Astrophysics
Stellar evolution works like Hollywood. A star is born, burns brightly during multiple complex stages of life, and eventually runs out of fuel, becoming negligible because of brighter stars.
Ultimately, what determines a star's fate is its initial mass. This is because the light we see from a star is a product of nuclear fusion inside a star, which emits tons of radiation, some of which we see as light.
Fusing two nuclei together requires a huge amount of energy. Why? Well, nuclei have neutrons and protons. Neutrons are neutral but protons have a positive charge. As we know, positive charges repel each other, so getting them to fuse is a problem. A Hydrogen nucleus, for example, is a single, solitary proton. Fusing two of them means that they must have enough energy to collide into each other despite their like charges, and then one turns into a neutron, forming deuterium. Two deuterium nuclei colliding—again, only with sufficient energy—create a helium nucleus.
Fusing hydrogen atoms can be done, but to do this, the core of the star has to reach very high temperatures, to the order of 5 million Kelvin. All this heat provides the necessary energy for fusion to occur. As it occurs, a star then exerts an outwards gas pressure. Since stars don't expand instantaneously, another force balances out this gas pressure. This other force is the star's own gravity based on its mass. The heavier the star, the stronger its gravity.
Think of a cute little chickadee trying to hatch out of an egg. It tries to get out and explore the world, but the eggshell gets in the way. There is a bit of a struggle there for a while. Stars are like that.
In this simplistic analogy, the eggshell is a star's gravity, and the chickadee provides the outward gas pressure. That means that until it goes super nova a star is in hydrostatic equilibrium, shown in the following diagram:
For example, our Sun is currently fusing hydrogen into helium. When all hydrogen is consumed, the Sun will no longer have the ability to counteract gravity with gas pressure. Fear not, however; as it shrinks down a bit, it becomes denser, and its core heats up a little more. Soon enough, the temperature is high enough to start fusing helium. When that happens, then the star exerts pressure again, and reaches a new hydrostatic equilibrium.
The star plays this game several times. Elements that end up being fused are, in order, hydrogen, helium, carbon, neon, oxygen, silicon and iron. How far the chain a star goes depends once again on its initial mass, since the initial mass determines what temperatures the core can reach. Small stars generally don't have enough mass to fuse elements heavier than oxygen.
Larger stars, however, are massive enough to provide temperatures that can fuse elements all the way up to iron. Once a star has a core of iron, it can no longer resort to nuclear fusion to stay alive. This is because fusing iron together would require more energy than would be released, breaking a law of thermodynamics, but that's not our concern at the moment.
Since iron cannot be fused, the star can no longer counteract its own gravitational pull with gas pressure. The star collapses upon itself, triggering the production of a supernova. This means all elements heavier than iron (including minerals that make up the human body) are and were created as a result of a star imploding into a supernova.
Kinda cool, huh? We are children of supernova.
Below is a summary of stellar evolution and the types of stars that can be created. Here, small stars are assumed to weigh less than 4 solar masses.
Our Sun will eventually turn into a Red Giant, which will expand in size and gobble up Earth.
Don't worry, though; this will only take place in a few billions of years, so we've got plenty of time to solve this problem. Maybe we can populate Mars or one of the moons of Jupiter, or leave the entire solar system by then.