29:50 Modern Astronomy
Fall 2002
Lecture 22...November 4, 2002
The Search for Black Holes

Observational highlights: the long period variables R Andromedae and W Andromedae are coming to peak brightness this month. They will be near apparent magnitude 7.0 at maximum. The Search for Black Holes

(1)Black Holes defined as an object inside its Schwarzschild Radius,

(2) Are they out there? Just because something is permitted by the laws of physics does not mean that it will occur naturally. But in the case of black holes, they do exist.

(3) How can we tell if there are black holes? What do we look for? Consider the possibilities of degenerate objects based on their masses.

• If it could be a white dwarf.
• If it could be a neutron star. According to the best estimates, neutron stars can be no more massive than or they will collapse under their own weight.
• If , we know of no other possibility except a neutron star.

So it is a process of elimination: if we find a degenerate object more massive than , it must be a black hole.

We can determine the masses of these degenerate objects by measuring the properties of binary star systems in which the degenerate objects are located.

(4) The case of Cygnus X-1. In the early 1970's a strong and variable source of x-rays was found in the constellation of Cygnus. Some of its characteristics were:

• There is an O supergiant moving in an orbit with an unseen companion.
• There is strong, bursting x-ray emission from this binary system.
• The O star is deformed into a teardrop shape by the strong gravity of the companion.
• Obviously the companion must be a massive object to exert that sort of gravitational influence on an object as massive as an O star.
• The clincher is that the analysis of the orbit indicates that the unseen companion has a mass in the range of 10-15 .

It is generally thought that Cygnus X-1 must be a black hole. Through the years, other examples of massive X-ray binaries like Cygnus X-1 have been discovered.

(5) A second class of black hole binaries have been found, of which there are several examples. This second class is called soft x-ray transients. In this case, the visible star is a main sequence star of spectral class K or M.

Additional considerations

(6)The black holes mentioned above are the ``low mass'' or stellar mass black holes. Incredibly, there are ones much more massive. It has been convincingly shown that there is a black hole at the center of the Milky Way with a mass of .
Journal article by Andrea Ghez.
It has become increasingly clear that there are probably mega-black holes at the center of every major galaxy.

(7) Why do black holes shine? Black holes are brilliant sources of radiation because they are surrounded by hot, spiraling disks of matter called accretion disks. Feeding a black hole through an accretion disk is the most efficient energy scheme known. If an amount of mass M is thrown into an accretion disk, the amount of energy radiated away is

By contrast, in the proton-proton cycle that powers stars, the energy yield is only . Thus black hole accretion disks are the most efficient engines in the universe.

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