Lecture 8: The Death of Stars White Dwarfs, Neutron Stars ...

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

Holes

Elizabeth Charlton, 2016

!  “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



!  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


!  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


!  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


!  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 Elizabeth Charlton, 2016

!  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 Elizabeth Charlton, 2016

!  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



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Lecture 8: The Death of Stars White Dwarfs, Neutron Stars ...

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