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How do we know?? • Luminosity = (apparent brightness) x distance2 • Apparent brightness: • Discovered quasars from their radio emission. Orion nebula Radio position 3C 273 • But… which object??? Apparent brightness Æ redshift = 0 Quasar 3C 273 Redshift = 0.158 Parkes, Australia … 210’ radio telescope. • Distance: Now measure optical spectrum Wavelength Æ • Doppler shift of wavelength of light Î velocity of recession (redshift) due to expansion of Universe. Î huge distance Î huge luminosity! How do we know?? • Size of quasars: • They change brightness in short time (weeks, days. hours). Î Size = light-weeks, light-days, light-hours. 1 Quasars: events in centers of galaxies • Hubble Space Telescope images. • Quasars are bright star-like objects at centers of faint galaxies. Most Quasars Lived and Died Long Ago Rate at which stars are formed in galaxies. Number of Quasars per unit volume Observations show: • • • • huge luminosity from a tiny volume at centers of galaxies happened early in history of galaxies Conclusion: Quasars are events in young galaxies. What sort of thing can do that? Now = 14 billion yrs Å time Formation of universe 2 Galaxy Formation Gravity Î Material Falls to Center Galaxy Formation Gravity Î Material Falls to Center 3 High density Î Strong Gravitational Field Î Black Hole Escape Velocity: vescape = Schwarzschild radius 2GM R Singularity Schwarzschild radius: vescape = c (speed of light) RS = 2GM c2 Empty space Accretion Disks Accretion disk • Well-studied phenomena in local binary star systems • “cataclysmic variables” • Angular momentum Î material cannot fall directly onto central mass. • Binary stars Î “thin” accretion disks • Material works its way in toward center due to viscosity. 4 The Source of the Luminosity: • Matter falls onto accretion disk. • Disk heats up & glows. • Also, Blandford-Znajek mechanism. • extract BH rotational energy [Fig 26.17] Accretion disk + Black Hole + Jets Black Hole Energy Source: • Gas, stars fall into 108 M black hole. • Gravitational potential energy Î thermal energy Î light Accretion disk + Black Hole + Jets 5