Most people view nuclear reactors with grave disapproval, yet we all owe our very existence to a nuclear reactor, the sun, our only external source of significant energy. Like all stars, the sun is a nuclear fusion reactor and radiates massive amounts of energy into space, some of which we benefit from here on earth. Dr William Reville writes
Stars are born when vast clouds of mostly hydrogen gas collapse under gravitational attraction. Eventually the hydrogen atoms get so close together and the temperature and pressure gets so high that nuclear fusion of hydrogen into helium begins. In the fusion process, some mass is converted directly into energy and this is the origin of the massive release of energy from stars.
The great heat generated in a star prevents it from totally collapsing in on itself under gravity - the heat keeps the star expanded. Eventually all the nuclear fuel in a star gets used up and the star collapses in on itself and dies. The way in which the star dies depends on its size.
Stars smaller than - and up to eight times the size of - our sun collapse into white dwarf stars at the end of their lives. Before collapsing, however, stars of this size first swell into red giants, about 100 times bigger than the original star. Before and during their collapse these stars lose tremendous mass. A star eight times the mass of our sun will lose mass equivalent to 6.8 suns, leaving a mass equivalent to 1.4 of our suns.
About five billion years from now our sun will swell into a red giant, swallowing up Mercury and Venus and engulfing the earth, destroying all life than has not found refuge in another planetary system. Eventually the outer layers of the red giant blow away leaving a white dwarf star, about 100 times smaller than the original sun. The white dwarf is very hot and radiates heat as it cools down over billions of years until it eventually disappears from view.
Heavyweight stars, much bigger than our sun, end their lives in a dramatic explosion called a supernova. They first of all expand into red giants, but after some time become unstable and explode. The pieces that survive the explosion collapse under gravity. If a piece is between 1.4 and 3.2 times the mass of our sun, it will collapse into a neutron star. If it is more than 3.2 times the mass of our sun it will collapse into a black hole.
A neutron star is as dense as the nucleus at the centre of an atom and is composed mainly of neutrons (one of two kinds of sub-atomic particles present in the atomic nucleus). If it could be brought to earth, a pinhead-sized piece of a neutron star would have a mass of one million tonnes. Neutron stars are about 20 km in diameter and have a mass about the same as our sun. The temperature of a neutron star can be as high as one million degrees Celsius. The surface is composed of many heavy nuclei including iron. Neutron stars have extremely strong magnetic fields.
Pulsars (pulsating radio stars), discovered in 1967 by Irish scientist Jocelyn Bell Burnell, are rapidly rotating neutron stars that give off regular pulses of radio waves. Neutrons at the surface of the star turn into energetic protons and electrons which spiral around the magnetic field giving off the radio waves.
A collapsing star more than 3.2 times the size of our sun continues to collapse until it becomes a mathematical point - a singularity, with no size, and an infinite density. Around the point is a region a few km in diameter where gravity is so strong that nothing, not even light, can escape. For any object to escape from a celestial body it must travel at least fast enough to overcome the gravitational attraction of the celestial body. This velocity is called the escape velocity.
The escape velocity from earth is seven miles per second. The escape velocity from a neutron star is about 120,000 miles per second. The escape velocity from a black hole exceeds the speed of light which is 186,282 miles per second. Nothing can accelerate beyond the speed of light and therefore nothing can escape from a black hole.
The event horizon is the spherical region around the singularity enclosing the space from which nothing can escape. Its radius is proportional to the mass of the black hole.
If nothing escapes from a black hole, how can we detect it? Matter can be gobbled up by a black hole and, in the process, it emits radiation that we can detect. It often happens that two stars circle each other, rotating around a common centre of gravity - a binary system. Consider two large stars in a binary system. One explodes in a supernova and collapses into a black hole. As the other star ages it swells up and some of its matter spirals into the black hole. This trapped matter heats up enormously and emits copious X-rays.
There is a powerful X-ray source (Cyg X-1) in the Cygnus constellation. There is an ordinary star as this point in the sky which cannot be the X-ray source. However this star swings around a companion star that is invisible to ordinary telescopes.
The invisible star exerts a gravitational pull of an object as massive as 10 suns. It is too heavy to be a neutron star and it must be a black hole. We also know that a massive black hole resides at the centre of our own galaxy and the centres of many others.
William Reville is associate professor of biochemistry and director of microscopy at UCC