Ken Tapping, 29th December, 2015
Over the last few years space probes have been revealing amazing things about our Solar System. However, sending manned or unmanned spacecraft to other stars is currently far beyond any technologies we have, although we will get there one day. We continue to learn about the universe beyond the Solar System by studying the electromagnetic waves emitted by distant objects.
Radio waves, infrared radiation, light, ultra-violet radiation, X-rays and gamma rays are all forms of electromagnetic waves. The only difference between them is their wavelength – the distance between two consecutive wave crests. Radio waves are long, having wavelengths of many meters down to a few millimetres; shorter waves, down to a wavelength of a millionth of a metre or so we call infrared. At 800 nanometres (billionths of a metre) the waves become visible as red light. As the wavelength shortens we pass through the colours of the rainbow, through orange, yellow, green, blue, indigo and violet, which has a wavelength of about 400 nanometres. Then the light becomes invisible again, as ultraviolet. At wavelengths less than 10 nanometres we call the waves X-rays, and below a thousandth of a nanometre, gamma rays.
Observing the different wavelengths of radiation we receive from the universe tells us a lot, mainly because of the nature of the electromagnetic radiation. It is produced in little packets or pulses called quanta. It is not possible to have half of a quantum, just whole ones. Moreover the wavelength of a quantum depends upon the amount of energy that went into making it. A poker in a fire can glow red-hot only because there is enough energy going into the poker to make quanta of red light. So if we see X-rays coming from some object in the sky, it means something very high-energy is going on, such as high-energy particle collisions or million-degree temperatures. To make gamma rays, the highest-energy waves of all, we need nuclear processes: the sorts of things that happen in the cores of stars and in supernova explosions. Very hot objects make X-rays; less hot objects, like stars, radiate ultraviolet, visible light, infrared and less energetic radiation. Interestingly, even the cold, dark clouds between the stars, a few degrees above absolute zero (which is –273 C) give off infrared and radio waves. Anything with a temperature of absolute zero will give off no emissions whatsoever. However, nothing in the universe is that cold, so everything in the universe gives off electromagnetic waves of some kind, so with an appropriate kind of telescope we can observe and even make images of the objects producing them: but there is a problem, our atmosphere.
Our atmosphere is of critical importance to us: we breathe it, and the greenhouse effect makes our planet warm enough to live on; otherwise it would hover around –24 C. It also filters out ultraviolet and other harmful radiations. It blocks everything other than visible light and radio waves. So we can do visible light and radio astronomy from the ground; everything else we have to observe from above the atmosphere, using spaceborne telescopes. There are some very long-wavelength cosmic radio emissions that don’t even penetrate to the Inner Solar System. We only know about them from space probes that have penetrated out beyond the planets, into interstellar space. It will be a while before we have the capability to study those radio emissions, which will have their own unique things to tell us about the universe.
This is my 1250th weekly article, and my last for 2015. This is thanks to the steady stream of new discoveries and ideas coming out of astronomy. Happy New Year!
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