Keeble Observatory
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 well.)
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 pm.
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.
Copyright 2002
George Spagna