Stardate:06:18:98
Nebula Hopping in the Eastern Sky
Gravity has finally won the battle. The weakest
of nature's forces, gravity waits for perhaps billions of years for the
fuel to run out in stars.
Mighty hydrogen bombs, the stars are stable for
many millions or billions of years because the force of the explosions
pushing the star outward exactly balance the gravity trying to collapse
it inward. Then comes a day when the fuel runs out. After a series of contractions
and expansions, stars the size of our Sun release their outer shell to
dance among the stars forever.
A much larger star will explode in a climatic
burst, and release it's outer shell in the process. Either way, much of
the star will expand forever, receding away from the now shrunken remains.
In a few cases, where this has happened more recently,
in the last few thousand years perhaps, we can still see the leftover gas
from the original star, slowly dimming over many thousands of years. There
are around 1500 of these planetary nebulae that we know of so far. but
perhaps the two most famous of these are the Dumbell Nebula, and the Ring
Nebula. Both of these objects are well in view this week in the eastern
sky.
To find the Dumbell Nebula. look just about ten
degrees north of due east about 10 pm, and travel nearly 24 degrees above
the horizon. This nebula looks like a Dumbell, and stretches 1/4 of a degree
across the sky. At magnitude 8.1, it is over fifteen times too dim to see
with the naked eye, but is an easy find with even a small backward telescope.
Also known as M27, it's NGC catalog is 6853. Many wonderful pictures have
been taken throughout the years by amateur astronomers of this object,
and many of those are published in astronomy magazines. The Dumbell Nebula
is an older nebula, whose material is now mixing with the interstellar
medium.
Less than 18 degrees away from M27 lies the famous
Ring Nebula in Lyra. Also known as M57 or NGC 6720, it is straight above
the Dumbell, 41 degrees above the horizon. It is bright enough to find
easily. About 40 percent as bright as the Dumbell Nebula, it is fairly
bright, but it appears very small. The Ring Nebula is a mere two and a
half arcminutes, or 1/24th of a degree across. Still, even with a small
instrument you should be able to see a central star. This is the remains
of the star from which.the Ring Nebula formed. It is now slowly cooling
and shrinking to form a white dwarf.
At one time, that star was about as massive as
the Sun, and probably about the same size. Long ago it ran out of fuel,
and started contracting, losing it's battle with gravity. The collapse
will not stop until the star has shrunken to about the size of the Earth.
Another type of nebula are globular clusters.
Unlike planetary nebulas, they are not the remains of recently departed
stars, but are instead collections of many thousands of stars happily living
out their lives, locked in gravitational embrace with one another.
One of the most dramatic examples of globular
clusters, the Great Cluster in Hercules, can be found less than 27 degrees
away from the Ring Nebula, in the constellation Hercules.
After viewing the Ring Nebula, turn your telescope
about 7-1/3 degrees to the south, and raise it by nearly 26 degrees. Here
lies the Great Cluster in Hercules, also known as M13 and-NGC 6-205.
Only once in the history of the human race have
we purposely sent a radio message to the stars. In November. 1974 a message
of greetings from the human race was sent toward M13. It will still be
nearly 26,000 years before our message arrives there.
Lastly, turn your gaze about 8-1/2 degrees toward
the north, and 4-1/4 degrees down toward the horizon. Here lies a fairly
bright globular cluster, M92. It is about 60 percent as bright as M13,
and appears about 2/3rd as wide across.