Great question! Your answer is below.
They were created prior to the formation of our solar system, thus...
Huge, gigantic stars had to have existed, perished, and burst, releasing the materials they produced into the universe. These materials subsequently gathered via gravity to form our solar system and perhaps dozens or hundreds of others.
Our Sun is currently fusing hydrogen into helium; when hydrogen runs out, the Sun will compress a little, and the added pressure on the core will cause the helium to start fusing into carbon for a short while; then, the sun will die, and that will be that. Stars fuse lighter materials into heavier ones.
But with large stars, the outer layer may be made of hydrogen, which will be fusing into helium underneath it along a boundary. The helium layer will fuse into the carbon layer, the carbon layer will fuse into the calcium layer, and so on, up to the iron layer.
When iron production in a star's core starts, the star is doomed. The creation of elements *heavier* than Iron *takes* energy* Iron is the heaviest element ever formed that will release energy. Therefore, the star will eventually stop fusing, and its powerful gravitational pull will lead it to collapse. The abrupt in-falling mass generates the energy required for fusing the heavier metals, such as lead, gold, and uranium, after which the star's mass rebounds, resulting in the explosion known as a supernova. Before the formation of our solar system, all of this took place.'
Thanks!
- Eddie