A core-collapse supernova occurs when a massive star, typically more than eight times the mass of the Sun, exhausts its nuclear fuel. As the core runs out of energy, it can no longer support itself against gravitational collapse. This leads to a rapid implosion, causing the outer layers of the star to explode outward, resulting in a brilliant burst of light and energy. These supernovae are crucial for the universe, as they distribute heavy elements, such as iron and gold, into space. The remnants can form neutron stars or black holes, depending on the mass of the original star.