Black Holes
Physical theory predicts black holes. Are they there?
There is evidence for the existence of such an object. The best case
is an xray source called Cygnus X1. Cygnus X1 appears
to be a black hole with a mass of about 7 Msun that is part
of a binary system. The other star in the binary system is a B class
supergiant star.
- Size.
- The X-ray emission flickers on a time scale that is as short as
0.01 second.
- This means that the object must be no larger than the distance
light can go in 0.01 sec, which is 3 x 103 km.
- So Cygnus X1 is smaller than the Earth.
- Mass.
- Since Cyngus X1 is part of a binary system, one can learn about its
mass.
- The doppler shift pattern for the big star gives its orbital period
and orbital speed, or at least the component of its orbital velocity that
is in our line of sight.
- The mass of the big star is known approximately from the mass-luminosity
relation for main sequence stars.
- The result is that the most likely mass for Cygnus X1 is 7 Msun.
A small size and a mass bigger than 3 Msun would mean that Cygnus
X1 must be a black hole. However, there is some uncertainty in the mass measurement. Moreover, if you didn't believe in black holes, it seems to me that this would not be enough evidence to convince you.
Davison E. Soper, Institute of Theoretical Science,
University of Oregon, Eugene OR 97403 USA
soper@bovine.uoregon.edu