How Radio Waves Help Us Understand the Universe

radio telescopes
The Karl Jansky Very Large Array of radio telescopes is located near Socorro, New Mexico. This array focuses on radio emissions from a variety of objects and processes in the sky. NRAO/AUI

There's more to the universe than the visible light that streams from stars, planets, nebulae, and galaxies. These objects and events in the universe also give off other forms of radiation, including radio emissions. Those natural signals fill in the whole story of how and why objects in the universe behave as they do.

Tech Talk: Radio Waves in Astronomy

Radio waves are electromagnetic waves (light) with wavelengths between 1 millimeter (one thousandth of a meter) and 100 kilometers (one kilometer is equal to one thousand meters).

In terms of frequency, this is equivalent to 300 Gigahertz (one Gigahertz is equal to one billion Hertz) and 3 kilohertz. A Hertz is a commonly used unit of frequency measurement. One Hertz is equal to one cycle of frequency.

Sources of Radio Waves in the Universe

Radio waves usually are emitted by energetic objects and activities in the universe. Our Sun is the closest source of radio emissions beyond Earth. Jupiter also emits radio waves, as do events occurring at Saturn.

One of the most powerful sources of radio emission outside of our solar system, and indeed our galaxy, comes from active galaxies (AGN). These dynamic objects are powered by supermassive black holes at their cores. Additionally, these black hole engines will create massive jets and lobes that glow brightly in the radio. These lobes, which have earned the name Radio Lobes, can in some bases outshine the entire host galaxy.

Pulsars, or rotating neutron stars, are also strong sources of radio waves. These strong, compact objects are created when massive stars die as supernovae. They're second only to black holes in terms of ultimate density. With powerful magnetic fields and fast rotation rates these objects emit a broad spectrum of radiation, and their radio emissions are particularly strong.

Like supermassive black holes, powerful radio jets are created, emanating from the magnetic poles or the spinning neutron star.

In fact, most pulsars are usually referred to as "radio pulsars" because of their strong radio emission. (Recently, the Fermi Gamma-ray Space Telescope characterized a new breed of pulsars that appears strongest in gamma-ray instead of the more common radio.)

And supernova remnants themselves can be particularly strong emitters of radio waves. The crab nebula is famous for the radio "shell" that encapsulates the inner pulsar wind.

Radio Astronomy

Radio astronomy is the study of objects and processes in space that emit radio frequencies. Every source detected to date is a naturally occurring one. The emissions are picked up here on earth by radio telescopes. These are large instruments, as it is necessary for the detector area to be larger than the detectable wavelengths. Since radio waves can be larger than a meter (sometimes much larger), the scopes are typically in excess of several meters (sometimes 30 feet across or more).

The larger the collection area is, compared to the wave size, the better the angular resolution a radio telescope has. (Angular resolution is a measure of how close two small objects can be before they are indistinguishable.)

Radio Interferometry

Since radio waves can have very long wavelengths, standard radio telescopes need to be very large in order to obtain any sort of precision. But since building stadium size radio telescopes can be cost prohibitive (especially if you want them to have any steering capability at all), another technique is needed to achieve the desired results.

Developed in the mid 1940s, radio interferometry aims to achieve the kind of angular resolution that would come from incredibly large dishes without the expense. Astronomers achieve this by using multiple detectors in parallel with each other. Each one studies the same object at the same time as the others.

Working together, these telescopes effectively act like one giant telescope the size of the whole group of detectors together. For example the Very Large Baseline Array has detectors 8,000 miles apart.

Ideally, an array of many radio telescopes at different separation distances would work together to optimize the effective size of the collection area as well improve the resolution of the instrument.

With the creation of advanced communication and timing technologies it has become possible to use telescopes that exist at great distances from each other (from various points around the glob and even in orbit around the Earth). Known as Very Long Baseline Interferometry (VLBI), this technique significantly improves the capabilities of individual radio telescopes and allows researchers to probe some of the most dynamic objects in the universe.

Radio's Relationship to Microwave Radiation

The radio wave band also overlaps with the microwave band (1 millimeter to 1 meter). In fact, what is commonly called radio astronomy, is really microwave astronomy, although some radio instruments do detect wavelengths much beyond 1 meter.

This is a source of confusion as some publications will list the microwave band and radio bands separately, while others will simply use the term "radio" to include both the classical radio band and the microwave band.

Edited and updated by Carolyn Collins Petersen.