Ken Tapping, January 12, 2011
For the last couple of weeks the astronomical headlines around the world have been spreading the word that a 10-year old girl from New Brunswick has discovered a new supernova, the explosive demise of a giant star. It lies in a galaxy 240 million light years away. Thanks to the high standard of equipment available to amateur astronomers and the accessibility of observations made at major observatories, amateurs continue to be an important part of astronomical progress.
Stars shine because of nuclear fusion: combining light elements into heavier ones. Starting with the hydrogen it collected from its birth cloud, it obtains energy by making helium and other light elements. When fuel runs low, the core cools and the star shrinks, which drives up the core temperature again. If the temperature gets high enough, the by-product elements from previous energy production can be fused into heavier elements, releasing more energy. Small stars like the Sun won't get far along this chain; eventually they will run out of fuel, sneeze off their outer layers, forming white dwarf stars which very slowly cool off.
Any star massive enough to get to the point of making iron is destined for a different fate. Iron cannot be persuaded to release energy by turning into heavier elements. Instead, changing iron into anything else requires energy. By the time a star is making iron in its energy production process, it is doomed. When it starts to run out of fuel, it tries its old solution of shrinking and driving up the temperature in its core. However, this time the consequence is its catastrophic demise.
Solving energy production problems by driving up the core temperature brings problems. As the temperature increases, so does the rate at which energy is lost by producing neutrinos, which leave the star, taking energy with them. However, the coup de grace comes when the iron in the core gets hot enough. It turns back into helium or other light elements. It obtains the energy it needs to do this by cooling the core, which removes the pressure holding up the rest of the star. It collapses and then explodes, producing one of the biggest explosions in the modern universe, a supernova. For a brief while it will outshine all the billions of other stars in its galaxy combined. In the explosion, heavy elements like lead, gold, silver, platinum and uranium are formed, mixed with the other elements formed during the star's life, and blasted off to enrich the interstellar gas and dust clouds, maybe triggering them to collapse to form new stars.
Supernovae are interesting in themselves. However, they have another important application. Because we can estimate fairly accurately the energy released in a supernova, we can also estimate the brightness of the explosion. By measuring how bright the explosion looks, we can estimate how far away it and its host galaxy happen to be. This makes discoveries of supernovae in distant galaxies very important. They become rulers for measuring the universe, at distances where nothing much else works.
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