For amateur astrophotographers, supernovas are like cats. You know they're doing it, but it's really hard to catch them in the act. Even though there are as many as 30 million supernova explosions occurring in the visible universe every day, the vast majority of them are too far away to capture with a camera. 

A supernova is, put simply, an exploding star. In the main, this generally occurs for one of two reasons. The most energetic form occurs when a star with three times the mass of the sun or more fuses all the hydrogen in its core into helium, fuses that helium into carbon, oxygen, and other heavier elements, and then attempts to fuse those heavier elements into iron. Unlike lower levels of thermonuclear fusion, fusing iron consumes more energy than it produces. This means that the star is no longer producing enough energy to hold up the weight of its outer layers. Those outer layers suddenly come crashing down onto the iron core at velocities as high as 30% of the speed of light. That material slams into the star's core and then rebounds in a massive explosion—a supernova explosion. This then seeds the surrounding area with heavier elements that may someday go on to become new stars, planets, guitars, and football teams. These are generally called Type II supernovas, although there are a number of subcategories. Type II supernovas typically leave behind either a neutron star or, in the case of more massive exploding stars, a stellar-mass black hole.

The second way a supernova may occur begins with a white dwarf star—a husk left behind when a less massive star runs out of fuel and (more gently) expels its outer layers. The white dwarf is the remnant core of the dead star and is largely made of carbon—a diamond in the sky. But if the white dwarf happens to be closely orbiting a larger, still-active star or it lives in a dense molecular cloud, it may start to accrete mass from one of these sources due to its strong gravitational pull. But once the mass of the white dwarf grows to around 1.44 solar masses—called the Chandrasekhar Limit after its progenitor, the great physicist Subrahmanyan Chandrasekhar—the star becomes a gigantic carbon bomb. This thermonuclear explosion is classified as a Type Ia supernova.

Type Ia supernovas have been very important to our study of the cosmos. Because they all explode with the same amount of energy and brightness, dictated by the Chandrasekhar limit, they serve as standard candles that allow us to calculate their distance. So when we detect a Type Ia supernova event in a distant galaxy, we are then able to calculate the distince to that galaxy. We know the inherent brightness of the explosion and can plug its apparent brightness to us here on earth into the inverse-square law to determine that distance.