Chapter 19: The Evolution of Stars Why do stars evolve? (change from one state to another) Energy Generation fusion requires fuel, fuel is depleted [fig 19.2] at higher temperatures, other nuclear process are possible (carbon cycle [fig 19.1 vs fig 17.2]) some using other fuels gravitation collapse and contraction also provides energy Opacity affects energy transfer from stars interior to surface depends upon density, chemical composition a1c19:1 equation of state: describes how pressure is related to temperature and density normal gases (aka, ideal gas) degenerate gases high pressure is independent of temperature (liquid-like) results from Pauli Exclusion Principle (QM) Vogt-Russel Theorem The entire evolution of a star is determined by its mass and its chemical make up. (...but watch out for oddities in close binary systems!) models for stellar evolution depend upon knowledge of physical processes a1c19:2
Evolutionary Tracks and H-R Diagrams Analogy: height-mass diagrams for humans height (adults) sequence: birth adolescent adult senior aged mass position on diagram indicates physical state path = evolutionary track (duration of each stage will vary) a1c19:3 Stellar Evolutionary Tracks Sneak Preview: evolutionary track for a 1 M star [Fig 19.4] Star clusters stars with same age and chemical composition provides snap shot of stellar evolution [Fig 19.5, 6(young,~10 Myr) fig 16.20, 19.15] different clusters: different ages and composition tests of stellar models Evolutionary tracks for pre-main sequence stars [Fig 19.5] isochrone: line on H-R diagram indicating stars of the same age Main Sequence Stars: hydrogen burners (in core) Mass:.08 M to 60 M [Fig 19.7,8] below.08 M degenerate core prevents core collapse fusion can start above 60 M intense fusion radiation pressure significant outflow of material Size:.1 R to 15 R Temperature: 2400K to 50,000 K Luminosity:.001 L to 1,400,000 L a1c19:4
Main Sequence Stars [table 19.1] M/M 60 40 18 3.2 1.7 1.1 1 0.8 0.5 0.1 Spectral Type O3 O5 B0 A0 F0 G0 G2 K0 M0 M8 Temp. (K) 50,000 40,000 28,000 10,000 7,400 6,000 5,900 4,900 3,500 2,400 R/R 15 12 7 2.5 1.3 1.05 1 0.85 0.6 0.1 L/L 1,400,000 500,000 20,000 80 6 1.2 1 0.4.06.001 a1c19:5 Internal Structure of Main Sequence Stars [Figure 19-8] Nuclear Burning core Radiative Region Convective Region Main Sequence Lifetime lifetime x how fast fuel is consumed = amount of fuel t L M mass-luminosity relation more mass more power L M 3.5 lifetime [Fig 19.9] more massive stars have shorter main sequence lifetimes! a1c19:6
After the Main Sequence [Fig 19.10,11 19_10.mov] core burning ceases star contracts: gravitational energy converted to heat hydrogen shell burning begins star expands (increased radiation pressure) + surface cooler up and to right on H-R additional helium buildup in core Next Stage => Helium core ignites! (life as a Red Giant) Red Giant Stars [fig 19.13] Hydrogen shell burning Helium added to (temporarily degenerate) helium core, increasing temperature Helium ignites in triple alpha process 3 Helium => 1 Carbon helium flash nearly explosive burning of helium in core about 100x energy output for entire Milky Way in the core (for a few minutes) energy consumed in expansion of stars interior, not visible outside star radius ~ 1 AU for 1 M star [fate of the earth?] star moves horizontally across H-R diagram => stars ~ 1 to 1.5 M group in horizontal branch [fig 19.14,15, fig 16.20] a1c19:7 Pulsating Stars [Fig 19.16] δ Cephei varies in intensity by a factor of two over a period of two days unstable equilibrium oscillations energy production in core is steady instability strip where (on H-R diagram) stars will oscillate [Fig19.17,18,20] well defined period-luminosity relation [Fig19.19] variables provide calibration for yardstick beyond the limits of stellar parallax types of variables: Cepheid: cycle is on order of days RR Lyrae: dimmest, hottest variables, period from 1.5 hours to 1 day a1c19:8
Asymptotic Giant Branch AGB [fig 19.21] helium core burning ceases as helium is consumed helium shell burning begins (stars move to asymptotic giant branch [Fig 19.22,21] helium shell burns in surges thermal pulses solar mass star ~ 100 year pulses every 1000 years strong stellar winds outflow of material large size of stars low surface gravity dust and gas obscure light converts visible light to infrared Infrared Stars hundreds of times more luminous than sun star sheds most of outer layer, leaving hot inner core core ionizes ejected material planetary nebula ( looks like a planet) [fig 19.24,25,26,28 ] a1c19:9 Formation of heavy elements natural products of nuclear fusion (up to iron-56) heavier elements require energy (and neutrons) heavier elements much rarer reactions with neutrons in star core s-processes ~ slow neutron capture rates (up to Bizmuth) r-processes ~ rapid neutron capture rates, need large number of neutrons to form heaviest nuclei (only available during supernova) formation of heavy elements occurs in the cores of stars, and are distributed through space by strong stellar winds/explosions we are stardust 209 83 Bi a1c19:10