By Joshua N. Winn, Princeton University
Until the 19th century, all astronomy was confined to the optical band. William Herschel had discovered infrared radiation, and Johann Ritter discovered ultraviolet. Then in the 1860s, James Clerk Maxwell revealed light to be an example of electromagnetic waves, which could have any wavelength, in principle, and it became clear that there were unexplored orders of magnitude on either side of the optical band. The truth is very few astronomers anticipated radio astronomy or X-ray astronomy.
The Sun’s spectrum peaks in the optical range, as do the spectra of other stars, if you include infrared and ultraviolet. And what else is there, besides stars? Many astronomers thought that those far-flung regions of the electromagnetic spectrum were probably dark wastelands.
One way to see their point is to ask: What would be the temperature of an object whose spectrum peaks at a given wavelength? For that, we use Wien’s law: the wavelength at the peak of the Planck spectrum is about one-fifth of hc over kT. Solving for T gives hc over 5k times lambda-peak.
In the optical range, the temperatures are of order 5000 Kelvin, and that’s where we find stars. The Sun’s outer layers are at 5800 Kelvin. The reddest, coolest stars are a few thousand Kelvin, and the bluest, hottest ones are around 30,000 Kelvin.
So, to produce X-rays with a photon energy of 1000 electron volts, you’d need a temperature of 2,000,000° Kelvin. But, what would be that hot?
And, at the other extreme, for the peak to be in the radio range, at, say, a gigahertz, the temperature would be about 0.01 Kelvin, which seemed equally impossible. At such low temperatures, the total flux, which is proportional to T to the 4th, would be pathetically low, so you wouldn’t be able to detect something so cold.
This article comes directly from content in the video series Introduction to Astrophysics. Watch it now, on Wondrium.
Radio Astronomy: A Product of Electrical Engineering?
Now, everybody knew at the time there is more to life than just the Planck spectrum. The Sun, for example, emits strong, non-thermal radio waves due to magnetic activity at its surface. But even so, the Sun is a very feeble radio source.
If there were a twin of the Sun 10 parsecs away, we’d barely be able to detect it at 1 GHz, even with a modern radio telescope. That’s why radio astronomy did not originate with astronomers. It grew out of electrical engineering.
Karl Jansky, a radio engineer at the Bell Telephone Laboratories, was the first person to detect the radio static coming from astronomical sources, in 1931. And for nearly a decade after that, the world’s leading radio astronomer—essentially the only radio astronomer—was Grote Reber, an engineer who built his own backyard radio telescope, as a hobby. He had a grand old time mapping all the brightest radio sources in the sky. He was the first person to see the radio constellations, so to speak.
Longer Wavelengths in Radio Astronomy
What distinguishes radio astronomy from the other types is that the wavelengths are longer. This has some important implications.
First, it’s easier to build a focusing mirror. As a rule of thumb, the mirror needs to be polished with an accuracy of at least a 10th of a wavelength, that is, it needs to conform to the shape of a parabola or whatever surface has been chosen to within lambda over 10. That is a whole lot easier when lambda is a meter, than when it’s a millionth of a meter.
That’s why Grote Reber could make a 9 meter radio dish as a hobby, whereas an optical telescope that big costs tens of millions of dollars and takes years to build. That’s also why the world’s largest telescopes are radio telescopes. The big dish at Arecibo, in Puerto Rico, is 305 meters across. And in 2016, China completed the construction of a dish 500 meters across.
The second implication of the long wavelengths of radio waves is that their wave nature is much more conspicuous than their particle nature. In fact, nobody ever calls them radio photons; we always say radio waves. We can use purely wave-based methods to detect, amplify, and to combine radio waves. It’s not really like counting photons, it’s more like following the rise and fall of ocean waves.
Findings of the Radio Telescopes
The radio telescopes can reveal wonders. For example, if we have an optical image of a galaxy in the constellation Hercules, the galaxy appears to be a blob of starlight at the center of the image. Now let’s overlay an image from the Very Large Array, or VLA, in New Mexico (an interferometer with 27 radio dishes) at radio wavelengths, which we will color red.
What we see is, there are jets of intense radio waves shooting out in opposite directions, and then they billow out into giant clouds many kiloparsecs away from the galaxy itself. What is going on here?
What’s happening is there’s a vortex of gas surrounding a giant black hole in the center of that galaxy. The gas is getting swallowed up and disappearing into the black hole, but some of the gravitational potential energy that’s released when the gas falls from far away ends up getting thrown outward in both directions at nearly the speed of light. It’s probably a consequence of the rapid spin of the black hole.
These jets might be aligned with the rotation axis of the black hole. The jets consist of streams of charged particles that are whirling in circles because of a strong magnetic field that threads through the jet. And remember, when charged particles accelerate, they radiate. In this case a lot of the radiation comes out at radio wavelengths.
Common Questions about the Discovery of Radio Astronomy
William Herschel discovered infrared radiation, and Johann Ritter discovered ultraviolet rays.
Karl Jansky, a radio engineer at the Bell Telephone Laboratories, was the first person to detect the radio static coming from astronomical sources in 1931.
What distinguishes radio astronomy from the other types is that the wavelengths are long. With longer wavelengths, it gets easier to build a focusing mirror and their wave nature is much more conspicuous than their particle nature.