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Crashing comets
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"Victoria crater" Image Credit: NASA/JPL/Cornell SPACE
HOME (Space)
NASA spots
'gamma ray-only'
pulsar
The Indian's Space
Program
Spaceman drinks
urine
Water Purification
Black Hole
Apophis
Planets
Mars-Methane
NASA spots
'gamma ray-only'
pulsar
The Indian's Space
Program
Spaceman drinks
urine
Water Purification
Black Hole
Apophis
Planets
Mars-Methane
How many hits from comets
Astronomers at the University of Washington used computer simulation to
model the evolution of comet clouds in the solar system over the past 1.2
billion years. The simulation allowed them to peer into the Oort Cloud, a
remnant of the nebula from which our solar system was formed. The
simulation found the Earth has likely only sustained two or three significant
hits from comets in the past 500 million years. [1]
About Nebula
A nebula (NEHB yuh luh) is a cloud of dust particles and gases in space. The
term nebula comes from the Latin word for cloud. Early astronomers also
used the term for distant galaxies outside the earth's galaxy, the Milky Way.
Such galaxies, called extragalactic nebulae, looked like hazy patches of light
among the stars. But modern telescopes showed that extragalactic nebulae
are actually systems of stars similar to the Milky Way.
Today, most astronomers use the term nebulae only for the clouds of dust
and gases in the Milky Way and other galaxies. They classify these masses
into two general types: diffuse nebulae and planetary nebulae. Both types are
also called gaseous nebulae.
Diffuse nebulae are the larger of the two types. Some diffuse nebulae contain
enough dust and gases to form as many as 100,000 stars the size of the sun.
A diffuse nebula may occur near an extremely hot, bright star. The intense
ultraviolet light from the star energizes the gas atoms of the nebula and
enables the mass to emit light. A diffuse nebula of this kind is called an
emission nebula.
Astronomers believe some emission nebulae are places where new stars
are forming. Gravity causes a portion of a nebula's dust and gases to
contract into a much smaller, denser mass. Pressure and temperature build
up within the mass of dust and gases as contraction continues for millions of
years. In time, the mass becomes hot enough to shine -- and forms a new
star.
A diffuse nebula also may occur near a cool star. In this case, the ultraviolet
light from the star is too weak to make the nebula's gas atoms give off light.
But the dust particles in the diffuse nebula reflect the starlight. Astronomers
refer to this kind of diffuse nebula as a reflection nebula.
If a diffuse nebula occurs in an area that has no nearby stars, it neither emits
nor reflects enough light to be visible. In fact, its dust particles blot out the light
from the stars behind them. Astronomers call such a diffuse nebula a dark
nebula.
Planetary nebulae are ball-like clouds of dust and gases that surround certain
stars. They form when a star begins to collapse and throw off the outer layers
of its atmosphere. When viewed through a small telescope, this type of
nebula appears to have a flat, rounded surface like that of a planet. [2]
Reference
[1] Comets probably won't cause the end of life as we know it: study AFP Thu Jul 30, 2:09 pm
ET [2] O'Dell, C. R. "Nebula." World Book Online Reference Center. 2005. World Book, Inc.
http://www.worldbookonline.com/wb/Article?id=ar385540.
Astronomers at the University of Washington used computer simulation to
model the evolution of comet clouds in the solar system over the past 1.2
billion years. The simulation allowed them to peer into the Oort Cloud, a
remnant of the nebula from which our solar system was formed. The
simulation found the Earth has likely only sustained two or three significant
hits from comets in the past 500 million years. [1]
About Nebula
A nebula (NEHB yuh luh) is a cloud of dust particles and gases in space. The
term nebula comes from the Latin word for cloud. Early astronomers also
used the term for distant galaxies outside the earth's galaxy, the Milky Way.
Such galaxies, called extragalactic nebulae, looked like hazy patches of light
among the stars. But modern telescopes showed that extragalactic nebulae
are actually systems of stars similar to the Milky Way.
Today, most astronomers use the term nebulae only for the clouds of dust
and gases in the Milky Way and other galaxies. They classify these masses
into two general types: diffuse nebulae and planetary nebulae. Both types are
also called gaseous nebulae.
Diffuse nebulae are the larger of the two types. Some diffuse nebulae contain
enough dust and gases to form as many as 100,000 stars the size of the sun.
A diffuse nebula may occur near an extremely hot, bright star. The intense
ultraviolet light from the star energizes the gas atoms of the nebula and
enables the mass to emit light. A diffuse nebula of this kind is called an
emission nebula.
Astronomers believe some emission nebulae are places where new stars
are forming. Gravity causes a portion of a nebula's dust and gases to
contract into a much smaller, denser mass. Pressure and temperature build
up within the mass of dust and gases as contraction continues for millions of
years. In time, the mass becomes hot enough to shine -- and forms a new
star.
A diffuse nebula also may occur near a cool star. In this case, the ultraviolet
light from the star is too weak to make the nebula's gas atoms give off light.
But the dust particles in the diffuse nebula reflect the starlight. Astronomers
refer to this kind of diffuse nebula as a reflection nebula.
If a diffuse nebula occurs in an area that has no nearby stars, it neither emits
nor reflects enough light to be visible. In fact, its dust particles blot out the light
from the stars behind them. Astronomers call such a diffuse nebula a dark
nebula.
Planetary nebulae are ball-like clouds of dust and gases that surround certain
stars. They form when a star begins to collapse and throw off the outer layers
of its atmosphere. When viewed through a small telescope, this type of
nebula appears to have a flat, rounded surface like that of a planet. [2]
Reference
[1] Comets probably won't cause the end of life as we know it: study AFP Thu Jul 30, 2:09 pm
ET [2] O'Dell, C. R. "Nebula." World Book Online Reference Center. 2005. World Book, Inc.
http://www.worldbookonline.com/wb/Article?id=ar385540.