Death of Stars III Physics 113 Goderya Chapter(s): 14 Learning Outcomes:

Black Holes Just like white dwarfs (Chandrasekhar limit: 1.4 M sun ), there is a mass limit for neutron stars: Neutron stars can not exist with masses > 3 M sun We know of no mechanism to halt the collapse of a compact object with > 3 M sun. It will collapse into a single point – a singularity: => A Black Hole!

Escape Velocity Velocity needed to escape Earth’s gravity from the surface: v esc ≈ 11.6 km/s. v esc Now, gravitational force decreases with distance (~ 1/d 2 ) => Starting out high above the surface => lower escape velocity. v esc If you could compress Earth to a smaller radius => higher escape velocity from the surface.

The Schwarzschild Radius => There is a limiting radius where the escape velocity reaches the speed of light, c: V esc = c R s = 2GM____ c2c2 R s is called the Schwarzschild Radius. G = Universal const. of gravity M = Mass

Schwarzschild Radius and Event Horizon No object can travel faster than the speed of light  We have no way of finding out what’s happening inside the Schwarzschild radius. => nothing (not even light) can escape from inside the Schwarzschild radius  “Event horizon”

Schwarzschild Radius of Black Hole (SLIDESHOW MODE ONLY)

Black Holes in Supernova Remnants Some supernova remnants with no pulsar / neutron star in the center may contain black holes.

Schwarzschild Radii

“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: Mass Angular Momentum (Electric Charge)

General Relativity Effects Near Black Holes An astronaut descending down towards the event horizon of the BH will be stretched vertically (tidal effects) and squeezed laterally.

General Relativity Effects Near Black Holes (2) Time dilation Event Horizon Clocks starting at 12:00 at each point. After 3 hours (for an observer far away from the BH): Clocks closer to the BH run more slowly. Time dilation becomes infinite at the event horizon.

General Relativity Effects Near Black Holes (3) Gravitational Red Shift Event Horizon All wavelengths of emissions from near the event horizon are stretched (red shifted).  Frequencies are lowered.

Observing Black Holes 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 M sun => Black hole!

End States of Stars (SLIDESHOW MODE ONLY)

Candidates for Black Hole Compact object with > 3 M sun must be a black hole!

Compact Objects with Disks and Jets Black holes and neutron stars can be part of a binary system. => Strong X-ray source! Matter gets pulled off from the companion star, forming an accretion disk. Heats up to a few million K.

X-Ray Bursters Several bursting X-ray sources have been observed: Rapid outburst followed by gradual decay Repeated outbursts: The longer the interval, the stronger the burst

The X-Ray Burster 4U In the cluster NGC 6624 Optical Ultraviolet

Black-Hole vs. Neutron-Star Binaries Black Holes: Accreted matter disappears beyond the event horizon without a trace. Neutron Stars: Accreted matter produces an X-ray flash as it impacts on the neutron star surface.

Black Hole X-Ray Binaries Strong X-ray sources Rapidly, erratically variable (with flickering on time scales of less than a second) Sometimes: Quasi-periodic oscillations (QPOs) Sometimes: Radio-emitting jets Accretion disks around black holes

Radio Jet Signatures The radio jets of the Galactic black- hole candidate GRS

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-shifted Line systems shift back and forth across each other due to jet precession

Gamma-Ray Bursts (GRBs) Short (~ a few s), bright bursts of gamma-rays Later discovered with X-ray and optical afterglows lasting several hours – a few days GRB of May 10, 1999: 1 day after the GRB 2 days after the GRB Many have now been associated with host galaxies at large (cosmological) distances. Probably related to the deaths of very massive (> 25 M sun ) stars.