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Comets
Comets are small, easily
broken, erratically shaped bodies composed mostly of a mixture
of water ice, dust, and carbon- and silicon-based compounds.
They have highly oval orbits that frequently bring them very
close to the Sun and then swing them into space. Comets have
three distinct parts: a nucleus, a coma, and a tail. The solid
core is called the nucleus, which develops a coma with one
or more tails when a comet sweeps lock to the Sun. The coma
is the dirty, nebulous cloud around the nucleus of a comet,
and the tail extends from the comet and points away from the
Sun. The coma and tails of a comet are brief features, present
only when the comet is near the Sun.
Formation Of Comets
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Our entire
solar system, counting comets, formed from the collapse
of a giant, disperse cloud of gas and dust about 4.5
billion years ago. When cloud was continuing its collapse,
it was revolving very slowly. But the cloud began to
heat up and spin faster as it shrank, just as twirling
ice skaters spin earlier by bringing their arms lock
to their bodies. The fast rotation helped make sure
that not all of the material chops into the core. Instead,
the material in the fast-spinning cloud spread out into
a trodden disk. Meanwhile, the temperature in the dense,
central core was heating up. |
The core ultimately became
so hot that it ignited nuclear fusion, creating the Sun. The
disk's external regions, however, were quite cold. The low
temperatures permitted water to ice over onto dust grains,
which grew in size to make clumps. Some clumps in time reached
a size of numerous kilometers in diameter. The clumps then
began amalgamation, most likely by collisions, and formed
the planets. Many theories abound about how these clumps became
planets. This topic is at the forefront of technical research.
Whatever the details, large planets were created from the
buildup of clumps of matter and gas from the nearby cloud.
But some of this matter did not merge into planets.
Within the last decade, for example, astronomers discovered
leftover clumps, called planet, in a region beyond Neptune,
although no large planets formed beyond that planet. These
bodies form an outer asteroid belt at the edge of the solar
system, called the Edgeworth-Kuiper belt, named for the scientists
who proposed its existence in the 1950's. Recent calculations
show that this asteroid-rich Kuiper belt (as it is now known)
is most likely the source of most of the short-period comets,
such as Halley's comet, which orbits the Sun every 76 years.
Comet’s
Tail
A comet's tail is its most typical feature.
As it approaches the Sun it develops an enormous tail
of polished material that extends for millions of kilometers
away from the Sun. When far from the Sun, a comet's
nucleus is very cold and its fabric is frozen. Water
ice, as well as other compounds such as carbon dioxide
and carbon monoxide ice, may be found in the nucleus.
This icy nucleus changes fundamentally when a comet
approaches the Sun. The strong solar wind from the Sun
transforms the solid nucleus directly into a vapor,
bypassing the liquid phase. This process is called sublimation.
The vapor helps stir things up in the nucleus, forcing
the core to form a cloud-like mixture of gas and dust
around it, called the coma. There, sunlight and the
solar wind relate with the ingredients, creating the
tails. |
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Features
Of Comet
The ingredients in the coma determine the types and number
of tails. Some comets may emerge to have no tails, but they
really do. They are simply very faint. Scientists can identify
these tails by using special filters that are aware to dust
or gas emissions. Other comets, which could be seen from Earth
in 1997, have very prominent tails. Although Hale-Bopp's tails
could be seen apparently from Earth, scientists using sensitive
cameras familiar a much more difficult tail structure. One
of these images revealed a long, curving dust tail. Other
pictures showed dust and gas ion tails. There was even an
image of a dust tail and two gas ion tails. The different
tails provide scientists with important information about
the internal chemistry and structure of a comet's nucleus.
Types
Of Comets
There are two types of comet tails: dust and gas ion. A dust
tail, which is usually yellow, contains small, solid particles
that are about the same size as those found in cigarette smoke.
This tail forms because sunlight acts on these small particles,
gently pushing them away from the comet's nucleus. Because
the pressure from sunlight is relatively weak, the dust particles
end up forming a diffuse, curved tail. A gas ion tail, which
is usually blue, forms when ultraviolet sunlight rips one
or more electrons from gas atoms in the coma making them into
ions (a process called ionization). A solar wind then carries
these ions straight outward away from the Sun. The resulting
tail is straighter and narrower. Both types of tails may extend
millions of kilometers into space. As a comet heads away from
the Sun, its tail dissipates, its coma disappears, and the
matter contained in its nucleus freezes into a rock-like material.
Recent observations of the very bright comet Hale-Bopp pinpointed
a tail made of sodium (Na), a relative of the gas ion tail.
This tail forms when sunlight pushes on sodium atoms released
from the nucleus.
vAlso
see the Meteors.
There you can find about meteors in details.
vAlso about
the Regions
Where Comets Found and
Orbits Of Comets.
vAbout
Asteroids
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