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Astronomy 22-24.docx - Astronomy 22-24 Zheng Yu 2.The first...

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Astronomy 22-24Zheng Yu2.The first event that happens to a star with roughly the mass of our Sun thatexhausts the hydrogen in its core and stops the generation of energy by the nuclearfusion of hydrogen to helium is that the core starts shrinking due to gravity. Energy canno longer be generated by hydrogen fusion in the stellar core because the hydrogen isall gone, and the fusion of helium requires much higher temperatures. Since the centraltemperature is not yet high enough to fuse helium, there is no nuclear energy source tosupply heat to the central region of the star. The long period of stability now ends,gravity again takes over, and the core begins to contract. Once more, the star’s energyis partially supplied by gravitational energy. As the star’s core shrinks, the energy of theinward- falling material is converted to heat.As the core contracts helium moves inward, producing heat. This increase oftemperature brings the hydrogen to the limit where it can undergo a fusion reaction.More heat causes more hydrogen fusion which causes further expansion of the upperlayer. New energy produced by fusion of this hydrogen now goes outward from this shelland begins to heat up layers of the star farther out, causing them to expand. Meanwhile,the helium core continues to contract, producing more heat right around it. This leads tomore fusion in the shell of fresh hydrogen outside the core. The additional fusionproduces still more energy, which also flows out into the upper layer of the star. Theprocess goes on and it becomes a Red Star.4.Protostar is a very young star still in the process of formation, before nuclearfusion begins. A star with the mass of our sun is form from the nebula at first. Then it willbecome "pro-stars ". During the protostar stage, gravity gathers gas and dust toward acentral location, which increases in temperature and pressure. Eventually, thetemperature and pressure at the center of the gas and dust will reach critical thresholdsand the nuclear fusion of hydrogen into helium will begin in what is now called the core.When this happens, the protostar officially becomes a zero-age main sequence star.After that, it becomes main sequence star in adulthood phase. During middle age,Oncethe hydrogen gas runs out in the core, gravity will begin to re-collapse the stellaratmosphere above the core, which in turn increases the temperature and pressurewithin the core again. While this occurs, a hydrogen shell around the core will beginnuclear fusion, causing the star to swell and become more luminous as it enters the redgiant phase. It will form "red giant star". In the old age phase, its temperature will cooldown and its lights will also get down.It will become "white dwarf" and after during thedying phase the" black dwarf". (The increased surface area of a star will cause itssurface temperature to decrease but since the core of the star is collapsing it providesadditional heat energy which allows the star to appear more luminous.)

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Term
Winter
Professor
Biezen
Tags
Energy, Mass, White dwarf

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