lecture17_ch12b - Life Stages of Low-Mass Star 1 Main...

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Unformatted text preview: Life Stages of Low-Mass Star 1. Main Sequence: H fuses to He in core 2. Red Giant: H fuses to He in shell around He core Lives and Deaths of Stars 3. Helium Core Burning: He fuses to C in core while H fuses to He in shell 4. Double Shell Burning: H and He both fuse in shells Not to scale! 5. Planetary Nebula leaves white dwarf behind High-Mass Stars > 8 MSun Reasons for Life Stages Core shrinks and heats until itʼs hot enough for fusion Nuclei with larger charge require higher temperature for fusion Low-Mass Stars < 8 MSun Core thermostat is broken while core is not hot enough for fusion (shell burning) Not to scale! Brown Dwarfs Core fusion canʼt happen if degeneracy pressure keeps core from shrinking Life Stages of High-Mass Star 1. Main Sequence: H fuses to He in core 2. Red Supergiant: H fuses to He in shell around He core What are the life stages of a high mass star? 3. Helium Core Burning: He fuses to C in core while H fuses to He in shell 4. Multiple Shell Burning: Many elements fuse in shells Not to scale! 5. Supernova leaves neutron star behind 1 CNO cycle is just another way to fuse H into He, using carbon, nitrogen, and oxygen as catalysts High-mass stars become supergiants after core H runs out CNO cycle is main mechanism for H fusion in high mass stars because core temperature is higher Luminosity doesnʼt change much but radius gets much larger How do high mass stars make the elements necessary for life? CNO cycle can change C into N and O Helium-capture reactions add two protons at a time Helium capture builds C into O, Ne, Mg, … 2 Advanced nuclear fusion reactions require extremely high temperatures Only high-mass stars can attain high enough core temperatures before degeneracy pressure stops shrinkage Advanced reactions make heavier elements Evidence for helium capture: Higher abundances of elements with even numbers of protons How does a high mass star die? Iron is dead end for fusion because nuclear reactions involving iron do not release energy (Fe has lowest mass per nuclear particle) 3 How a high mass star dies. Iron builds up in core until degeneracy pressure can no longer resist gravity Death seq of high mass star Core then suddenly collapses, creating supernova explosion Electron degeneracy pressure vanishes when electrons combine with protons, making neutrons and neutrinos. Neutrons collapse to the center, forming a neutron star. Neutron stars are held up by neutron degeneracy pressure. Energy and neutrons released in supernova explosion enables elements heavier than iron to form Elements made during supernova explosion Crab Nebula: Remnant of supernova observed in 1054 A.D. 4 before Keplerʼs supernova (1604) X-rays from the remnant of Keplerʼs supernova after Supernova 1987A is the nearest supernova observed in the last 400 years The next nearby supernova ? So how do we know this “really happens”? • We see supernova happen, and have identified the star that blew up. • We observe red giants & planetary nebula & white dwarfs & neutron stars, and compare measurements to predictions from stellar models. • We detect neutrinos from the Sun & supernovae. • Stars can’t live forever - finite fuel source. • Simple calculations about fusion fuel and fuel burning rates gives us star lifetimes. • Physics calculations predict the interiors of stars. • Studies of star clusters test the evolution models. How does a star’s mass determine its life story? 5 How are the lives of stars with close companions different? Life of a 20 M Sun star Life of a 1 MSun star STUDY FIG. 12.22 Stars in Algol are close enough that matter flows from subgiant onto main-sequence star The star that is now a subgiant was originally more massive than its companion. As it reached the end of its life and started to grow, it began to transfer mass to its companion Now the companion star is more massive How are the lives of stars with close companions different? Transfer of mass from one star to the other can alter the evolution of both. 6 ...
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