Paul murdin institute of astronomy cambridge
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Paul Murdin Institute of Astronomy, Cambridge. Closing in on Black Holes – why this conference is important for me. Black holes as theoretical entities. John Michell 1783; Pierre Laplace 1796 Posed a theoretical question about a star

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Paul Murdin

Institute of Astronomy, Cambridge

Closing in on Black Holes – why this conference is important for me


Black holes as theoretical entities

  • John Michell 1783; Pierre Laplace 1796

    • Posed a theoretical question about a star

      • At what radius of star, like the Sun, would the escape velocity exceed the speed of light?

  • Karl Schwarzchild 1915; Roy Kerr 1963

    • Calculation in general relativity of the gravitational field of a point or spherical mass, with/without rotation

  • Well-developed solutions looking for their problem


Does nature make stellar black holes?

  • S. Chandrasekhar 1930

    • Maximum mass of (Newtonian) self gravitating stars supported by degenerate electron pressure (white dwarfs)

  • J. R. Oppenheimer and G. Volkoff 1939

    • Ditto for general relativity and a neutron fluid (neutron stars)

    • “..unlikely that static neutron cores can play any great part in stellar evolution”

    • (However: Bell 1967: pulsars)

  • J.R. Oppenheimer and H. Snyder 1939

    • Neutron star’s “continued gravitational contraction,” asymptotically to the gravitational radius

  • → Possibly Nature does make black holes, yes

    • (but Nature doesn’t make neutron stars! – not a confidence-enhancing, mistaken judgement)


Do stellar black holes exist?

  • X-ray sources

    • Luminosities imply accretion

    • Temperatures imply accretion on to a compact object like a neutron star or black hole

  • Rocket and balloon-borne detectors: Cygnus XR-1

    • Huge positional uncertainty

  • Uhuru 1971 reduced positional uncertainty of Cygnus X-1

  • Hjellming and Wade 1971; Braes and Miley 1971: radio source with precise position

  • Optical ID with HDE226868

  • Correlated variability in X-ray/radio/optical effectively settled the identification

    • But a post hoc argument


Cyg X-1 - can we definitively say it is a neutron star or black hole?

  • Uhuru 1971 X-ray fluctuations at ~10 Hz frequencies → neutron star

  • Webster and Murdin 1971, Bolton 1971 HDE 228868 with a massive companion (>6 Msun), so not a neutron star → BH

  • Pringle and Rees 1972 → quasi periodic oscillations at inner edge of accretion disc

  • So black hole found? or some other sort of star with an unexplained small source of X-rays?


How close is the evidence to the black hole?

  • Cygnus X-1

    • Variability at 100 light milliseconds

    • Companion at 0.2 AU

    • Evidence is 1,000 to 1,000,000 Schwarzchild radii from the black hole

  • Evidence connecting Cygnus X-1 to a black hole has weak points

    • It is credible and consistent to say that Cyg X-1 is a black hole, but not, as far as I can see, unassailable

    • Perhaps this conference will tell me differently


Does nature make massive black holes?

  • Martin Rees 1971

    • Routes to a massive black hole

  • Looks like nature can indeed make them


Identifications of galactic black holes

  • Seyfert 1943 – explosive nuclei

  • Dent 1965 – variability with 1 year, < 1 l.y. extent

  • Salpeter, Zeldovich 1964 – powered by accretion onto black hole?

  • Lynden Bell 1971 – consistent physical model

  • Redhead, Cohen, Blandford 1978 – aligned jets from radio sources, maintained by rotating black hole

  • Richstone, Kormendy … 1990-95

    • large “black masses” in quiescent galaxies

  • Miyoshi et al. 1995, many others

    • NGC 4258 contains a central mass of 3×107 Mo

  • Genzel 1996, Ghez 1998

    • mass of Galaxy’s black hole by motion of a star cluster is 3×106 Mo


How close is the evidence to the black hole?

  • Galactic centre

    • S2 is 2×1010 km distant

    • 1,000 Schwarzchild radii

  • Evidence connecting these phenomena to a black hole is circumstantial

    • It is credible and consistent to say that they are, but this evidence is not unassailable

    • However…


Close to a black hole

  • High speeds

    • special relativity

      • e.g. relativistic beaming

  • Strong gravitational fields

    • general relativity

      • e.g. gravitational redshift

  • The MCG–6-30-15 Fe line profile provides the direct connection with the BH Schwarzchild radius (1995)

  • Very strong and convincing evidence, now very robust

  • Likewise for galactic sources? Not so robust?

Tanaka et al., 1995


Black hole history - a cynical view of astronomers, from the 1970’s

R McCray 1977


Black hole history now – a positive view

Astronomers investigate and their ideas clash

Viewing boundary

The indifferent and the ignorant

Imagination and new technology sees what lies inside

Boundary of interest


  • I hope to learn more…


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