Presentation on theme: "Neutron Stars and Black Holes"— Presentation transcript:
1 Neutron Stars and Black Holes Chapter 11:Neutron Stars and Black Holes
2 The central core will collapse into a compact object of ~ a few Msun. Neutron StarsA supernova explosion of an M > 8 Msun star blows away its outer layers.Pressure becomes so high that electrons and protons combine to form stable neutrons throughout the object.The central core will collapse into a compact object of ~ a few Msun.Typical size: R ~ 10 kmMass: M ~ 1.4 – 3 MsunDensity: r ~ 1014 g/cm3 Piece of neutron star matter of the size of a sugar cube has a mass of ~ 100 million tons!!!
3 Discovery of Pulsars Angular momentum conservation Angular momentum conservation=> Collapsing stellar core spins up to periods of ~ a few milliseconds.Magnetic fields are amplified up to B ~ 109 – 1015 G.(up to 1012 times the average magnetic field of the sun)=> Rapidly pulsed (optical and radio) emission from some objects interpreted as spin period of neutron stars
4 The Crab Pulsar Remnant of a supernova observed in A.D. 1054 Pulsar wind + jetsRemnant of a supernova observed in A.D. 1054
7 The Lighthouse Model of Pulsars A pulsar’s magnetic field has a dipole structure, just like Earth.Radiation is emitted mostly along the magnetic poles.
8 Images of Pulsars and other Neutron Stars The Vela pulsar moving through interstellar spaceThe Crab Nebula and pulsar
9 The Effects of Pulsar Winds Pulsars blow off a constant stream (wind) of high-energy particles:pulsar winds
10 Proper Motion of Neutron Stars Some neutron stars are moving rapidly through interstellar space.This might be a result of anisotropies during the supernova explosion forming the neutron star.
11 Binary PulsarsSome pulsars form binaries with other neutron stars (or black holes)Radial velocities resulting from the orbital motion lengthen the pulsar period when the pulsar is moving away from Earth… and shorten the pulsar period when it is approaching Earth.
12 Neutron Stars in Binary Systems: X-ray binaries Example: Her X-1Star eclipses neutron star and accretion disk periodically2 Msun (F-type) starNeutron starOrbital period = 1.7 daysAccretion disk material heats to several million K => X-ray emission
13 Compact Objects with Disks and Jets Black holes and neutron stars can be part of a binary system.Matter gets pulled off from the companion star, forming an accretion disk.=> Strong X-ray source!Heats up to a few million K.
14 The X-Ray Burster 4USeveral bursting X-ray sources have been observed:Rapid outburst followed by gradual decayOpticalUltraviolet
15 Some pulsars have planets orbiting around them. Pulsar PlanetsSome pulsars have planets orbiting around them.Just like in binary pulsars, this can be discovered through variations of the pulsar period.As the planets orbit around the pulsar, they cause it to wobble around, resulting in slight changes of the observed pulsar period.
16 Neutron stars can not exist with masses > 3 Msun Black HolesJust like white dwarfs (Chandrasekhar limit: 1.4 Msun), there is a mass limit for neutron stars:Neutron stars can not exist with masses > 3 MsunWe know of no mechanism to halt the collapse of a compact object with > 3 Msun.It will collapse into a single point – a singularity:=> A black hole!
17 Escape VelocityVelocity needed to escape Earth’s gravity from the surface: vesc ≈ 11.6 km/s.vescNow, gravitational force decreases with distance (~ 1/d2) => Starting out high above the surface => lower escape velocity.vescIf you could compress Earth to a smaller radius => higher escape velocity from the surface.vesc
18 The Schwarzschild Radius => There is a limiting radius where the escape velocity reaches the speed of light, c:2GM____Vesc = cRs =c2G = gravitational constantM = massRs is called the Schwarzschild radius.
19 Schwarzschild Radius and Event Horizon No object can travel faster than the speed of light=> nothing (not even light) can escape from inside the Schwarzschild radiusWe have no way of finding out what’s happening inside the Schwarzschild radius.“Event horizon”
21 “Black Holes Have No Hair” Matter forming a black hole is losing almost all of its properties.black holes are completely determined by 3 quantities:massangular momentum(electric charge)
22 The Gravitational Field of a Black Hole Gravitational PotentialDistance from central massThe gravitational potential (and gravitational attraction force) at the Schwarzschild radius of a black hole becomes infinite.
23 General Relativity Effects Near Black Holes An astronaut descending down towards the event horizon of the black hole will be stretched vertically (tidal effects) and squeezed laterally.
24 General Relativity Effects Near Black Holes (II) Time dilationClocks starting at 12:00 at each point.After 3 hours (for an observer far away from the black hole):Clocks closer to the black hole run more slowly.In a finite amount of time for an in falling clock to the events horizon and infinite amount of time will pass for an outside the event horizon observer.Time dilation becomes infinite at the event horizon.Event horizon
25 General Relativity Effects Near Black Holes (III) gravitational redshiftAll wavelengths of emissions from near the event horizon are stretched (redshifted). Frequencies are lowered.Event horizon
26 Observing Black Holes Mass > 3 Msun => Black hole! No light can escape a black hole=> Black holes can not be observed directly.If an invisible compact object is part of a binary, we can estimate its mass from the orbital period and radial velocity.Mass > 3 Msun=> Black hole!
27 Compact object with > 3 Msun must be a black hole! Compact object with > 3 Msun must be a black hole!
28 Jets of Energy from Compact Objects Some X-ray binaries show jets perpendicular to the accretion disk
29 Model of the X-Ray Binary SS 433 Optical spectrum shows spectral lines from material in the jet.Two sets of lines: one blue-shifted, one red-shiftedLine systems shift back and forth across each other due to jet precession
30 Gamma-Ray Bursts (GRBs) Short (~ a few s), bright bursts of gamma-raysGRB of May 10, 1999: 1 day after the GRB2 days after the GRBLater discovered with X-ray and optical afterglows lasting several hours – a few daysMany have now been associated with host galaxies at large (cosmological) distances.Probably related to the deaths of very massive (> 25 Msun) stars.