Dr. Whitney Shane, MIRA's Charles Hitchcock Adams Fellow
Fixed Stars
Most of the bright galaxies are to be found in the spring sky, and of these the Sombrero Galaxy (Messier 104) is one of the most beautiful. Despite its brightness, it was not one of Messier's original 100 bright nebulae. It is an early type spiral galaxy (with tightly wound and well ordered spiral arms) embedded in an unusually large and bright bulge. It is seen almost edge on, so that the disk, which contains a large amount of dust, is seen as a dark band dividing the bulge into two parts. It is located on the very southern edge of Virgo, about 10 degrees due west of Spica, and it is an interesting object even for a telescope of modest size. It is probably the first external galaxy for which the rotation was determined in 1916 by Pease, using the Mt. Wilson 60-inch telescope and exposing for several nights.
An odd combination of circumstances makes the Sombrero a particularly tempting object for some radio astronomers. It is seen edge on, it lies near the equator, and its plane is oriented almost exactly east-west. Thus the interstellar gas, which most interests radio astronomers, lies almost entirely on an east-west line and resolution in declination is not required. An important class of radio telescopes gives only east-west resolution for objects near the equator, so the combination is perfect. Add the fact that, the disk being viewed edge on, we observe rotation velocities directly, so that we have no need for uncertain projection factors.
Your correspondent was one of many astronomers who were unable to resist this temptation. A single scan in the radiation of neutral hydrogen made along the plane of the galaxy is sufficient to give all available observational information, leading to great economy of effort. From this scan we can determine the rotation speed of the galaxy by looking at the maximum velocity in each direction, and we can see the individual spiral arms, which are optically almost impossible to resolve, because they are well separated in velocity. The rotation speed gives a measure of the total mass distribution in the galaxy, and, according to the density wave theory of galactic structure, the spacing of the arms tells us the degree of concentration of this mass in the thin gas-filled disk.
Most spiral galaxies, even those without apparent bulges, seem to be embedded in large mass concentrations. It is not clear what form this mass takes, but it does not seem to produce any observable radiation. We might expect that a galaxy like the Sombrero, which has a very bright bulge, would be surrounded by an even larger mass concentration. This does not seem to be the case. The mass distribution of the Sombrero is very like that of any other large galaxy of its type. The composition of the disk is also quite like that of the Milky Way. The only difference seems to be that the surrounding mass, at least in its inner regions, has lead to the formation of far more stars than is usually the case, giving the bright visible bulge. We have, at present, no idea why this should have happened.
The best observing opportunity for Mercury for this year will be around the middle of May, when it is quite high in the north-west evening sky. Thereafter it will sink rapidly into the evening twilight as it approaches inferior conjunction in June.
Venus will emerge from the morning twilight early in April, but it will remain low in the eastern sky during the entire quarter, reaching maximum brightness in early May and maximum elongation in early June.
Mars will brighten rapidly during the quarter as it approaches opposition on June 13, when it will be a little brighter than Jupiter (which, however, is so close to conjunction as to be unobservable). This will be a favorable opposition, in that Mars is quite close to the Earth. However, such oppositions always occur during the summer months, when Mars remains low in the southern sky. For this reason, southern observers always get the best views of Mars.
Jupiter will disappear into the evening twilight during the quarter. It will be useful mainly as a guide to finding Mercury, which will stand 3 degrees north of Jupiter on May 16.
Saturn vanishes into the evening twilight at the beginning of the quarter and does not reappear in the morning twilight until the end of June.
The most promising meteor shower in this quarter is the Lyrids, visible for about 10 days around April 22. Although usually not very active, the Lyrids can exhibit sudden peaks, and these can make observations very worthwhile. The Moon is new on April 23, so the sky will be dark. The morning hours will be best.
The pi Pupids, a young stream first seen in 1972, can be observed for about 10 days around April 23. Although usually quite inactive, it has shown a few strong peaks. It is best visible in the south in the evening, so dedicated observers will be in for some long nights.
The eta Aquarids, visible for a few days around May 5, coincide with full Moon, and thus will not be rewarding observing.
Comet McNaught-Hartley, which we mentioned last time, will become circumpolar in March and remain visible, but steadily fainter during the remainder of the quarter.
Of the remaining faint comets currently visible, one, Comet Utsunomiya-Jones was discovered in November, 2000, by real live comet observers as a fast-moving eighth magnitude object. It is now much fainter, but we note with satisfaction that not all comets are necessarily discovered by machines.
There will be a total eclipse of the Sun on June 21, but it will be visible almost exclusively from the southern hemisphere. The path of totality will pass over the south Atlantic Ocean and the southern part of Africa.
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| What you don't see on TV: security is tight at shuttle launches, for obvious reasons. NASA photo. | The men who actually make it go (note absence of jumpsuits and hard hats) make a last-minute repair to the shuttle transporter. |
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| Endeavour, STS-97, stands poised for its nighttime launch last November. Our correspondents Kim Cohan and Brian Jacobson traveled to Florida to cover the event for the MIRA Newsletter. Photo by Kim Cohan. | Our man at the Cape, Kim Cohan. Kim and the other spectators retreated to a safe distance for the launch. Photo by Brian Jacobson. |
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Last updated 3/18/02 DMC