Karl Jansky: Father of Radio Astronomy


By Emily LevesqueUniversity of Washington

When Karl Jansky first discovered “star noise”, he had trouble getting astronomers to pay attention. Today, more than 80 years later, radio astronomy is an immense field that has given rise to enormous radio dishes, planet-wide arrays of radio telescopes, and some of the most exciting scientific discoveries of the last century.

Jansky’s radio antenna
Karl Jansky’s antenna was enormous, 100 feet across and 20 feet tall. (Jarek Tuszyński/Public domain)

A Great Discovery at the Wrong Time

Jansky wasn’t an astronomer himself; he was a physicist and engineer at Bell Labs in New Jersey, working on designing a new antenna that could detect long-wavelength radio waves. His antenna was enormous, 100 feet across and 20 feet tall.

When Jansky began using it, he quickly discerned that he could detect static produced by thunderstorms and something else, a faint but constant hiss in his data. That “something else” ultimately proved to be radio waves emitted by the center of the Milky Way, a discovery he reported in 1933. 

Image of Grote Reber
Grote Reber was the first to build a radio telescope after Jensky’s discovery built the foundation for radio astronomy. (Image: NRAO/Public domain)

It was a phenomenal finding; the first clear detection of radio emission from space. Still, he had trouble obtaining support to explore his discovery further. The country was in the midst of the Great Depression, astronomers didn’t work with radio detectors, and they were hesitant to sink limited funds into a brand-new endeavor. Bell Labs also couldn’t justify funding a project that wasn’t directly related to their main goal of long-distance communication.

It wasn’t until several years later that an engineer named Grote Reber built the first dedicated radio telescope, a parabolic 31-foot dish in his backyard, and began surveying the sky for signs of astronomical radio waves.

The Electromagnetic Spectrum

Radio waves lie at one extreme of the electromagnetic spectrum, with low energies and long wavelengths. Most of the telescopes utilize visible light, with wavelengths of only a few hundred nanometers. 

By comparison, the shortest radio wavelengths are a millimeter and the longest wavelengths that we can detect are tens of thousands of kilometers. As we move from one end of the electromagnetic spectrum to the other, we can use different wavelengths of light to probe different physical phenomena.

This article comes directly from content in the video series Great Heroes and Discoveries of AstronomyWatch it now, on Wondrium.

Centre of the Milky Way

When Karl Jansky first detected radio waves with his Bell Labs antenna in the early 1930s and pinpointed their location, he thought he was detecting radiation from the Sun. However, as he continued to follow the signal for months, he noticed that the most intense source of this signal was sliding away from the Sun’s location, following the motion of background stars across the sky rather than the motion of the Sun. 

He compared his signal’s location with astronomical maps and realized that his signal was coming from the Milky Way itself and was strongest near the constellation Sagittarius, which lay in the direction of the Milky Way’s center.

Jansky didn’t realize it at the time, but he was detecting a signature of our Milky Way’s magnetic field. Our galaxy’s magnetic field is thousands of times weaker than our own planet’s, but astronomers today believe that it impacts how stars form in our galaxy along with the flux of strange particles known as cosmic rays.

Radio Light from Cosmic Rays

Cosmic rays are high-energy protons, atomic nuclei, and solitary electrons, streaking through space at close to the speed of light—even today astronomers still don’t fully understand where cosmic rays come from. Still, we do know that when cosmic ray electrons travel through our Milky Way, they can be slowed down a bit as they follow the curves of our galaxy’s magnetic field lines. 

Image of Karl Jansky
Karl Jansky had initial trouble gaining the attention of astronomers to his discovery. (Image: Saber1983/Public domain)

To slow down, these electrons must release a small amount of energy in the form of low-energy light: radio light. This radio light was what Jansky detected, emitting from the entire Milky Way and appearing the most intense (or brightest) near the Galactic Center, where the magnetic field is the strongest.

Jansky’s Legacy in Radio Astronomy

In the 1930s, Jansky’s discovery was met with scientific curiosity but not much else. His antenna was the only one of its kind at the time; observatories weren’t building radio antennae, and the very idea of a radio telescope was foreign to most astronomers. 

The antenna itself also looked decidedly unlike a telescope; it looked more like a gigantic version of the radio aerials you may have seen on cars, an ungainly array of straight-line antennae arranged in a series of squares. A crucial decision made by Jansky when designing his antenna was to mount it on a set of Ford Model T tires, allowing him to spin the antenna in a complete circle. 

This mounting led his colleagues to nickname the antenna “Jansky’s merry-go-round”. But ridicule aside, it allowed him to point the detector wherever he wanted and hone in on precisely where a signal was coming from—including the center of the galaxy. It was effective, and the discovery was intriguing, but still observatories weren’t exactly lining up to build more antennae.

Today, Karl Jansky is recognized as the founder of radio astronomy. The National Radio Astronomy Observatory grants a postdoctoral research fellowship named in his honor, and even the physical unit used to describe brightness in radio astronomy is named after him: the Jansky.

Common Questions about Karl Jansky and Radio Astronomy

Q: Why did Jansky have trouble finding support for his initial finding?

Though Jansky’s discovery later turned out to be an important step in the establishment of radio astronomy, he himself wasn’t an astronomer. The country was in the midst of a Great Depression and astronomers weren’t willing to sink their limited funds into an endeavor they were not accustomed to. Also, Bell Labs wasn’t willing to fund a project not related to their goals.

Q: What are cosmic rays?

Scientists still don’t understand where cosmic rays exactly come from. They are high-energy protons, atomic nuclei, and solitary electrons that travel in space at speeds close to the speed of light.

Q: How did Karl Jansky influence the field of radio astronomy?

Karl Jansky is recognized as the founder of radio astronomy. Though his initial discovery wasn’t met with a lot of enthusiasm from the astronomy community, he was the first to discover radio waves coming from the center of the Milky Way. Even today, a unit in radio astronomy is named after him.

Keep Reading
Henrietta Leavitt and the Discovery of Stellar Distances
Cepheids and the Standard Candle Technique
The Great Debate over the Spiral Nebulae