The Birth of a Star: In space, there exists huge clouds of gas and dust. These clouds consist of hydrogen and helium, and are the birthplaces of new stars. Gravity causes these clouds to shrink and become warmer. The body starts to collapse under its own gravity, and the temperature inside rises. After the temperature reaches several thousand degrees, the hydrogen molecules are ionized (electrons are stripped from them), and they become single protons. The contraction of the gas and the rise in temperature continue until the temperature of the star reaches about 1,000,000 degrees Celsius (18,000,000 degrees Fahrenheit). At this point, nuclear fusion occurs in a process called proton-proton reaction. Briefly, proton-proton reaction is when four protons join together and two are converted into neutrons; an 4He nucleus is formed. During this process, some matter is lost and converted to energy as dictated by Einstein's equation. At this point, the star stops collapsing because the outward force of heat balances the gravity.

The Hydrogen Burning Stage: The proton-proton reaction occurs during a period called the hydrogen-burning state, and its length depends on the star's weight. In heavy stars, the great amount of weight puts a large amount of pressure on the core, raising the temperature and speeding up the fusion process. These heavy stars are very bright, but only live for a short amount of time. After the energy from this deuteron-hydrogen fusion process ends, the star begins to contract again, and the temperature and pressure subsequently increase. Nuclear fusion occurs between the hydrogen and lithium & other light metals in the star, but this process soon ends. Contraction starts again, and the extreme high temperature and pressure cause the hydrogen to transform into helium through the carbon-nitrogen-oxygen cycle. When all the hydrogen has been used up, the star is at its largest size, and it is called a red giant. Different things can happen to the star now.

Scenario 1: Planetary Nebulas:
One scenario is that the star will continue to make energy by using hydrogen and helium outside of the core; its surface will rise and fall and the star will become a variable star. After it gets out of control, the layers of gas will pull away, forming a shell of gas known as a planetary nebula.

Scenario 2: White dwarf
The other scenario is that the star will continue to shine through the fusion of helium nuclei, in the triple alpha process. The star is now a white dwarf, and further contraction is prevented by the repulsion of electrons in the core.

Supernova: Very heavy stars will continue to fuse heavy elements in order to produce more energy. However, once iron is formed, it cannot be fused to make more energy since it has such a high binding energy and is therefore very stable. The core will collapse under gravity and huge amounts of gas on the surface of the star will explode out. This star is now called a supernova.

Neutron Star: After a supernova explosion, the iron core of the star may be extremely heavy, and the force of gravity may be extremely large. It then becomes a neutron star, where the repulsion between neutrons stops the contraction caused by gravity. Neutron stars consist of matter that is 100 million times denser than white dwarf matter.

Pulsars: A neutron star may spin rapidly after a supernova explosion, and it may emit two beams of radio waves, light, and X-rays. These beams radiate in a circle because the star is spinning, and it appears that the star is pulsing on and off. Thus, it is given the name Pulsar.

Black Holes: Neutron-neutron repulsion can only counteract the force of gravity if the core of the dead star weighs less than three times the weight of the sun. In an extremely heavy core, no force can stop the matter from being squeezed into a smaller and smaller space. Nothing can escape these black holes; not even light.