Guest Author - Barbara Melville

The term “black hole” refers to a region of space with a powerful gravitational field, so powerful in fact that light cannot escape its pull. Black holes merit their name because their thirst for light renders them invisible, blending in with the blackness of space. These mysterious objects have sparked much interest in the field of science, and are a popular plot device in science fiction.

Black holes are predicted when massive old stars run out of fuel and collapse under their own gravity. The singularity is the center of the black hole – a point object where the surface gravitational force is infinite. The event horizon refers to an imaginary spherical shell around the singularity, representing the boundary beyond which nothing can ever return. Although black holes are sometimes still referred to as theoretical, there is much evidence to support their existence.

But how can we see what cannot be seen? There are actually a number of techniques used for locating black holes. The gas and dust that are pulled into black holes heat up to several million degrees, and the result is the emission of x-ray light. X-ray telescopes are used to observe this action. Large gravitational fields also distort the appearance of stars behind them, giving the impression of a lens. We can also learn a great deal about black holes by observing the motion of stars.

Our own Milky Way is thought to contain several million black holes. Perhaps the most interesting of these lies in a region called Sagittarius A*, the center of our galaxy. There is evidence that a supermassive black hole occupies this region, that is, a black hole several million times the mass of our Sun. It is orbited by a star traveling at incredibly fast speeds, which is only possible if the object it is orbiting is a black hole.

Who discovered black holes? In 1916, Albert Einstein published the theory of general relativity, offering equations that ground a basis for black holes. However, the earliest known reference to them was in a paper written by geologist and astronomer John Mitchell in 1783. A great number of people have played a role in our understanding of black holes, including mathematician Roy Kerr and physicists Stephen Hawking and Karl Schwarzchild.

Sources

Feature: Black Holes, NASA website, accessed: 20th August 2008, no author specified

Black Holes, Universe, published by Channel 4 books in 1999, authors: Couper, H and Henbest, N