Black holes are among the most exotic objects in the universe. They represent finality and
inescapability. They are the remnants of once giant stars - and they are a cosmic drain
in the centers of galaxies.
Black holes are celestial bodies so dense, their gravity so strong, that nothing can
escape their pull.
The idea of black holes is generally attributed to the French mathematician, Pierre Simon
Laplace, who, in 1796 was studying the subject of escape velocity. This is the speed something
must be accelerated to in order to prevent being pulled back by the gravity of a larger
body.
For example, to escape the Earth's gravitational pull, we must accelerate our rockets to over
11 kilometers per second. Any slower and they would fall back to Earth, victim of the pull
of Earth's gravity. Any Faster, and it will never return.
While working on this problem, Laplace noticed a relationship between the size of an object
and its mass. He noticed that if you made an object smaller, but kept its mass the same,
the escape velocity increased.
While playing with the calculations, he found that if you made the Earth half as small,
but kept the mass the same, the escape velocity doubled to 22 km per second. Taking things
further, he reasoned that if you kept squeezing the Earth smaller and smaller, eventually
the escape velocity would equal the speed of light. For the Earth to be this heavy,
Laplace calculated it would be only 1.8 millimeters across.
The idea of the black hole was born.
Over one hundred years later, the publication of Einstein's general theory of relativity
in 1915 brought the prediction that gravity could bend light rays.
A German mathematician/astronomer named Karl Schwarzchild took that notion and worked out
that for an object of any given mass, there was a specific radius at which light would
be unable to escape.
This distance has become known as the Schwarzchild radius and the formula he came up with defines
the size any object of mass M would need to be in order for its escape velocity to equal
the speed of light.
The concept of black holes remained a theoretical construct for decades. Astronomers had no
real use for them because at the time the universe was considered relatively simple:
stars collected in galaxies and (perhaps untold billions) of planets collected around the
stars. No one considered black holes possible in nature.
All that changed in 1962 with the launch of an Aerobee rocket with x-ray detectors on
board. Designed to look for x-rays from the Moon, the detector found not only a faint
background glow of of x-rays from all over the sky, there was also one strong source
of x-ray emission in the constellation of Scorpius, known as Scorpius X-1 it was the
first x-ray source ever detected.
Astronomers hadn't anticipated this. Creating x-rays requires an emormous amount of energy
and an extremely hot gas. For a source that was as many light years aways as Scorpius
X1 was, to have the quantity of x-ray emissions that was detected implied that it must be
generating huge quantities of x-rays.
The simplest explanation for this was if material was heated to extreme temperatures as it was
accelerated by gravity onto a nearby object. For this to be true, the companion object
had to be very small and very dense.
The era of high energy astrophysics was born.
The next pivotal discovery came with the launch of the Uhuru x-ray satellite on December 12th,
1970. Given the job of mapping the x-ray sky, this satellite found 339 sources; some were
galaxies, some supernova remnants, but most were x-ray binary stars.
Here, ordinary stars stream their material onto nearby neutron stars which gets extremely
hot as it falls, emitting x-rays.
One of these sources however, a source known as Cygnus X1, did not fall into this category.
Neutron stars are formed from the collapse of stars less than three times the mass of
the Sun. From uhuru data, The companion to this star appeared to be five to eight times
the mass of the Sun, this object was too large to be a neutron star. Current, more recent
estimates put the mass of this companion at least 10 solar masses.
This could be nothing other than a black hole.
Since their discovery and association with x-rays, black holes have been discovered primarily
in the center of galaxies with sizes of hundreds of millions of solar masses, identified by
enormous jets in the radio and x-ray wavelengths.
After hundreds of years of theory and speculation, black holes have moved into the realm of observation.
Their discovery was as unexpected as it was strange. Luckily for us, the truth of the
universe isn't limited by our feeble ability to understand it.