INVISIBLE ASTRONOMIES

Infrared Astronomy

Radio Astronomy

Utraviolet Astronomy

RADIATIONS FROM SPACE

An innovation as crucial as the invention of the telescope swept astronomy in the late 20^th century. New technology enabled astronomers to tune in to all the radiation coming from objects in space – and not simply light. Capturing light tells only part of the story. It is like hearing a single note from a melody: to experience the music fully you need to listen to all the notes, from the highest to the lowest. Light forms just one part of a whole range of electromagnetic radiation. Tuning in to invisible waves of energy, such as radio waves and X – rays, reveals a startlingly different picture of the Universe.

BEYOND OUR ATMOSPHERE

Stars, galaxies, and other objects in space all give off electromagnetic radiation. Whether it is in the form of light or radio waves, it consists of a stream of vibrating electric and magnetic fields spreading outwards. Travelling at 300,000 km/s (the speed of light), this radiation may travel thousands or even millions of light years towards us, but most is then absorbed by Earth’s atmosphere. Invisible astronomy has only come of age since scientists have been able to intercept radiation in space.

 ELECTROMAGNETIC SPECTRUM

All radiation moves like a wave at sea, and the distance between the crests of the waves is known as the wavelength. Different radiations are distinguished by different wavelengths: those with the shortest wavelengths have the highest frequencies (number of waves per second) and carry the most energy.

INFRARED ASTRONOMY

If our eyes were sensitive to infrared, or heat, radiation the night sky would appear quite different. It would be filled with glowing cosmic clouds and scattered, distant galaxies ablaze with newborn stars. We would be able to pick out young stars and the centre of our Galaxy, which are normally hidden by tiny grains of dust in space – infrared can travel straight through interstellar dust. Everything in the Universe cooler than normal stars (around 3,000°C) emits infrared. By using infrared telescopes, astronomers can reveal information invisible to the optical telescope.

INFRARED WAVELENGTHS

As its name suggests, infrared lies just beyond the red end of the visible spectrum. It covers a much wider part of the electromagnetic spectrum than visible light: from 700 nanometres (billionths of a millimetre) to 1 millimetre, where radio waves begin. Astronomers divide infrared into four bands: near, mid-, and far infrared, and submillimetre waves. Observing infrared radiation is always a struggle within Earth’s atmosphere, where carbon dioxide and water vapour absorb infrared. Some of the shorter and longer wavelengths, though, do reach mountain tops.

By tuning in to radio waves from space, astronomers have discovered many of the most energetic objects and most explosive events in the universe. These include the remains of supernovas, magnetic whirlpools around supermassive black holes, and even the radiation from the big bang in which the universe was born. Radio telescopes can also track down molecules in space, the raw material of new planets and life. No one is allowed to broadcast at the wavelengths used to study the universe. Even so, radio telescopes increasingly suffer from radio pollution – from mobile phones, for example.

Radio waves have the longest wavelengths of any electromagnetic radiation, covering all wavelengths longer than 1 millimetre. Most radio waves can penetrate the atmosphere down to the earth’s surface, although radio waves longer than 100 m are reflected back into space by the ionosphere, a layer at the top of the atmosphere. Scientists often refer to radio waves by frequency – the number of waves that pass every second. The shorter the wavelength, the higher the frequency.

To track down the hottest stars – 50 times hotter than the sun – astronomers must use ultraviolet radiation. A star that is hotter than 10,000°C shines most brightly at ultraviolet wavelengths. Ultraviolet can also reveal what is in the hot, invisible gas clouds between the stars. Ozone in earth’s atmosphere, however, makes observing difficult. In everyday life, the ozone layer protects us from the sun’s ultraviolet radiation, and we worry about the ozone hole, but the ozone layer blocks astronomers’ view of sources of ultraviolet radiation in the universe.