Introductory Astronomy: Supernovae

Supernovae were first written about as early as A.D. 1006. In 1054, Chinese astronomers noticed a new star in the constellation of Taurus. When astronomers today look in the same direction, what they see is the Crab Nebula, a supernova remnant of the explosion that the ancient chinese astronomers witnessed. Supernovae (the plural of supernova) are explosions. They occur when the iron core of a massive supergiant collapses under the effect of gravity. Because this happens so rapidly (on the order of one millisecond), the core "falls out" from under the surrounding layers. With nothing to hold them up, the surrounding layers begin to fall inward at high speeds. When they hit the core, they "bounce" off, and are expelled back out. The shock waves that occur cause the star to explode, thus causing the supernova.

There are two types of supernovae, Type I and Type II. Type II supernovae occur naturally as the end result of the evolution of massive stars (as described above). Type I supernovae aren't as well understood, but it is clear that they have nothing to do with the normal evolution of single stars. Scientists believe that type I supernovae occur in binary systems that contain a White Dwarf star (see the reference to cataclysmic variables at the end of the white dwarf section). Due to the effect of gravity, material from the companion star accretes onto the white dwarf (in other words, material leaves the companion star and falls towards the white dwarf). As more mass is added to the white dwarf, it may happen that the mass of the white dwarf exceeds the Chandrasekhar limit. When this happens, the white dwarf collapses, causing the carbon to ignite, which then causes an explosion. This is a Type I supernova.

When the star goes supernova, gas is thrown off in the explosion. This gas (referred to as a supernova remnant ) expands outwards at high speeds. As it encounters ambient material in the interstellar material it slows down. The lifetime of a supernova remnant is only a few tens of thousands of years. With the outer layers expelled, all that is left is the small, hot core. The mass of this core will determine the final stage of the massive star.

An observation of a supernova.