Alice Kemp, of King's High School, asks :-

What would happen to earth if there was a supernova on the star Alpha Centauri?

Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.

Alpha Centauri is four light years away, or 41 million million kilometres. So if it blew up as a supernova now we wouldn't know about it for another four years.

A supernova at Alpha Cen's distance would be blindingly bright. Depending on the type of supernova it would appear anything from 1/800th to 1/20th the sun's brightness, 800 to 30,000 times brighter than the full moon. And all this light would be coming from a star-like point.

Soon after the supernova was seen we would be hit by a burst of radiation, atomic particles traveling at near the speed of light. A nearby supernova would likely produce enough radiation to kill off all land-dwelling animals.

Fortunately, Alpha Centauri cannot become a supernova. It is two sun-size stars orbiting each other. Sun-sized stars don't become supernovae. There are two broad types of supernovae. Type I supernovae result from old white dwarf stars orbiting close to a companion star. The white dwarf gathers new fuel by slurping gas off the companion. When it reaches a critical mass it explodes. Type Is are brightest and are used to estimate the distances of remote galaxies.

Type II supernovae are the last gasp of stars much heavier than the sun. The core of the star collapses suddenly. The outer layers fall onto it, compress, heat up and explode into space. The explosion produces in seconds an amount of energy that would have taken the star millions of years to produce otherwise. The collapsed core becomes a neutron star, billions of times denser than ordinary matter. Spinning neutron stars are called pulsars because they emit pulses of radio noise.

Supernovae are the source of all the chemical elements heavier than iron. The sun and planets (and us) are made partly from material that came from a supernova.