Fall 1997
Supernovae,
which were mentioned in passing last time, are among the most spectacular
events in the universe. A bright supernova, near maximum, can easily outshine
its entire host galaxy. We recognize two types of supernovae, called type
I and type II, along with many sub-types. Surprisingly, the most massive
stars become the fainter type II supernovae. This happens when the inert
core of the star, which has exhausted all of its nuclear fuel, collapses
under its own weight. The outer layers fall inward, bounce off the remains
of the core and are scattered into space.
The radio source Cassiopeia A, the strongest radio source in the sky except the sun, is probably the remnant of such a supernova. It is conveniently located in the northern sky during the fall months, but don't bother to look for it with your telescope. It is visible only on very deep photographs as a bit of weak and fragmented nebulosity. Presumably it is heavily obscured by intervening dust, which may also explain why there is no record of the original explosion, late in the seventeenth century. The exceptional strength of the radio source may tell us more about the surroundings than about the supernova itself. Since the progenitor stars are very massive, they are also young and are located in dusty star-forming regions.
Type II supernovae originate from very ordinary old stars, with masses
not much larger than the sun. Normally such stars become white dwarfs and
expire quietly. However, if such a star has a less massive companion which,
after slower evolution, expands to become an inflated giant star, much
of the mass from this star will be transferred to the more massive white
dwarf. (Just to make matters complicated, there may well have been a mass
transfer in the opposite direction in an earlier stage of evolution.) Eventually
the white dwarf may exceed its stability limit and collapse, ejecting its
outer layers into space. Often the remains of the core will become a neutron
star. The Crab Nebula in Taurus, Messier 1, which was observed to explode
in the year 1054, is the most famous example of this kind of event. It
is easily observable in the morning sky during the fall months.
Taurus
Mercury will become visible in the evening sky in November, reaching greatest elongation on November 28, but it remains a difficult object because of its southern declination. It will reappear in the morning sky at the end of December.
Venus is visible in the evening sky for the remainder of the year, reaching greatest elongation on November 6, but until mid-December it is less than 20 degrees above the horizon at sunset.
Mars remains low in the southwest evening sky during the fall. Because of this and its relatively large distance from the earth, observations are apt to be unrewarding. It remains close to Venus until the end of the year, starting out to the east, then passing 2 degrees north on October 26, moving on to 7 degrees west and then returning to pass 1 degree south on December 22.
Jupiter is visible in the evening sky during the fall, but as the year draws to a close, it will begin to move into the south-western twilight. It will pass its stationary point on October 8, marking the end of this year's opposition.
Saturn reaches opposition on October 10 and remains visible in the evening sky until well into the new year. Conditions for observing should be favorable. Finally one of the series of lunar occultations may be marginally visible from our area during the night of December 8/9.
EclipsesThe diamond ring visible around the total eclipse, taken during the MIRA Eclipse Cruise to Mazatlan, Mexico, in 1991 (photo by Ray Dyck).
