December 2002 Sky from the Keeble Observatory
Black holes seem like the exotic stuff of science fiction, yet today we acknowledge
them as only too real. Einstein's General Theory of Relativity tells us that every
object with mass warps space and time in its vicinity. We aren't usually aware of
this, because the distortion is very small except for very massive objects, like
stars or galaxies. In 1919 it was confirmed that starlight passing near the surface
of the Sun is deflected a tiny amount - exactly as predicted by Dr. Einstein's theory.
(When first told of this confirmation, he was asked how he would have felt if the
observations had not supported his calculations. His reply, typically wry, was that
It would have been a pity, because the theory is correct.)
The strength of the gravitational field (i.e. the amount of curvature of space-time
it causes) is proportional to the mass of the object, and inversely proportional
to the square of the distance from its center. At any distance from a star or planet
(or a person, for that matter) we can calculate the escape speed necessary for a
projectile to recede forever. The more massive the object, the greater speed required.
(Escape from Earth's surface requires a speed of about 7 miles per second. Escape
from the Sun's surface takes about 125 miles per second. Compress the Sun into an
Earth-sized ball, and the escape speed will be 300 times greater (same mass, smaller
radius). Compress it to a ball a few kilometers in radius, the escape speed is greater
still. Keep compressing, and eventually that escape speed is the speed of light
. the object is now a black hole!
Where do we find them? Massive black holes, in excess of a million solar masses,
are found at the cores of many galaxies - including our own Milky Way, whose core
regions are about 25,000 light years away in the constellation Sagittarius. They
are detected by observing orbital motions of stars near the core, and by the x-ray
emissions from gas heated as it falls into the black hole. (And, no, they are not
cosmic vacuum cleaners sucking in their surroundings. Outside the event horizon
they are just massive gravitational centers, and it's possible to orbit or escape.
It's from the event horizon inward that they represent an inescapable potential
And, now, one has been detected of far smaller mass and much closer to Earth. The
object, with the catalog designation GRO J1655-40, is some 6000 - 9000 light years
away, heading in our general direction at about 250,000 miles per hour. (Don't sell
your real estate just yet - it will take about 15 million years to get here!) It
is detected because it has a close companion star, which orbits it every 2� days.
Gas escaping the atmosphere of this companion is falling into the black hole, emitting
copious x-rays in the process. Some of that gas is being ejected from the system
in extremely energetic jets of debris, streaming away at nearly the speed of light.
This find is exciting, because it seems to confirm the theoretical notion that one
way to make a black hole is from a supernova explosion - essentially compressing
the core of the exploding star at the same time the envelope is blasted into space.
This particular black hole probably formed in the inner part of the Galaxy's disk,
where star formation proceeds at a faster rate than in our vicinity. Other than
a supernova, it is difficult to imagine any real phenomenon which could propel a
stellar mass object to such speed. Thus, it provides a link between our theories
of stellar evolution and our ideas about black holes.
Lunar phases for December: New Moon at 2:34 am on the 4th; First Quarter at 10:49
am on the 11th; Full Moon on the 19th at 2:10 pm; Last quarter on the 26th at 7:31
Saturn will be at opposition on the 17th, so this is a good month to look for the
ringed planet virtually all night long. It rises ENE about an hour after sunset
at the beginning of the month, just about sunset at mid-month. This year's opposition
is only 7 months before it makes perihelion (its orbital period is just over 29
years, so this is about as good as it gets). Mercury joins the evening sky at mid-month,
but will be low to the southwest and not easy to pick out from ground clutter before
it sets. Jupiter rises about 5 hours after sunset and remains visible till dawn.
Venus is brilliant in the predawn sky, high to the southeast, joining Mars, Jupiter
and Saturn as good morning objects. These 4 planets span about 120 degrees at the
beginning of December, spreading out to almost 150 degrees by month's end.
An overhead view at mid-month (about 9:00) finds the Andromeda Galaxy west of zenith.
Recall that it is the most distant object directly visible without a telescope,
some 2 million light years distant. It's headed this way, but don't worry about
a collision with the Milky Way for the next few billion years! The plane of our
own Galaxy makes an arc slightly north of zenith, now crossing the horizon at southeast
and northwest. To the northwest Cygnus shows itself as the Northern Cross, with
the upright marking the plane of the Milky Way. Deneb is the bright star at the
top, the binary Albireo marks the cross's base. To the east we see bright Capella
high over the horizon, above Castor and Pollux in Gemini. Orion is to the southeast,
with the distinctive stars of the belt vertical. Sirius is below Orion. To the southwest
we see few bright stars, save only Fomalhaut, low on the southwest horizon. This
is the direction out of the plane of the Galaxy. In the north, the Big Dipper (Ursa
Major) is low on the horizon. Cassiopeia is high above the horizon, now looking
like an irregular letter M.
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.