Nova, (plural: Novae), a star that rapidly increases in brightness and then fades again. The term comes from the Latin for new. Before the invention of the telescope a nova was thought to be a new star because it suddenly appeared where no star had been seen before. However, the novae were simply stars that had been too dim to be seen with the unaided eye. Astronomers estimate there are as many 30 stars in the Milky Way that become novae in any given year, but generally only one or two are visible from the earth. A few novae briefly appear as bright as the brightest stars in the sky, but most can be observed only with a telescope.
A typical nova increases in brightness by 10,000 to 1,000,000 times within a few days. It then gradually declines to its original brightness over a period of several months or years. Novae that flare up again, typically decades later, are called recurring novae.
A nova is usually a relatively old star that is a member of a binary system. Astronomers believe the sudden brightness of a nova occurs when a thin shell of gas is thrown outward by an explosion near the star's surface. In the case of a binary system, the explosion is probably caused by infalling matter drawn from the nova's companion star.
A supernova reaches a maximum brightness much greater than that of a nova; it can increase in brightness by as much as a billion times. A supernova recorded by the Chinese in 1054 A.D. could be seen in full daylight for many weeks. The last known supernova in the Milky Way was observed in 1604. However, using telescopes, astronomers have discovered and studied supernovae in many other galaxies since then. In 1987 a supernova in the Large Magellanic Cloud, a nearby galaxy, became bright enough to be seen with the unaided eye. Astronomers believe that a supernova results from the collapse of the core of an old, dying star. The collapse generates an explosion that ejects most of the matter from the outer portions of the star, and the core forms either an extremely dense object called a neutron star or a black holea region in space from which no radiation (not even light) can escape.