Exam 3 Study Guide
Exam 3 Study Guide AY 101
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This 4 page Study Guide was uploaded by Alyssa Hendrixson on Thursday March 26, 2015. The Study Guide belongs to AY 101 at University of Alabama - Tuscaloosa taught by Dean Martin Townsley in Winter2015. Since its upload, it has received 141 views. For similar materials see Introduction to Astronomy in Physics 2 at University of Alabama - Tuscaloosa.
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Date Created: 03/26/15
Measuring Stars The apparent brightness of an object increases as its luminosity increases and decreases as its distance away increases In the magnitude system brighter objects have smaller magnitudes and vice versa The luminosity of a star increases with both area and temperature The only way to clearly measure a stars mass is for it to be in a binary with another star so that the orbit can be measured and the mass of the system inferred Surface Temperature amp Luminosity Notable Stars Luminosity increases with temperature and size Thus among two stars with the same luminosity but different temperatures the lower temperature one must be larger Among mainsequence stars lower surface temperature stars are also lower in mass By contrast although they are low temperature giants have high luminosities compared to the main sequence White dwarfs are both cooler and less luminous than solarlike stars The ionization state or number of electrons on atoms depends on temperature This causes stellar spectra of different temperature stars to show different absorption lines Higher mass stars have shorter lifetimes as main sequence or normal stars fusing hydrogen in their centers That is lifetime decreases with increasing mass Notable Stars Types of Stars and Clusters of Stars The presence of sustained nuclear fusion reactions in the core is the defining characteristic of a star in contrast to for example gas giant planets and sub stellar brown dwarfs Higher mass stars have shorter lifetimes as main sequence or quotnormalquot stars fusing hydrogen in their centers That is lifetime decreases with increasing mass Among mainsequence stars lower surface temperature stars are also lower in mass By contrast although they are low temperature giants have high luminosities compared to the main sequence White dwarfs are both cooler and less luminous than solarlike stars In order to determine if a star of a given temperature is a giant or main sequence star knowledge of either the radius or luminosity is necessary The luminosity of a star can be obtained from its apparent brightness and distance Stars born together in a given cluster will be the same age composition and distance from Earth but with a variety of masses This makes clusters very useful for comparing the properties of stars of different masses The age of a star cluster must be less than the lifetime of all stars still present as normal stars in their corehydrogenburning phase and greater than all stars already gone Higher mass stars in their central hydrogen fusion phase are higher temperature therefore bluer than lower mass stars Inside the Sun Label location of the major structures of the sun core radiative zone convection zone photosphere and corona The turbulent motion in the convection zone is driven by the transport of energy from the interior of the sun to the surface as heated gas rises and cooler gas sinks The equation Emcquot2 expresses the equivalence of energy and matter An amount of energy E can under certain circumstances change into matter particles with an amount of mass m and vice versa Transforming nuclei to be more similar to Iron releases energy by reducing the overall mass of the combined nuclei Stars are powered for nearly all of their lives by nuclear fusion of hydrogen to form helium in their centers The Sun is currently in this core hydrogen fusion stage Nuclear fusion the energy source for the Sun occurs in the solar core where the temperature is highest For fusion to take place particle motion such as thermal motion must overcome the electromagnetic repulsion between nuclei How the Sun Works Solar activity constitutes changes in the magnetic field structure at the surface and in the corona of the Sun Sunspots form at the footpoints of magnetic loops protruding from the sun s surface The spots are cooler than the surrounding solar surface because they are prevented from mixing by the magnetic field The stretching of magnetic fields by the nonuniform rotation of the sun drives the constant changes in magnetic field structure that manifest as solar activity As the thermal and gravitational energy content of a normal star increases its central temperature decreases due to its expansion and vice versa This is the opposite of everyday objects Nuclear fusion is stable inside stars because too much energy input leads to a decrease in central temperature and therefore a decrease in fusion rate and energy input Birth of Stars In order to form stars a molecular cloud must have high enough density for self gravity to act and must be low enough temperature that the selfgravity will win out over the thermal pressure Within any cluster or similar grouping of stars after formation there are many lower mass stars than higher mass stars This extends all the way down to substellar objects As a star contracts the core is compressed and the temperature in the core increases The general stages of star formation in order of their occurrence are collapse from molocular cloud formation of protester and growth via accretion contraction of protester and ignition of hydrogen fusion Objects below about 01 solar masses never become hot enough in their cores for hydrogen fusion Because of this they are considered substellar objects Lives of Low Mass Stars The categorization of high and low mass stars is based on their end products high mass stars become neutron stars or black holes after a supernova low mass stars become white dwarfs without making a supernova The end of the main sequence phase of stellar evolution is marked by the depletion of hydrogen in the star s core As the mass in the inert core of a giant star increases with time the luminosity of the shell source surrounding it also increases A low mass star finally finishes its life and becomes a white dwarf star when it has exhausted its nuclear fuel through a combination of fusing it to heavier elements and losing it in a stellar wind The basic evolution of a low mass star begins with the mainsequence phase followed by the red giant phase and ending as a white dwarf Lives of High Mass Stars When a high mass star dies it will leave behind a neutron star or black hole A low mass star will leave behind a white dwarf Stars are powered for nearly all of their lives by nuclear fusion of hydrogen to form helium in their centers This is also called the quotmain sequencequot phase of their evolution The Sun is currently in this core hydrogen fusion stage Heavier nuclei have higher electric charge than lighter nuclei therefore fusing them together requires higher temperatures in order to overcome the stronger electromagnetic repulsion The creation of a new core burning phase inside a star proceeds through the following events fuel depletion in the core formation of a burning shell around the core contraction of the now inert core raising its temperature eventual ignition of burning in the core Nuclear energy production ceases at the center of a highmass star because an iron core has formed and fusing iron to heavier elements does not release energy The star cannot release any more energy by nuclear fusion When electrons begin to be captured onto protons in the iron core of a massive star the pressure support due to these electrons is lost eventually causing the collapse of the core Stellar Remnants review When a high mass star dies it will leave behind a neutron star or black hole A low mass star will leave behind a white dwarf A white dwarf star is a stellar remnant and differs from a normal star in that it is no longer generating energy from nuclear fusion A white dwarf star is still hot and radiating energy to space and can have more mass than very low mass star Pressure in degenerate matter like that making up a white dwarf star does not depend much on temperature Stars are held up against gravity by their internal pressure Therefore as the temperature of a white dwarf star decreases it stays approximately the same size When a white dwarf or the core of a highmass giant star collapses the protons and electrons merge to form the neutrons that make up the neutron star that is made Since the star was mostly held up by the electrons which are merge with the protons the star becomes much smaller Neutrons have no charge and therefore no electric repulsion This makes it easier for neutrons to get close to each other and to other nuclei Protons and neuclei in general are positively charged and therefore repel one another unless very close From a typical planetary orbit a black holes gravitational field is no different than that of a star with the same mass The difference is that a black hole is very small so that very close to it gravity is quite strong
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