Lecture - Ann Arbor Earth Science

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Lecture - Ann Arbor Earth Science

Unit 2: The Sun and Other StarsObjective:E5.2g - Explain how the balance between fusion and gravity controlsthe evolution of a star (equilibrium).E5.2h - Compare the evolution paths of low, moderate, and highmass stars using the H-R diagram.


Fusion vs. GravityThe nuclear fusion processreleases energy, causing a star toexpand. At the same time,gravitational forces are working atcompressing a star into a smallnucleus.Most stars reach an equilibriumbetween these two forces whilenuclear fusion continuesthroughout the major part of thestars life.


Fusion vs. GravityMain sequence stars are stable stars because the nuclear fusionforce is balanced by the force of gravity. Eventually, when thestar runs out of an ample supply of hydrogen to fuse into helium,these forces will no longer be in balance.At this point, a star will begin to expand, moving into the nextstage of its life cycle.


Mass of StarsThe mass of a star is thedeciding factor that determineswhere a star will be located onthe H-R diagram.High mass stars burn hydrogenvery fast in order to keep fromcollapsing. Thus, high massstars are very hot, have highluminosities, and have shorterlife spans.


Mass of StarsLow mass stars burn their fuelmore slowly since they have asmaller gravitational force.These stars are cooler and havesmaller luminosities.Because they don’t burn fuel sofast, low mass stars have alonger life span.


DeathStars that are 2x the size of the sun will end their lives byblowing up in an explosion called a supernova. Largerstars may have remnants of the core left over. These arereferred to as Neutron Stars. Still larger stars will shrinkeven smaller and take themselves out of the knownuniverse. These are known as Black Holes.

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