Stellar Physics

1. Stellar Physics Dr Martin Hendry Dept of Physics and Astronomy University of Glasgow martin@astro.gla.ac.uk 10 lectures, exploring the development of cosmology, and some of the key ideas of Big Bang theory Access PPT slides at http://www.astro.gla.ac.uk/users/martin/teaching/aberdeen.ppt

2. Surface temperature (K) 25000 10000 8000 6000 5000 4000 3000 106 -10 100 RS 1000 RS We can plot the temperature and luminosity of stars on a diagram Stars don’t appear everywhere: they group together, and most are found on the Main Sequence 10 RS Supergiants 104 -5 1 RS 102 Giants 0 0.1 RS Luminosity (Sun=1) Absolute Magnitude Main Sequence 1 +5 0.01 RS 10-2 +10 0.001 RS White dwarfs 10-4 +15 O5 B0 A0 F0 G0 K0 M0 M8 Spectral Type

3. Surface temperature (K) 25000 10000 8000 6000 5000 4000 3000 . . . 106 . -10 . Deneb . . . . Stars on the Main Sequenceturn hydrogen into helium. Blue stars are much hotter than the Sun, and use up their hydrogen in a few million years Rigel . . . . Betelgeuse . . . . . Antares 104 . -5 . . . . . . . . . . . . . Arcturus . . . Aldebaran . . . . . . . . . . . . Regulus . . . . . . . 102 Vega . . . . . 0 . . . Mira Sirius A . . . . . . Pollux . Procyon A . . . . Luminosity (Sun=1) . . Altair Sun Absolute Magnitude 1 . . . +5 . . . . . . . . . . . . . . . 10-2 . +10 . . . . . . . . . . . . . . . . Barnard’s Star . . . . Sirius B . . . . 10-4 . +15 . . Procyon B . . . . O5 B0 A0 F0 G0 K0 M0 M8 Spectral Type

4. Observational Evidence for Compact Objects • White Dwarfs • Neutron Stars • Black Holes

5. White Dwarfs Small but very luminous (because of high T) Can be directly observed

10. Important Type of White Dwarf for Cosmology: Type Ia Supernovae Excellent for measuring cosmological distances – good “Standard Candle”

11. Type Ia Supernova White dwarf star with a massive binary companion. Accretion pushes white dwarf over the Chandrasekhar limit, causing thermonuclear disruption Good standard candle because:- Narrow range of luminosities at peak brightness; Observable to very large distances

13. Will the Universe continue to expand forever? To find out we need to compare the expansion rate now with the expansion rate in the distant past… Is the Universe speeding up or slowing down?

14. Answer depends on the geometry of the Universe Closed Open Flat

15. We can measure this using Type Ia Supernovae

18. Results: The geometry of the Universe is FLAT The Universe will continue to expand indefinitely The expansion is accelerating

19. What is driving the cosmic acceleration?… Dark Energy Cosmological Constant? Quintessence?

20. Neutron Stars Very much smaller: (almost) invisible at optical, but can be seen in X-Rays if their surfaces are very hot

22. Crab Nebula: supernova of 1054

26. There exist large numbers of compact objects in binary systems. These are powerful emitters of X-rays, many sources are concentrated near the Galactic plane. X-Ray Binaries: compact source orbiting a massive star

28. Chandra has revealed many more X-ray binary sources in the Milky Way, globular clusters and external galaxies. Chandra (launched 1999): high-resolution X-ray map of the Galactic Centre

29. XRB’s: How do we get so much energy out? 2 E = mc Need something approaching Gravitational energy from accretion

30. For how long might we expect such an X-ray binary source to shine?... Suppose we could completely annihilate a source of, say, So if we want a source lifetime of, say, we would need to extract around 10% of the source’s rest mass energy (same efficiency would give longer lifetime for a less luminous source) Is this realistic? Energy source believed to be gravitational infall (accretion) of matter onto a neutron star from a binary companion. Energy yield / unit mass

31. Matter falls in via an accretion disk. Some orbital angular momentum is lost by viscous friction. XRB luminosity comes from disk as well as the central source.

32. Accretion Luminosity and the Eddington Limit If matter accretes at rate then we expect, at radius But if is large, the accretion process becomes self-limiting, because the emitted luminosity exerts a significant radiation pressure force on the infalling material. Consider a proton of mass at radius Radiation force Thomson cross-section

33. Radiation force reduces the effective gravitational force to We can write this as where the critical, or Eddington, luminosity is Putting in some numbers we find that which is close to the maximum observed

34. Pulsars Discovered by Jocelyn Bell, in 1965.

35. Pulsars Discovered by Jocelyn Bell, in 1965. Extremely accurate ‘clocks’ Rapidly rotating NS, with beams of radiation

36. Pulsars Synchrotron radiation

37. Pulsars Observe: High spin rate High B field Electron acceleration

38. Binary neutron stars Very strong gravity provides a test of GR. Advance of periastron, Production of GWs Source of GRB’s?

39. Gravity in Einstein’s Universe Gravity and acceleration are completely equivalent: both cause spacetime to become curved or ‘warped’ Gravity is not a force propagating through space and time, but the result of mass (and energy) warping spacetime itself

40. Gravity in Einstein’s Universe “Spacetime tells matter how to move, and matter tells spacetime how to curve”

41. Gravity in Einstein’s Universe v Differences between Newtonian and Einsteinian gravity are tiny, but can be detected in the Solar System – and Einstein always wins!

42. Gravity in Einstein’s Universe v 1. Precession of orbits – observed for Mercury, matching GR prediction

43. Gravity in Einstein’s Universe v 1. Precession of orbits – observed for Mercury, matching GR prediction 2. Bending of light close to the Sun – visible during total eclipse, measured in 1919

46. Gravity in Einstein’s Universe ‘Ultimate’ case of light deflection = ‘Black Hole’: warps spacetime so much that light can’t escape

47. Lines of central Pressure, constant mass Rel. Proton degeneracy pressure Pressure, P N.R. Proton degeneracy pressure Rel. Electron degeneracy pressure E D N.R. Electron degeneracy pressure C B A Density,

48. Evidence for stellar black holes from binary systems: e.g. Cygnus X-1 Inferred mass far exceeds OV limit