In simple terms, a supernova is a star that explodes. During a supernova, a star brightens considerably over a period of about a week and then starts to fade slowly, over a period of a few months or a year or two before it disappears completely.
One kind of supernova is called a Type I supernova. This kind of supernova occurs in a double star system in which one of the stars has become a white dwarf. A double star system, or a binary star, is a pair of stars that are held together by the force of gravity and orbit around each other; a white dwarf is a formerly medium-sized star in the last stages of its life, a star that has run out of fuel and has collapsed into a small, dense star that is smaller than our planet. A Type I supernova occurs only in this very specific situation, when a white dwarf is part of a double star system.
A Type I supernova occurs in a double star system in a situation when a white star’s companion star has grown too big. The companion star is always growing, and the white dwarf’s companion star will continue to grow in size until its proximity to the white dwarf causes its growth to halt. When the companion star can grow no further, material from the companion star flows from the companion star to the white dwarf. When the white dwarf reaches a certain critical mass, a mass equal to approximately 1.4 times the mass of the Sun, the white dwarf explodes catastrophically in a supernova event.
Only two Type I supernovae have been visible to the naked eye in recorded history, one in 1572 and the other in 1604. Since then, numerous other Type I supernovae have been observed using the telescope, which was invented by Galileo in 1610.