Transcript: For a few days after a supernova explosion, the dying star rivals in brightness the entire Milky Way galaxy. The expanding gas cloud moves outward at a speed of ten thousand kilometers per second or over twenty million miles per hour. The light curve of Type I Supernova has a characteristic exponential decay that’s powered initially by the decay of radioactive nickel-56 with a half-life of 55 days and subsequently by the decay of radioactive cobalt-56 with a half-life of 78 days. Years later a colossal expanding nebula is seen in the sky; it’s called a supernova remnant.