Introduction to Astronomy Lecture 8: The Death of Stars White Dwarfs, Neutron Stars, and Black Holes
Continued from Last Week Lecture 7 Observing Stars
Clusters of stars Some clouds start breaking into multiple protostars Can form open or globular clusters http://commons.wikimedia.org/wiki/file:a_swarm_of_ancient_stars_- _GPN-2000-000930.jpg#mediaviewer/File:A_Swarm_of_Ancient_Stars_-_GPN-2000-000930.jpg Open clusters can drift apart over time http://commons.wikimedia.org/wiki/file:pleiades_large.jpg#mediaviewer/file:pleiades_large.jpg
Main Sequence 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
Mass verses MS
Main Sequence 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
M-S lifetime examples 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
Post Main Sequence 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
Beyond H Luckily H-He fusion isn t the only type of fusion He -> heavier elements Process called nucleosynthesis
Heavier element fusion As each element, runs out Radiative pressure drops Core collapses Temp and density increases Next type of fusion becomes possible
Post MS path 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
Very Low Mass Stars Masses less than 0.5M Stars are entirely convective Core never reaches temperature to required for helium fusion Slow collapse into white dwarf
Evolution on the HR diagram We can track the changes in size and luminosity by a changing position on HR diagram
Low Mass Stars 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
Low Mass Stars 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
Higher Mass Stars 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
High mass stars
Extremely High Mass Stars 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
The end? Eventually the star can not continue fusion Low mass verses high mass Once fusion stops Star dies But what happens to material?
Low mass stars
Planetary nebula 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
Planetary nebula "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
Low mass core 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
White dwarf 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
Black dwarf 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
Other Remnants White dwarfs aren t the only type that form Type depends on mass
Remnants of stars (Exact boundaries are uncertain) white dwarf initial star < 8-10 M neutron star initial star < 15 M black hole initial star >15 M
Higher mass stars 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
High mass stars
Supernova 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
Supernova "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
Supernova Remnant After the supernova explosion Gas is thrown out into space Often violent and chaotic
Supernova Remnants "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
Remnant core 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
Neutron star 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
Neutron stars Predicted by theory Then observed as pulsars
Neutron star density! So dense that a teaspoon full weighs about 100 million tons
Black Hole 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
Black Hole 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
Black hole density So dense We don t really know how compact the material is or what it is like
NASA/CXC/SAO
Gravity s affect on light 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
Deflection of light