Novas and supernovas are both massive explosions that occur within stars. However, they actually signal different stages of a star’s life. Novas are like occasional fireworks on the surface of a living star, while supernovas are the grand finales.
Novas are explosions caused by a fusion reaction on the surface of a white dwarf star. The white dwarf is usually the smaller of the two stars in a binary system. The nova occurs in the star’s outer layer built up from material pulled from the second star. Gas accumulates on the surface until it ignites a nuclear reaction.
During a nova, the star's brightness increases by several thousand times. This flare is caused by a gas shell that forms around the star. Historically, when people saw one in the sky, they assumed it was the birth of a new star and called it "nova" or Latin for "new".
Novas are much more common than supernovas, with about 40 novas per year in our galaxy alone. Most must be seen with a telescope, but occasionally one is visible with the naked eye. The brightness lasts between 25 and 80 days, during which time it slowly fades.
Every so often, a white dwarf star that has been producing novas will reach such a critical point that it goes supernova and explodes to its death.
A supernova is the explosion and death of a large star. A supernova occurs when the star runs out of internal nuclear fuel and collapses under the weight of its outer layers. Its self-destruction leaves behind either a black hole or a neutron star.
Stars with mass greater than three times that of our sun form black holes. Black holes trap all the matter that remains from the dead star into a single point. All other matter and light within a certain distance of the hole is sucked in by a massive gravitational pull. Incredible as it sounds, at the center of the black hole, mass lacks volume and time stops.
Neutron stars are the remains of stars with mass of up to three times greater than our sun. They are tiny, dense, and fast.
Supernovas are rare and visible from Earth once every few hundred years. History was made in September 2011 when a spectacular supernova in the Pinwheel galaxy became visible to Earth. Any astronomy buff with a good telescope could capture the explosion from the tropical comfort of a Lahaina hotel and share the photos with the world.
Supernovas contribute critical building blocks of energy and elements to the galaxy. The type 1a supernova produces heavy metals and massive shock waves that help form new stars. The core collapse type of supernova creates neutrino articles. The light and radiation produced can last months and is brighter than an entire galaxy.