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NATURE’S
OWN FIREWORKS SHOW
by Frank Snively (AVAS Co-President) - 11/18/01
Article About the 2001 Leonid Meteor Shower
Insomniacs, graveyard
shift workers, and people who keep up with the astronomical news
were treated to a real treat on Sunday morning, November 18, 2001
between 3:00 AM and dawn. There were many bright streaks in the
sky as the Leonid Meteor shower proceeded. Those who remained asleep
missed quite a show.
Anyone who goes
outside when there is little moon at night, and who happens to be
in an area where taxpayer dollars are not being wasted on misdirected
outdoor lights, can see an occasional bright streak in the sky.
Usually, it can
be pretty frustrating if there is more than one person present.
“Hey, did you see that?” is the usual question, with the
almost universal answer “Did I see what?” That wasn't
a problem for the sleep deprived on Sunday morning. The conversations
went more like: “There's one.” “Hey, I saw two.”
Let's try to
explain what is going on. The bright meteor streak you see in the
sky is a small bit of material, no bigger than a grain of coarse
sand, which is going so fast that it heats itself and the surrounding
air to incandescence. Yes, the air itself. The air cant move out
of the way faster than the speed of sound, so it piles up in front
of the object and is compressed mightily. The compression heats
the air so much it glows brightly. The object itself is usually
vaporized by the intensely heated air, leaving what appears to be
a smoke trail.
There are always
a certain number of bits of matter in interplanetary space. But
for a real show, it is necessary to have a collection of objects.
The source is usually a comet.
What is a comet?
Comets are remnants of the formation of the solar system, a frozen
collection of the gases and dust which condensed to form the sun,
the planets and you and me. Not all was collected into planets,
and there are remnants in the outer reaches of the solar system,
beyond even the furthest planets. Every so often, the repeated tug
of gravity from a passing planet - usually Jupiter, which is the
heaviest - or the even more gentle nudge of a passing star causes
one of these objects to come toward the sun. The heat of the sun
causes some of the frozen gases in the comet to evaporate and carry
off some of the dust. The gases, such as the oxygen and nitrogen
in the air we breathe, methane, carbon monoxide, etc. are blown
away by the pressure of the particles streaming from the sun, and
by the pressure of the sunlight itself. The gas mixture is made
to glow by the sunlight and forms the“tail” of the comet.
But the dust
particles are too heavy to get blown far away. They tend to follow
(approximately) the orbit of the parent comet. They don't spread
that much to the side, at least not much by astronomical standards,
a few million miles, typically. However they spread out hundreds
of millions of miles ahead and behind along the comet's path. A
very long thin cloud!
When the earth
flies into that cloud in its orbit around the Sun, we have a meteor
shower. That's all there is too it. (The Editor asks “Why is
this article so long?”. So do the readers.) While that cloud
is long, it isn't uniform, and is thickest near the parent comet.
There is a bit of a meteor shower every year, when the earths orbit
passes through the cloud. It is thickest for a few years after the
comet has gone by.
Sometimes the
parent comet which formed the cloud has been located and named.
That is the case with the meteor shower of Sunday, November 18.
Trivia buffs should remember the name “Temple-Tuttle”,
which is the parent comet. It passed by about five years ago, leaving
the debris we saw. It returns every 33 years, but will not be well
placed for bright showers until 2099. If you missed this one, well,
too bad.
Of course, there
is a related question, why are the meteor showers named after constellations?
Besides Leonid (constellation Leo), we have Perseids, Geminids,
and so on. The reason is that the meteors seem to originate in the
named constellation. It was really obvious with the Leonids. Meteor
trails seemed to come from a common point, that was easy to locate.
Just draw an imaginary line back along the path of each of several
meteor trails that appear in different parts of the sky. The lines
intersect at a common point, which seemed to be in the “sickle”
shape pattern of stars that form Leo. In fact, last night was the
first time this writer knew for sure where Leo was located in the
heavens.
On rare occasions,
the object making the meteor is so large it strikes the ground.
Then it is called a meteorite. Some samples will be shown at the
December meeting of the Arkansas Valley
Astronomical Society.
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