December 2004 Sky from the Keeble Observatory
Think of a 100 watt light bulb. Now, imagine that same bulb seen from a distance
of 93 million miles ... you wouldn't be able to read your Herald-Progress by that
light! But, reading in full sunlight is, if anything, uncomfortably bright. The
Sun is 93 million miles away, and is the equivalent of hundred watt bulbs (that's
4 followed by 24 zeroes - 4 trillion trillion), delivering about 1000 watts to each
square meter at the Earth's distance. Yet, the Sun is an average star, brighter
than many, but fainter than others. The most massive stars are more than a million
times brighter, the lowest mass stars a mere .05% as bright. Stars appear faint
in the night sky because of their great distances.
Now, imagine a light source times brighter than the Sun, but lasting only a few
milliseconds to a few minutes, but shining only at wavelengths shorter than x-rays.
Such a source is called a gamma ray burst, and NASA's latest probe has been launched
to find and study them. Bursts occur all over the sky, at a rate of perhaps several
hundred per year. Since they are such short-lived phenomena, by the time one has
been detected, it's typically too late to bring other instruments to bear before
the event fades to oblivion.
SWIFT should change that. Launched on November 20, this probe carries a wide angle
survey telescope to find the bursts, and a pair of x-ray and gamma ray telescopes
to make detailed observations. It also has the capability of swiftly repoint itself
(hence the name!) so that the narrower field, higher precision instruments can study
the event and its afterglow. The mission also includes real-time internet alerts
so that other observers can study the phenomenon.
What it is that is being studied is still a mystery. Most theorists believe that
each gamma ray burst represents the creation of a black hole, probably through the
collision of a pair of neutron stars. That's some birth announcement!
Lunar phases for December: Last Quarter on the 4th, at 7:53 pm; New Moon at 8:29
pm, on the 11th; First Quarter on the 18th, at 11:40 am; Full Moon on the 26th,
at 10:06 am.
Evening skies finally have a planet, with Saturn rising to the east about 3 hours
after sunset. It should be visible all night, moving to 20 degrees off the west-northwest
Mercury is back in the predawn sky, about 10 degrees above the southeast at sunrise.
Jupiter and Venus have moved apart after last month's conjunction. Venus is now
only 20 degrees above the southeast horizon at sunrise, about 5 degrees below Mars,
while Jupiter is almost due south almost 50 degrees above the horizon.
Three hours after sunset we find the Milky Way nearly dividing this sky from west
to east, bowed slightly to the north. Cygnus has slipped another 10 degrees towards
the northwest horizon, compared to last month, and looks every bit the "northern
cross" with bright Deneb at the top, and Albireo at the foot of the cross.
To the north, Cassiopeia is high above the horizon (Ursa Major is low, and may not
even show above the clutter and haze), making a distinctive M shape. Recall six
months ago? Then we described this constellation as a W.
Almost directly overhead, you can now easily find the Andromeda Galaxy, perhaps
a twin of our own Milky Way, but some 2 million light years distant. The chart below
shows where Andromeda lies in relation to Cassiopeia. It was produced with Sienna
Software's "Starry Night Pro" software.
Best viewing will be on clear, cold nights with no moon. Stick around until the
Sun dies (another 5 billion years) and Andromeda will be in a collision with our
home Galaxy. The collision will be slow, and will eventually merge the two spirals
into a single elliptical galaxy.
Saturn rises to the east, near the bright pair Castor and Pollux. Towards the southeast,
Orion rises majestically on its side, the familiar belt almost perpendicular to
the horizon. These familiar constellations will dominate the night sky for the next
several months. Winter is here!
For your own monthly star chart, you can direct your web browser to
http://www.skymaps.com. You will find extensive descriptions of what's worth
looking for, and you can download and print a single copy for your personal use.