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Animation of a supernova explosion, ending
in a pulsing neutron star |
Supernovae
Supernovae (the plural of supernova) are extremely important for
understanding our Galaxy. They heat up the interstellar medium, distribute
heavy elements throughout the Galaxy, and accelerate cosmic rays. But just
what is a supernova? And is there more than one type?
Indeed, there seems to be two distinct types of supernovae -- those which
occur for a single massive star and those which occur because of mass
transfer onto a white dwarf in a binary system. As you will see, however, it is only what gets
the process started toward the explosion which differs between the two
types.
Supernovae from Single, Massive Stars
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The Life Cycle of a Massive Star
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Stars which are 8 times or more massive than our Sun end their lives in a
most spectacular way; they go supernova. A supernova explosion will occur
when there is no longer enough fuel for the fusion process in the core of
the star to create an outward pressure which combats the inward gravitational
pull of the star's great mass. First, the star will swell into a red
supergiant...at least on the outside. On the inside, the core yields to
gravity and begins shrinking. As it shrinks, it grows hotter and denser. A
new series of nuclear reactions begin to occur....temporarily halting the
collapse of the core... but alas, it is only temporary. When the core
contains essentially just iron, it has nothing left to fuse (because of
iron's nuclear structure, it does not permit its atoms to fuse into
heavier elements). Fusion in the core ceases. In less than a second, the
star begins the final phase of gravitational collapse. The core temperature
rises to over 100 billion degrees as the iron atoms are crushed together.
The repulsive force between the nuclei overcomes the force of gravity.
So the core compresses, but then recoils. The energy of the recoil
is transferred to the envelope of the star, which then expodes and
produces a
shock wave.
As the shock encounters material in the star's outer layers, the material is
heated, fusing to form new elements and radioactive isotopes. The shock
then propels the matter out into space. The material that is exploded away
from the star is now known as a supernova remnant.
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Cygnus Loop in X-rays
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Crab Nebula in X-rays
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All that remains of the original star is a small, super-dense core composed
almost entirely of neutrons -- a neutron star. Or, if the original star was
very massive indeed (say 15 or more times the mass of our Sun), even the
neutrons cannot survive the core collapse...and a black hole
forms.
The hot material given off by the supernova, the radioactive
isotopes, and the free electrons moving in the
strong magnetic field of the
neutron star... all of
these things produce
X-rays and gamma rays. These high energy photons are
used by astrophysicists studying the phenomena of neutron stars and supernovae.
A White Dwarf Goes Thermonuclear
Another type of supernova involves the sudden explosion of a white
dwarf star in a binary star system. A white dwarf is the
endpoint for stars of up about 5 times that of the Sun. The remaining
white dwarf has a mass less than 1.4 times the mass of the Sun, and is
about the size of the Earth.
A white dwarf star in a binary star system will draw material off its
companion star if they are close to each other. This is due to
the strong gravitational pull of an object as dense as a white dwarf.
Should the in-falling matter from the companion star cause the white
dwarf to approach a mass of 1.4 times that of the Sun (a mass called the
Chandrasekhar limit after the scientist who discovered it), the
pressure at the center will exceed the threshold for the carbon and
oxygen nuclei to start to fuse uncontrollably. This results in a
thermonuclear detonation
of the entire star. Nothing is left behind, except whatever elements
were left over from the white dwarf or forged in the supernova
blast. Among the new elements is radioactive nickel, which
liberates huge amounts of energy, including visible light. The
evolution of these supernovae tend to all be similar.
Last Modified: November 2004
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