I want to talk about star death!
My last post made me think more what I was really talking about–being more “special” than a meteorite. Sometimes writing things at 1 am do not come out as polished.
Most people are familiar with the idea that stars are massive hydrogen fusion centers; making helium from hydrogen releasing tons of energy. But what you may not know is stars of significant mass actually fuse elements up to Iron.
What we see on the surface of our Sun and all other stars is actually a vast supermassive ocean of heated non-burning hydrogen.
Our star along with many others actually contain shells of fusion; zones where the heat and pressure have reached levels high enough to begin fusing the elements within to higher atomic masses. As a star ages, a significant amount of heavier elements accumulate in their zones until the heat and pressure again reach a high enough level to begin fusing the next shell. The force and energy from fusing the next shell causes the star to expand. Imagine millions of helium fusion bombs exerting pressure and all of a sudden millions of carbon fusion bombs go off, all more powerful and forceful than the shells above it.
The force of those fusion reactions pushes out while balancing the force of gravity. In effect, the star swells.
This cycle continues repeatedly down each shell of the star until (unfortunately for the star) it reaches the element Iron. As we found out earlier, the heat of the fusion reactions from the shells above release a lot of energy, in fact more than the energy required to fuse it. But the energy needed to fuse Iron is actually more than the energy released from the reaction. This spells the death of any star within seconds.
By this time the star has swelled immensely by sometimes billions of kilometers. The reaction of Iron fusion causes a deficit in the energy of the core of a star; as Iron fusion reactions absorb the energy of the surrounding fusion shells, depleting the core of enough energy to sustain fusion. This causes the outward pressure of fusion to drop instantly, causing the core to collapse.
It is within this brief moment, the few seconds of collapse, the last breath of the star’s life, that every element beyond Iron and Nickel are created. The force of collapse is so great, the heat and pressure from this instant alone are significant enough to fuse most of the elements past Iron, and everything else that we have around us today. The core then rebounds, sending a massive shockwave outward. This is what we know as a supernova explosion.
I said in my first post that at least two stars had to live and die before our Sun came into existence. In reality it could be at least one because one supernova explosion was needed to create all the elements we have now beyond Iron before the creation of our Sun could happen. But in the early universe, hydrogen was the prominent matter source everywhere. More than likely, a supermassive star–which typically live on the order of a few million years to a couple billion years, and keeping in mind the age of the universe at approximately 14 billion years, it makes sense that at least one supermassive star lived and died, providing a portion of the material for planets and life, while another star much like our Sun (which tend to live longer) lived and died before it but after the first. (This also supports the idea of nebulae in star formation. Stars are born by sweeping up enough material, then die spewing matter in all directions to be used in the life of another star.)
Our Sun is a typical main sequence star, meaning it is neither too massive to die as a supernova and neither too small to fall in the red M region and fizzle out as a brown dwarf (at least not immediately). Our Sun most likely will swell as usual, but not attain enough energy and pressure to begin fusing Iron* and explode violently in a supernova explosion. Most models predict it will recede from a maximum diameter of about the distance we are from the Sun**, and live for billions and billions of more years as a white dwarf until eventually most of the heat will have escaped and it will be a lowly brown dwarf at last.
*Some evidence suggests our Sun will only make it to carbon fusion before it dies.
**Scientists can’t seem to agree completely whether we will be absorbed into the Sun, or if we will just be fried for millions of years before it begins to retreat (which depends whether it will swell past our orbit).