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 dies. 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 top 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).

FIND YOUR OWN CV!
Cataclysmic Variables have a whole repertoire of cool ways to change their appearance, and best of all, you can see them do it with an ordinary telescope, unlike X-ray binaries. One of their favourite tricks is to let the accreted matter accumulate and explode it all off at once. Some of them blow off a little and often, having an outburst every few months where the brightness increases by a factor of about 10 for a few days: these are called dwarf novae. Then there are the classical novae, that are thought to explode only once every 3000 to 10000 years but become up to 100 million times brighter. Amateur astronomers discover many new dwarf novae and classical novae each year. The real fireworks happen when the white dwarf piles on so much extra mass that it collapses into a neutron star - the result is a second type of supernova, like the brightest one ever in 1006.