It has a Flamsteed number of 111 (Uranometria), or 112 (old SA 2000). The new SA 2000 labels it 112 and Beta - and gives us the name, "Alnath", no doubt to the delight of those Americans who will always prefer an obscure naming to the clear and readily accessible Greek letter and Constellation name. With names such as Alnath, you need either a photographic memory or a dictionary of equivalents to know what star is being referred to. We could all have lived more happily without knowing this.

To finish, I will note the nebula NGC 1985. This is absent from the new SA 2000, is shown as a diffuse nebula on the old SA 2000, and as a planetary nebula on Uranometria. Sky Catalog 2000 does not list it; Steven Hynes book Planetary Nebulae does not have it in the catalogue list; an object at its position is given in the list of "misclassified planetary nebulae". The RNGC unsurprisingly lists it as a planetary nebula, no doubt the origin of Uranometria's error. NGC 2000.0 has its position, gives an emission or reflection nebula code, and reproduces a description from Dreyer's original NGC. Whether it is bright enough to list for the typical backyard observer is unclear - neither Hartung, nor Luginbuhl and Skiff, list it in their observing guides. Such are the vagaries of catalogues and atlases.