The Life and Death of Stars
The night sky is the silent backdrop of human experience.
It is populated by an incredibly large number of stars, separated by vast distances and mostly empty space. We now know that there is a profound connection between these objects and our own existence: Not only stars lighten up an otherwise dark Universe, but through their lifecycle also produce and shed the fundamental elements required by biological life. All the elements we are made of, except for hydrogen, have been forged inside the hot and dense cores of stars. The calcium in our bones, the iron in our blood, the nitrogen and oxygen in the air and in our DNA. Basically every important ingredient of life came from the stars. We are literally made of stardust.
Studying stars humans discovered that, aside from hydrogen, all of the elements we are made of have been synthesized inside stars. A star is a self-regulating system, with the amount of energy released by nuclear burning is exactly the amount needed to counteract the gravitational force.
If the equilibrium is perturbed, the star readjust its structure, such that the nuclear reactions provide again the right amount of energy. In this way stars can be stable for long timescales during hydrogen burning (the so-called main sequence). This phase cannot last forever, since the amount of fuel inside of stars is ﬁnite, and energy is released only from the fusion of isotopes lighter than iron.
When a sufficient amount of energy can no longer be extracted from the rest mass of the star, the battle against gravity is lost. Then the final fate of the star depends on the mass of the object: low mass stars end their lives as white dwarfs, while massive stars (more massive than about 8 solar masses) die in spectacular explosions or disappear quietly forming a black hole.