Decmber 2001 Sky from Keeble Observatory

Early in the 19th Century, chemists were beginning to look at the light emitted from various chemicals and to analyze that light with prism spectrometers. These instruments take advantage of the slight variations with color of the speed of light in glass to disperse light from a source into its component wavelengths. The "Bunsen burner" was developed initially to produce a hot, colorless flame in which chemicals could be vaporized for just this purpose. Gustav Kirchhoff, a German chemist formulated some simple rules for interpreting spectra:

Hot, opaque surfaces emit a continuous spectrum (i.e. all the colors are present) which can be analyzed to determine the temperature of the surface.

Hot, transparent gases emit a discrete spectrum (i.e. only some specific colors are present - called "spectral lines") which is characteristic of the gas's chemical composition.

A hot, transparent gas in front of a hot, opaque surface will produce a continuous spectrum minus the lines which would have been observed from the gas alone, a so-called "absorption spectrum.

Note, that Kirchhoff had no idea why these rules apply! He was just a good enough observer to recognize that they did. Astronomers quickly began to apply these rules to their study of stars and nebulae. One thing they noted was that stars typically show absorption spectra (due to the stars' hot atmosphere) and most nebulae show emission line spectra. In the 1920s we added radio observations to our astronomer's tool kit, and found radio frequency line emission and absorption in the interstellar gas.

The 1960s brought the first observations of the continuous microwave background radiation, a relic of the Big Bang. Now we have another continuum radio source to observe. Researchers using the 140' radio telescope at Green Bank, West Virginia (not the largest one there, by any means!) have detected continuum radio emission from dust grains in the interstellar medium.

These grains are very small, less than a thousandth of a millimeter, and have apparently been ionized by ultraviolet radiation from nearby stars or by cosmic ray bombardment. This ionization leaves them with a residual electric charge. Further interaction with charged cosmic ray particles can spin them up to billions of revolutions per second, and we know that a spinning charge will radiate. Once the stuff of theory, this radiation has now been detected. Its observation will give us yet another probe into the state of matter in these dust clouds which are not penetrated by visible light. And, we may have to correct for its contribution to the cosmic background radiation as we seek to unravel the story it tells about the early universe.

Lunar phases for December: Last Quarter on the 7th, 2:52 pm; New Moon on the 14th, 3:47 pm; First Quarter on the 22nd, 3:56 pm; Full Moon on the 30th, 5:40 am. The New Moon on the 14th will be accompanied by a partial solar eclipse. "First Contact" for observers in this area will be about 4:15. Unfortunately, the Sun will set before the eclipse is too far advanced. Observers on the West Coast will see the whole thing. Observers just south of Hawaii will see this as an annular eclipse, in which the Moon is too far away to perfectly match the size of the Sun on the sky. The Full Moon on the 30th will be accompanied by a penumbral lunar eclipse, which will probably not be noticeable.

Evening planet watchers have both Jupiter and Saturn as good targets almost all night. Jupiter will be rising at sunset by the end of the month. Mars is still visible, but it's fading fast. Mercury will appear as an "evening star" towards the end of the month, setting about an hour after sunset by New Year.

Andromeda and its famous galaxy will be almost directly overhead at 8:00 pm mid-month. Cassiopeia looks like a capital M scrawled high above the northern horizon. Cygnus is a little north of west, standing almost like a cross, it looks less like a swan at this time of year, hence its other common name, the Northern Cross. Vega and its home constellation Lyra are below and to the left, amost directly above the northwest horizon. Altair is setting almost due west - you may not see it if there's any clutter on the horizon. Rising to the east we see both Orion and Gemini. The constellations are about the same distance above the horizon, Gemini on the right, Orion to the left, and both "lying down." Jupiter spends the month in Gemini. Above Orion is Taurus, marked by the bright star Aldebaran and the planet Saturn. Also prominent is the open cluster known as the Pleiades. The Japanese call this group "Subaru" - which explains the seven stars in the logo of that manufacturer.