A supernova is a massive explosion generated by a dying star. A star in our galaxy goes supernova approximately once every 100 years. The explosion hurls matter into space and can shine as brightly as an entire galaxy for short periods. Stars are giant nuclear reactors, producing energy by fusing hydrogen into helium. Eventually, however, stars run out of hydrogen to fuse. When this occurs, the star fuses helium into heavier elements, including iron. The star’s core shrinks while its outer layers expand, creating a red giant that consumes surrounding planets.
What happens when the sun runs out of helium? Fusion simply ends in small stars like our sun. The star becomes a white dwarf and slowly fades away. When a large star depletes its helium reserve, the core collapses within seconds, causing temperatures in excess of 100 billion degrees. The outer layers of the star collapse as well, only to explode outwards in a massive explosion known as a supernova. Supernovas generate enough energy to fuse elements heavier than iron. Gold, silver, uranium and other elements with high atomic numbers are created in supernovas. What’s left of the star’s core becomes a dense neutron star or forms a black hole, an area of such densely packed matter that not even light
escapes its gravitational pull.
After a supernova, material expelled in the explosion may form nebulae–massive interstellar clouds of gas and dust. Over millions of years gravity pulls nebulae material together into a dense, hot core called a protostar.