The Stellar Lives of Stars
Stars are giant nuclear reactors. In the center of stars, atoms are taken apart by tremendous atomic collisions altering the atomic structure and releasing an enormous amount of energy that makes stars hot and bright. Nuclear fusion is an atomic reaction that fuels stars. In fusion, many nuclei combine together to make a larger but different element, and the result of this process is the release of a lot of energy. Stars are powered by nuclear fusion in their cores, mostly converting hydrogen into helium (Fusion in Stars, 2012).
The production of new elements by nuclear reactions is called nucleosynthesis. A star's mass determines what type of nucleosynthesis occurs during explosive changes in its life cycles (Fusion in Stars, 2012).
The smallest stars only convert hydrogen into helium. Medium stars, such as our Sun, can convert helium into oxygen and carbon. Both small stars and medium stars are considered to be low mass stars. “A low mas star like the Sun never gets hot enough to fuse carbon in its core, because degeneracy pressure stops the gravitational collapse of its core. The star expels its outer layers into space as a planetary nebula, leaving its exposed core behind as a white dwarf that is supported by degeneracy pressure” (Bennett, 2013, p. 357).
A high mass star fuses hydrogen into helium by way of the carbon, nitrogen, and oxygen cycle (CNO cycle). After exhausting its core hydrogen, a high mass star begins hydrogen shell fusion. This is a series of stages fusing heavier elements and include: hydrogen burning, helium burning, carbon burning, oxygen burning, and silicon burning. Massive stars can convert helium atoms into carbon and oxygen, and is followed by the fusion of carbon and oxygen into neon, sodium, magnesium, sulfur and silicon. Later reactions transform these elements into calcium, iron, nickel, chromium, copper and others.
A nova is a strong, rapid increase in the brightness of a star. The word comes from the Latin for "new star," because often a star previously too dim to be seen with the naked eye can become the brightest object in the sky (besides the sun and the moon) when it becomes a nova. A supernova occurs when a high mass star dies in a cataclysmic explosion and scatters newly produced elements into space.
The supernova occurs after fusion begins to pile up iron in the high mass star’s core. An inert iron core builds up and successive layers above the core consume the remaining fuel of lighter nuclei in the core. Since iron does not act as a fuel, the burning stops.
“The sudden stoppage of energy generation causes the core to collapse and the outer layers of the star to fall onto the core. The in falling layers collapse so fast that they `bounce' off the iron core at close to the speed of light. The rebound causes the star to explode as a supernova. The energy released during this explosion is so immense that the star will out shine an entire galaxy for a few days. Supernova can be seen in nearby galaxies, about one every 100 years” (StellarDeath, 2013)
Electron Degeneracy Pressure:
The Pauli Exclusion Principle states that no two electrons with the same spin can occupy the same energy state in the same volume. Once the lowest energy level is filled, the other electrons are forced into higher and higher energy states resulting in them travelling at progressively faster speeds. These fast moving electrons create an electron degeneracy pressure capable of supporting a star (COSMOS, 2013).
Electron degeneracy pressure is what supports white dwarfs against gravitational collapse, and the Chandrasekhar limit (the maximum mass a white dwarf can attain) arises naturally due to the physics of electron degeneracy. As the mass of a white dwarf approaches the Chandrasekhar limit, gravity attempts to squeeze the star into a smaller volume, forcing electrons to occupy higher energy states and...
References: Bennett, D. S. (2013). the esential comic perspective 6th edition. San Francisco: Addison-Wesley.
COSMOS. (2013, November 23). Retrieved from Swinburn University of Technology Web site: atronomy.win.edu.au/cosmos/E/electron
Fusion in Stars. (2012, November). Retrieved from enchanted learning : www.enchantedlearning.com/subjects/astronomy/stars/fusion.shtml
Krimm, H. (2000, November 6th). Imagine the Universe. Retrieved from NASA Web site: http://imagine.gsfc.nasa.gov/docs
Neutron Degeneracy Pressure. (2013, November). Retrieved from Virginia Education Web site: www.astro.virginia.edu/~jh8h/glossary/neutron
Zimmerman, A. (2013, November 23rd). Physics. Retrieved from About.com Web site: http://physics.about.com/od/astronomy/f/BlackHole.htm
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