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 8X 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 (pre - main sequence) Stage 7 - main sequence Stages (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 10 8 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 12.8L G-Type Star Evolution

Figure 12.8R 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 kg/m 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 White Dwarf on an H-R Diagram

Table 12.2 Sirius B – A Nearby White Dwarf

Figure Sirius Binary System

Figure Distant White Dwarfs

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

Figure 12.13ab Nova Herculis a) March 1935 b) May 1935

Figure 12.13c 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 Close Binary System

Figure 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 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 Heavy-Element Fusion

Figure Mass Loss from Supergiants

Examples in Orion Rigel - blue supergiant 70 R , 50,000X luminosity of sun Originally 17 M  Betelgeuse - red supergiant 10,000X 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 kg/m 3 Neutrinos escape, taking away energy Further collapse to kg/m 3 Neutrons packed together slow further collapse Overshoots to kg/m 3, then rebounds Shock wave ejects overlying material into space Core collapse supernova

Figure Supernova 1987A

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 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 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 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 Cluster Evolution on the H-R Diagram

Figure Newborn Cluster H-R Diagram

Figure Young Cluster H-R Diagram

Figure Old Cluster H-R Diagram

Figure Stellar Recycling