A Nuclear Furnace:
A star is like a gigantic nuclear furnace. The nuclear reactions inside convert hydrogen into helium by means of a process known as fusion. It is this nuclear reaction that gives a star its energy. Fusion takes place when the nuclei of hydrogen atoms with one proton each fuse together to form helium atoms with two protons. A standard hydrogen atom has one proton in its nucleus. There are two isotopes of hydrogen, which also contain one proton, but contain neutrons as well. Deuterium contains one neutron while Tritium contains two. Deep within the star, A deuterium atom combines with a tritium atom. This forms a helium atom and an extra neutron. In the process, an incredible amount of energy is released. When the star’s supply of hydrogen is used up, it begins to convert helium into oxygen and carbon. If the star is massive enough, it will continue until it converts carbon and oxygen into neon, sodium, magnesium, sulfur and silicon. Eventually, these elements are transformed into calcium, iron, nickel, chromium, copper and others until iron is formed. When the core becomes primarily iron, the star’s nuclear reaction can no longer continue. This is because the temperature required to fuse iron is much too great. The inward pressure of gravity becomes stronger than the outward pressure of the nuclear reaction. The star collapses in on itself. What happens next depends on the star’s original mass.
The Circle of Life:
Stars begin their lives as clouds of dust and gas called nebulae. The gravity of a passing star or the shock wave from a nearby supernova may cause the nebula to contract. Matter in the gas cloud will begin to coalesce into a dense region called a protostar. As the protostar continues to condense, it heats up. Eventually, it reaches a critical mass and nuclear fusion begins. This begins the main sequence phase of the star. It will spend most of its life in this stable phase. The life span of a star depends on its size. Very large, massive stars burn their fuel much faster than smaller stars. Their main sequence may last only a few hundred thousand years. Smaller stars will live on for billions of years because they burn their fuel much more slowly. Eventually, the star’s fuel will begin to run out. It will expand into what is known as a red giant. Massive stars will become red supergiants. This phase will last until the star exhausts its remaining fuel. At this point, the pressure of the nuclear reaction is not strong enough to equalize the force of gravity and the star will collapse. Most average stars will blow away their outer atmospheres to form a planetary nebula. Their cores will remain behind and burn as a white dwarf until they cool down. What will be left is a dark ball of matter known as a black dwarf. If the star is massive enough, the collapse will trigger a violent explosion known as a supernova. If the remaining mass of the star is about 1.4 times that of our Sun, the core is unable to support itself and it will collapse further to become a neutron star. The matter inside the star will be compressed so tightly that its atoms are compacted into a dense shell of neutrons. If the remaining mass of the star is more than about three times that of the Sun, it will collapse so completely that it will literally disappear from the universe. What is left behind is an intense region of gravity called a black hole.
The nebula that was expelled from the star may continue to expand for millions of years. Eventually, the gravity of a passing star or the shock wave from a nearby supernova may cause it to contract, starting the entire process all over again. This process repeats itself throughout the universe in an endless cycle of birth, death, and rebirth. It is this cycle of stellar evolution that produces all of the heavy elements required for life. Our solar system formed from such a second or third generation nebula, leaving an abundance of heavy elements here on Earth and throughout the Solar System. This means that We are all made of star stuff. Every atom in our bodies was created either in the nuclear furnace of a star or in the cataclysmic explosion of a supernovas.