1 Reading: Chapter 24, Sect. 24.5-24.6; Chap. 20, Chap. 25, Sec. 25.1 Exam 2: Thursday, March 22; essay question given on Tuesday, March 20 Last time:death of massive stars - supernovae & neutron stars massive stars (> 8Mo) can fuse C, O all the way up to Iron Iron fusion eats energy - accelerates gravitational collapse collapse -> rebound -> catastrophic explosion -> supernova heavy elements synthesized and spread back into the ISM neutron stars as an end stage of massive stars Today: More bizarre endings - black holes etc. even neutron degeneracy can t stop collapse of most massive remnants gravity s ultimate victory: black holes finding black holes by their influence on their surroundings Discovery of Neutron Stars - Pulsars (1967) 1967: S. Jocelyn Bell discovers a radio signal: regularly pulsing rapid (once every 1.33 seconds) extraterrestrial aliens? LGM1, LGM2,... no. why not? Neutron stars! PSR 0329: 0.72 s Vela: 0.089 s 1974 Nobel Prize to... Tony Hewish (Bell s advisor?!#@) 3 Discovery of Neutron Stars - Pulsars (1967) 1966-67: Tommy Gold (and Franco Pacini) why does the Crab nebula shine?? supernova leaves a rapidly rotating neutron star neutron star has an intense magnetic field (10 11 Gauss) for comparison: Earth magnetic field = 1/2 G (compass needle) Strongest permanent magnet ~ 14,000 G Strongest magnetic field produced ~ 4x105 G light produced by motion of e - in magnetic field light energy derived from NS rotation spin rate should decrease with time Pulsars - a new tool for astronomy and physics Binary Pulsars: precise tests of general relativity (1993 Nobel, Hulse & Taylor) Pulsars with Planets timing jitter -> planet-sized companions (Alex Wolsczan) Millisecond Pulsars fast pulsar, small dp/dt Crab Pulsar (0.033 s) PSR 1937 (0.00167s) recycled pulsars spun-up by companion (Don Backer) should have companion - most do, many do not! Black Widow Pulsars pulsar blasts away its companion (Dan Stinebring) 2 4
M > 25 M sun iron core > 5 Msun even neutron degeneracy can t stop collapse complete gravitational collapse to... A Why black? BLACK Escape velocity: HOLE from Earth = 11.3 km/s = 25,400 mph from Sun = 600 km/s = 1,350,000 mph from a WD = 5000 km/s from a NS= 200,000 km/s = 2/3 the speed of light! 1783: John Mitchell escape velocity > speed of light when R < R sch = 3 km x M/M sun 5 6 1905: Albert Einstein NOTHING can travel faster than light... the ultimate speed limit is 300,000 km/s near Black Holes, light cannot escape no light = no information EVENT HORIZON= point of no return within Event Horizon = central singularity Anything that enters the Event Horizon loses contact with the rest of the Universe... forever Life near a Black Hole far outside: (d >> R sch ): feel gravity as elsewhere just outside: tidal forces... inside: (d < R sch ): cut off from Universe The Black Hole Environment 7 8 The Black Hole Environment Dangerous - tidal forces are enormous Dangerous - tidal forces are enormous
The Black Hole Environment 9 10 Dangerous - tidal forces are enormous strange views - gravitational bending of light warped time - gravity near a black hole slows down time (with respect to outside observers) Black holes and time machines Monster Black Holes in Galaxy Cores what lies beneath? non-rotating: d < r s fall @ speed of light to form a singularity viewed from afar, collapse stops at event horizon Cosmic Censorship time stops loses meaning is irrelevant! cut off from the rest of the Universe (almost) forever How can we find a black hole? 11 12 They exert a gravitational force find a binary star with an unseen companion, M > 5 M sun They suck (sort-of) surrounding matter may fall into the black hole gas falling in gets squeezed heats up glows as X-Rays Best candidate: Cygnus X-1 visible star: O9 (~20 M sun ) spectroscopic binary; companion ~10 M sun copious X-ray emission
13 Gamma Ray Bursts Gamma Ray Bursts short-duration, high energy flashes evenly distributed across the entire sky cosmological distances - intensely bright supernovae 2704 BATSE bursts 14 Gamma Ray Bursts short-duration, high energy flashes evenly distributed across the entire sky cosmological distances - hypernovae rapid rotation -> accretion disk relativistic jet collimated by accretion disk beam points towards us, we see gamma ray burst 15 neutron star mergers binary with 2 massive stars after both go SN, left with binary neutron stars orbit decays (gravitational radiation) stars eventually merge, collapse, and explode as a kilonova synthesize and disperse the heaviest elements extremely rare but essential consequence of how we know stars (and physics) works 16
neutron star mergers 17 neutron star merger first observed in August 2017 gravitational radiation seen by LIGO (LIGO won Nobel Prize in Physics in October for last year s detection of merging black holes ) gamma ray burst observed by Fermi optical light seen from Chile radio pulse observed multimessenger astronomy 18 Black hole in galaxy center (APOD 3/12/2013) 19 20