Lecture 16: Stellar Structure and Evolution – I

1. 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 TC = 15 million K, TS = 5800 K •  energy (heat) must flow from core  surface • but what physical processes are involved ? Additional reading: Kaufmann (chap. 21-22), Zeilik (chap. 16) PHYS1005 – 2003/4

2. Energy Transport: • possibilities are: • radiation • convection • 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 = R2 • temperature gradient = TC / R • conductivity = κ PHYS1005 – 2003/4

3. in very hot gas, electrons impede (scatter) photons • and since neαρ then • and hence • recall that TC ~ M / R • and since fusion is very TC-sensitive then TC ~ 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: • L generated in very small region • and/or material is very opaque (as at low T) PHYS1005 – 2003/4

4. Stellar Structure • from basic physics described so far  detailed computer models of stars • results  stars have 2 basic structures: High Mass (>2 MO) Low Mass (<1.5 MO) • TC > 18 x 106K  CNO cycle fusion • rate αT17  large L in small region • core is convective • outer layers hot  not very opaque •  envelope stable, radiative • TC < 18 x 106K  P-P chain fusion • rate αT4  small L in large region •  core is radiative • outer layers cool and opaque •  envelope is convective PHYS1005 – 2003/4

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

6. Evolution of 1MO star in H-R Diagram Stellar Evolution: • 34Core H-burning • H fuses in core • star on Main Sequence • as H fraction drops, T ↑ to compensate  more energy generated  L ↑ • 456Shell 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7He ignition • T in He core reaches 108 K • He ignites (the Helium Flash) • core expands, envelope contracts • star smaller, hotter, on Horizontal Branch PHYS1005 – 2003/4

7. 78Shell He-burning • He runs out in core • core contracts until He ignites in shell • envelope expands  Asymptotic Giant Branch star • 78Loss of envelope • fusion now unstable • huge mass loss in wind (red giant has R ~ 100 RO, so surface gravity g= G M / R2 is ~ 10,000 times weaker than Sun  easy to drive off matter) • core exposed  Planetary Nebula • 89End of the line • fusion dies away • White Dwarf (remnant hot core) emerges • cools (eventually) to a black dwarf (as all energy sources now exhausted) Evolutionary sequence is: • MS  RG  HB  AGB  PN  WD PHYS1005 – 2003/4

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