Presentation on theme: "Stellar Physics 10 lectures, exploring the development of cosmology, and some of the key ideas of Big Bang theory Access PPT slides at"— Presentation transcript:
Stellar Physics 10 lectures, exploring the development of cosmology, and some of the key ideas of Big Bang theory Access PPT slides at Dr Martin Hendry Dept of Physics and Astronomy University of Glasgow
Surface temperature (K) O5 B0 A0 F0 G0 K0 M0 M8 Luminosity (Sun=1) Spectral Type Absolute Magnitude R S 0.01 R S 0.1 R S 1 R S 10 R S Main Sequence White dwarfs Supergiants 1000 R S 100 R S Giants We can plot the temperature and luminosity of stars on a diagram Stars dont appear everywhere: they group together, and most are found on the Main Sequence
Surface temperature (K) O5 B0 A0 F0 G0 K0 M0 M8 Luminosity (Sun=1) Spectral Type Absolute Magnitude Regulus Vega Sirius A AltairSun Sirius B Procyon B Barnards Star Procyon A Aldebaran Mira Pollux Arcturus Rigel Deneb Antares Betelgeuse Stars on the Main Sequence turn hydrogen into helium. 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 expansion is accelerating The geometry of the Universe is FLAT The Universe will continue to expand indefinitely
Cosmological Constant? Quintessence? What is driving the cosmic acceleration?…
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 (launched 1999): high-resolution X-ray map of the Galactic Centre Chandra has revealed many more X-ray binary sources in the Milky Way, globular clusters and external galaxies.
XRBs: How do we get so much energy out? Need something approaching E = mc 2 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 sources 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 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
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
Pulsars Discovered by Jocelyn Bell, in 1965.
Pulsars Discovered by Jocelyn Bell, in 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 GRBs?
Gravity in Einsteins 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
Gravity in Einsteins Universe Spacetime tells matter how to move, and matter tells spacetime how to curve
Gravity in Einsteins Universe v Differences between Newtonian and Einsteinian gravity are tiny, but can be detected in the Solar System – and Einstein always wins!
Gravity in Einsteins Universe v 1. Precession of orbits – observed for Mercury, matching GR prediction
Gravity in Einsteins 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 Einsteins Universe Ultimate case of light deflection = Black Hole: warps spacetime so much that light cant escape
Pressure, P Density, N.R. Electron degeneracy pressure Rel. Electron degeneracy pressure N.R. Proton degeneracy pressure Rel. Proton degeneracy pressure Lines of central Pressure, constant mass A B C D E
Evidence for stellar black holes from binary systems: e.g. Cygnus X-1 Inferred mass far exceeds OV limit