Chapter 12 Stellar Evolution Infrared Image of Helix

Chapter 12 Stellar Evolution

Infrared Image of Helix Nebula

Mass and Stellar Fate • Low mass stars end life quietly • Massive stars end life violently • Massive - more than 8 X M

Core-hydrogen burning • Main sequence stars fuse H into He • On main sequence for over 90% of life • Hydrostatic equilibrium - pressure and gravity balance

Figure 12. 1 Hydrostatic Equilibrium

Evolution of a sun-like star • Stages 1 - 6 (pre - main sequence) • Stage 7 - main sequence • Stages 8 - 14 (post main sequence)

Stages 8 and 9 • • • Stage 8 - Subgiant branch Stage 9 - Red Giant branch H depleted at center, He core grows Core pressure decreases, gravity doesn’t He core contracts, H shell burning increases Star’s radius increases, surface cools, luminosity increases

Figure 12. 2 Solar Composition Change

Figure 12. 3 Hydrogen Shell Burning

Figure 12. 4 Red Giant on the H-R Diagram

Stage 10 - Helium Fusion • Red Giant core contracts (no nuclear burning there) • Central temperature reaches 108 K • Fusion of He starts abruptly - Helium flash for a few hours • Star re-adjusts over 100, 000 years from stage 9 to 10 • H and He burning with C core - horizontal branch

Figure 12. 5 Horizontal Branch

Figure 12. 6 Helium Shell Burning

Stage 11 - Back to Giant Branch • C core contracts (no nuclear burning there) • Gravitational heating • H and He burning increases • Radius and luminosity increases

Figure 12. 7 Reascending the Giant Branch

Table 12. 1 Evolution of a Sun-like Star

Figure 12. 8 L G-Type Star Evolution

Figure 12. 8 R G-Type Star Evolution

Death of a low mass star • For solar mass star, core temperature not high enough for C fusion • Outer layers drift away into space • Core contracts, heats up • UV radiation ionizes surrounding gas • Stage 12 - A planetary nebula • (nothing to do with planets)

Figure 12. 9 Planetary Nebulae

Other elements • As red giant dies, other elements created in core • O, Ne, Mg • Enrich interstellar medium as surface layers ejected

Dense matter • • Carbon core shrinks and stabilizes Core density 1010 kg/m 3 1000 kg in one cm 3 Pauli Exclusion Principle keeps free electrons from getting any closer together • This is a different sort of pressure

Stage 13 - White Dwarf • • • Red giant envelope recedes C core becomes visible as a white dwarf Approximately size of earth, 1/2 mass of sun White-hot surface, but dim (small size) Glow by stored heat, no nuclear reactions Fades in time to a black dwarf - stage 14

Figure 12. 10 White Dwarf on an H-R Diagram

Table 12. 2 Sirius B – A Nearby White Dwarf

Figure 12. 11 Sirius Binary System

Figure 12. 12 Distant White Dwarfs

Novae • Plural of nova • Some white dwarfs become explosively active • Rapid increase in luminosity

Figure 12. 13 ab Nova Herculis a) March 1935 b) May 1935

Figure 12. 13 c Nova

Nova explanation • White dwarf in a binary • Gravitation tears material from companion, forming accretion disk around white dwarf • Material heats until H fuses • Surface burning brief and violent • Novae can be recurrent

Figure 12. 14 Close Binary System

Figure 12. 15 Nova Matter Ejection

Evolution of High-Mass Stars • All main sequence stars move toward red-giant phase • More massive stars can fuse C and other heavier elements • Evolutionary tracks are more horizontal • 4 M star can fuse C • 15 M star can fuse C, O, Ne, Mg and become a red supergiant

Figure 12. 16 High-Mass Evolutionary Tracks

Evolution of 4 M star • • No He flash Hot enough to fuse C Can’t fuse beyond C Ends as a white dwarf

Evolution of 15 M star • • Rapid evolution Becomes red supergiant Fuses H, He, C, O, Ne, Mg, Si Inner core of iron

Figure 12. 17 Heavy-Element Fusion

Figure 12. 18 Mass Loss from Supergiants

Examples in Orion • Rigel - blue supergiant • 70 R , 50, 000 X luminosity of sun • Originally 17 M • Betelgeuse - red supergiant • 10, 000 X luminosity of sun in visible light • Originally 12 to 17 M

High mass fast evolution • • Consider 20 M star Fuses H for 10 million y Fuses He for 1 million y Fuses C for 1000 y Fuses O for one year Fuses Si for one week Fe core grows for less than a day

Death of high mass star - 1 • • • Fe fusion doesn’t produce energy Pressure decreases at core Gravitational collapse Core temperature reaches nearly 10 billion K High energy photons break nuclei into protons and neutrons - photodisintegration • Reduced pressure, accelerated collapse

Death of high mass star - 2 • • • Electrons + protons neutrons and neutrinos Density 1012 kg/m 3 Neutrinos escape, taking away energy Further collapse to 1015 kg/m 3 Neutrons packed together slow further collapse • Overshoots to 1018 kg/m 3, then rebounds • Shock wave ejects overlying material into space • Core collapse supernova

Figure 12. 19 Supernova 1987 A

Table 12. 3 End Points of Evolution for Stars of Different Masses

Novae and Supernovae • Nova - explosion on white dwarf surface in a binary system • Supernova - exploding high mass star • Million times brighter than nova • Billions of times brighter than sun • Supernova in several months radiates as much as our sun in 10 billion years

Types of Supernovae • Type I - very little H • Sharp rise in brightness, gradual fall • Type II - H rich • Plateau in light curve • Roughly half Type I and half Type II

Figure 12. 20 Supernova Light Curves

Type II Supernovae • Core collapse as previously described • Expanding layers of H and He

Type I Supernovae • Accretion disk around white dwarf can nova • Some material adds to white dwarf • Below 1. 4 M (Chandrasekhar mass), electrons support white dwarf • Above 1. 4 M , white dwarf collapses • Rapid heating, C suddenly fuses throughout • Carbon-detonation supernova • Also possible for two white dwarfs to merge

Figure 12. 21 Two Types of Supernova

Supernovae summary • Type I - carbon detonation of white dwarf exceeding 1. 4 M • Type II - core collapse of massive star, rebound and ejection of material • All high mass stars Type II supernova • Only some low mass stars Type I supernova • Low mass stars much more common than high mass • Type I and II about equally likely

Figure 12. 22 Supernova Remnants

Heavy Element Formation • All H and most He is primordial • Other elements produced through stellar evolution

Stellar evolution in star clusters • All stars the same age • Snapshot at one time

Figure 12. 23 Cluster Evolution on the H-R Diagram

Figure 12. 24 Newborn Cluster H-R Diagram

Figure 12. 25 Young Cluster H-R Diagram

Figure 12. 26 Old Cluster H-R Diagram

Figure 12. 27 Stellar Recycling