LIFECYCLES OF STARS

1. LIFECYCLES OF STARS Option 2601

2. Stellar Physics • Observational properties of stars • Stellar Spectra • The Sun • Stellar Structure • Stellar Evolution • Stars of particular interest

3. Unit 3 The Sun

4. The Sun • Basic physical parameters • Structure of interior and atmosphere • Surface features • Magnetic field • Solar activity, flares and pulsations • Relationship to other stars

5. Our nearest star • Nearest, therefore studied in most detail • Standard against which other stars are compared • Radius = 6.96x105 km (~109R) • Mass = 1.99x1030 kg (~333,000M) • Luminosity = 3.86x1026 W • Spectral type/luminosity class = G2 V

6. Structure • Core – region of nuclear burning • Radiative zone • Convection zone • Photosphere • Chromosphere • Corona Only regions directly observable

9. Solar interior Helioseismology • Solar oscillations used to study the structure similar to seismology on Earth • Periods range from 5 min to 2h 40min • Detected by periodic changes in Doppler shifts of spectral lines

11. Photosphere Granulation • Base of photosphere is deepest region observable • Patchwork of granules • d~700 km • Transient (5-10mins) • Bright irregular formations surrounded by darker lanes • Top layer of convection zone

12. Photosphere • Bright regions are rising hot gas (convection cells) • Dark regions are falling cooler gas

13. Sunspots • Cooler regions (appear darker) than surrounding photosphere • Temperatures ~3800K cf. 5800K elsewhere • Associated with high magnetic fields • More later…

15. Limb darkening • Brightness of solar disk decreases from centre to limb (edge of disk) • Arises because we see deeper hotter gas at centre, cooler layers at limb

17. Limb darkening

18. Optical depth: Absorption in photosphere In a slab of thickness dx, density , fraction of flux Fl absorbed is: Units of opacity κ are m2/kg (or cm2/g) Main source of opacity in solar photosphere is H¯

19. Absorption lines • Discussed in detail in last Unit • First mapped by Fraunhoffer (1787-1826)

20. Absorption lines • Alphabetic designation… capital letters for strong, lower case for weak lines • Hence… Na D lines, CaII H & K, Mg b

21. Chromosphere • Spectrum contains emission features from highly excited/ionized species (e.g. H Balmer, HeII) • High temperatures (see last lecture)

22. Chromosphere

23. Chromosphere • Photospheric continuum absorbed by Chromospheric gas • Absorption lines projected against solar disk • Emission lines seen against dark space

24. Chromospheric fine structure • Some absorption lines have large optical depth (e.g. H, CaII H & K) • Monochromatic photos show large bright and dark patches… plages and filaments

25. Chromospheric fine structure • Structure appears over whole disk • Bright network associated with magnetic fields at boundaries of supergranules • Brightening (i.e. less absorption) of CaII K  increasing magnetic field strength

26. Chromospheric fine structure • See spicules at the limb, jets of glowing gas emerging at 20-25km/s • 500-1500km across, 10000km high • Form a network following supergranule boundaries • Probably play a significant role in mass transport… chromosphere  corona  wind

27. Transition Region

28. Transition Region • Gives rise to UV spectral features • e.g. Lyman , CIII, NIII, OVI • Network continues through this region • Disappears at ~1.6x106K in MgX images

29. Solar Corona

30. Solar Corona • In visible light… • K Corona – dominates near the Sun • Light scattered by (1-2)x106K electrons • Strongly affected by solar activity • F Corona – visible at a few solar radii • Light scattered from dust

31. Solar Corona

32. Solar Corona • Radio Corona: arises from free-free transitions of free electrons & atoms/ions • Line emission: “forbidden” lines due to high temperature & low density • EUV lines: e.g. FeVIII-XVI… high ionization states

33. Solar Corona

34. Coronal Loops & Holes • Coronal gas hot enough to emit low energy X-rays • X-ray images show irregular gas distribution • Large loop structures  hot gas trapped in magnetic loops

35. Coronal Loops & Holes • Dark regions (gas less hot and dense)  coronal holes • Holes correspond to magnetic field lines that do not reconnect with the surface

36. Solar Corona

37. Solar Wind • Solar gravity is insufficient to retain high temperature coronal gas • Gas is a plasma (ionized but electrically neutral on a large scale) • Thermal conductivity high  high T prevails out to large distances

38. Solar Wind • Wind accelerates as it expands • 300km/s at 30R  400km/s at 1 AU • Proton/electron energy ~103eV • Density at Earth ~(0.4-8.0)x106m-3

39. Solar Wind

40. Solar Activity • We can easily observe transient phenomena • These are manifestations of Solar Activity • Linked through solar rotation and magnetic field

41. Sunspots • Field strengths (deduced from Zeeman effect) ~0.1T (up to 0.4T) • Fields may inhibit convective energy transport • Any given spot has an associated magnetic polarity • May be paired with spot of opposite polarity (or diffuse region no observed as a spot)

42. Sunspots

43. Sunspots • Can measure solar rotation rate by following spots • Pequator~25d • P40o~27d • P70o~30d

44. Sunspots • Spot numbers vary with an 11 year cycle (except Maunder Minimum) • Spot latitudes vary during cycle • Spot lifetimes days to months

45. Sunspots

46. Sunspots • Bipolar spot pair – preceding spot always has same polarity through cycle • Polarities are opposite in each hemisphere • Reverse at end of 11yr cycle – overall 22 year cycle • Spots always follow constant latitude

48. Sunspot Cycle