The Chandrasekhar limit

At the age of 20, when most college sophomores are still picking their majors, Harvard student Subrahmanyan Chandrasekhar proved that white dwarf stars would explode after reaching a mass 1.4 times that of our sun. The year was 1931, and astrophysicists have based their research on his findings ever since. An exception (in the form of a white dwarf estimated to have reached 2 times the sun's mass before exploding) observed in 2006 is still being studied.

Types of Supernovae

Stars that have enough heft to go out with a bang are separated into two supernova classes -- Type I and Type II. Astronomer Rudolph Minkowski laid out these classifications in 1941. Astronomers learn a lot about stars from the colors of light that they emit. Using a device called a spectrograph, they can get a clear picture of exactly what elements are burning inside a star.

By using a spectrograph, Minkowski noticed that some supernovae (Type I) don't contain hydrogen, but the others (Type II) do. In the 1980s, as observational technology improved, scientists further divided Type I supernovae into three subcategories: Type Ia (which contain silicon in their spectra), Type Ib (which contain helium) and Type Ic (which contain neither) [source: Swisburne University of Technology]. Stars lose elements when stellar winds rip their outer layers away long before they go supernova.

Type Ia supernovae work differently than all other types. A Type Ia supernova results from a white dwarf that's part of a binary system (that is, one that shares an orbit with another star) and was about twice the size of our sun during its life. This white dwarf's mass allows it to fuse elements slightly heavier than hydrogen, so it has a stable core of carbon and oxygen.

Left to its own devices, this white dwarf would eventually decay into a black dwarf. But since it's not alone, it has access to resources that other stars don't. The more massive of the two stars acts like an opportunistic sibling, using its gravitational pull to steal matter from the other star. This gluttonous star grows until it exceeds the Chandrasekhar limit -- a mass 1.4 times that of our sun, otherwise known as 1.4 solar masses. At this size, the white dwarf suddenly has enough heat and pressure in its core to fuse carbon, and all of that carbon fuses at once like a thermonuclear bomb going off, blowing the star to bits [source: Atkinson]. It leaves behind a gaseous remnant that's symmetrical in shape and contains a great a deal of iron created in the heat of the explosion [source: Chandra X-ray Observatory].

Because Type Ia supernovae all explode at the same point in their stellar deaths, they all peak at almost exactly the same brightness. It's so consistent that Type Ia supernovae are also called standard candles: Once astronomers find one in a region of space, they can use it as a baseline with which to compare other objects around it.

Type Ib, Type Ic and Type II supernovae, despite showing different elements in their spectra, all explode the same way. Find out how they work on the next page.