It's a Star! It's a Nova! It's Super-Nova!
The primary message of this article is that stellar explosions can be
classified into two types: novae and super-novae. And that super-novae
can be split up into two different kinds – Type I that show no signs of
hydrogen in their spectrum and Type II that do show hydrogen.
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:
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