Stars form in vast galactic clouds called nebulae which contain mostly hydrogen, the simplest and most abundant element in the universe. Over millions of years, gravitational attraction causes hydrogen and other trace elements in a nebula to group together to form an increasingly dense sphere of gas. The high temperature and pressure at the core of this gas sphere pushes hydrogen atoms close enough together to form helium atoms. Called nuclear fusion, this process generates an immense amount of energy in the form of heat and sunlight.
A hydrogen atom consists of a single, positively charged proton nucleus orbited by a single, negatively charged electron. Normally, electromagnetic repulsion prevents protons from fusing; however, the extreme temperatures and pressures at the center of a star cause two protons to come close enough together for the strong nuclear force to take effect. This force works only over a very short distance, but it is far stronger than electromagnetism.
One of these fused protons then changes into the third subatomic particle, a neutrally charged neutron, to give a deuterium atom. There is a slight difference in the mass between two protons and a deuterium nucleus; this difference is liberated as energy as described by Albert Einstein's equation E=mc^2 (where E is energy, m is mass, and c the speed of light).
The deuterium nucleus will undergo further fusion reactions, either with a proton to form helium 3 or another deuterium nucleus to form helium 4. Each reaction produces more energy and keeps the core of the star at an extremely high temperature. The sun's core, for example, has a temperature of 27,000,000 degrees F.
How long a star lives depends on how much hydrogen it has and the rate at which it converts its hydrogen into helium. Our sun is about 5 billion years old and has enough hydrogen left to burn for another 5 billion years. Proxima Centauri, the nearest star to our solar system, is a red dwarf which will live for trillions of years as it consumes hydrogen very slowly. Conversely, a hotter, blue star like Sirius A will only live for around 1 billion years before its hydrogen is used up.
The fate of a star once its hydrogen has been depleted depends on its mass. Our sun will expand to become a red giant with greatly increased diameter; it will also be much brighter than at present. The even higher pressure and temperature in the core of a red giant allows helium atoms to combine to form heavier elements such as lithium, beryllium and boron.
Once the helium in the Sun has been depleted, further fusion is impossible. Gravity will cause the star's core to collapse to form what is called degenerate matter, while its vestigial outer layers will be shed into space. The compacted core is called a white dwarf. A white dwarf has no fuel source, yet will emit light for trillions of years because of its extremely high temperature. This can be up to 45,000 degrees F.
Heavier stars will expand to become red supergiants and undergo further reactions to form elements such as carbon, nitrogen and oxygen. As the complexity of the elements increases, so does the extremity of the conditions required to form them, with the heaviest supergiants being hot enough to produce chromium, manganese or even iron, the most stable of all elements. These stars will have more explosive deaths in the form of supernovae with their cores being compressed enough to form neutron stars or even black holes. The gaseous remnants of red supergiants will also shed their outer layers. These gases will drift through space and could one day become part of a nebula and of a new star.