Dr. Whitney Shane, MIRA's Charles Hitchcock Adams Fellow
The apparently brightest star clusters on the whole sky are both in Taurus and thus well placed for winter observation. Everyone immediately recognizes the Pleiades, but the nearby Hyades is equally bright and considerably larger, and this large size makes it a little less prominent. Its apparent diameter is 6 to 7 degrees and Aldeberan, which is not a part of the cluster, is located on its eastern edge. For observing the Hyades you will want nothing larger than binoculars.
The Hyades has played a very important part in building up the cosmic distance ladder. This is the series of steps which we go through in order to extend the astronomical distance scale to the edge of the universe. One of the critical steps on this ladder has been the determination of distances to relatively nearby objects in the Milky Way. Some of these objects contain Cepheid variables or other standard candles which, as we mentioned last time, we must calibrate in order to extend the distance scale.
There are no Cepheids in the Hyades, but there are plenty of normal stars, and we can use these to calibrate the color-magnitude relation, which we can then apply to more distant clusters of similar age. Since the Hyades is the closest of all the prominent clusters, determining its distance is very important.
Trigonometric parallaxes, where we measure the change in the apparent position of a star as the earth moves around the Sun, are used to determine the distances to the closest stars, but even the Hyades is too distant for reliable results from this method when earthbound telescopes are used. The Hipparcos satellite has now been able to extend reliable trigonometric parallaxes to more than ten times their previous limits. This finally makes the trigonometric parallax a useful tool in building up the distance ladder.
The best distance to the Hyades has been determined using the moving cluster method. We assume that all of the stars in the cluster have the same velocity in space, except for small random individual motions. We then measure the apparent motion of many cluster stars and determine the point on the sky toward which (or from which) all these motions converge. This is the direction toward (or from) which the cluster is moving. The proper motions of the stars give us the apparent velocity across the line of sight. We can measure the absolute velocity along the line of sight spectroscopically. We know geometrically, from the direction of motion, what the ratio of these two velocities should be. Applying this ratio, we finally divide the absolute velocity by the corresponding apparent velocity to calculate the distance.
In order to work, this method requires that the cluster be large enough that the convergent point be well defined and that the motion be large enough to give accurate measurements and that it be directed neither directly along nor perpendicular to the line of sight. The Hyades happens to satisfy all of these requirements. It is also important that the cluster not be expanding or contracting, and happily the Hyades is old enough that it is fairly close to
equilibrium. We are fortunate that there is at least one cluster which has all of the required properties.
Mercury will be visible in the evening twilight during the first half of January and, after passing quickly through inferior conjunction, in the morning twilight starting early in February. It will be brightest at the end of the quarter, but will remain far south of the equator so that observations from the northern hemisphere will be difficult.
Venus, having passed behind the Sun in January, will become visible in the evening sky toward the end of February.
Mars will remain visible in the evening sky during the whole quarter, moving into the north-western part of the sky, from Aquarius into Aries.
Jupiter passed opposition on January 1, when it was well placed for observation because of its northern declination. It then moved into the evening sky where it will be visible until the end of the quarter and beyond.
There will be three occultations, but they will be visible only from far northern latitudes.
Saturn will be in Taurus during the quarter, where it will gradually become an evening object. An occultation on the early evening of February 20 may be observable from our region.
The only important meteor shower during the first quarter is the Quadrantids, which peaked sharply on January 3. Unfortunately the Moon, which was a few days past full, made observations unrewarding.
Among the minor showers, the delta Cancrids, expected during the first three weeks of January, might be the most interesting. Except in the beginning of this period the Moon will not be a problem.
Comet C/2000 WM1 (LINEAR) is expected to be a naked eye object by the first of the year, although visible mainly from the southern hemisphere. Northern observers will have a chance to see it in early January and again later in the quarter as it fades and moves north once more.
A remarkable recent discovery is periodic comet P/2001 (Petriew), discovered in August by a Canadian amateur astronomer. It has been in the morning sky, about magnitude 12, and moving south. It may still be visible in the winter quarter.
The distant comet C/2000 SV74 (LINEAR) should remain visible as a magnitude 13 circumpolar object well into 2002.
The year 2002 will be a poor one for eclipses, particularly for those of us living in North America. The year starts off with no eclipse of any sort during the first quarter.
| Winter 2002 Contents | Newsletter Index | Mira Home Page |
Last updated 2/17/02 DMC