Stars explode when they run out of fuel.

A star is a balance between gravity that sucks everything IN and high temperature due to nuclear reactions that push everything OUT. So, when a star runs out of FUEL, gravity wins the balance and the star collapses in (implodes), and then the imposition turns itself inside out and blows the star apart. This is called a supernova and it is very destructive.

As stars get older, they waste material from the nuclear reactions that power stars builds up in the stars' cores. The sun is burning hydrogen and accumulating helium, and will eventually begin burning helium to create carbon and oxygen. The sun isn't massive enough to burn oxygen, so the oxygen will sit there like a lump and never burn. More massive stars can burn the oxygen to create silicon, and the silicon to make iron, but no star, no matter how massive, can burn iron. If a star builds up a lump of iron with a mass of 1.44 times that of the sun, then the pressure that the iron atoms make on each-other will not be enough to hold them up, and they will collapse. This collapse releases energy, so much energy in fact that the rest of the star can't contain it, and it explodes.

Very large stars explode when they run out of fuel. Stars burn hydrogen (H) in their core throughout their lifetime. We call it "burning", but what is really happening is that H atoms are crashing together to form helium (He) atoms. H and He are what make up stars. Eventually a star will run out of H fuel in the core, and will have nothing left to burn. This is just like when you have a bonfire and you run out of wood to burn - the fire dies out. When this happens in a star, some of the mass from the outer parts of the star sink to the middle. This new mass in the middle of the star is very hot and heavy, so it also starts burning to create the heavier elements like carbon, oxygen, and every element up to the mass of iron. These elements are much denser than hydrogen, so it causes the star to become unstable and it collapses on itself under its own gravity in an explosion called a supernova.

Inside a supernova, even more elements are born! The periodic table is arranged by mass of the elements: everything from H to Fe is made in a star, and everything heavier than Fe is made during the supernova explosion. So all of the elements that make up the earth and even you were made inside a star or during a supernova! Only stars that are 5x heavier than our sun will go through a supernova.