Stars are giant balls of hot gas. Their range of size, colour, temperature and brightness varies enormously. They can be members of a pair, triplet or a huge cluster of hundreds or thousands of stars. The colour of a star indicates how it is: cool stars are red, hot stars are bluish. Many of the stars studied by astronomers are in pairs and orbit each other. The "binary stars" often differ in brightness and colour: a dim white dwarf, for example, might orbit a red giant. Stars that make up a binary pair are usually a great distance from each other, but some are so close they almost touch. These stars are called contact binaries and as they are so close, they have to orbit each other very rapidly. The smaller star is very dense and its gravity constantly sucks hydrogen gas away from the larger star. The big star becomes distorted and turns into a distinctive teardrop shape.
What happens to all the hydrogen that the small star takes from the large star in the binary system? The small star has no use for it and the hydrogen collects in a disc. When enough hydrogen has built up, over months, decades or possibly centuries, it blows up a huge nuclear explosion. The star brightens 100 times or more for a few days. Once the explosion has died away, the process begins again.
Neutron Star
The star is composed entirely of particles called neutrons and is therefore called a neutron star. Because so much matte is compressed into such a small volume, the force of gravity at the star's surface is enormous, perhaps 100,000 million times the force of gravity on Earth.
The collapse of a massive star that forms a neutron star is so sudden that the neutron star spins very quickly - as fast as 1,000 times per second! Particles escaping from the star's surface are caught up in the intense magnetic field that surrounds it. They give out radio waves which are squeezed by the field into two beams. As the star spins, the radio beams sweep around the sky like beams of light from a lighthouse. Astronomers can detect them on earth. The rate at which the star blinks on and off at radio wavelengths shows how quickly it is spinning.