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SOLAR
SYSTEM
STARS
BLACK
HOLES
METEORS AND
METEORITES
ASTEROIDS
COMETS
GALAXIES
NEBULAE
GLOSSARY
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Stars
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BIRTH OF A STAR:
A star is formed when a large amount of gas (mostly hydrogen) starts
to collapse in on itself due to its gravitational attraction. As it
contracts, the atoms of the gas collide with each other more and more
frequently and at a higher and greater speed - the gas heats up. And
eventually the gas will be so hot that when the hydrogen atoms collide,
they will no longer bounce off each other, but instead coalesce to
form helium.The heat released in this reaction, which is like a controlled
hydrogen bomb explosion, is what makes the stars shine.The additional
heat also increases the pressure of the gas until it is sufficient
to balance the gravitational attraction, and the gas stops contracting.Stars
will remain stable like this for a long time, with the heat from the
nuclear reactions balancing the gravitational attraction.Eventually,
the star will however run out of hydrogen and other nuclear fuels.Paradoxically,
the more fuel a star starts off with, the sooner it runs out of fuel.This
is because,the more massive the star is, the hotter it needs to be
to balance the gravitational attraction, and the hotter it is, the
faster it will use up its fuel.When a star runs out of fuel, it starts
to cool and so to contract.
(Extract from the book - A brief history
of Time by Stephen Hawkings) |
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PULSARS:
In August, 1967 Radio Astronomers, in Cambridge
and England working with a new specialized type of radio telescope,
noticed something quite out of the ordinary. A large burst of radio
energy was coming from a certain spot in the sky every one thirtieth
(1/30 th) of a second. The new radio source was called a Pulsar
because of the characteristic pulsating signal received from it.
When these extremely precise radio signals were examined in detail,
it was found that the pulses were composed of sub pulses.
In physics, it is well known that no object as whole can produce
a pulse of energy shorter in length of time than it would take of
or what pulse of energy to travel across the object. The shortness
of the pulses indicated that the Pulsars could not be bigger that
a few thousand km across. A Pulsar is located in the Crab Nebula.
Like the radio and the x-ray pulses, the light pulses from it were
also discovered flashing on and off 30 times per second.
Recently, intensive studeis are being made to detect x-ray pulses
from other known pulsars especially those that seem to be associated
with the remnants of ancient supernovae.
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NEUTRON STARS:
About 40 years before the first Pulsar was
discovered, physicists and astronomers had hypothised the existence
of a 'Neutron Star' - a star with particles so densely packed that
the electrons and neutrons are as if squashed together so that entire
mass would be composed only of neutrons.
Neutron Stars are about a million times denser than a White Dwarf.Neutron
stars are so dense that on cubic inch of this material would weigh
more than a billion tonnes on Earth. For instance a six-foot man
squashed to such high densities would barely be visible through
an electron microscope.
There is convincing evidence that the Pulsars are the theoretically
predicted 'Neutron Stars'. After careful observations of the Pulsars,
for several years, it was found that their pulse rates decreased
with time but very slowly. This supported the theory that as the
Neutron Star rotates, its rotational speed decreases as it loses
energy through radiation. The pulse rate could be used as a rough
indicator of its age.
But whether these pulsars are indeed the same objects as those theoretically
postulated Neutron Stars and whether they are originated from Supernovae
explosions are questions that are still not fully answered.
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