"The Three-Tailed Comet"
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For Sky Watchers the event of the year was clearly the arrival
of comet Hale-Bopp, which burned bright in northwestern skies from late
March through May. The biggest and brightest comet since the Great Comet
of 1811, it was visible even through murky city air. The first sightings,
in July 1995, had been made by two experienced observers, Alan Hale and
Thomas Bopp, and two years later their peers came out in force. Astronomy
clubs planned Hale-Bopp parties; amateur photographers flooded the Internet
with snapshots and viewing tips. And on another plane entirely, members of a
financial New Age cult committed mass suicide, hoping to ride the comet
to a higher form of existence.
In the ensuring uproar the astronomy seemed almost secondary.
But Hale-Bopp was also of great scientific interest: it was the first
comet with three tails ever seen.
Even lazy observers could easily spot the bright but squat dust
tail, and a little more effort revealed the narrower tail of ionized gas.
The first reports of the third tail came on April 18 from a team of
European astronomers at the Isaac Newton Group of Telescopes on the
Canary Islands. Their images of the comet revealed a thin , faint jetlike
tail some 30 million miles long and nearly 400,000 miles wide, pointing in
the direction almost exactly opposite the sun --- and made entirely of
sodium atoms. By May, other astronomers had processed their own images
of the comet, and thy saw a sodium tail too --- although not necessarily of
the same shape and size. "In our data it is wide --- roughly the same
sizes as the dust tail --- and it gets wider as it moves away from the
comet, and it's also slightly curved," says astronomer Jody Wilson of
Boston University
"The European team has suggested that the tail is produced by
sodium gas escaping directly from the nucleus," Wilson goes on. "But
that is how you get narrow tail." He and his colleagues have another
theory to explain the wide sodium tail: the sodium atoms are coming off
the grains in the comet's wide-fanning dust tail. But whereas the dust
is pulled by the sun's gravity into a curving wake behind the comet
nucleus, the lighter sodium atoms are influenced more strongly by the
pressure of solar radiation and are pushed away from the sun. Thus they
separate from the dust and form a distinct third tail --- or at least
it was distinct if you had a large telescope.
Astronomers don't expect to find hidden sodium tails in all the
comets that cross our skies. Last year's comet Hyakutake, for one, was
noticeably lacking in sodium. "You would expect more sodium to come from
comets that have more dust," Wilson says. "That is consistent with
Hyakutake. It was almost all ices that turned into gas, and we say very
little sodium."
By Kathy A.Svitil
以上的內容是從discover中找到的...
雜誌裡有圖片，可以去翻翻看~~~
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未燃的蠟燭怎能明亮?
未燃的蠟燭怎能溫暖?
但它若被火光點上,就是寒夜裏最真實的太陽.....
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※ 來源: 師大精靈之城 <bbs.ntnu.edu.tw> ◆ From: PSHSU.m8.ntu.edu.t