As stars age, many use up their fuel and fade away to oblivion. Others, however, go out with a bang as supernovae, releasing energies of up to 1044joules—an amount of energy equivalent to 30 times the power of a typicalnuclear bomb. The explosions of low-mass stars can be triggered by the accretion of mass from a companion star in a binary system to create classical, or Type Ia, supernovae. These supernovae show no hydrogen in their spectra. Massive stars, on the other hand, proceed through normal nuclear fusion but then, when their energy supply runs out, there is no outward pressure to hold them up and they rapidly collapse. The core is crushed into a neutron star or black hole, and the outer layers bounce and are then hurled outward into the surroundings at many million kilometers per hour. These are Type Ib and II supernovae. The Type II supernovae still eject some hydrogen from the unprocessed atmosphere of the star. During a supernova explosion, temperatures are so high that all the known elements can be produced by nuclear fusion.
The center of the Crab Nebula as viewed by the Hubble Space Telescope. The Crab was created by a supernova explosion on July 4, 1054, and is located approximately 6,500 light years from Earth.
The most recent supernova that was close enough to be seen without a telescope occurred in early 1987 within a nearby galaxy, the Large Magellanic Cloud. Known as 1987A, it is the only supernova for which there is accurate data on the progenitor star before it exploded. It has been a tremendous help in understanding how stars explode and expand.
The rapidly growing surface of the star can brighten by up to 100 billion times. Then, as the material gets diluted, it becomes transparent andthe brightness fades on time scales of a few years. The ejecta are still moving rapidly, however, and quickly sweep up surrounding matter to form a shell that slows down as mass gets accumulated, an action similar to that of a snowplow. This is the beginning of the supernova remnant that can be visible for tens of thousands of years. 1987A is starting to show such interaction with its surroundings.
Supernova remnants emit various forms of radiation. The material is moving highly supersonically and creates a shock wave ahead of it. The shock heats the material in the shell to temperatures over 1 million degrees, producing bright X rays. In the presence of interstellar magnetism, shocks also accelerate some electrons to almost the speed of light, to produce strong synchrotron radiation at radio wavelengths. Sometimes, even high-energy gamma rays can be produced. Dense areas can also cool quickly and we observe filaments of cool gas, at about 10,000 degrees, in various spectral lines at optical wavelengths.
In 1054 astronomers in China and New Mexico observed a famous example of the explosion of a massive star. What remains is a large volume of material that, with a lot of imagination, looks like a crab and, hence, is named the Crab Nebula. The object is being stimulated by jets from a rapidly spinning (about thirty times a second) neutron star called a pulsar. In most supernova remnants, this pulsar wind nebula is surrounded by the shell discussed above, but remarkably, no one has yet detected the shell around the Crab Nebula. Oppositely, the young supernova remnant Cassiopeia A has a shell and a neutron star but no pulsar wind nebula. Astronomers hope to explain these and many other mysteries about supernovae and their remnants using more multiwavelength observations with new telescopes.
