COMPACT
BINARIES - THE MOST EFFICIENT POWER HOUSES IN THE UNIVERSE?
After stars die, the kind of corpse they leave behind
depends on the mass of the original star. From lightest
to heaviest the possibilities are: white dwarf, neutron
star or black hole. In a binary system (one that has two
stars circling each other) an interesting event occurs
if one of the two stars die. If they remain in position
after the explosion, then a compact binary could be formed,
allowing the corpse star to feed on the companion star,
like a cosmic vampire! They are powered by the gravitational
energy gained by the gas while "falling" from
the companion to the compact star, often via an accretion
disc. This is formed when the material from the companion
star overshoots the dead star and forms a graceful curve
as it is sucked in [see picture to the right].
An apple falling onto a neutron
star would make a bigger bang than 500 million Hiroshima
bombs- not something Isaac Newton would want to fall
on his head! Neutron stars are about twelve miles across
and about one and a half times heavier than the Sun,
while white dwarfs are much bigger, about 7500 miles
across and half as heavy as the Sun. This means that
the gravity at the surface of a neutron star is lots
more than on a white dwarf.
Compact binaries with neutron
stars or black holes are called X-ray binaries because
the gas becomes X-ray hot as it falls in and most of
the energy is produced as X-rays - an X-ray
binary with a combined mass of only two times that of
the Sun can be up to 500,000 times more powerful.
Compact binaries with white dwarfs produce mostly ultraviolet
and visible light and are called cataclysmic variables
(CVs).
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