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1、Chapter 12 The deaths of stars Fig.12-1 This schematic H-R diagram summarizes the three major modes of stellar demise(死亡死亡).Low-mass red dwarfs live long lives and become white dwarfs.Sun-like stars become giants with cores collapse to become white dwarfs.The most massive stars become supergiants an
2、d explode as supernovae.12.1Red dwarfs (Fig.12-1)Main-sequence stars with M 0.4 solar mass;they are cool,small and faint.lower right portion of the main sequence-The stars are convective from their centers to their surfacesgas is constantly mixed Hydrogen,helium ashes distributed uniformly inside th
3、e starno hydrogen-burning shellwill not form giants-Low mass burns hydrogen very slowly very long lives on main sequence(100s billions years)-They are not massive enough to burn helium(not hot enough at the core)contract slowly when fuels are used up getting smaller,hotter move from right to left on
4、 H-R diagrambecome white dwarfs red dwarfs 12.2 Sun-like stars 0.4 solar mass mass of the star 4 solar massescontinue burning of heavier and heavier elements until iron-the most tightly bounded elements(Fig.12-5)cannot produce any more energy by fusing ironan inert(不活动不活动)iron core is formed in whic
5、h no energy is generatedthe star core cannot resist the contracting force of gravitycollapses rapidly Fig.12-5 Schematic interior of a massive star approaching the end of its life.This cool giant star,about the size of the orbit of Jupiter,contains a core about the size of the earth in which concent
6、ric layers fuse heavier fuels around the growing iron core.Not even electron degeneracy pressure can stop gravitythe core contracts faster and fasterdensity goes higher and higher.When density 1014 gcm-3,nucleons(neutrons and protons in atomic nuclei)are pressed together,exerting strong pressure to
7、resist,reacting like a hard core.inner core rebounds,building up powerful shock waves which propagate outward,triggering nuclear reactions in the outer layers.An extremely high energy explosion results(equivalent to 1028 Mton of TNT).complete destruction of the stars outer layers in a very short tim
8、esupernova:sudden increase in the brightness of a star by many magnitudese.g.,guest star in Taurus recorded by Chinese astronomers in Shung dynasty(1054 AD);it was visible during daytime,and it faded after a month and vanished in two years.Supernova remnant:Nebulous remnant of a supernova explosion
9、e.g.,Crab Nebula M1 is the remnant of the supernova in 1054A small,extremely dense core of neutron matter may remain after the explosion neutron star 12.5 Evolution of binary starsFig.12-6 Evolution of a close binary system.(a)One of the stars(right)is more massive.(b)The more massive star evolves i
10、nto a giant,transferring mass to its companion,which then becomes more massive but is still on the main sequence.(c),(d)The giant dies and becomes a compact object(e.g.a white dwarf).(e)Mass is transferred to the compact object from the companion,which(f)later also becomes a giant.Fig.12-6 Two stars
11、 revolve around each other under their mutual force of gravityMore than half of all stars in the universe are members of binary systems.When one member of the system swells into a red giant,matter in its outer layers is loosely held and may be transferred to its companion.the evolution paths of both
12、 stars are changedAlgol paradox:In some binary systems,the less massive star is a giant while the more massive one remains on the main sequence.Explanation:The more massive star leaves the main sequence first and swells to a giant,spilling(溢出溢出)materials over to its companion(Fig.12-6).the companion
13、 becomes relatively more massive but is still on the main sequenceIf both members swell into giants,they could merge together to form one rapidly-spinning supergiant.If one member becomes a white dwarf(or other compact stars),it can form an accretion disk(吸吸积积盘盘).Mass transferring from the companion
14、 forms a whirlpool(漩漩涡涡)structure surrounding the white dwarf(Fig.12-7).friction and tidal forces make the disk very hot(1 million K)emits high energy radiation(e.g.ultraviolet radiation,X-rays)Fig.12-7 Matter falling into a compact object forms a whirling accretion disk.Friction and tidal force can
15、 make the disk very hot.Nova:(新星)Materials accreted onto a white dwarf become hotter and hotter until nuclear fusion is igniteda sudden explosion blows off the surface of the white dwarfThe expanding shells are very luminous sudden increase in brightness-The star fades when the shells grow larger and coolMass transfer may resume afterward the process repeats,similar explosions occur laterthe star may become a recurrent nova(再发新再发新星星);some become irregular variables(stars that change their brightness irregularly with time)