Supernovae of Type I and Type II

The theory of supernovae is reasonably well developed. Fortunately, the sun is not likely to suddenly explode violently to wipe out all terrestrial life. On the other hand, it is certain that when the sun enters its dotage, somewhere between 2 to 5 billion years from now (depending whose estimate one adopts—neither of which should give us cause for concern), the earth's atmosphere will be destroyed.

Stars containing more than four solar masses of gas are likely explode at the end of their lives, but not all of them will do so in the same way. The Tau A and Cas A represent a different class of event compared to Kepler's supernovae, or the one seen with the naked eye in 1572 by another famous astronomer of old,

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FIGURE 4.2. Radiograph of the Cassiopeia A (Cas A) supernova remnant created by a star that exploded in 1680, the date inferred from the current rate of expansion of the nebula. Investigators: RE. Angerhofer, R. Braun, S.F. Gull, R.A. Perley,and R.J. Tuffs. (VLA image courtesy of NRAO/AUI.)

FIGURE 4.2. Radiograph of the Cassiopeia A (Cas A) supernova remnant created by a star that exploded in 1680, the date inferred from the current rate of expansion of the nebula. Investigators: RE. Angerhofer, R. Braun, S.F. Gull, R.A. Perley,and R.J. Tuffs. (VLA image courtesy of NRAO/AUI.)

Tycho Brahe. The latter two are examples of Type I supernovae, believed to be the destruction of what was originally a white dwarf star—a highly mature star which, in its old age, shrinks to a mere shadow of its former self. These white dwarfs were probably members of binary star systems. (Approximately half the stars in our Galaxy are paired in binaries, unlike our sun, which has no close companion.) The interaction between binary stars can be very dramatic. If one member is a dwarf star, evolving slowly, and the other a more massive star, aging rapidly, the larger star may enter a phase in its life when it swells to enormous size. Some of its material then falls onto the dwarf and if this process continues for long enough the white dwarf may suddenly become incapable of absorbing any more of its neighbor's debris. Its surface layers overheat and explode outward. The Tycho and Kepler supernovae are believed to have been produced in this manner. Study of their light gives much information about the chemical constituents of the exploding material. Type I supernovae are regularly observed to occur in distant elliptical galaxies, which contain no interstellar matter, nor any young, massive stars are capable of becoming supernovae on their own. A Type II supernova, on the other hand, may involve the explosion of a single massive star due to a catastrophic incrca.se in the amount of heat generated in its core as part of its "normal" evolution.

4.5. Supernovae and Life 37

4.5. Supernovae and Life

The filaments within supernova remnants produce nonthermal radiation. As the remnant expands, ages, and runs out of energy, the electrons responsible for that emission escape into surrounding space, where they become cosmic rays. These cosmic rays fill the Galaxy, all the while encountering magnetic fields between the stars. The electrons gain energy from such encounters and then, rejuvenated, transmit nonthermal radiation that we pick up as radio waves from the Milky Way (Chapter 5). The heavier particles, such as the protons created in the supernova explosions, may later strike earth as cosmic rays.

The total energy generated by supernova explosions over billions of years provides enough energy to propel interstellar clouds hither and thither, causing them to collide with each other. Supernovae keep interstellar matter stirred up, and when clouds collide the birth of a new star is triggered. It is likely that a nearby supernova remnant probably spawned the formation of the solar system.

Supernovae play a direct role in assuring our existence. The early universe contained none of the heavy elements, the basic constituents of matter, which make up our world. However, tantalizingly, the Hubble Space Telescope is revealing the products of stellar death in the very earliest galaxies seen at the edge of space-time. Those elements include oxygen, nitrogen, and carbon, elements essential for life, as we know it. All the atoms of which we are made (except hydrogen) were formed in stars and then injected into space and made available for future planetary formation because stars exploded. The heaviest elements, such as the gold and silver in rings or pendants, were formed during the supernova explosions themselves.

Today the material within the supernova remnant Cas A is being fed into space and sometime, somewhere, in the very distant future, some of it now radiating radio and light signals at us may be incorporated into alien living entities.

In imagination I often wonder what it would be like to live on another planet that happens to be located within 30 light-years of a star that explodes as a supernova. Imagine a technologically sophisticated civilization like ours. One day a new star appears that signifies the initial flash of the explosion. Within days a fatal dose of gamma rays and X rays from the event causes widespread death and triggers mutations that lead to radiation sickness. Knowing the exploded star is 30 light-years away and knowing how fast the shell of ejected matter is streaming out into space, their scientists estimate that they have about 300 years before the shock waves carrying lethal doses of high-energy particles strike their planetary system. There is only one thing to do and that is to plan to live underground where they have some protection from the cosmic rays and the X rays generated in the nebula. And that is where they will have to stay for thousands of years while the nebula envelops them. It seems likely that the sun formed as the result of the compression of interstellar matter triggered by a nearby supernova some 5 billion years ago. Fortunately there is no likely candidate star that might become a supernova within a thousand light-years of earth, assuming astronomers know enough about stellar evolution for us to be comforted by this conclusion!

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