Spring 2000

The Spring Sky
by Dr. Whitney Shane, MIRA’s Charles Hitchcock Adams Fellow

M81, one of the prettiest spiral galaxies.

Fixed Stars
The spring, when the north galactic pole is overhead in the evening, is the time of the year for observing galaxies. The brightest galaxy in the north galactic hemisphere is Messier 81. This galaxy and its companion, Messier 82, are known as Bode’s Galaxies, a designation which astronomers seem never to use. Since, like many galaxies, it is in a rather empty part of the sky, it is not easy to find. Starting with the two stars on the outer edge of the bowl of the Big Dipper, Merak and Dubhe, which point to Polaris, we proceed toward the northwest (down and left) by about twice the separation of the two stars. The galaxy is large and diffuse, so that it is not a very dramatic object, but it can be observed easily even with the smallest telescope provided the sky is reasonably dark.

The visual observer will usually see only the bright inner part of the galaxy. The spiral arms are faint and narrow, so that they can be seen only with the greatest difficulty, if at all. For observing spiral arms, the more distant Whirlpool Galaxy is much the better candidate. However, photographs show that Messier 81 has very regular and well-defined spiral arms. In fact, it is one of the most regular-appearing spiral galaxies in the sky. This fact and its large apparent size make it one of the favorite objects for astronomers interested in the dynamics of galaxies, almost all of whom (including your correspondent) have studied it at one time or another.

Radio astronomy, particularly using the 21-centimeter line of neutral hydrogen, is an important tool in studying galaxies like Messier 81. This was the first galaxy that could be studied (about 1980) in sufficient detail to test current theories of galactic dynamics. This test showed that the favored theory at that time, the so-called density wave theory, was consistent with the observations, but competing theories could not be eliminated. This situation still exists, but recent observations with the Very Large Array in New Mexico, giving much improved resolution in angle and velocity, are now being analyzed. They show that, when examined in detail, Messier 81 is much less regular and symmetrical than it at first appeared. The controversy between advocates of the major theories continues, but it should not surprise us if it turns out that all of the theories are valid, each within limits, and all work together to produce the very complicated picture that we observe.

Planets
Mercury will no longer be visible in the morning sky as the quarter begins. It passes behind the Sun in early May and by the middle of June.

Venus will be in superior conjunction in June and will not be visible at any time during the quarter.

Mars is low in the evening sky during April and will disappear into the twilight at the beginning of May. It will not be seen again until September.

Jupiter will overtake Mars and will be lost in the western twilight during April. After passing conjunction on 8 May, it will reappear in the morning twilight early in June.

Saturn follows closely after Jupiter and will also be lost in the evening twilight before the end of April. It will be in conjunction on 10 May and will become visible again in the morning twilight before the end of June.

Observers with larger telescopes will want to take note of the opposition of Pluto on 1 June.

By this time the attentive reader will have noticed that this is an exceptionally bad quarter for planetary observers. In fact, for a couple of days around 4 May there will be no naked-eye planet visible in the sky. Even the Moon, which is new on 4 May, will be absent. This is the first time that this has happened in 38 years!

Meteor Showers
Only two weak, but rather extended, meteor showers are expected during the quarter. The Lyrids, around 21 April will be much hindered by a nearly full Moon. The Eta Aquarids on 4 May will not suffer from moonlight, but they will be visible only in the morning hours.

Eclipses
There are no eclipses this quarter, and the remainder of the year has no more to offer than a total lunar eclipse and two partial solar eclipses, all visible only to a small degree from our area.