Supernova remnant Cassiopeia A, Chandra X-ray image. Shells of gas are expanding from the supernova remnant Cassiopeia A (Cas A). This is the remains of a star which exploded around 320 years ago. The outrushing gas is about 10 light years in diameter and is colliding with the stellar medium, which causes a shock wave. This heats the gas to around 50 million degrees Celsius and causes it to emit X-rays. Supernovae are useful in understanding the origins of life. The only way heavier elements necessary for life are made is in massive stars, and it is only when those stars explodes that the elements are spread elsewhere.