Fixed Stars
The strongest radio source in the sky (excepting the sun) is in Cassiopeia, and it is still known by its old name, Cassiopeia A, where the A indicates that it is the brightest source in the constellation. Visually it is anything but bright, as I pointed out two years ago in this column, at least partly as a result of heavy obscuration. It appears to be the remainder of a supernova which exploded in about 1667, although there is no record of its having been observed at the time. In the early days of radio astronomy, when accurate flux measurements were very difficult, it was selected as a standard against which other sources could be measured. A special instrument was built to make accurate measurements of this one object, which was conveniently located at high declination, but it was soon discovered that it was slowly fading, as supernovae do, so it was quite unsatisfactory as a standard.

Fortunately not all strong radio sources are so faint optically. Most of them are either old supernovae or the centers of galaxies. A rewarding example of the first is the Crab Nebula (Messier 1), which radio astronomers call Taurus A. It is visible virtually all night during the fall months.

The second strongest radio source is Cygnus A, which is an example of a galaxy. It is in the evening sky during the fall, but don’t bother to look for it as it is also extremely faint. When first identified in 1954 it was thought to be a pair of colliding galaxies, but it is probably more accurately described as a large galaxy capturing a small satellite. The large galaxy must have a black hole in its center, and the tidal disturbance of the capture is allowing material to fall into this gravitational trap, producing the observed energy. Because of its great distance, this turns out to be a remarkably powerful radio source. The best nearby example of such a galaxy is Centaurus A, which is quite bright, but located, unfortunately for us, in the southern sky.

Collisions do not seem to be necessary to create strong radio sources. Virgo A (Messier 87), which will become visible in the morning sky toward the end of fall, looks much like an ordinary elliptical galaxy, although a jet extending out from the center is visible on photographs. Actually, it is a huge radio source, and it appears that the nucleus is sending out powerful streams of very energetic charged particles in opposite directions. These interact with a magnetic field to produce the radio energy. Even the nucleus of our own Galaxy is a strong radio source, part of the complex source Sagittarius A. There is probably also a black hole here into which a small amount of matter is falling. This seems to be the rule for large galaxies. Unfortunately Sagittarius A is hidden from our optical view by very heavy obscuration.

Planets
Mercury will be easily observable in the morning sky in early December. At about 4 P.M. on November 15 there will be a transit of Mercury (in which Mercury passes in front of the solar disk). These events occur once in seven years, on average. This will be a grazing transit, with Mercury passing across the north-eastern limb of the sun. It should last for and hour or so, depending on the location of the observer. Please don’t neglect to take all necessary precautions when making observations of the sun.

Venus will be very prominent and high in the morning sky through the whole quarter. It will reach maximum elongation at the end of October, but will still be well above the horizon at dawn at the end of the year.

Mars will not be favorably located for observation, remaining fairly low in the south-western sky during the whole quarter. A lunar occultation on December 12 will unfortunately not be visible from the western U.S.

Jupiter, on the other hand, is well placed for observation, passing opposition on October 23. A day later it will be found 4 degrees north of the full moon. It will continue moving west until it reaches its stationary point in the second half of December.

Saturn is currently following Jupiter across the sky, and a few degrees to the north. It will pass opposition on November 6 and will thus be well placed for observation during the whole quarter.

The current series of occultations of Uranus and Neptune continue, and one of these, the occultation of Uranus on October 19, will be visible from our region.

Meteor Showers
The Leonids will return this year on November 17. The shower (or storm) should be brief but still quite intense. However, it will peak in the late afternoon, and in the early evening the radiant will still be below the horizon. The Orionids (October 22), S Taurids (November 3), N Taurids (November 13) and Ursids (December 22) are all minor showers. Only the prominent Geminids on December 14 should provide us with a satisfying performance.