A supernova is a star that is very dim then suddenly it becomes very bright. In the past, novae were at first thought to be new stars. But they are not new. Instead, they are variable stars, meaning their brightness changes. It can be a star too faint to see and then suddenly become very bright. This change in brightness has to do with the star exploding and the debris being illuminated by radiation from the blast. The star is not destroyed and after a time its brightness fades. There are stars in which nova explosions have occurred more than once.
The photo shown was taken in 1987 by the Hubble Space Telescope. Astronomers were delighted to have a record of an exploding star. At the center the debris from the explosion is expanding at a speed of 6 million mph. he material in the rings were ejected by the star before it exploded. Radiation from the blast illuminates the material.
For more information about novae, see Janice VanCleave’s Constellations for Every Kid.