As it starts to collapse, the crush of in-falling matter slamming into the iron core sends the temperature rocketing to 50 billion degrees. A supernova is a massive explosion. Huge quantities of material are blown into space, but only from the outer regions of the star. Most of the star has actually imploded, with the core being given a gravity bear hug so extreme that the protons and electrons have been squeezed into a ball of superdense subatomic particles called neutrons. The resulting neutron star would weigh about one and a half times as much as the sun but would measure only about 20 kilometres across - about the size of Brisbane.
Astronomers can prove that neutron stars exist, because they give off a unique distress signal.
Like a lighthouse warning of a dangerous shore, a neutron star sweeps space with a blinding beam of radiation, generated by a magnetic field more than a trillion times greater than the Earth s. Such a neutron star is called a pulsar. To astronomers, the pulsing beam sweeping the darkness of space is an unmistakable warning that extreme gravity lurks nearby. A neutron star resist the ongoing crush of gravity, only with its neutrons packed in like sardines in a tin. But if the remnants of the star after supernova weigh more than three times the mass of the sun, even neutrons cannot hold back the inexorable force of gravity.