A supernova is the catastrophic death of a star, characterized by a massive output of energy. In the Milky Way, supernovae are relatively rare, with a few notable incidences of historical supernovae recorded as far back as 185 CE. Many previous supernovae were probably topics of conversation and concern among the people who witnessed them. Around the universe, several hundred are observed and recorded each year, providing information about the formation of the universe and the objects within it.
There are two basic types of supernova, although each type is broken up into subtypes. In the case of a Type One, an instability arises in the chemical makeup of the star, leading to a thermonuclear explosion of formidable power. The core temperature of the star rises as a result of pressure and the imbalance, ultimately igniting the star in an explosion which can sometimes be visible with the naked eye from Earth.
A Type Two supernova involves the collapse of the core of a star, triggering a chemical reaction which causes the center of the star to essentially implode. The core of the star compresses into a neutron star, while the outer layers of the star are blown away into the surrounding space. A neutron star is an extremely dense star, all that remains of the compressed core of a star which has exploded in a Type Two supernova. Neutron stars have a number of unusual properties which make them highly intriguing for astronomers.
Astronomers study supernovae because they can provide valuable information about the universe. When stars explode, they initially form a cloud of plasma, creating a shock wave which leaves behind a distinct signature. The star also distributes heavy metals throughout the universe, and the large amount of energy behind a supernova can make it very easy to spot for an astronomer. By identifying and studying supernovae, astronomers can learn more about the size of the universe and the bodies in it. Supernovae created the materials which later came to become the Solar System, and a supernova will probably ultimately destroy our solar system as well.
Historically, the appearance of a supernova within the Milky Way has sparked discussion and debate. Supernovae helped early scientists to learn about the world around them, but also stimulated a general response among the populace. A supernova can last for weeks, and a close supernova would burn brighter than the sun. Many cultures feared that the appearance of a supernova signaled the end of the world, or the wrath of an angry God.