A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed to form a diffuse nebula. The peak optical luminosity of a supernova can be comparable to that of an entire galaxy before fading over several weeks or months. The last supernova to be directly observed in the Milky Way was Kepler's Supernova in 1604, appearing not long after SN 1572, but the remnants of more recent supernovae have been found. Observations of supernovae in other galaxies suggest they occur in the Milky Way on average about three times every century. These supernovae would almost certainly be observable with modern astronomical telescopes, while the supernovae of 1604 and 1572 were visible to the naked eye. The most recent naked-eye supernova was SN 1987A, which was the explosion of a blue supergiant star in the Large Magellanic Cloud, a satellite of the Milky Way. Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star such as a white dwarf, or the sudden gravitational collapse of a massive star's core. In the first class of events, the object's temperature is raised enough to trigger runaway nuclear fusion, completely disrupting the star. Possible causes are an accumulation of material from a binary companion through accretion, or a stellar merger. In the massive star case, the core of a massive star may undergo sudden collapse once it is unable to produce sufficient energy from fusion to counteract the star's own gravity. While some observed supernovae are more complex than these two simplified theories, the astrophysical mechanics are established and accepted by the astronomical community. Supernovae can expel several solar masses of material at velocities up to several percent of the speed of light. This drives an expanding shock wave into the surrounding interstellar medium, sweeping up an expanding shell of gas and dust observed as a supernova remnant. Supernovae are a major source of elements in the interstellar medium from oxygen to rubidium. The expanding shock waves of supernovae can trigger the formation of new stars. Supernovae are a major source of cosmic rays.