Week 4 The Universe and the Earth

Week 4 The Universe and the Earth - The Universe and the...

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The Universe and the Earth The Universe | Earth in Space | Earth | Conclusion | Practice Quiz Please note, all words that are bold are rollovers that will display additional text in the form of a pop-up box. The Universe Stars are theoretically born in clouds of hydrogen gas and dust in the space between other stars. Gravity pulls huge masses of hydrogen gas together into a protostar that contracts, becoming increasingly hotter at the center durign the process. Eventually, the temperature inside the core of the star is high enough to start nuclear fusion reactions between hydrogen atoms. Pressure from hot gases balances the gravitational contraction The result is a newborn star that will shine quietly for billions of years. Our sun has sufficient hydrogen in the core to continue shining for another 4 or 5 billion years. Like most stars, the sun has a dense, hot core where nuclear fusion releases radiation, a less dense radiation zone where radiation moves outward, and a thin convection zone that is heated by the radiation at the bottom then moves to the surface to emit light to space. The brightness of a star is related to the amount of energy and light produced, the size of the star, and the distance from the Earth to the star. The apparent magnitude is the brightness of a star as it appears to you. To compensate for differences in brightness due to distance, astronomers calculate the brightness that stars would have at a standard distance. This is called the absolute magnitude . Not all stars are alike. Stars appear to have different colors because they have different surface temperatures as depicted on a graph of temperature by spectral types and brightness by absolute magnitude, called the Hertzsprung- Russell diagram as shown below. This graph shows that normal, mature stars fall on a narrow band called the main sequence . Other groups of stars on the H-R diagram have different sets of properties that are determined by evolutionary age. The life of a star consists of several stages, the longest of which is the main sequence stage. This stage begins after a relatively short time as a protostar . After using up the hydrogen in the core, a star with an average mass expands to a red giant , and then blows off the outer shell to become a white dwarf star . The white dwarf slowly cools to a black lump of carbon. The blown-off outer shell forms a planetary nebula that disperses over time to become the gas and dust of interstellar space. More massive stars collapse into neutron stars after a violent supernova explosion. A supernova produces a brilliant light in the sky that may only last for a few months before new elements are formed that diffuse into space. Galaxies
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This note was uploaded on 12/01/2009 for the course SCI 214 taught by Professor Bodus during the Spring '09 term at DeVry Cincinnati.

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Week 4 The Universe and the Earth - The Universe and the...

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