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

Humans perceive the universe using visible light that we can see with our eyes. Yet, there's more to the cosmos than what we see using 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 an important part of the cosmic of how and why objects in the universe behave as they do.

Tech Talk: Radio Waves in Astronomy

Radio waves are electromagnetic waves (light), but we can't see them. They have 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 (abbreviated as Hz) is a commonly used unit of frequency measurement. One Hertz is equal to one cycle of frequency. So, a 1-Hz signal is one cycle per second. Most cosmic objects emit signals at hundreds to billions of cycles per second.

People often confuse "radio" emissions with something that people can hear. That's largely because we use radios for communication and entertainment. But, humans do not "hear" radio frequencies from cosmic objects. Our ears can sense frequencies from 20 Hz to 16,000 Hz (16 KHz). Most cosmic objects emit at Megahertz frequencies, which is much higher than the ear hears. This is why radio astronomy (along with x-ray, ultraviolet, and infrared) is often thought to reveal an "invisible" universe that we can neither see nor hear.

Sources of Radio Waves in the Universe

Radio waves usually are emitted by energetic objects and activities in the universe. The  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 the solar system, and beyond the Milky Way 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 of material that glow brightly with radio emissions. These can often outshine the entire galaxy in radio frequencies.

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

Many pulsars are referred to as "radio pulsars" because of their strong radio emission. In fact, data from the Fermi Gamma-ray Space Telescope showed evidence of a new breed of pulsars that appears strongest in gamma-rays instead of the more common radio. The process of their creation remains the same, but their emissions tell us more about the energy involved in each type of object. 

Supernova remnants themselves can be particularly strong emitters of radio waves. The Crab Nebula is famous for its radio signals that alerted astronomer Jocelyn Bell to its existence. 

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). Some wavelengths can be as large as a mountain, and so astronomers have built extended arrays of radio telescopes. 

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 globe 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.