Page 51 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 6

Figure 6:3

For this mechanism to work, the unseen object has to be very small, like a white dwarf, neutron star, or black hole.
From the observed orbit of the visible star, one can determine the lowest possible mass of the unseen object. In the
case of Cygnus X-l, this is about six times the mass of the sun, which, according to Chandrasekhar’r result, is too great
for the unseen object to be a white dwarf. It is also too large a mass to be a neutron star. It seems, therefore, that it
must be a black hole.
There are other models to explain Cygnus X-1 that do not include a black hole, but they are all rather far-fetched. A
black hole seems to be the only really natural explanation of the observations. Despite this, I had a bet with Kip Thorne
of the California Institute of Technology that in fact Cygnus X-1 does not contain a black hole! This was a form f
insurance policy for me. I have done a lot of work on black holes, and it would all be wasted if it turned out that black
holes do not exist. But in that case, I would have the consolation of winning my bet, which would bring me four years of
the magazine Private Eye. In fact, although the situation with Cygnus X-1 has not changed much since we made the bet
in 1975, there is now so much other observational evidence in favor of black holes that I have conceded the bet. I paid
the specified penalty, which was a one-year subscription to Penthouse, to the outrage of Kip’s liberated wife.

We also now have evidence for several other black holes in systems like Cygnus X-1 in our galaxy and in two
neighboring galaxies called the Magellanic Clouds. The number of black holes, however, is almost certainly very much
higher; in the long history of the universe, many stars must have burned all their nuclear fuel and have had to collapse.
The number of black holes may well be greater even than the number of visible stars, which totals about a hundred
thousand million in our galaxy alone. The extra gravitational attraction of such a large number of black holes could
explain why our galaxy rotates at the rate it does: the mass of the visible stars is insufficient to account for this. We also

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