Save our starry night skies

Be a considerate neighbor: Reduce nighttime glare.
Shield all your outside lights downward
(or turn them off completely)
and enjoy the beautiful, starry night sky.

December 2007 Night Sky

—CHARLIE CHRISTMANN

Something wonderful, spectacular, and completely unexpected happened on October 23rd. A small, unremarkable comet called P17/Holmes exploded. But this is not the first time P17/Holmes has had an outburst.

In November 1892, astronomer Edwin Holmes was observing the Andromeda galaxy. On the 6th of the month, an object brightened over the course of several evenings to about 4th magnitude. Then, during the next several weeks, it faded away. Holmes was the first to observe the comet that now bears his name. More observations in 1892 showed that P17/Holmes is a short-period comet orbiting the Sun every 6.9 years. The comet was observed again in 1899 and 1906, but it was then lost for more than fifty years. Using a computer-aided orbital prediction, the U.S. Naval Observatory in Flagstaff was able to locate it again in 1964.

On the evening of October 23rd of this year, P17/Holmes exploded, going from a very dim magnitude seventeen to bright magnitude 2.5 in just a few hours (smaller magnitude numbers are brighter). It became easy to find in the northern constellation Perseus as a bright yellow “star.” By October 25th, 17P/Holmes appeared as the third brightest “star” in that constellation. As October closed, its comet-like nature started to appear as the cloud of dust and gas expanded, giving the object a fuzzy quality. The cloud surrounding the rocky nucleus of a comet is called the coma.

In late October, the coma appeared to be about half the diameter of a full moon. By November 9th, the coma’s diameter was 869,900 miles, based on measurements from the University of Hawaii Institute for Astronomy. By comparison, the sun’s diameter is about 864,900 miles. P17/Holmes currently has the largest extended atmosphere in the solar system.

How far away is the comet now? P17/Holmes is 149 million miles away from Earth. As it circles the Sun, its orbit never goes inside Mars’ orbit; nor does it ever go outside Jupiter’s orbit.

How big is the nucleus? The actual solid body of the comet is estimated to be 2.2 miles in diameter.

How long will the comet be visible? Well, it is already fading as the cloud expands and dissipates into space. The dust and gas expelled from the nucleus is racing away at eleven hundred miles per hour. So the show is coming to a close—unless there is another outburst.

What caused the outburst? Scientists can only speculate. The blast may have resulted from a collision with a meteoroid, or, more likely, a build-up of gas inside the comet’s nucleus eventually broke through the surface. The asymmetrical nature of the coma suggests a large chunk broke off the main nucleus body and disintegrated into tiny dust particles.

Where is the tail? Most comets form a tail as the gas and dust boil off the nucleus and the coma is pushed away from the Sun by the solar wind. Recent images of P17/Holmes are beginning to show a tail forming. Don’t expect to be able to see a tail unless you have a professional telescope.

When will the comet be back? Look for P17/Holmes to be in our neighborhood in 2013. Who knows, perhaps it will put on another spectacular show.

METEORS

One of the most predictable meteor showers of the year is scheduled to peak on December 14th before dawn. Astronomers are expecting one to two meteors every minute for North American observers with dark, clear skies. The Geminids are named for the constellation of Gemini, the Twins. On the nights of December 13th and 14th, the meteors appear to emanate from a spot in the sky near the bright star Castor in Gemini. This year’s event should not be hampered much by a crescent moon.

THE PLANETS AND THE MOON

• Mercury will be very low in the eastern sky about thirty minutes before sunrise. It will be difficult to spot.

• Venus is rising in the east about 4:45 this month below Virgo. It is hard to miss bright Venus at -3.8 magnitude.

• Mars rises about two hours after sunset in the east, just above the constellation Gemini. Look for Mars just to the lower left of the full moon on the 23rd near the northeastern horizon. Mars is at its closest approach to Earth on the 24th—a good time to look at the red planet through a telescope.

• Jupiter will set in the west about 6:30 p.m. If you look with binoculars in the southeast, you might just be able to spot the giant planet near the horizon.

• Saturn is rising around midnight in the east. Look for it in the constellation Leo. The Moon will be three degrees to the lower left of Saturn on the 1st. The bright star to the right of Saturn is Regulus. The event almost repeats again on the 28th.

• The Moon is new on the 9th at 10:40 a.m. MST. The Full Moon occurs at 6:16 p.m. on the 23rd.

• Winter officially begins on December 21st at 11:09 p.m. MST.

Interested in meeting me in person? Come to the Placitas Library on December 13 at 7:00 p.m. I’ll be giving an hour-long talk on two diverse subjects: the Geminid meteor shower and our nearest star, the Sun.

If you have a question or comment for Charlie, you may email him at: k5cec@yahoo.com.