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Stellar Physics Dr Martin Hendry 10 lectures, exploring the Dept of Physicsof andcosmology, Astronomy development University of Glasgow and some of the key ideas of Big Bang theory [email protected] Access PPT slides at http://www.astro.gla.ac.uk/users/martin/teaching/aberdeen.ppt Surface temperature (K) 25000 10000 8000 6000 5000 4000 3000 106 -10 We can plot the temperature and luminosity of stars on a diagram Supergiants -5 102 0 Giants 1 +5 10-2 +10 10-4 +15 O5 B0 A0 F0 G0 Spectral Type K0 M0 M8 Absolute Magnitude Luminosity (Sun=1) 104 Stars don’t appear everywhere: they group together, and most are found on the Main Sequence Surface temperature (K) 25000 106 10000 8000 6000 . 5000 4000 3000 . . . . .. . . . . .. . . .. .. . ... .. . .... .. . .. ..... ... . . .. . ... . . ... ........ . .. .... . .. . . ..... ...... ...... .. .. ..... . ...... . . ... . . . ... ... .. Deneb -10 Stars on the Main Sequence turn hydrogen into helium. Rigel Betelgeuse Antares Luminosity (Sun=1) Arcturus 102 Aldebaran Regulus Vega Procyon A Altair 10-2 Pollux Sun Procyon B O5 B0 +5 +10 Barnard’s Star Sirius B 10-4 0 Mira Sirius A 1 -5 A0 F0 G0 Spectral Type K0 M0 M8 +15 Absolute Magnitude 104 Blue stars are much hotter than the Sun, and use up their hydrogen in a few million years Observational Evidence for Compact Objects 1. White Dwarfs 2. Neutron Stars 3. Black Holes White Dwarfs Small but very luminous (because of high T) Can be directly observed Important Type of White Dwarf for Cosmology: Type Ia Supernovae Excellent for measuring cosmological distances – good “Standard Candle” 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 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? Answer depends on the geometry of the Universe Closed Open Flat We can measure this using Type Ia Supernovae Results: The geometry of the Universe is FLAT The Universe will continue to expand indefinitely The expansion is accelerating What is driving the cosmic acceleration?… Cosmological Constant? Quintessence? Neutron Stars Very much smaller: (almost) invisible at optical, but can be seen in X-Rays if their surfaces are very hot Crab Nebula: supernova of 1054 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 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 XRB’s: How do we get so much energy out? 2 Need something approaching E = mc Gravitational energy from accretion 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 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. Accretion Luminosity and the Eddington Limit If matter accretes at rate then we expect, at radius r GMM Lacc ~ r 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 mP at radius Radiation force Frad L T 4 r 2c Thomson cross-section 6.651029 m2 Radiation force reduces the effective gravitational force to Fgrav We can write this as Fgrav GMmP L T 2 r 4 r 2c GMmP r2 L 1 L crit where the critical, or Eddington, luminosity is Putting in some numbers we find that Lcrit Lcrit 4 GMmP c T M 3 10 4 LO ~ MO which is close to the maximum observed LX Pulsars Discovered by Jocelyn Bell, in 1965. Pulsars Discovered by Jocelyn Bell, in 1965. Extremely accurate ‘clocks’ Rapidly rotating NS, with beams of radiation Pulsars Synchrotron radiation Pulsars Observe: High spin rate High B field Electron acceleration Binary neutron stars Very strong gravity provides a test of GR. Advance of periastron, Production of GWs Source of GRB’s? Gravity in Einstein’s Universe Matter causes space to curve or warp 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 Einstein’s Relativity Gravity in Einstein’s Universe “Spacetime matter how Matter tells causes space to move, and matter tells to curve or warp spacetime how to curve” 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! Gravity in Einstein’s Universe v 1. Precession of orbits – observed for Mercury, matching GR prediction 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 Gravity in Einstein’s Universe ‘Ultimate’ case of light deflection = ‘Black Hole’: warps spacetime so much that light can’t escape Lines of central Pressure, constant mass Pc 4 / 3 Rel. Proton degeneracy pressure 4/3 P N.R. Proton degeneracy pressure 5/ 3 P Rel. Electron degeneracy pressure 4/3 E D C B P N.R. Electron degeneracy pressure P 5/ 3 A Density, Evidence for stellar black holes from binary systems: e.g. Cygnus X-1 Inferred mass far exceeds OV limit