Download Our Cosmic Neighborhood From our small world we have gazed

Our Cosmic Neighborhood
From our small world we have gazed upon the cosmic
ocean for thousands of years. Ancient astronomers
observed points of light that appeared to move among
the stars. They called these objects "planets," meaning
wanderers, and named them after Roman deities—
Jupiter, king of the gods; Mars, the god of war; Mercury,
messenger of the gods; Venus, the goddes of love and
beauty, and Saturn, father of Jupiter and god of
agriculture. The stargazers also observed comets with
sparkling tails, and meteors or shooting stars apparently
falling from the sky.
Since the invention of the telescope, three more planets
have been discovered in our solar system: Uranus (1781),
Neptune (1846), and, now downgraded to a dwarf
planet, Pluto (1930). In addition, there are thousands of
small bodies such as asteroids and comets. Most of
the asteroids orbit in a region between the orbits of Mars
and Jupiter, while the home of comets lies far beyond
the orbit of Pluto, in the Oort Cloud.
The four planets closest to the sun—Mercury, Venus,
Earth, and Mars—are called the terrestrial planets
because they have solid rocky surfaces. The four large
planets beyond the orbit of Mars—Jupiter, Saturn,
Uranus, and Neptune—are called gas giants. Tiny,
distant, Pluto has a solid but icier surface than the
terrestrial planets.
Nearly every planet—and some of the moons—has
an atmosphere. Earth's atmosphere is primarily
nitrogen and oxygen. Venus has a thick atmosphere of
carbon dioxide, with traces of poisonous gases such as
sulphur dioxide. Mars's carbon dioxide atmosphere is
extremely thin. Jupiter, Saturn, Uranus, and Neptune
are primarily hydrogen and helium. When Pluto is near
the sun, it has a thin atmosphere, but when Pluto travels
to the outer regions of its orbit, the atmosphere freezes
and collapses to the planet's surface. In that way, Pluto
acts like a comet.
Moons, Rings, and Magnetospheres
There are 140 known natural satellites, also
called moons, in orbit around the various planets in our
solar system, ranging from bodies larger than our own
moon to small pieces of debris.
From 1610 to 1977, Saturn was thought to be the only
planet with rings. We now know that Jupiter, Uranus,
and Neptune also have ring systems, although Saturn's
is by far the largest. Particles in these ring systems range
in size from dust to boulders to house-size, and may be
rocky and/or icy.
Most of the planets also have magnetic fields, which
extend into space and form a magnetosphere around
each planet. These magnetospheres rotate with the
planet, sweeping charged particles with them. The sun
has a magnetic field, the heliosphere, which envelops
our entire solar system.
Ancient astronomers believed that the Earth was the
center of the universe, and that the sun and all the other
stars revolved around the Earth. Copernicus proved that
Earth and the other planets in our solar system orbit our
sun. Little by little, we are charting the universe, and an
obvious question arises: Are there other planets where
life might exist? Only recently have astronomers had the
tools to indirectly detect large planets around other stars
in nearby solar systems.
—Text courtesy NASA/JPL
A star is born when atoms of light elements are squeezed under enough
pressure for their nuclei to undergo fusion. All stars are the result of a
balance of forces: the force of gravity compresses atoms in interstellar gas
until the fusion reactions begin. And once the fusion reactions begin, they
exert an outward pressure. As long as the inward force of gravity and the
outward force generated by the fusion reactions are equal, the star remains
stable.
Clouds of gas are common in our galaxy and in
other galaxies like ours. These clouds are called
nebulae. A typical nebula is many light-years
across and contains enough mass to make several
thousand stars the size of our sun. The majority
of the gas in nebulae consists of molecules of
hydrogen and helium--but most nebulae also
contain atoms of other elements, as well as some
surprisingly complex organic molecules. These
heavier atoms are remnants of older stars, which
have exploded in an event we call a supernova.
The source of the organic molecules is still a
mystery.
Irregularities in the density of the gas causes a
net gravitational force that pulls the gas
molecules closer together. Some astronomers
think that a gravitational or magnetic disturbance
causes the nebula to collapse. As the gases collect,
they lose potential energy, which results in an
increase in temperature.
Image: Hubble Space
Telescope
STAR BIRTHS are
started when the
interstellar matter in gas
clouds, such as the Eagle
Nebula shown here,
As the collapse continues, the temperature
compresses and fuses.
increases. The collapsing cloud separates into
many smaller clouds, each of which may eventually become a star. The core
of the cloud collapses faster than the outer parts, and the cloud begins to
rotate faster and faster to conserve angular momentum. When the core
reaches a temperature of about 2,000 degrees Kelvin, the molecules of
hydrogen gas break apart into hydrogen atoms. Eventually the core reaches
a temperature of 10,000 degrees Kelvin, and it begins to look like a star
when fusion reactions begin. When it has collapsed to about 30 times the
size of our sun, it becomes a protostar.
When the pressure and temperature in the core become great enough to
sustain nuclear fusion, the outward pressure acts against the gravitational
force. At this stage the core is about the size of our sun. The remaining dust
envelope surrounding the star heats up and glows brightly in the infrared
part of the spectrum. At this point the visible light from the new star cannot
penetrate the envelope. Eventually, radiation pressure from the star blows
away the envelope and the new star begins its evolution. The properties and
lifetime of the new star depend on the amount of gas that remains trapped.
A star like our sun has a lifetime of about 10 billion years and is just
middle-aged, with another five billion years or so left.
Margaret M. Hanson, an assistant physics professor at the
University of Cincinnati, gives this response:
Stars form from the gravitational collapse of large clouds of interstellar
material. In fact, the space between stars is not empty; it is nearly empty,
but not entirely. Interstellar matter, that found lying between the stars, is
made from gas and dust. Granted, only about 10
percent of the mass in our Milky Way galaxy is made up
of interstellar matter. But this material, as tenuous as it
is, exerts a gravitational force, and as a result, it will
begin to pull itself together.
As this accretion continues, the gravity becomes
increasingly strong because its strength rises as the
mass increases and the distance of the individual atoms
decreases. Eventually this interstellar matter entirely
collapses in on itself. The material at the very center is
compressed by the infalling material on the outside,
pushing down to get to the center. And this
compression heats up the center of the collapsing cloud.
Image: Hubble
Space Telescope
GALACTIC
NURSERY.Many
stars are born in
the beautiful
Orion Nebula.
At some point, the temperature gets so extremely high
at the center, it triggers a fusion reaction. All the material that has fallen in
then evolves into a hot, bright star. The star will continue to shine as long as
there is hydrogen gas to fuse through nuclear reactions, and the
gravitational pressure pushing inward keeps the atoms very hot and tightly
packed at the center. For a more advanced, elaborate description, with
wonderful pictures, see the Web site A Star Is Born, put together by Lee
Carkner of the University of Colorado.
Solar sytem
our solar system is nearly five billion years old. It is made up of eight planets,a
handful of so-called dwarf planets, and more than 170 moons, as well as dust, gas,
and thousand of asteroids and comets, all orbitting around a central sun.