Hobbies And Interests
Most Stars
Astronomers have classified stars by their color and the amount of energy they put out. When astronomers plot stars on a graph of energy versus color, most of them appear in a broad swath going from blue, bright stars in the upper left to red, dim stars in the lower right. There are a few patches of stars that lie off that broad swath, but most stars are in that region, called the main sequence. Although it took many years to develop the understanding, now it's known that main sequence stars are those that are still in the prime of their lives, burning hydrogen and creating helium.
Nuclear Fusion
Within the core of main sequence stars, neutrons and protons crash together. Usually they bounce off, but sometimes they stick together. A couple more collisions, and the hydrogen is now helium. When stars get older and no longer have enough hydrogen to sustain fusion, if they're heavy enough they start burning helium. The helium fuses into carbon, which then fuses into oxygen. The oxygen fuses into silicon, and the silicon then fuses into iron. Along the way, other elements in between are created. So older, massive stars still consist mostly of hydrogen and helium (in their outer layers), but they can also have elements as heavy as iron.
Supernova
The steps of nuclear fusion described in the previous section happen in phases, one after another. Each successive stage burns hotter and more quickly. When iron is reached, the star is done for. The pressures of even the largest stars are not enough to move past iron---an extremely stable nucleus. When fusion stops, the star collapses. But when it suddenly collapses it reaches such a high density at the core that the star "bounces." The star explodes in a burst of energy---so much energy that additional fusion reactions take place. In that explosion the heavier elements---things like germanium, tin, lead, silver, gold, zinc, and dozens of others---are all created. The explosion shoots all those elements into the universe.
Full Circle
The material from supernova explosions mixes with hydrogen and helium throughout the universe. The particles collect in clouds, and the clouds condense to stars and planets. So what elements are in the new stars? Again, hydrogen and helium are by far the most prevalent elements in stars. These second or third generation stars---so-called because they are born from materials ejected from other stars---also contain tiny amounts of other elements. In general, those tiny amounts of other elements are so insignificant as to have no effect on the star's life or death.