10/17/2012. Stellar Evolution. Lecture 14. NGC 7635: The Bubble Nebula (APOD) Prelim Results. Mean = 75.7 Stdev = 14.7
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1 /17/2012 Stellar Evolution Lecture 14 NGC 7635: The Bubble Nebula (APOD) Prelim Results Mean = 75.7 Stdev =
2 Energy Transport in Stars How does the energy produced get out? Energy can be transported by Conduction Convection Radiation Stars mainly use the latter two methods Trade between convection and radiation depends on the star and region within a star White dwarfs and neutron stars are exceptions A Model of the Sun CORE Convective Zone Radiative Zone Temp. Density Energy (10 6 K) g/cm 3 Transport T surface ~ 6000 K Core ~ Convective Rad. ~ 3 1 Radiative Conv. ~ Convective 2
3 3.5 solar masses The Interiors of Stars Convection Zone 1 solar mass Radiative Zone Radiative Zone 0.5 solar masses Energy Transport Summary Massive stars (> 2 M sun ) have small convective cores and large radiative envelopes. Low mass stars (< 1 M sun ) have small radiative cores and large convective envelopes. 3
4 The Balance of Stellar Life Hydrostatic Equilibrium is the balance of gravity and pressure in each layer of a star. This keeps a star from collapsing (or expanding). This balance is maintained as stars age, so that the size might shrink or grow to maintain it. The Life Cycle of Stars Birth: Life: Death: Grav. Collapse of Interstellar Clouds Hayashi Contraction of Protostar Stability on Main-Sequence Long life - energy from nuclear reactions in the core (E = mc 2 ) Lack of fuel, instability, variability, expansion (giants, supergiants), explosions!! 4
5 Luminosity (L sun ) 10/17/2012 Main-Sequence Evolution Fusion is occurring in the cores of stars. H is being converted to He. Since 4 particles are converted to 1, the pressure drops. The core collapses and heats up. This heats the outer layers which expand outward. 5x10 6 yrs Stars evolve, even on the Main-Sequence 30 M 6x10 8 yrs 3 M yrs Zero Age Main-Sequence Present Sun Initial Sun Ref: Seeds O B A F G K M Temperature 5
6 Sun: On the Main-Sequence 5 billion years ago: Beginning of its life on main-sequence Sun had 1/3 luminosity it has now. 5 billion years from now: End of its life on main-sequence Sun will have twice the luminosity it has now. Stellar Evolution When H is exhausted, the core shrinks. It heats up but can not yet burn He, which needs 100,000,000 K! The high temperatures ignite a shell of H around the core. The increased pressure drives the envelope of the star outward. Creating a giant or supergiant. 6
7 Giant and Supergiant Stars Luminosity steadily climbs as shell fusion of H accelerates Expanded star: very large radius => large luminosity ( L = 4pR 2 st 4 ) Uneasy stellar evolutionary stage Variability Mass loss since gravity on the surface is weak Very high, and increasing temperature in the core L 10 3 L! 7
8 Stellar Evolutionary Tracks Late stages of Stars The Helium Flash: - When T core ~ 10 8 K, He begins to fuse: He 4 + He 4 (Be 8 )* Be 8 is highly unstable another He 4 must come along within 10-8 seconds! He 4 + (Be 8 )* C 12 Triple process: 3 He 4 C 12 First realized by Salpeter (Cornell) 8
9 Late stages of Stars Onset of He 4 fusion is explosive in solar mass stars Core was largely supported by electron degeneracy pressure not dependant on T As T 10 8 K, reactions take off T, but P stays the same! Reactions run away since T 40 bomb! L ~ L but we don t see this! L lifts the core and enables stable He 4 fusion at core: Luminosity stabilizes at L ~ 30 to 100 L H fusion in shell continues, but at a much slower rate L goes down Late stages of Stars Eventually He in core is exhausted Core then must begin contracting again, raising its temperature Ignites He shell burning around core We now have twin layers of He and H shell burning at ever increasing rates Eventually, for solar mass stars, core stabilizes under electron degeneracy pressure Envelope is ejected as a planetary nebula Core remains as a white dwarf 9
10 Evolution of a star in the H- R diagram 10
11 11
12 Luminosity (L sun ) Luminosity (L sun ) 10/17/2012 Evolution of the Sun Instability Red Giant Explosion Protostar White Dwarf Hayashi Contraction Cloud Collapse O B A F G K M Temperature Evolution of a 20 Solar Mass Star Supernova SuperGiant Protostar Hayashi Contraction Cloud O B A F G K M Temperature 12
13 13
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