Our eyes are insensitive to electromagnetic radiation whose wavelength is shorter than 4x10-5 cm (Ultraviolet, X-rays and Gamma rays), and longer than 7x10-5 cm (Infrared, Microwave and Radio). While this radiation is invisible to us, we have experience with it in our day-to-day lives: doctors use X-rays to take pictures of the bones in our bodies; we use microwave ovens to cook with, and infrared goggles to give us "night vision"; Ultraviolet light from the Sun is responsible for some pretty uncomfortable sunburns if we don't wear our sunscreen. Even though our experience with low frequency radiation such as radio waves or microwaves is dramatically different from our experience with X-rays, all electromagnetic radiation is fundamentally the same. All electromagnetic radiation travels at the speed of light.
ALL objects emit electromagnetic radiation! This radiation is spread over some region of the electromagnetic spectrum. The energy of any object peaks at a wavelength which is dependent on the temperature of the object. Cooler objects emit most of their energy at lower energies, while the hottest objects emit most of their energy at the highest frequencies. The Sun emits most of its energy in the visible portion of the spectrum, as do all objects with similar temperatures (6000 degrees Kelvin). The Earth's atmosphere lets in visible light, but effectively blocks much of the harmful higher energy radiation, and much of the infrared. The atmosphere allows radio waves to pass through it and reach the surface of the Earth. Still, it wasn't until the 1950s that astronomers began to seriously study the universe at radio wavelengths.
In astronomy, the only way we obtain information about objects in the sky is by collecting and studying the electromagnetic radiation they emit. Until someone thought of looking at the sky in a new way, by collecting radio waves from it, no one knew if or how objects emitted radio waves.
In 1931, a researcher at Bell Labs, Karl Jansky, was interested in finding the source of shortwave radio interference. This noise affected long-range radio communications. The purpose of this project was practical: how could radio communications be improved with a better understanding of this noise? But what he discovered as a result of building a special radio detector, and taking measurements that spanned years, would profoundly affect our picture of the universe. Jansky found no source on Earth for the constant hiss, but he did notice that the time at which this noise was strongest was four minutes earlier from one day to the next. He knew that stars in the sky also rise four minutes earlier each day. This is known as a sidereal day. By linking the peak of the noise to the rising times of stars in the sky, Jansky thought a celestial source must be responsible for the hiss. He eventually realized the radio waves came from the center of our Milky Way galaxy. It took about 10 years from the time of Jansky's discovery for astronomers to understand that interstellar gas would emit observable radio radiation. Once this was known, the idea of studying the universe in the radio was born. With Jansky's accidental discovery of a cosmic source of radio waves, our view of the Universe suddenly grew, from the relatively limited visible window into the radio.
Radio telescopes are often quite large. The largest, Arecebo, in Puerto Rico, is a radio observatory built right into a hillside, and is about 300 meters across. Because of its size it can detect long wavelength radio waves. Radio waves are detectable day and night, since the Sun is not a very strong source of radio waves when it is up during the day. Radio waves are longer than raindrops and snowflakes, and many pass right through storm clouds to be detected on Earth even during the murkiest days and nights.
An early technical difficulty of radio astronomy was resolution. Since radio waves are so long, it is difficult to accurately determine where in the sky they come from. Only with very large dishes could accurate position measurements be made. A clever way around this obstacle is to combine observations of a single object taken at the same time at two or more radio dishes. This is called interferometry. The effect is that the resolving power increases as if the dish were the diameter of the separation of the dishes. The further apart the dishes are, the higher the resolution. With the Very Large Baseline Array (VLBA) in which telescopes spanning the continental US, Alaska, and Hawaii observe together, we can achieve resolution about 100 times better than the best optical telescope in space (HST).
Some sources in the sky give off lots of visible radiation and lots of radio. Others give off mostly one or the other. Very "loud" radio sources may not have a bright optical counterpart, thus studying the sky in the radio has introduced us to new sources in the sky, invisible to the naked eye.
Distant galaxies are also sometimes relatively strong sources of radio waves. Because of what we know about the source of radio waves at the center of the Milky Way, we can speculate that these "radio loud" galaxies also harbor supermassive black holes at their centers. Material falling into the black hole is accelerated to such a high speed that it gives off intense radio radiation. (This is known as synchrotron radiation.)
Most of the matter in the Milky Way is cool hydrogen gas. Because it is cool it gives off no visible radiation. It is composed of hydrogen atoms, which in turn are composed of one proton in the nucleus, with one electron in orbit about it. Protons and electrons can be thought of as spinning like a top, with one main difference. The proton and electron in a hydrogen atom can spin in only two opposite directions. They either align, so both spin in the same direction, or they spin each in the opposite direction from the other. This slight difference in the orientation of the two subatomic particles has a very slight difference in energy. When an electron in a hydrogen atom "flips" its spin from the higher to the lower energy state, it emits a very low energy photon with a wavelength of 21 centimeters.
Cool clouds of gas are located in the galaxy very near star-forming regions. In fact, it is these clouds of hydrogen that compress to form stars, so we expect to find cool hydrogen near areas where stars are forming. Stars are constantly forming in the spiral arms of our galaxy. Gas and dust between the stars absorbs much of the visible radiation, but 21 cm radiation passes through gas and dust virtually undisturbed. This means that we can see much further out into the Milky Way when we look at radio wavelengths.
Astronomers can look out into the sky in any given direction, and detect many clouds of cool hydrogen emitting 21 centimeter radiation. By measuring the Doppler shift of the 21 cm line, the speeds of these clouds can be measured. By combining radio observations of the 21 cm radiation from many different directions, a map of the spiral structure of our galaxy can be formed. This is one way that our picture of the Milky Way, and our place in it, is determined from our position within the galaxy.
The first observations of pulsars, neutron stars with high magnetic fields, spinning rapidly and firing off high energy radiation, were made in the radio. When electrons are accelerated to very high speeds, which occurs near the intense magnetic fields in pulsars, they give off lots of radio waves. The discovery of pulsars proved the theoretical prediction of very dense objects resulting from the death of massive stars.
In the 1960's a very important observation was made, again at Bell Labs, that would have a profound effect on our interpretation about the birth of the Universe. Penzias and Wilson, again searching for the source of noise that had no known origin, discovered that the universe as a whole was emitting weak microwave radiation in every direction! In other words, even empty space is radiating, with a temperature of about 3 degrees above absolute zero. Earlier theorists had predicted that if the universe started out much hotter and was expanding as the result of an initial explosion, the fingerprints of that event would be a very low temperature radiation in all directions of the sky. The COsmic Background Explorer (COBE) was launched in 1989 to take detailed measurements of this radiation, and we have learned much about the universe and the Earth from COBE's results. The spectrum of the universe is just as predicted by a Big Bang beginning, with a temperature of 2.74 degrees. By studying the slight variations from perfect homogeneous distribution, we can accurately measure our speed and direction in the universe (we are racing toward the Virgo cluster). On an even smaller scale, the seeds of formation of galaxies are also evident in this radiation.
There truly is a Universe in the Radio, one which we have only very recently begun to observe and understand. By observing the sky in the radio, we gain new insight into the galaxy, discover new objects like pulsars, "see" the invisible clouds of cool hydrogen, watch the violent turbulence at the centers of distant galaxies, and uncover a mind-bending piece of information about the beginning of the universe. In this century the Cosmic Radio Show has just begun. It is an exciting and amazing time to be alive, watching as the plot continues to unfold.