Do black holes really exist? Dr Marek Kukula, Royal Observatory Greenwich

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<ul><li><p>Do black holes really exist?Dr Marek Kukula, Royal Observatory Greenwich</p></li><li><p>Now have strong evidence for two classes of black holeStellar-mass black holes:few times the mass of the sun. Found throughout our own Galaxy.Supermassive black holes:up to 10 billion times the mass of the sun. Found only in the centres of large galaxies.</p></li><li><p>What is a black hole?A region of space with such intense gravity that not even light can escape.First suggested in the 18th Century by Laplace.Idea confirmed by Einsteins General Theory of Relativity.</p></li><li><p>Escape velocity If enough mass is concentrated into a small enough volume its gravity will be so strong that even light will not be able to escape.Strength of gravity depends on:</p><p>Mass of objectDistance from centre of massA Black Hole</p></li><li><p>Event horizonBackground light distortedby intense gravitational fieldclose to the black hole.Event horizon: escape velocity = speed of lightNothing can escape the gravitational pull insidethis radius.Singularity: allmatter inside theevent horizon iscrushed to a pointof ZERO SIZE andINFINITE DENSITY.</p></li><li><p>How might black holes form?</p><p>Where should we look for them? very large mass very small volumeTo be sure that weve found a black hole astronomers need to demonstrate the object has:Physicists and mathematicians might also like to see evidence for: an event horizon a singularity()</p></li><li><p>Stellar mass black holesNuclear reactions in the stellar core support a star against the inward force of its own gravity. When the stars nuclearfuel runs out it should begin to collapseIs this a way to form a black hole?</p></li><li><p>Death of a star like the sunWhen the the Suns helium fuel is exhausted it will have no further source of energyThe outer layers of the star are gently expelled into space, forming a glowing planetary nebulaThe hot, dense stellar core is left behind to cool slowly over billions of years a White Dwarf starEverything depends on the mass of the star</p></li><li><p>White Dwarf starThe mass of the sun in a volume the size of a planet.Composed of degenerate matter. but its not a black hole</p></li><li><p>Planetary nebulae</p></li><li><p>Stars more massive than the Sun end their lives in Supernova explosions:</p></li><li><p>Much of the stars mass is lost in the explosionA dense, compact coreis left behind.</p></li><li><p>If the remaining stellar core has a mass less than 3 times the mass of the sun it will form a Neutron Star:</p></li><li><p>Neutron star: a ball of subatomic particles supported by nuclear forces</p><p>Mass: 1.4 3 times the SunRadius: 10 km</p><p> Density: Ben Nevis per teaspoonful!</p></li><li><p>Do neutron stars really exist?Radio signals from the centre of supernova remnants:Lovell radio telescope, Jodrell Bank pulsars</p></li><li><p>The discovery of pulsarsJocelyn Bell-Burnell &amp; Anthony Hewish 1967Such rapid radio pulsations could only come from a verysmall, dense object with anintense magnetic field.</p><p> Exactly the properties expected for a rapidly spinningneutron star. But this still isnt a black hole!</p></li><li><p>For really massive stars (&gt; 10 solar masses) the remaining stellar core will have a mass more than 3 times that of the sun. even neutrons cannot support this amount of mass.</p><p>The core is crushed down to a point of INFINITE DENSITY witha gravitational field so intense that even light cannot escapeA Black Hole</p></li><li><p>How can we detect them?Cant see the black hole directlyBut can try to observe the effects of its gravity on its surroundings</p></li><li><p>Binary star systemsMany stars occur in binary pairs, orbitingeach other.If one of the stars goes supernova, the collapsed core of the star will remain in orbit around its companion.</p></li><li><p>X-ray Binary SystemsThe collapsed stellar core is too small to be directly detected but we can infer its presence from its effect on the visible companion star.Gas is stripped from the companion starand heated as it spirals in towards the neutron star or black hole.</p><p>This gas emits huge amounts of X-rays.</p></li><li><p>Anatomy of an X-ray binary systemGravity of compact objectpulls matter off companion starAccretion disc: shines in X-raysJets of material ejected at high speed, giving off radiowaves</p></li><li><p>Measuring mass in X-ray BinariesBinary orbit around common centre of mass causes a wobble in the position of the visible star:If the mass of the compact companion is greater than 3 times the mass of the sunit CANNOT be a neutron star.The object must be a black hole.Speed of wobble gives mass ofinvisible compact companion.</p></li><li><p>Cygnus-X1: the best candidate for a stellar-mass black holeFrom the wobble of the visible star we can weigh the mass of the companion to be ~10 solar masses. Astronomers are 95% certain that Cyg-X1 is a black hole.</p><p>X-ray source associated with a binary star. 1 billion timesmore luminous in X-raysthan the Sun.</p></li><li><p>8 such black hole candidates are now known, with masses estimated at &gt;3 solar massesThe case for stellar-mass black holes looks good</p></li><li><p>The evidence for stellar mass black holesIntense X-ray emission from gas falling onto an extremely compact object (&lt; 3km across)Wobble of companion star indicates a mass of over 3 times the mass of the Sun Physics suggests such an object can only be a black hole</p></li><li><p>Supermassive Black Holes</p></li><li><p>1963: radio source 3C273 associated with a blue star-like object. Implied distance is 2 billion light years. Optical luminosity 250 times brighter than the milky way. Quasi-stellar radio sources (Quasars) Many similar objects soon discovered, all with highly unusual properties. 3C273</p></li><li><p>Imaging quasars with Hubble Quasars lie at the centres of distant galaxies</p></li><li><p>Quasar propertiesLuminous at all wavelengthsJets compact, stable energy sourceRapid variability object is small</p></li><li><p>Powering quasarsExtremely luminousExtremely smallOnly plausible energy source is an accretion disc around a black hole with millions of times the mass of the Sun.</p></li><li><p>The black holes accretion disc is only the size of the solar system, yet it emits more light than the 100 billion stars in the Milky Way.</p></li><li><p>X-rays from iron atoms Gas moving with velocities up to 100,000 km/s - exactly the speed wed expect at the Event Horizon</p><p> Broad emission tail evidence for gravitational redshift predicted by General Relativity close to a BH High temperatures cause iron atoms to give off X-rays High speeds close to the black hole change the frequency of these X-rays Doppler ShiftX-Ray frequency </p></li><li><p>More evidence from the Hubble Space TelescopeHubble finds signs of dormant black holes in most large galaxies, not just quasarsStellar velocities:very massive,very compactobject ingalaxy centre.</p></li><li><p>Is there a Supermassive Black Hole in the Milky Way?Radio image of the Galactic CentreSag A*</p></li><li><p>Infrared images Reveal the central star cluster:</p></li><li><p>The La Silla Observatory Chile</p></li><li><p>The SHARP-1 Camera(Speckle-Interferometry)Special technique counteracts atmospheric blurringto give accurate positions for the stars in the Galactic centre.</p></li><li><p>High resolution infrared imaging of the galactic centre199419972000 can track the motions of individual stars</p></li><li><p>Stellar motions in the Galactic centre mass of central object = 3 million suns</p></li><li><p>Chandra launch, July 23 1999Measure X-ray emission from the Galactic centre</p></li><li><p>Our black hole takes a snackBefore:After:</p></li><li><p>What does the black hole look like?</p></li><li><p>The Evidence for Supermassive Black Holes Energy source for quasars Quasar variability Stability of radio jets X-rays from iron atoms at the Event Horizon</p><p> Motion of gas in nearby galaxies Stellar motions in centre of Milky Way only plausible explanation is a black hole</p></li><li><p>So do black holes really exist?Extremely compact stellar-mass objects in X-ray binary systemsExtremely massive compact objects in the centres of most galaxiesWe have found:Their properties are exactly what wed expectif they are powered by black holes(BUT we still havent seen a black hole directly!)</p></li><li><p>Answer: yes (probably)</p></li><li><p>The End</p><p>Need to edit film*</p></li></ul>