Ancient astronomers assigned the word nova, Latin for "new," to any bright star that suddenly appeared in the sky. They called an extremely bright new star a supernova.
Modern astronomers now know that a supernova, one of the most violent events in the universe, is the massive explosion of a star. Only relatively large stars (those having 1.5 times the mass of our Sun or more) explode in supernovae at the end of their lives. Once a star has used up all its nuclear fuel, it begins to collapse in on itself. During this process, energy is released and the outer layers of the star are pushed out. These layers are large and cool, and the star at this point is considered a red giant. The star continues to expand, however, and soon explodes outward with great force. As a result of the explosion, the star sheds its outer atmospheric layers and shines more brightly than the rest of the stars in the galaxy put together.
What happens next depends on the original mass of the star. Stars up to three times the mass of the Sun end up as densely packed neutron stars or pulsars (rapidly rotating stars that emit varying radio waves at precise intervals). Stars more than three times the mass of the Sun collapse, in theory, to form a black hole (an infinite abyss from which nothing can escape).
Black hole: Remains of a massive star that has burned out its nuclear fuel and collapsed under tremendous gravitational force into a single point of infinite mass and gravity.
Chandrasekhar's limit: Theory that determines whether an exploding supernova will become either a neutron star or a black hole depending on its original mass.
Neutrino: High-energy subatomic particle with no electrical charge and no mass, or such a small mass as to be undetectable.
Neutron star: Extremely dense, neutron-filled remains of a star following a supernova.
Nuclear fusion: Merging of two hydrogen nuclei into one helium nucleus, with a tremendous amount of energy released in the process.
Pulsar: Rapidly spinning, blinking neutron star.
Radio waves: Electromagnetic radiation, or energy emitted in the form of waves or particles.
Astronomers did not know what causes a star to explode in a super nova until the 1939, when Indian-born American astrophysicist Subrahmanyan Chandrasekhar (1910–1995) pieced together the sequence of events leading up to a supernova. He also calculated a figure for the mass of a star (known as Chandrasekhar's limit) that would determine if it would end up as a neutron star or a black hole.
Various theories have been proposed to explain the reasons a star explodes outward while collapsing inward. One theory is that the explosion is caused by a final burst of uncontrolled nuclear fusion. A more recent theory is that the explosion is due to the ejection of a wave of high-energy subatomic particles called neutrinos (electrically neutral particles in the lepton family). The neutrino theory gained greater acceptance following the 1987 supernova in the Large Magellanic Cloud, our galaxy's closest companion. Just before the supernova came into view, a surge of neutrinos was detected in laboratories around the world. This supernova, called Supernova 1987A, was the first visible to the naked eye since 1604.