Nope, Galileo didn't observe a supernova (an exploding giant star). But Tycho Brahe did. Tycho was an arrogant, rude, and gluttonous man with a false nose (he had lost his in a sword fight). Tycho also happened to be a great observational astronomer. In 1572, he observed a brilliant new star (a supernova) appear in the sky. Artistotle, and subsequently the Catholic Church, had argued that the starry sphere was perfect and unchanging, so how could a "new star" appear? The supernova discovery started Tycho and his followers (but not the Catholic Church) thinking and questioning about the Church-proscribed models of the cosmos.