Dr. Whitney Shane, MIRA's Charles Hitchcock Adams Fellow
Even those of us who are constellationally challenged have no difficulty recognizing Orion, which dominates the winter sky. The prominent belt, which does not contain even one of the three brightest stars in the constellation, and the sword are familiar to everyone. The sword is of particular interest, being the home of the famous Orion Nebula, Messier 42. The sword is outlined by the bright stars iota Orionis, just south of the nebula, and theta 1 Orionis, right in the middle of the nebula. The latter is actually four bright and very hot stars (and some fainter ones as well), known as the trapezium cluster. These stars provide most of the energy to the bright nebula. They are hot enough to ionize the surrounding gas so that much of it is seen in emission lines. The contrast in brightness between the bright stars and the surrounding nebulosity is so great that it is very difficult to produce an image in which both are seen clearly.
The large dark mass in this (negative) MIRA image is M42, the Orion Nebula, with M43 on its upper edge. The smaller nebula above M43 is NGC 1977. The blank area (white, in this negative image) between the two is a dark cloud.Almost every process related to star formation can be observed somewhere in or near the Orion Nebula. Stars form in regions rich in interstellar gas and dust. The dust is particularly important because it provides cooling, leading to the high densities needed for star formation. In some regions, such as the dark clouds in Taurus, star formation proceeds quietly and only stars of modest mass are formed. In others, such as the Orion Nebula, the process is much more violent, and we find many massive and very hot stars. This can be explained if massive stars can form only in regions of exceptionally high density. But once such a star forms, its powerful radiation will compress the surrounding gas, leading to the formation of more massive stars. Thus a sort of contagion sets in, as it has in Orion but not, or not yet, in Taurus.
Dust is essential for star formation, so what we do not see in the Orion Nebula is fully as important as what we do see. Anyone doubting the presence of dust in the Orion region need look no further than the nearby Horsehead Nebula, Barnard 33. Here we see the emission nebula, IC434, surrounding the bright belt star zeta Orionis. The nebula is obscured on its east side by a dark cloud. Extending westward from this cloud, so that it is seen projected upon the bright nebula, is the dark form of the Horsehead. With such examples abounding, it is remarkable that it took astronomers so long to realize that dust was ubiquitous in the Milky Way, and that the light of every star is obscured and reddened, to some degree, by this dust.
Messier 42 itself is the most spectacular member of a group of bright nebulae located in the sword region. Just to the north, and separated only by a narrow band of obscuration, is the much smaller Messier 43. About a half degree further north we find a complex region, dominated by the nebula NGC1977 and containing several smaller nebulae and some bright stars, the brightest of which are the double stars 42 and 45 Orionis. A half degree further on is the most northerly member of the sword complex, the unspectacular open cluster NGC1981.
This Hubble Space Telescope image shows clearly that the Horsehead Nebula is composed of dark material obscuring the bright nebula behind it. (Unlike the image on page 6, this is a positive image.)
The region between M43 and NGC1977 is of particular interest. A small-scale picture shows that this region is almost devoid of stars and that the faint nebulosity which surrounds M42 is obscured here by a foreground cloud. In the middle of this is the remarkable Becklin-Neugebauer object. This must be a very bright star, but it is visible only in the infrared. The star must be embedded in a dense cloud, which absorbs most of the light and converts it into heat. This and the foreground cloud, which is probably a separate object, must together produce about 50 magnitudes of extinction in visible light. Another object in the same region, which we do not see, is the Kleinmann-Low nebula. This is a very cool and dense cloud, which we can detect only at radio and far infrared wavelengths. It is full of dust and complex molecules and is a prime site for future star formation. It, or a nearby region, is also imaginatively known as Orion Molecular Cloud number 2 (OMC-2). Its companion object, OMC-1, is located on the western edge of M42.
Northern observers will see Mercury only briefly around the end of January, when it will be visible low in the southeast during morning twilight.
During January Venus, which is at greatest elongation on the eleventh, is well visible in the southeastern sky before dawn, but by the end of the quarter it will be too low to be observed.
Although the elongation of Mars will be increasing during the quarter, observing conditions will not improve because Mars will be approaching its most southerly declination while the Sun is moving north. It will be observable in the southeastern morning sky.
Jupiter will be visible all night during the whole quarter. It will be at opposition, in Cancer, on February 2. Because of its high declination, this will be an excellent observing opportunity for northern observers.
Saturn is also well placed for observation, particularly from the northern hemisphere. It passed opposition in December, but is visible for practically the whole night through March. The ring system is currently close to its maximum tilt, so this is a good year for observation.
The year opens with one of the best of the meteor showers, the Quadrantids. The peak should occur in the very early evening of January 3. The radiant will be very low in the north at that time, so observers in Europe will have the best view, if it should happen not to be raining. The Moon will be only one day old, so it will be no problem.
The maximum of the delta Cancrids on January 17 nearly coincides with full Moon, so that these are not suitable candidates for observation this year.
The southern hemisphere will be treated to the spectacular alpha Centaurids in early February. Unfortunately these are not visible from our latitude.
Finally, the very sparse and faint delta Leonids will peak around February 25. These are easily confused with the very diffuse Virginids, which can be found at any time from January through April, and of which they may be a part. The Moon will be favorable. Comets
It is becoming increasingly difficult to find good current comet information. The only prediction that we have for 2003 is that Comet C/2001 HT50 (LINEAR-NEAT) will pass perihelion on July 8 at 2.8 AU. It is possible that this comet will be visually observable.
There will be no eclipses during the first quarter. In fact, the whole year will not provide much of interest in our region.
| Winter 2002 Contents | Newsletter Index | Mira Home Page |
Last updated 12/14/02 DMC