It's a Star! It's a Nova! It's Super-Nova!

Astronomers had long been observing new stars, or novae. These are newly visible stars whose brightness changes by hundreds or millions of times (increasing between 5 to 15 magnitudes). They had been observed in our galaxy, and in the "spiral nebulae". Once Hubble determined the distance to the Andromeda Nebula, he realized that the bright nova that had been observed to have occurred there in 1885 must have been much more luminous than those occurring in our own galaxy. By 1934 there was growing evidence from other bright novae in distant galaxies that there were two types of phenomena. Walter Baade and Fritz Zwicky were apparently the first to coin the term "super-nova". Minkowski studied the spectra from these supernovae and determined there were two types.

We now know novae to be an event that happens on a white dwarf. A white dwarf is the remnant of a star like our sun after it has used all its nuclear fuel. White dwarfs may be at the center of planetary nebulae, or in orbit around another star. When in a close orbit around another star, the white dwarf can accumulate matter from its companion. With enough material on the surface of the white dwarf, there is an episode of nuclear burning of the accumulated hydrogen. This happens rather quickly, and the star brightens.

Supernovae are much more violent events, and hence much brighter.

• Type Ia supernovae (a subclass of the Type Is discussed in the article) is the total destruction of a particular type of white dwarf. Here enough material accumulates on the white dwarf that it reaches the upper allowed limit of its mass, 1.4 times the mass of the sun (a limit discovered by Chandrasekhar). Once this limit is reached, carbon and oxygen in the core of the white dwarf fuse, completely detonating the star.
• Type II supernovae result from massive stars that have come to the end of their lives. Stars that start with more than 8 times the mass of the sun go supernova when their cores start to fill with iron, the last step in a long chain of fusion processes. Since iron does not generate energy by nuclear fusion, the core collapses under its own weight. The rest of the star falls onto the collapsing core, and then bounces back, creating the supernova explosion. The energy of the explosion is enough to create heavy elements beyond iron. The explosion leaves behind either a neutron star or a black hole (depending on the original mass), as well as a gaseous remnant (known as a supernova remnant).

Side Note

The style of this article plays off the popularity of Superman in the mid-1950s. The Superman comic books debuted in the 1930s, and were wildly popular by the early 1940s. In 1955, the television series "Adventures of Superman", starring George Reeves, was in its heyday.

Other resources

The following web pages have more detailed information:

• Supernovae - basic information about supernovae.
• Supernovae - more information about supernovae.
• Teachers Guide to the Life Cycles of Stars
• "What is Your Cosmic Connection to the Elements?" - a teachers guide to the origin of the chemical elements, including a section on supernovae.