Dr. Whitney Shane, MIRA's Charles Hitchcock Adams Fellow
Many of us, when examining the night sky, find it difficult to associate the accidental configurations of the stars that make up the constellations with the figures they traditionally represent. The same is true of some of the nebulae, whose common names are only remotely suggested by their appearances. Happily the North American Nebula and its companion the Pelican Nebula, do not belong to this class. Both have a really remarkable resemblance to the objects for which they are named.
Although the brightest nebulae seem to be located in the Winter Milky Way (the Orion Nebula comes to mind), the Summer Milky Way also offers good, and somewhat more comfortable, hunting. For an observer with a small telescope or binoculars and a dark sky, the North American Nebula, known officially by the easily remembered name of NGC 7000, is a good candidate. It is easily located, lying 3.5 degrees due east of Deneb. It is large, between 1 and 2 degrees in diameter, but quite faint, so it may be hard to identify in this crowded region of Cygnus. The Pelican Nebula, which lies about 1.5 degrees to the west, is smaller and so faint that it can be seen only on photographs.
The visual observer will probably be unable to distinguish the form of the North American Nebula, but on a photograph the correspondence with a map of the continent is striking. An observer with a sufficiently vivid imagination will no doubt succeed in identifying the brighter stars with urban agglomerations. Even the orientation is correct, with Canada to the north and California to the east. (Remember that when looking upward to the sky the directions are the mirror image of what they are when looking downward upon the Earth. Most of you will have no idea how difficult it is to explain this to beginning students!) The Pelican Nebula, located somewhere in the Atlantic Ocean, is about where one would expect to find the mythical continent of Atlantis, leading to its alternate name, the Atlantis Nebula.
The North American Nebula, NGC 7000
Both of these nebulae are of the emission kind, meaning that we see radiation from hot gas. This gas must be heated, and ionized, by one or more hot stars, and in the case of the North American nebula there is just one hot star, HD 199579, and it is very hot indeed, about as hot as a nomal star can get. The gas density can be estimated from a study of the spectrum of the nebula, and is found to be quite low, only about 10 atoms per cubic centimeter which will help to explain the low surface brightness. Combined with the distance, which is about 1000 parsecs, this gives a mass estimate of a few thousand solar masses.
During the whole summer quarter Venus will be visible in the north-east morning sky, rising 2 or 3 hours before the Sun, but it will remain low in the sky during these early morning hours and is not favorably placed for observation.
Mars will be moving into the evening sky during July, where it will still be well placed for observation. It will move lower into the southwestern evening sky as the quarter progresses.
Saturn is now more than an hour ahead of Jupiter and is thus visible in the morning sky during the whole quarter. It starts its retrograde motion on September 27.
Uranus and Neptune will both pass opposition during the quarter, but both will be low in the southern sky and thus not well placed for observation.
Observers who insist upon a dark sky will have to be content with three very weak showers, the kappa Cygnids and the Northern iota Aquarids in the middle of August and the poorly studied Piscids in mid September.
I am often reminded of the fact that living on Chew's Ridge is like living in the fourth world (it might qualify for the third world except for having to truck in the water during the summer). Off the power grid & unwired, I've felt like those kids attending school by shortwave radio in the outback of Austrailia (Editor's note: being off the power grid has had certain advantages this year).
At long last OOS has been elevated from its informationally challenged status. Two years ago, we bartered a year's worth of time as an excellent radio-repeater site for the eventual use of three 900 Mhz spread-spectrum wireless modems. It has taken some effort to establish another workable down-link site after the successful conclusion of the original project, but I am pleased to report that this wireless link is now up and running. The result is that the OOS has a speedy 115, 200 Kbps connection to the Internet, with no extra recurring monthly charges.
The downlink site and ace networking expertise came courtesy of indefatigable FOM volunteer Bill Bishop, to whom we owe a mjor debt of gratitude. Beyond the greater efficiency for those of us toiling away in the Nether Regions, such connectivity offers other interesting possibilites. One is that the link may be utilized for digital remote control of the MIRA 36" telescope.
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Stan Karas, MIRA's steely-eyed observatory engineer, looks up from his work on the 36" telescope. |
| Indefatigable FOM Volunteer Bill Bishop |  |
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Last updated 3/8/02 DMC