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DSST Astronomy notes 1

Containing the majority of stars and they run

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containing the majority of stars and they run diagonally from the upper left to the lower right (a.i.1) About 23,000 stars are plotted on the H-R diagram to show relationship between absolute magnitude, luminosity, classification and the effective temperature b) A star on the main sequence as long as it has enough Hydrogen to burn c) Life of a Main Sequence Star c.i) Becomes a main sequence star once it starts nuclear fusion (c.i.1) Main sequence star is converting hydrogen to helium through nuclear reactions (c.i.2) Left over mass escapes as electromagnetic radiation (light and heat) (c.i.3) Once the hydrogen supply is gone, it leaves the main sequence and becomes a Red Giant 113) Stars are born in Molecular Clouds a) Dense clouds of gas and dust b) Molecular clouds are also referred to as a “Stellar Nursery” c) Usually seen as a nebula of high density 114) Protostar – the stage before becoming an actual star a) Large cool mass of gas b) If it’s too small it may never be a star – instead a Brown Dwarf 115) Nuclear Fusion of Hydrogen– must occur for a protostar to become an actual star a) Must have enough mass to reach 10 million degrees Celsius b) Nuclear fusion begins to turn hydrogen into helium so the protostar can become an actual main sequence star, determined by mass c) Star collapses after the star turns all of the hydrogen in its core into helium d) The core keeps compressing until it reaches a temperature and density that is high enough to begin fusion of helium to form carbon e) Meanwhile the outer layers are expanding rapidly f) Result is a bigger star with a smaller core – Red Giant f.i) Red because its cooler than a main sequence star, but due to its size, its more luminous 116) Summary- a.i) The hydrogen in a molecule cloud collapses to form a weakly glowing protostar. a.ii) A protostar collapses until it gets hot and dense enough to start nuclear fusion to begin turning its hydrogen into helium. a.iii) As soon as this nuclear fusion begins, the core stops collapsing and stabilizes--it is now a main sequence star
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b) Some protostars don't have enough mass to ever reach the 10 million Celsius required for nuclear fusion to start. These little protostars end up becoming brown dwarfs instead--too big to be a planet and too small to be a star. c) Summary II: c.i) the main sequence star burns until it's used up all of the hydrogen in its core c.ii) Once the hydrogen in the core has all been converted into a core of helium, the nuclear fusion stops. c.iii) As a result, the core destabilizes and resumes its collapse d) The core of a star maintains its stability only while nuclear fusion is going on to counter the pressure of gravity. If there's no nuclear fusion, then the core is going to keep compressing until it gets hot and dense enough for nuclear fusion to occur. e) Summary III e.i) Now that the star is no longer burning hydrogen, its time as a main sequence star is done. e.ii)
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containing the majority of stars and they run diagonally...

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