ASTR 1120General Astronomy:Stars & Galaxies• Homework #4 on Mastering Astronomy, due on Thursday of next week, 10/08, by 5pm• Next Extra Credit Observing Night:–Thursday, 10/08 at Sommers-BauschLast class: When a Low-Mass Starruns out of Hydrogen in its Core1. With fusion no longeroccurring in the core,gravity causes corecollapse2. Hydrogen shell startsfusion, lifting outer layers.Red Giant increasesluminosity.3. Under degenerateconditions, core becomeshot enough to fuseHelium: Helium Flash4. Outer layers fall and heat,luminosity decreases.Helium burns stably:Horizontal Branch StarA star moves upwards and to theright on the HR diagram. What isprobably happening in the core?A. The core has just started to burn a new elementB. All nuclear burning is slowing downC. The inner core temperature is coolingD. The inner core is collapsing and heating up;shell burning is increasingClicker QuestionA star moves upwards and to theright on the HR diagram. What isprobably happening in the core?A. The core has just started to burn a new elementB. All nuclear burning is slowing downC. The inner core temperature is coolingD. The inner core is collapsing and heating up;shell burning is increasingClicker QuestionHR Diagram for an old globular clusterHelium Fusion• Temperatures ~ 100million K• He + He + He ! C +energy– Triple-alpha processWhat Will Happen When ThereIs No More Helium in the Core?A. The core will cool down.B. Carbon fusion will start immediately.C. The star will explode.D. The core will start to collapse.E. The hydrogen fusing shells will go out.Clicker QuestionWhat Will Happen When ThereIs No More Helium in the Core?A. The core will cool down.B. Carbon fusion will start immediately.C. The star will explode.D. The core will start to collapse.E. The hydrogen fusing shells will go out.Clicker QuestionWhen helium runs out….• Carbon core collapses and heats up• Burning in helium AND hydrogen shells– Shell burning now unstable = Thermal Pulses• Energy generationbecomes much higheragain• Outer layers lift andcool again• Star becomes veryluminous red giant– Class II• Degeneracy pressurereached before T=600million K– No carbon fusion• Stellar winds blowmaterial from the outside– including some carbon• Outer layers thrown off ina big puff around theinert carbon core– Big puff = Planetary Nebula– Inert carbon core = WhiteDwarf• Slowly cools and fadesuntil it becomes a nearlyinvisible “black dwarf”Spirograph nebulaIC 418: The Spirograph nebulaNGC 6826NGC 6826: The Blinking Eye NebulaEskimo NebulaNGC 2392: The Eskimo NebulaHelix NebulaNGC 7293: The Helix NebulaMore Planetary Nebulae!HAS IS A WHITE DWARF? Exposed core of a low-mass star that has diedNo fusion to maintain heat and pressure to balance gravity pullElectron degeneracy pressure balancesinward crush of its own gravityMostly made of Carbon and Oxygen Very high density and hence gravityMaximum mass=1.4 Msun (Chandrasekar limit)Size Of A White DwarfSize ofEarthFunky properties of white dwarf material1 Kg chocolate cake2 Kg chocolate cake0.4 Msun white dwarf0.8 Msun white dwarf• Hubble SpaceTelescope spies12-13 billion yearold white dwarfs– Formed less than1 billion yearsafter the creationof the universe© HST and H. Richer(University of BritishColumbia)Which is correct order for somestages of life in a low-mass star?A. protostar, main-sequence star, red giant,planetary nebula, white dwarfB. protostar, main-sequence star, red giant,supernova, neutron starC. main-sequence star, white dwarf, red giant,planetary nebula, protostarD. protostar, main-sequence star, planetarynebula, red giantE. protostar, red giant, main-sequence star,planetary nebula, white dwarfClicker QuestionWhich is correct order for somestages of life in a low-mass star?A. protostar, main-sequence star, red giant,planetary nebula, white dwarfB. protostar, main-sequence star, red giant,supernova, neutron starC. main-sequence star, white dwarf, red giant,planetary nebula, protostarD. protostar, main-sequence star, planetarynebula, red giantE. protostar, red giant, main-sequence star,planetary nebula, white dwarfClicker QuestionTime scales for Evolutionof Sun-like StarH core burning Main Sequence 1010 yr10 billion yearsInactive He core, H shell burning Red Giant 108 yr100 million yearsHe core burning (unstable), ” Helium Flash HoursHe core burning (stable), ” Horizontal Branch 107 yr10 million yearsC core, He + H shells burning Bright Red Giant 104 yr10 thousand yearsEnvelope ejected Planetary Nebula 105 yr100 thousand yearsCooling C core White Dwarf -Cold C core Black Dwarf !Hubble image, visible lightChandra image, X-ray lightSirius A & BMain Sequence & White DwarfThe Big Bang produced only hydrogen andhelium. Suppose the universe containedonly low mass stars. Would elementsheavier than carbon exist?A. YesB. NoClicker QuestionThe Big Bang produced only hydrogen andhelium. Suppose the universe containedonly low mass stars. Would elementsheavier than carbon exist?A. YesB. NoClicker QuestionLives of Intermediate/High-MassStars• Low mass: < 2 times the Sun• Intermediate mass: 2-8 times the Sun• High mass: > 8 times the SunGeneral Principles Are the Same:Battle Between Pressure and Gravity• Main sequencelifetimes are muchshorter (H burns through CNO cycle - don’t worry about the details)• Early stages aftermain sequence– Similar to a low massstar, but happen muchfaster• No helium flashIntermediate-Mass Stars• May burn up to carbon but do not have enough massto get temperatures high enough to go any higher upthe periodic table• Degeneracy pressure stops the core from collapsingand heating enough: particles are squashed togetheras much as possible• End their lives with planetary nebulae, white dwarves• Sequence of expansion/contractionrepeats as higher and higherelements begin to fuse• Each heavier element requireshigher core temperatures to fuseHigh-Mass Stars• Core structurekeeps on buildingsuccessive shell- Like an onion• Lighter elementson the outside,heavier ones on the inside• Most elements are formed via Helium Capture– A helium (2 protons) nucleus is absorbed, energy isreleased• The elements are created going up the periodictable in steps of 2Other ReactionsCarbon (6), Oxygen (8), Neon (10)Magnesium (12)….!E ARE STAR STUFF!!No significant changes in luminosityStar travels back and forth on the HR diagramIn the most massive stars,changes happen so quickly that the