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nova, plural NOVAS, or NOVAE, any of a class of exploding stars whose luminosity temporarily increases from several thousand to as much as 100,000 times its normal level. A nova reaches maximum luminosity within hours after its outburst and may shine intensely for several days or occasionally for a few weeks, after which it slowly returns to its former level of luminosity. Stars that become novas are nearly always too faint before eruption to be seen with the unaided eye. Their sudden increase in luminosity, however, is sometimes great enough to make them readily visible in the nighttime sky.

Encyclopedia Britannica supernova, plural SUPERNOVAS or SUPERNOVAE, any of a class of violently exploding stars .. Supernovae resemble nova in several respects. Both are characterized by a tremendous, rapid brightening followed by slow dimming. .. When a star goes "supernova," considerable amounts of its material, equaling the material of several Suns, may be blasted into space with such a burst of energy as to enable the exploding star to outshine its entire home galaxy consisting of hundreds of billions of stars.

Encyclopedia Britannica

comet, any of a class of small celestial objects in orbit around the Sun that develop diffuse gaseous envelopes and often long luminous tails when near the Sun. Comets are commonly distinguished from other components of the solar system by their rather nebulous appearance and extremely elongated orbits. The nucleus, or core, of a comet resembles a large dirty snowball. It is thought to consist of about 25 percent dust and chunks of rocky or metalic material and 75 percent ice. The ice is mainly frozen water, with an admixture of compounds containing methane, ammonia, and carbon dioxide radicals. Encyclopedia Britannica