Lecture 8: The Death of Stars White Dwarfs, Neutron Stars, and Black Holes

Lecture 8: The Death of Stars White Dwarfs, Neutron Stars, and Black Holes

! the time a star is fusing hydrogen into helium in its core! stars spend most of their time in this stage! main-sequence stars in equilibrium " Gravity vs pressures in balance Elizabeth Charlton, 2015

Elizabeth Charlton, 2015

! amount of time depends on mass! more massive stars! Higher temperature and pressure in core! Fusion reaction proceeds very rapidly! Short life span! low mass stars! Fusion proceeds very slowly! Long life span Elizabeth Charlton, 2015

! Star like our Sun (1 M ) " ~10 billion years! Higher mass star (2 M ) " ~ 1 billion years! Very high mass star (30 M ) " ~3 million years Elizabeth Charlton, 2015

! main-sequence lifetime - " fusing hydrogen into helium in its core! Eventually runs out of hydrogen in core "Star types" by Estrellatipos.png: The original uploader was Xenoforme at Spanish Wikipediaderivative work: Begoon - This file was derived from:estrellatipos.png. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/ File:Star_types.svg#mediaviewer/File:Star_types.svg Elizabeth Charlton, 2015

! Luckily H-He fusion isn t the only type of fusion! He -> heavier elements! Process called nucleosynthesis

! As each element, runs out " Radiative pressure drops " Core collapses " Temp and density increases " Next type of fusion becomes possible

! What type of fusion occurs and how long depends on mass! During these transitions, star not in equilibrium " Expansion or contraction " Surface cools or heats

NASA/CXC/SAO

! Masses less than 0.5M " Stars are entirely convective " Core never reaches temperature to required for helium fusion " Slow collapse into white dwarf

! We can track the changes in size and luminosity by a changing position on HR diagram

! masses between 0.5-10 M " Red Giant Phase! Star unstable! H shell burning around contracting core! Outer layers expand, surface cools! Core continues to contract and heat until HELIUM IGNITION at 100,000,000 K

! Once helium fusion is exhausted in core (C, O core) " Second red giant phase (asymptotic giant phase)! Core contracts and heats! He and H burning shells ignite around core! Star expands and surface cools! Temperature for next fusion stage is never reached! Star ejects outer layers

! masses more than 10M " Form red supergiants " Massive enough to continue heavier element fusion " Fusion continues until Fe-56! Now fusion consumes energy! Core Collapse "Evolved star fusion shells" by User:Rursus - R. J. Hall. Licensed under Creative Commons Attribution 2.5 via Wikimedia Commons - http://commons.wikimedia.org/wiki/ File:Evolved_star_fusion_shells.svg#mediaviewer/File:Evolved_star_fusion_shells.svg

! Masses above 40M " Mass loss due to stellar winds " Cannot expand to red supergiant " Will remain extremely hot and luminous on HR diagram " Heavier metal fusion continues to Fe-56! Core Collapse

NASA/CXC/SAO

! Eventually the star can not continue fusion " Low mass verses high mass! Once fusion stops " Star dies " But what happens to material?

! The ejected layer of gas from the outside of the low mass star! Glows due to heat and light of the remaining core! Shape forms as the gas expands away from star

"NGC6543". Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/ wiki/file:ngc6543.jpg#mediaviewer/file:ngc6543.jpg "Ngc2392" by NASA, ESA, Andrew Fruchter (STScI), and the ERO team (STScI + ST-ECF) - http:// www.spacetelescope.org/images/html/heic9910a.html. Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/file:ngc2392.jpg#mediaviewer/file:ngc2392.jpg

! The core of the star remains! No fusion though, so it isn t a star " No radiation pressure " Core collapses " Very hot and dense! Called a remnant! White dwarf - a dense remnant of a star which shines due to residual heat

! remaining core is very dense! supported against gravity via electron degeneracy pressure! mass around 1 M! radius around 1 earth radius (1/100 R )! density > 1 million times sun's density

! a collapsed star that has cooled to the point where it emits little or no visible radiation! all white dwarfs will become black dwarfs after cooling down for enough time

! White dwarfs aren t the only type that form! Type depends on mass

! (Exact boundaries are uncertain)! white dwarf initial star < 8-10 M! neutron star initial star < 15 M! black hole initial star >15 M

! after giant stages, star changing very rapidly " fusion ceases " eventually equilibrium completely breaks down! first get fast core collapse! then get a supernova explosion as a reaction " bounce

! a very bright explosion marking the end of some star's evolution! outer layers thrown off into space! sends out heavier elements that were generated in the star and in the explosion

"SN1994D" by NASA/ESA, The Hubble Key Project Team and The High-Z Supernova Search Team - http://www.spacetelescope.org/images/html/opo9919i.html. Licensed under Creative Commons Attribution 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/ File:SN1994D.jpg#mediaviewer/File:SN1994D.jpg

! After the supernova explosion! Gas is thrown out into space! Often violent and chaotic

"Crab Nebula" by NASA, ESA, J. Hester and A. Loll (Arizona State University) - HubbleSite: gallery, release.. Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/ File:Crab_Nebula.jpg#mediaviewer/File:Crab_Nebula.jpg

! Production of heavy elements in supernova explosions! First advanced by Fred Hoyle in 1954! Iron 56 is the last element that causes a net release of energy by nuclear fusion exothermically! Core collapse supernova produce heavy elements through endothermic fusion processes.! Type II Supernova explosion releases neutrons " synthesizes heavy elements via neutron-capture method called the r-process " lasts about 1s inside star as shockwave passes

! The cores of the some higher mass stars survive the supernova explosion " Depends on mass and metallicity of star " Pair-instability supernovas can entirely destroy the core! Becomes either a neutron star or black hole

! a very dense, compact star composed primarily of neutrons! after supernova core mass is still high! core is compressed even more than white dwarf! atoms break down and leave only dense neutrons! supported against gravity via quantum degeneracy pressure

! Predicted by theory! Then observed as pulsars

" So dense that a teaspoon full weighs about 100 million tons

! the most massive stars will form black holes! an object whose gravitational attraction is so strong that its escape velocity equals the speed of light! even light can not escape once it falls in

! Classical vs. Modern Description " Black holes have no hair! classical description! can only observe mass, angular momentum, and charge " Hawking radiation! quantum mechanical description! radiate as blackbodies

! So dense! We don t really know how compact the material is or what it is like

NASA/CXC/SAO

By cmglee, NASA Goddard Space Flight Center - File:star_life_cycles_red_dwarf.jpg, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=39174476

! Gravity can pull on light! Will only be noticed if gravity is very strong! Like a black hole! Can bend light from stars in the background