1. Lecture 15PHYS1005 – 2003/4 Lecture 16: Stellar Structure and Evolution – I Objectives: Understand energy transport in stars Examine their internal structure Follow their evolutionary paths in H-R diagram Energy Transport in Stars: Sun’s T C = 15 million K, T S = 5800 K  energy (heat) must flow from core  surface but what physical processes are involved ? Additional reading: Kaufmann (chap. 21-22), Zeilik (chap. 16)

2. Lecture 15PHYS1005 – 2003/4 Energy Transport: possibilities are: 1)radiation 2)convection 3)conduction but only radiation and convection are important in normal stars although “radiation” is really more like “conduction” 1) Radiative Diffusion: Photons follow a random walk from centre to surface of star –absorbed and re-emitted many times (called “radiative diffusion”) before escaping e.g. in Sun’s core, mean distance travelled by photon = 0.1 mm! Expect luminosity L to be proportional to: –area = R 2 –temperature gradient = T C / R –conductivity = κ

3. Lecture 15PHYS1005 – 2003/4 in very hot gas, electrons impede (scatter) photons and since n e α ρ then and hence recall that T C ~ M / R –and since fusion is very T C -sensitive then T C ~ constant  R α M and hence –which is the M-L relation for massive (hot) stars! 2) Convection: Convecting star has blobs rising, giving up heat, then descending again Large T gradients  convection –which occurs when: a)L generated in very small region b)and/or material is very opaque (as at low T)

4. Lecture 15PHYS1005 – 2003/4 Stellar Structure from basic physics described so far  detailed computer models of stars results  stars have 2 basic structures: High Mass (>2 M O ) Low Mass (<1.5 M O ) T C > 18 x 10 6 K  CNO cycle fusion rate α T 17  large L in small region  core is convective outer layers hot  not very opaque  envelope stable, radiative T C < 18 x 10 6 K  P-P chain fusion rate α T 4  small L in large region  core is radiative outer layers cool and opaque  envelope is convective

5. Lecture 15PHYS1005 – 2003/4 Solar convection: e.g. outer 1/3 of Sun convects  seen as surface granulation (taken by the Swedish Solar Tower on La Palma) granulation

6. Lecture 15PHYS1005 – 2003/4 Stellar Evolution: 3  4 Core H-burning –H fuses in core –star on Main Sequence –as H fraction drops, T ↑ to compensate  more energy generated  L ↑ 4  5  6 Shell H-burning –at 4, H runs out in core –without fusion, core contracts and heats up until H re-ignites in shell around core –higher ρ, g  H burns faster  increase in L  envelope expands as core contracts! –becomes Red Giant 6  7 He ignition –T in He core reaches 10 8 K –He ignites (the Helium Flash) –core expands, envelope contracts –star smaller, hotter, on Horizontal Branch Evolution of 1M O star in H-R Diagram

7. Lecture 15PHYS1005 – 2003/4 8  9 End of the line –fusion dies away –White Dwarf (remnant hot core) emerges –cools (eventually) to a black dwarf (as all energy sources now exhausted) 7  8 Loss of envelope –fusion now unstable –huge mass loss in wind (red giant has R ~ 100 R O, so surface gravity g = G M / R 2 is ~ 10,000 times weaker than Sun  easy to drive off matter) –core exposed  Planetary Nebula Evolutionary sequence Evolutionary sequence is: –MS  RG  HB  AGB  PN  WD 7  8 Shell He-burning –He runs out in core –core contracts until He ignites in shell –envelope expands  Asymptotic Giant Branch star

8. Lecture 15PHYS1005 – 2003/4 HST images of planetary nebulae: