ASTR 1020 1st Edition Lecture 16 Outline of Last Lecture I. The Stellar GraveyardA. White DwarfsB. NovaC. Neutron StarOutline of Current Lecture I. The Stellar Graveyard: Black HolesA. Surface of a Black HoleB. Space Travel Near a Black HoleC. Observational EvidenceD. Gamma-Ray Bursts E. Star OutcomeCurrent LectureI. The Stellar Graveyard: Black Holes- A black hole is an object whose gravity is so powerful that not even light can escape it.A. “Surface” of a Black Hole - “Surface” of a black hole is the radius at which the escape velocity equals the speed of light = the event horizon.- Nothing can escape from within the event horizon because nothing can go faster than light. - The radius of the event horizon is known as the Schwarzschild radius: 3 (M/Msun)km (shrink Earth to size of a dime)B. Space Travel Near a Black Hole- Far from a black hole, the force of gravity same as for any massive object. - Close to a black hole, enormous tidal forces exist that stretch, heat, and tear apart- Imagine a spacecraft nearing the event horizon of a black hole:- Outside observers would observe clocks slowing down and photons with greater gravitational redshift. The spacecraft would begin to turn orange, then red, then fade from view. - In the spacecraft itself, however, time would appear to pass normally.- What’s inside a black hole?- Theory predicts that the mass collapses until its radius is zero and its density infinite (singularity). - This is unlikely to be what actually happens; we need a combined theoryof gravity and quantum physics (big and small).C. Observational Evidence- Black holes cannot be directly seen BUT we can search for evidence of their gravitational tug on nearby stars and/or the emission of X-rays from the surrounding hot gas - First direct evidence from the X-ray binary system Cygnus X-1- First X-ray satellites flown in 1970s led to the discovery of many X-ray sources - Brightest source in constellation Cygnus named Cygnus X-1 - Very luminous and rapidly variable (suggesting a small size)- Accurate position not known until a sudden change occurred in X-ray and radio brightness- Many black hole binaries are known: some with massive companions and some with solar mass companions.D. Gamma-Ray Bursts- Observations in the 1990s showed that many gamma-ray bursts were coming from very distant galaxies - They must be among the most powerful explosions in the universe—could be the formation of a black hole- Two models for gamma-ray bursts: - (1) merging neutron stars or - (2) a hypernova- In both models the energy is restricted to narrow jets of emission (like pulsars).- Hypernova: explosion of a very massive star that leads to the birth of a blackhole.E. Star Outcome- M < 0.08 Msun Star cools as brown dwarf - 0.08 < M < 10 Msun White dwarf remnant - 10 < M < 40 Msun Neutron star remnant- M > 40 Msun Black hole