We have all heard of radio and the terms long wave, short wave and VHF (Very High Frequency). These refer to the wavelength itself. (See diagram below.) Radio waves are electromagnetic waves along with microwaves, infrared, visible light, ultra violet, x-rays and gamma rays. Radio waves have the longest wavelength in the (EM) electromagnetic spectrum.
Light is made up of tiny particles called photons. Photons in visible light have a medium amount of energy. When photons have a little bit more energy, they become ultraviolet radiation, which you cannot see but which can give you a sunburn. With more energy than that, photons become X-rays, which travel right through you. If photons possess even more energy, they become gamma rays, which come out of stars when they explode.
But when photons have a little less energy than visible-light photons, they are known as infrared radiation. You can feel them as heat. Finally, we call the photons with the least energy “radio waves.” Radio waves come from strange spots in space – the coldest and oldest places and the stars with the most material stuffed into a small space. Radio waves tell us about parts of the universe we would not even know existed if we only used our eyes or telescopes that see visible photons.
Between 1885 and 1889 German physicist Heinrich Hertz produced electromagnetic waves in his laboratory, measuring their length and velocity. It is now possible to tune a radio to a specific wavelength or frequency. The speaker converts these mechanical vibrations to sounds.
The basic building block of radio communications is a radio wave. Like waves on a pond, a radio wave is a series of repeating peaks and valleys. The entire pattern of a wave, before it repeats itself, is called a cycle. The wavelength is the distance a wave takes to complete one cycle. The number of cycles, or times that a wave repeats in a second, is called frequency. Frequency is measured in the unit hertz (Hz), referring to a number of cycles per second. One thousand hertz is referred to as a kilohertz (KHz), 1 million hertz as a megahertz (MHz), and 1 billion hertz as a gigahertz (GHz). The range of the radio spectrum is considered to be 3 kilohertz up to 300 gigahertz.
A radio wave is generated by a transmitter and then detected by a receiver. An antenna allows a radio transmitter to send energy into space and a receiver to pick up energy from space. Transmitters and receivers are typically designed to operate over a limited range of frequencies. – Thuy Mai
The most powerful natural source of Extremely Low Frequency / Very Long Frequency waves on Earth is lightning. Waves produced by lightning strikes can bounce back and forth between the Earth and the ionosphere, so they can travel around the world. Radio waves are also produced by artificial sources, including electrical generators, power lines, appliances and radio transmitters. So how can we hear astronomical objects? When astronomical objects have changing gravity waves they produce radio waves. The sun, Earth, Jupiter and billions of other astronomical objects have these gravitational fields, hence they produce radio waves which are transmitted through the universe.
Although few “adult” stars emit enough radio energy to be detected, the birth and death of stars is of interest to radio astronomers. Space is not an empty void and a great deal of matter, in the form of clouds of gas and dust, is strewn between the stars. Portions of these clouds that are large enough and dense enough may begin to condense, contracting into protostars. Within the protostar increasing temperature and pressure eventually ignites the nuclear furnace in the core – and a new star is born. Radio waves pass easily through the clouds, providing radio astronomers with an opportunity to study the events leading to star formation.
After some stars reach the end of their lives burning nuclear fuel, they collapse and explode in one of the most spectacular events witnessed by astronomers – supernova explosions. These produce churning clouds of stellar matter sometimes seen by optical astronomers as wisps of glowing gas. Many are invisible, but radio telescopes can portray their extent and characteristics from emissions generated by excited atoms within them. From these studies, astronomers can determine a star’s makeup before it collapsed, the forces involved in the explosions, and how matter reacts under such stresses.