Intro To Stars And Galaxies
Intro To Stars And Galaxies PHYS 1060
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Date Created: 09/30/15
113 What are the two types of star clusters Open cluster Afew thousand loosely packed stars Globular cluster Up to a million or more stars in a dense ball bound together by gravity How do we measure the age of a star cluster Massive blue stars die first followed by white yellow orange and red stars Pleiades now has no stars with a life expectancy less than around 100 million years The main sequence turnoff point of a cluster tells us its age Detailed modeling of the oldest globular clusters reveals that they are about 13 billion years old To determine accurate ages we compare models of stellar evolution to the cluster data How do we measure the age of a star cluster A star cluster s age roughly equals the life expectancy of its most massive stars still on the main sequence Chapter 12 Star Stuff 121 How do stars form Star Forming Clouds Stars form in dark clouds of dusty gas in interstellar space The gas between the stars is called the interstellar medium Gravity Versus Pressure Gravity can create stars only if it can overcome the force of thermal pressure in a cloud Gravity within a contracting gas cloud becomes stronger as the gas becomes denser Mass of a Star Forming Cloud A typical molecular cloud TN 30 K n N 300 particlescm3 must contain at least a few hundred solar masses for gravity to overcome pressure The cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons that escape the cloud Fragmentation of a Cloud This simulation begins with a turbulent cloud containing 50 solar masses of gas The random motions of different sections of the cloud cause it to become lumpy Each lump ofthe cloud in which gravity can overcome pressure can go on to become a star A large cloud can make a whole cluster of stars Glowing Dust Grains As stars begin to form dust grains that absorb visible light heat up and emit infrared light Long wavelength infrared light is brightest from regions where many stars are currently forming Thought Question What would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy It would continue contracting but its temperature would not change Its mass would increase Its internal pressure would increase Cloud heats up as gravity causes it to contract due to conservation of energy Contraction can continue if thermal energy is radiated away As gravity forces a cloud to become smaller it begins to spin faster and faster due to conservation ofangular momentum Gas settles into a spinning disk because spin hampers collapse perpendicular to the spin axis Rotation ofa contracting cloud speeds up for the same reason a skater speeds up as she pulls in her arms Flattening Collisions between particles in the cloud cause it to flatten into a disk Collisions between gas particles in a cloud gradually reduce random motions Collisions between gas particles also reduce up and down motions The spinning cloud flattens as it shrinks Formation ofJets Rotation also causes jets of matter to shoot out along the rotation axis Jets are observed coming from the centers of disks around protostars Thought Question What would happen to a protostar that formed without any rotation at all Its jets would go in multiple directions It would not have planets It would be very bright in infrared light It would not be round Protostar to Main Sequence A protostar contracts and heats until the core temperature is sufficient for hydrogen fusion Contraction ends when energy released by hydrogen fusion balances energy radiated from the surface It takes 30 million years for a star like the Sun less time for more massive stars Summary ofStar Birth Gravity causes gas cloud to shrink and fragment Core of shrinking cloud heats up When core gets hot enough fusion begins and stops the shrinking New star achieves long lasting state of balance How massive are newborn stars A cluster of many stars can form out of a single cloud Very massive stars are rare Low mass stars are common Upper Limit on a Star s Mass Photons exert a slight amount of pressure when they strike matter Very massive stars are so luminous that the collective pressure of photons drives their matter into space Models of stars suggest that radiation pressure limits how massive a star can be without blowing itself apart Observations have not found stars more massive than about 150M5un Lower Limit on a Star s Mass Fusion will not begin in a contracting cloud if some sort offorce stops contraction before the core temperature rises above 107 K Thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation Is there another form of pressure that can stop contraction Brown Dwarfs Degeneracy pressure halts the contraction of objects with lt008Msun before the core temperature becomes hot enough for fusion Degeneracy Pressure Laws of quantum mechanics prohibit two electrons from occupying the same state in the same place Thermal Pressure Depends on heat content The main form of pressure in most stars Degeneracy Pressure Particles can t be in same state in same place Doesn t depend on heat content Starlike objects not massive enough to start fusion are brown dwarfs Brown Dwarfs A brown dwarf emits infrared light because of heat left over from contraction ts luminosity gradually declines with time as it loses thermal energy Brown Dwarfs in Orion Infrared observations can reveal recently formed brown dwarfs because they are still relatively warm and luminous 122 Life as a Low Mass Star Thought Question What happens when a star can no longer fuse hydrogen to helium in its core ts core cools off Its core shrinks and heats up ts core expands and heats up Helium fusion immediately begins Life Track After Main Sequence Observations of star clusters show that a star becomes larger redder and more luminous after its time on the main sequence is over A star remains on the main sequence as long as it can fuse hydrogen into helium in its core Broken Thermostat As the core contracts H begins fusing to He in a shell around the core Luminosity increases because the core thermostat is broken the increasing fusion rate in the shell does not stop the core from contracting Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion larger charge leads to greater repulsion The fusion of two helium nuclei doesn t work so helium fusion must combine three He nuclei to make carbon Thought Question What happens in a low mass star when core temperature rises enough for helium fusion to begin Helium fusion slowly starts up Hydrogen fusion stops Helium fusion rises very sharply Helium Flash The thermostat is broken in a low mass red giant because degeneracy pressure supports the core The core temperature rises rapidly when helium fusion begins The helium fusion rate skyrockets until thermal pressure takes over and expands the core again Life Track After Helium Flash Models show that a red giant should shrink and become less luminous after helium fusion begins in the core Observations of star clusters agree with those models Helium burning stars are found in a horizontal branch on the H R diagram How does a low mass star die Thought Question What happens when a star s core runs out of helium The star explodes Carbon fusion begins The core cools off Helium fuses in a shell around the core Double Shell Burning After core helium fusion stops He fuses into carbon in a shell around the carbon core and H fuses to He in a shell around the helium layer This double shell burning stage never reaches equilibrium the fusion rate periodically spikes upward in a series of thermalpulses With each spike convection dredges carbon up from the core and transports it to the surface Planetary Nebulae Double shell burning ends with a pulse that ejects the H and He into space as a planetary nebula The core left behind becomes a white dwarf End of Fusion Fusion progresses no further in a low mass star because the core temperature never grows hot enough for fusion of heavier elements some He fuses to C to make oxygen Degeneracy pressure supports the white dwarf against gravity Life Track of a Sun Like Star 123 What are the life stages of a high mass star CNO Cycle High mass main sequence stars fuse H to He at a higher rate using carbon nitrogen and oxygen as catalysts A greater core temperature enables H nuclei to overcome greater repulsion Life Stages of High Mass Stars Late life stages of high mass stars are similar to those of low mass stars Hydrogen core fusion main sequence Hydrogen shell burning supergiant Helium core fusion supergiant How do high mass stars make the elements necessary for life Big Bang made 75 H 25 He stars make everything else Helium fusion can make carbon in low mass stars The CNO cycle can change C into N and O Helium Capture High core temperatures allow helium to fuse with heavier elements Helium capture builds C into 0 Ne Mg Advanced Nuclear Burning Core temperatures in stars with gt8Msun allow fusion of elements as heavy as iron Advanced reactions in stars make elements like Si S Ca and Fe Multiple Shell Burning Advanced nuclear burning proceeds in a series of nested shells Iron is a dead end for fusion because nuclear reactions involving iron do not release energy Fe has lowest mass per nuclear particle Evidence for helium capture Higher abundances of elements with even numbers of protons How does a high mass star die ron builds up in the core until degeneracy pressure can no longer resist gravity The core then suddenly collapses creating a supernova explosion Supernova Explosion Core degeneracy pressure goes away because electrons combine with protons making neutrons and neutrinos Neutrons collapse to the center forming a neutron star Energy and neutrons released in a supernova explosion enable elements heavier than iron to form including Au and U Supernova Remnant Energy released by the collapse of the core drives outer layers into space The Crab Nebula is the remnant ofthe supernova seen in AD 1054 Supernova 1987A The closest supernova in the last four centuries was seen in 1987 124 How does a star s mass determine its life story Role of Mass A star s mass determines its entire life story because it determines its core temperature High mass stars have short lives eventually becoming hot enough to make iron and end in supernova explosions Low mass stars have long lives never become hot enough to fuse carbon nuclei and end as white dwarfs LowMass Star Summary Main Sequence H fuses to He in core Red Giant H fuses to He in shell around He core Helium Core Burning He fuses to C in core while H fuses to He in shell Double Shell Burning H and He both fuse in shells Planetary Nebula leaves white dwarf behind Reasons for Life Stages Core shrinks and heats until it s hot enough for fusion Nuclei with larger charge require higher temperature for fusion Core thermostat is broken while core is not hot enough for fusion shell burning Core fusion can t happen if degeneracy pressure keeps core from shrinking Life Stages of HighMass Star Main Sequence H fuses to He in core Red Supergiant H fuses to He in shell around He core Helium Core Burning He fuses to C in core while H fuses to He in shell Multiple Shell Burning many elements fuse in shells Supernova leaves neutron star behind How are the lives of stars with close companions different Thought Question The binary star Algol consists of a 37Msun main sequence star and a 08Msun subgiant star What s strange about this pairing How did it come about Stars in Algol are close enough that matter can flow from the subgiant onto the main sequence star The star that is now a subgiant was originally more massive As it reached the end of its life and started to grow it began to transfer mass to its companion mass exchange Now the companion star is more massive Chapter 13 The Bizarre Stellar Graveyard 131 What is a white dwarf White dwarfs are the remaining cores of dead stars Electron degeneracy pressure supports them against gravity White dwarfs cool off and grow dimmer with time Size ofa White Dwarf White dwarfs with the same mass as the Sun are about the same size as Earth Higher mass white dwarfs are smaller The White Dwarf Limit Quantum mechanics says that electrons must move faster as they are squeezed into a very small space As a white dwarf s mass approaches 14M5un its electrons must move at nearly the speed of light Because nothing can move faster than light a white dwarf cannot be more massive than 14M5un the white dwarf limit also known as the Chandrasekhar limit What can happen to a white dwarf in a close binary system A star that started with less mass gains mass from its companion Eventually the mass losing star will become a white dwarf What happens next Accretion Disks Mass falling toward a white dwarffrom its close binary companion has some angular momentum The matter therefore orbits the white dwarf in an accretion disk Accretion Disks Friction between orbiting rings of matter in the disk transfers angular momentum outward and causes the disk to heat up and glow Thought Question What would gas in the disk do if there were no friction A It would orbit indefinitely B It would eventually fall into the star C It would blow away Nova The temperature of accreted matter eventually becomes hot enough for hydrogen fusion Fusion begins suddenly and explosively causing a nova Nova The nova star system temporarily appears much brighter The explosion drives accreted matter out into space Thought Question What happens to a white dwarf when it accretes enough matter to reach the 14Msun limit A It explodes B It collapses into a neutron star C It gradually begins fusing carbon in its core Two Types of Supernova Massive star supernova Iron core of massive star reaches white dwarfimit and collapses into a neutron star causing an explosion White dwarf supernova Carbon fusion suddenly begins as white dwarf in close binary system reaches white dwarf limit causing a total explosion One way to tell supernova types apart is with a light curve showing how luminosity changes with time Nova or Supernova Supernovae are MUCH MUCH more luminous than novae about 10 million times Nova H to He fusion of a layer of accreted matter white dwarf left intact Supernova complete explosion of white dwarf nothing left behind Supernova Types Massive Star or White Dwarf Light curves differ Spectra differ exploding white dwarfs don t have hydrogen absorption lines 132 What is a neutron star A neutron star is the ball of neutrons left behind by a massive star supernova The degeneracy pressure of neutrons supports a neutron star against gravity Electron degeneracy pressure goes away because electrons combine with protons making neutrons and neutrinos Neutrons collapse to the center forming a neutron star A neutron star is about the same size as a small city Discovery of Neutron Stars Using a radio telescope in 1967 Jocelyn Bell noticed very regular pulses of radio emission coming from a single part ofthe sky The pulses were coming from a spinning neutron star a pulsar Pulsar at center of Crab Nebula pulses 30 times per second Pulsars A pulsar is a neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis Pulsars The radiation beams sweep through space like lighthouse beams as the neutron star rotates Why Pulsars Must Be Neutron Stars Circumference of Neutron Star 2n radius N 60 km Spin Rate of Fast Pulsars N 1000 cycles per second Surface Rotation Velocity N 60000 kms N 20 speed of light N escape velocity from NS Anything else would be torn to pieces Pulsars spin fast because the core s spin speeds up as it collapses into a neutron star Conservation of angular momentum Thought Question Could there be neutron stars that appear as pulsars to other civilizations but not to us A Yes B No What can happen to a neutron star in a close binary system Matter falling toward a neutron star forms an accretion disk just as in a white dwarf binary Accreting matter adds angular momentum to a neutron star increasing its spin Episodes of fusion on the surface lead to X ray bursts Thought Question According to conservation of angular momentum what would happen ifa star orbiting in a direction opposite the neutron s star rotation fell onto a neutron star The neutron star s rotation would speed up The neutron star s rotation would slow down Nothing the directions would cancel each other out X Ray Bursts Matter accreting onto a neutron star can eventually become hot enough for helium to fuse The sudden onset offusion produces a burst ofX rays Neutron Star Limit Quantum mechanics says that neutrons in the same place cannot be in the same state Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass exceeds about 3M5un 133 Black Holes Gravity s Ultimate Victory What is a black hole A black hole is an object whose gravity is so powerful that not even light can escape it Some massive star supernovae can make a black hole if enough mass falls onto the core Thought Question What happens to the escape velocity from an object if you shrink it A It increases B It decreases C It stays the same Surface of a Black Hole The surface ofa black hole is the radius at which the escape velocity equals the speed of light This spherical surface is known as the event horizon The radius of the event horizon is known as the Schwarzschild radius The event horizon of a 3M5un black hole is also about as big as a small city Event horizon is larger for black holes of larger mass A black hole s mass strongly warps space and time in the vicinity ofthe event horizon No Escape Nothing can escape from within the event horizon because nothing can go faster than light No escape means there is no more contact with something that falls in It increases the hole s mass changes its spin or charge but otherwise loses its identity Singularity Beyond the neutron star limit no known force can resist the crush of gravity As far as we know gravity crushes all the matter into a single point known as a singularity Thought Question How does the radius of the event horizon change when you add mass to a black hole A Increases B Decreases C Stays the same What would it be like to visit a black hole If the Sun shrank into a black hole its gravity would be different only near the event horizon Time passes more slowly near the event horizon Thought Question Is it easy or hard to fall into a black hole A Easy B Hard Tidal forces near the event horizon of a 3M5un black hole would be lethal to humans Tidal forces would be gentler near a supermassive black hole because its radius is much bigger Do black holes really exist Black Hole Verification Need to measure mass Use orbital properties of companion Measure velocity and distance of orbiting gas It s a black hole if it s not a star and its mass exceeds the neutron star limit 3M5un Near the event horizon time slows down and tidal forces are very strong Do black holes really exist Some X ray binaries contain compact objects too massive to be neutron stars they are almost certainly black holes Some X ray binaries contain compact objects of mass exceeding 3M5un that are likely to be black holes 134 Gamma Ray Bursts Brief bursts of gamma rays coming from space were first detected in the 1960s What causes gamma ray bursts Observations in the 1990s showed that many gamma ray bursts were coming from very distant galaxies They must be among the most powerful explosions in the universe could be the formation of black holes Supernovae and Gamma Ray Bursts Observations show that at least some gamma ray bursts are produced by supernova explosions Some others may come from collisions between neutron stars
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