A “Jansky” is a unit for quantifying the intensity of radio waves coming from deep space.
The unit gets its name from Karl Jansky, a physicist who worked for Bell Laboratories in the first half of the last century. At the time, making international phone calls was a tricky business. If you were in New York and wanted to call someone in London, there was no way to do so. It’s not possible to string cable on poles across an ocean and burying a cable under the Atlantic was not feasible given the technological and financial hurdles of the era. Bell Labs was interested in solving this problem by using radio signals to transmit telephone conversations across the ocean.
The first transatlantic radio communications were notoriously unreliable. They were plagued with all manner of noises: clicks, pops, and hissing. Jansky was assigned the project of identifying the source of this interference. To do so, he built a large antenna near the laboratory’s home in New Jersey. The largest of its type at the time, it sat on a large turntable which allowed Jansky to pivot the antenna and roughly pinpoint the direction of any radio signals he picked up. In the course of his investigation, he identified a few primary sources of noise. A couple came from thunderstorms, which showed up in telephone conversations as loud, erratic clicks and bangs. The other was a steady hiss, the origin of which was not clear.
The intensity of the noise varied. It was stronger during the day and weaker at night. Using the directional capabilities of his antenna, Janksy noticed that the hiss appeared to be coming from the Sun. Could the Sun itself be the source of radio noise? The idea that anything from space would interfere with long distance telephone calls had not yet been considered!
More careful monitoring of this mysterious hiss revealed that it did not cycle every 24 hours exactly. The noise instead peaked every 23 hours and 56 minutes, just four minutes shorter than a solar day. While not much of a difference, those missing four minutes meant the Sun could not be the source of the noise. So what is the significance of this shorter time interval?
Jansky was not an astronomer. He was a radio engineer. For that reason, 23 hours and 56 minutes didn’t mean anything to him. But to an astronomer, it is a very important duration: it’s how long it takes the Earth to complete one rotation or what astronomers call a “sidereal day“. It’s a day measured relative to the distant stars. The reason that this is a bit shorter than the 24 hour day – or solar day – that most people know has to do with the Earth’s orbital motion around the Sun. The solar day tracks how long it takes the Sun to return to the same point on the sky (more or less). After 23 hours and 56 minutes, the Earth has completed one rotation about its axis, but it has also in that time moved about one degree around its orbit. The Sun is therefore in a slightly different part of the sky; it is a hair east of where it was 24 hours earlier. We have to wait an additional four minutes for the Sun to get repositioned. The solar day takes into account both the rotation and orbital motion of the Earth. The sidereal day tracks our planet’s rotation only.
If Jansky’s antenna kept picking up the signal every 23 hours and 56 minutes, then the source of the radio waves was coming from outside our Solar System – from somewhere out in the Galaxy! Jansky was able to identify that the source of the signal was coming from the summer constellation of Sagittarius. After consulting some star atlases, he learned that Sagittarius is a special place on the sky; it contains the center of the Milky Way Galaxy. It became clear that Jansky’s noise was radio emission in the Galactic Center some 28,000 light years from New Jersey.
With this discovery, Karl Jansky accidentally gave birth to the science of radio astronomy. In his honor, the unit that radio astronomers use to quantify the intensity of cosmic radio sources is named after him. One Jansky is equal to 10 to the -26 Watts per meter squared per Hertz.
Those are probably strange sounding units. “Watts” are no different than the watts used to describe the brightness of a lightbulb or how much electricity your refrigator consumes. It’s a measure of power, or how much energy you use or receive every second.
“Meter squared” tells you how much power you’re collecting in one square meter of area. If you were to point two differently sized radio telescopes at the sky, the bigger telescope would collect more energy than the smaller one in the same amount of time. This is no different than placing a small cup and a large bucket outside in a rain storm for a minute, bringing them back inside, and measuring how much water each captured. The bucket, being bigger, will naturally hold more water. But if you were using both to figure out how much rain fell, you’d need to account for the different bucket sizes. The same goes for radio telescopes: you need to divide how much power you receive by the area of the receiving dish. This gives you a measurement of Watts per meter squared.
A more subtle difference is the frequency range over which the telescope is capable of measuring. A few weeks ago, while talking about Fraunhofer lines, we talked about how light can be thought of as a wave and that any light source emits waves at many different wavelengths or frequencies. For visible light, our eyes interpret these different frequencies as colors. Radio telescopes are only sensitive to a specific range of frequencies, much like our ears can only hear certain frequencies of sound. The range of frequencies is called the telescope’s “bandwidth”. You might already know this word from thinking about how fast your internet connection is. Here it means something a little different: it’s the difference between the highest and lowest frequency a radio telescope can detect. For example, if a telescope can only record frequencies between 10 and 30 Hertz, then it’s bandwidth is 20 Hertz.
But say you had another telescope that had a bandwidth of 25 Hertz. If you pointed both telescopes at the same star, which would collect more energy? Well, just like having a larger collecting area will increase the power you measure, so will having a larger bandwidth. Therefore, astronomers need to divide by that as well. This gives us units of Watts per meter squared per Hertz – a measurement of what’s called the “spectral flux density”. Measuring things this way nicely accounts for variations in size and bandwidth of different telescopes. Astronomers then multiply by 10 to the 26 solely for convenience because the numbers from astronomical radio sources are extremely small. To give you an idea of how small, a cell phone sitting on the Moon would be third brightest radio source in the sky (after the Sun and a supernova remnant in the constellation Cassiopeia). To keep things simpler, these units and numbers are repackaged into a single unit: the Jansky.
Karl Janksy never did get to pursue his findings further. Bell Labs wasn’t interested in following up, especially once it was determined that the extraterrestrial radio signal wouldn’t significantly impact the quality of transatlantic radio communications. Karl moved on to other projects, but other individuals took up the torch. In 1937, amateur astronomer Grote Reber built a radio telescope in his backyard and, after duplicating Jansky’s discovery, made the first survey of the entire sky at radio wavelengths. Later generations of radio telescopes would go on to probe the hearts of galaxies, investigate the remains of supernovae, map the surfaces of planets, see the fading glow of the Big Bang, and even hunt for signals from alien life. Jansky never got to see how the field he accidentally started would mature. He died young, in 1950, at the age of 44 from a heart condition. But he lives on in the numbers we use to map the radio Universe and unravel cosmic mysteries unimagined during his lifetime.