Observing Black Holes

• What kinds of stars end up as black holes? They are the natural consequence of the evolution of massive stars. Neutron stars have an upper mass limit of 2 to 3 solar masses. A collapsed object of greater mass will continue to collapse indefinitely, forming a black hole. Either the core of the star weighs more than about 3 solar masses when it begins to collapse, in which case a black hole will form directly; or enough additional matter falls onto a newborn neutron star so that its mass increases above the maximum limit. How massive must a star be to become a black hole rather than a neutron star? We suspect that the star must have an initial mass greater than about 30 solar masses. Does a supernova explosion accompany the formation of a black hole? We don't know (lots to discover in this field!). Perhaps the collapse to form a black hole is a rather quiet event compared to the formation of a neutron star, calling to mind a well-known T. S. Eliot poem:
  This is the way the world ends,
  Not with a bang but a whimper.

• How can we detect and observe black holes? Isolated black holes, the result of the collapse of a single massive star, are too small to detect with any telescope that we could build today. There are probably millions of black holes in the Milky Way, weighing between 3 and 30 solar masses, but we can't find them without help. But there is another way ...

• What happens if your black hole is in a binary star system, orbiting with a normal star? If the orbit is small enough the normal star will evolve off the Main Sequence and expand in size to a red giant until it fills its Roche Lobe (and its outer atmosphere starts to be more attracted to the black hole companion than to the parent star). At this point it will begin to spill over, dumping gas into the Roche lobe of the black hole. As the gas falls toward the black hole, it swirls into a rapidly rotating accretion disk around the black hole, and as the gas spirals in it will heat up and emit high energy X-rays. The inner part of the disk heats up to 10⁷ - 10⁸ K, hot enough to create X-rays. Most come from well beyond the event horizon of the black hole (radiation might still be emitted by the gas after it falls within the event horizon, but we will never see it).

  The Cygnus X-1 system: a blue supergiant star (left) and a black hole (right) in orbit around each other. [ESA/Hubble European Space Agency Information Centre, M. Kornmesser, L. L. Christensen]

• What would happen if the Sun were replaced by a black hole of equal mass?