Dr. Whitney Shane, MIRA's Charles Hitchcock Adams Fellow
It will not have escaped the notice of the attentive reader of this column that there are lots of galaxies in the spring sky and not very much else. One of the most peculiar of these galaxies is the "Spindle", which we also know as NGC 2685. It is not a prominent object, and the visual observer will probably see no more than the very elongated elliptical core. On photographs we see something that suggests a cigar being smoked by a very clever smoker who is able to blow smoke rings around his cigar. To less imaginative folk it suggests a spindle. Deep photographs reveal that the whole is surrounded by a faint elliptical ring, nicely aligned with the central part. Observers wishing to test their visual skills can find this galaxy, probably with some difficulty, in an empty region near the western edge of Ursa Major. It is about 4 degrees ESE of the variable star Omicron Ursae Majoris.
The "cigar" and "smoke ring" are easy to see in this image of NGC-2685. The larger eliptical ring is also discernable, but with difficulty.
At one time it became your correspondent's task to map the neutral hydrogen in this galaxy. Although initially we had to work close to our sensitivity limit, we eventually got a good set of maps. Our first surprise was the discovery that almost all of the hydrogen was in the faint outer part, not in the smoke rings, which were known to be full of young stars and ionized gas. There was, however, enough in this part to enable us to measure the rotation speed. We found that in this part of the galaxy all of the gas was rotating around a completely different axis from the disk part of the galaxy and that there was no gas and very few stars in this part of the disk. In the central body, where most of the light originates, we found no gas at all. Combining these measures with published optical data we were able to construct a simple mass model of the galaxy. We found, among other things, that much of the mass was in the outer, almost invisible, part of the galaxy. This is now a well known property of galaxies, and it is related to the problem of the dark matter, but at the time (the late 1970s) it came as something of a surprise.
One thing was clear from the outset. The galaxy could not be constructed as it appeared, an elongated structure of stars, which had to be tumbling end over end in order to maintain its shape, surrounded by several rings of gas and stars. The tumbling bar would pass through the rings and wipe them out in no time. A much more attractive model started with a normal disk galaxy seen under a steep angle, with a strong concentration of stars in the inner part and an extended, but much weaker, outer part. We imagine now a small gas-rich galaxy plunging into this disk and being slowed down by the material already present there. The result could be rings of gas stretched out by friction and rotating about an entirely different axis from the rest of the galaxy. The material originally in this part of the galaxy would have been caught up in the rings leaving the gap in the disk which we observe. The difference between this model and the structure of the galaxy as it appears on photographs led us to think of it as a swindle rather than as a spindle.
Our model did not last long. It soon became clear that many elliptical galaxies have a long axis which is maintained by just the sort of tumbling motion which we had rejected. Calculations also showed that it was not difficult to con-struct a mass model in which rings around this long axis, once formed, would precess at the same rate as the tum-bling so that they would not be wiped out, as we had thought. So it appears that we were the ones being swindled after all. Nevertheless, your correspondent would not be surprised to see the earlier model rediscovered sometime in the distant future. [Dr. Shane's research on NGC 2685 was published in Astronomy and Astrophysics 82, 314-321 (1980)--Ed.]
Mercury will be very well placed for observation by northern observers around the end of April when it will be bright and high in the evening sky. However it will soon fade into the twilight and pass inferior conjunction on May 27. It will not be visible again until the end of June.
Venus will be visible low in the evening sky during the whole quarter.
Mars will be observable low in the west after sunset until June, when it will disappear into the evening twilight.
Jupiter is best observed early in the quarter when it is high in the western sky during the evening. As the quarter progresses it will move into the evening twilight until it becomes unobservable in late June.
Saturn starts the quarter low in the west after sunset and will disappear into the evening twilight around the middle of May. It will be in conjunction with the Sun on June 9.
From the above, it appears that all of the naked eye planets will be in the western evening sky at one time or another during the quarter. Between the middle of April and the middle of May all five will be visible simultaneously, although some with difficulty. This will result in many conjunctions between planets and some unusual groupings. The Moon will contribute to the festivities at the beginning and again at the end of the period.
Two meteor showers, both of secondary importance, are expected in this quarter. The Lyrids, around April 22, will suffer a little from moonlight in the evening hours. The Eta Aquarids, around May 5 in the morning hours, will have to contend with a Moon just past last quarter.
Six months ago your correspondent expressed his frustration at the consistent failure of the Leonids to perform according to plan. Last November they had their revenge, putting on an excellent show during the early morning hours of November 18. We had the good fortune to be observing gravitational lenses from Chews Ridge on that night, and during the somewhat lengthy exposures were able to enjoy the spectacle. At the height of the shower the inexperienced observer estimated a mean hourly rate of as much as 1000. During the not infrequent bursts of activity, sometimes several could be seen simultaneously. Although the weather was less than ideal for direct imaging with the 36-inch telescope, it was quite good enough for meteor watching, and we were very grateful for the opportunity.
Comet C/2000 SV74 (LINEAR) has passed its maximum brightness, while visible only from the southern hemisphere, and is now moving north. It should be well observable from the northern hemisphere during this quarter.
Periodic comet Pons-Winnecke will reach perihelion on May 15 and may have a magnitude of 10 to 11 during the quarter. It should be well observable.
A fairly deep penumbral lunar eclipse will occur in the early morning hours of May 26. On the west coast, maximum should occur shortly before moonset. A penumbral eclipse is never spectacular, but in this case the shading should be well observable.
An annular solar eclipse on June 10 will be seen from our area as a 70 per cent partial eclipse, which will reach maximum in the late afternoon.
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Last updated 4/14//02 DMC