|
|
|
3. Supernova Explosions
Stars that are born with masses greater than about eight solar masses end their lives in gigantic explosions called supernovae.
The Structure of an Evolved, Massive Star
Before discussing supernova explosions, let's look once more at the structure of the highly evolved star of 20 solar masses that we showed earlier:
|
|
Focus on the star's iron core. It's very compact, with a mass somewhat less than 1.4 times that of the Sun, but a size of only about two-thirds that of the Earth. Once a star has reached this structure, the nuclear burning history that we have discussed draws to a close. The iron core will not ignite to induce further nuclear burning as has happened earlier to the helium, carbon-oxygen, and other cores. The reason is that iron can't burn. |
Nuclear burning (just like chemical burning) is possible only if the reactions release energy. However, the fusion of iron with other nuclei to make still heavier nuclei requires the input of energy. This input of energy for making the heavier elements becomes available only during the explosion that is about to occur.
The fact that iron does not burn leads to the collapse of the star's iron core and the explosion of the star's outer envelope. We shall describe this development in three sections: