Night after night, people all over the world look up into the inky black sky and view the thousands of pinprick points of light called stars, wondering about their existence. Every single element in the entire universe, including the atoms that make up each human being, once belonged inside a star! In fact, all atoms, the tiniest particles that are the building blocks of everything we know, were made during the life cycle of stars.
Stars begin their lives in clouds of dust and hydrogen gas called nebulae. Movement of particles within the nebula causes the clouds of dust and gas to condense, and gravity continues this process. As the clouds of particles come together, the temperature in the center, or core, increases. As the temperature increases, nuclear fusion causes atoms of hydrogen to combine to form atoms of helium, a heavier element. Nuclear fusion is only possible under tremendous heat and pressure conditions. When helium atoms are formed from the fusing of hydrogen atoms, incredible energy is released, and the star shines. A protostar is formed.
Stars will continue to burn their hydrogen supply for a long while, ranging from a few million years for very hot stars to several billion years for average stars like our sun. This period in a star’s life is called the main sequence: this phase makes up most of the star’s lifespan. During this time in a star’s life, the gravity crushing the star inward is balanced by the energy created by nuclear fusion in the core radiating outward. A star stays in the main sequence period for about 90% of its life.
When a star begins to run low on its supply of hydrogen, a star is in the process of “dying.” Nuclear fusion slows down, so the pressure of gravity no longer is counter-acted by the power of the fusion reactions; thus, the star starts to collapse. The building pressure in the core of the star causes the helium to fuse into heavier elements such as oxygen and carbon. The star begins to expand, becoming a red giant (for stars less than 8 solar masses). A solar mass is defined as the mass of the Sun and is used to compare the masses of stars. The temperature on the surface of the star decreases greatly. The outer layers of the star drift away, forming a planetary nebula. The star’s core begins to cool down, and the star eventually becomes a white dwarf. The white dwarf cools even more until it is nothing more than a dark mass about the size of the Earth composed of a core of carbon and oxygen surrounded by helium and a little leftover hydrogen. The mass, though, is still very heavy, ranging from a bit less than half a solar mass to more than a solar mass. The star is officially dead now.
For a star that starts out at eight solar masses or greater, a different fate awaits. After the hydrogen fuel supply is depleted, helium atoms still fuse to form carbon and oxygen; however, the carbon and oxygen atoms also fuse to form heavier elements such as neon, magnesium, silicon, and sulfur. The silicon and sulfur combine to form iron in the core. The star swells to become a red supergiant. The iron in the center of the star does not fuse because there is not enough energy to support this reaction; therefore, with nothing to balance the inward pressure of gravity, the star dramatically shrinks from a huge burning ball the size of the Earth into a sphere that is only about twelve miles in less than a second! This collapse causes a tremendous release of energy termed a supernova. What is left of the star glows brighter than a billion suns for a day or two. The iron now is free to fuse to form heavier elements, thanks to the new surge of energy from the supernova.
Either a neutron star or a black hole is left of the once magnificent star. A neutron star forms if the leftover core is about one and a half to three solar masses. It is very small and dense, spinning very quickly in space. This spinning creates a magnetic field and causes the neutron star to give off radiation. If what remains of a supernova is more than four solar masses, a black hole forms. Gravity continues to crush what is left. A black hole’s gravity is so devastating that nothing, not even light, can escape its pull. Thus concludes the existence of a life-giving entity.
“Life Cycle of a Star.” Thinkquest. n.d. Web. 04 May 2010. http://library.thinkquest.org/26220/stars/formation.html>.
St. Cyr, Linda. “About the Formation of Stars.” EHow. n.d. Web. 04 May 2010. http://www.ehow.com/about_4606435_formation-stars.html>.
O’Brien, Stuart and Windsor, Michael. “The Life of a Star.” n.d. Web. 04 May 2010. http://www.astro.keele.ac.uk/workx/starlife/StarpageS_26M.html>.