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 by sunrise.
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
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
You will find extensive descriptions of what's worth looking for,
and you can download and print a single copy for your personal use.