The Discovery of Cosmic Radio Noise

Cosmic radio emission was discovered by accident in the 1930s by a physicist working as a radio engineer.  Why didn't "real" astronomers discover radio astronomy? In part because they knew too much.  Stars are approximately blackbody radiators at visible wavelengths. The spectral brightness  at frequency $\nu$ of an ideal blackbody radiator is given by Planck's Law
$$B_\nu(T) \equiv I_\nu(T) = {2 h \nu^3 \over c^2} \biggl({ 1 \over e^{h\nu \over kT} - 1} \biggr) , $$
where
$h \approx 6.63 \times 10^{-27}$ erg s $ = 6.63 \times 10^{-34}$ Joule s $ =$ Planck's constant,
$\nu=$ frequency (Hz = s$^{-1}$),
$k \approx 1.38 \times 10^{-16}$ erg K$^{-1} = 1.38 \times 10^{-23}$ Joule K$^{-1} =$ Boltzmann's constant,
$c \approx 3.00 \times 10^{10}$ cm s$^{-1} = 3.00 \times 10^8$ m s$^{-1} =$ the speed of light, and
$T$ is the absolute temperature (K) of the black body.

In the low-frequency radio limit, the dimensionless quantity $h \nu / (kT) \ll 1$ for most astronomical sources.  For example, the photosphere of the Sun has temperature $T \approx 5800$ K at visible wavelengths.  At $\nu = 1$ GHz, near the high-frequency limit of 1930s radio technology,
$${h \nu \over k T} \approx {6.63 \times 10^{-27}{\rm ~erg~s} \times 10^9 {\rm ~Hz} \over 1.38 \times 10^{-16} {\rm ~erg~K}^{-1} \times 5800 {\rm ~K}} \approx 8 \times 10^{-6}$$
Replacing the exponential in Planck's equation by its Taylor-series approximation
$$\exp\biggl({h \nu \over k T}\biggr) - 1 \approx 1 + { h \nu \over k T} + ... -1 \approx {h \nu \over k T}$$
yields the simple Rayleigh-Jeans approximation
$$B_\nu (T) \approx {2 h \nu^3 \over c^2} {k T \over h \nu} = {2 k T \nu^2 \over c^2} = {2 k T \over \lambda^2}$$
to the blackbody spectrum at low frequencies or long wavelengths. The radio flux from a star, which subtends a very small solid angle, would be undetectably low.  This argument is more-or-less correct; in fact, even modern radio telescopes with high sensitivity could not detect the 1 GHz blackbody emission from the photosphere of a star like the Sun at the distance of the nearest stars $d \approx 1$ pc.  So radio astronomy was discovered by accident.

Example:  What is the $\nu = 1$ GHz flux density of a $T = 5800$ K blackbody the size of the Sun (radius $R_\odot \approx 7 \times 10^{10}$ cm) at the distance of the nearest star, about 1 parsec ($d \approx 3 \times 10^{18}$ cm)?

The flux density $S_\nu$ of a source having brightness $B_\nu$ and subtending a solid angle $\Omega$ is
$$S_\nu = B_\nu \Omega$$
$$B_\nu = {2 k T \nu^2  \over c^2}$$
$$B_\nu \approx {2 \times 1.38 \times 10^{-16}~{\rm erg~K}^{-1} \times 5800~{\rm K} \times (10^9 ~{\rm Hz})^2 \over (3.00 \times 10^{10}~{\rm cm~s}^{-1})^2}$$
Since Hz = s$^{-1}$ and sr is dimensionless,
$$B_\nu \approx 1.78 \times 10^{-15}~{\rm erg~cm}^{-2}~{\rm sr}^{-1}$$
$$\Omega = {\pi R_\odot^2 \over d^2}\approx { \pi (7 \times 10^{10}~{\rm cm})^2 \over (3 \times 10^{18} {\rm ~cm})^2} \approx 1.71 \times 10^{-15}~{\rm sr}$$
$$S_\nu \approx 3.0 \times 10^{-30}~{\rm erg~cm}^{-2}$$
$$S_\nu \approx 3.0 \times 10^{-33}~{\rm J~m}^{-2} \approx 3.0 \times 10^{-33}~{\rm W~m}^{-2}~{\rm Hz}^{-1}$$
$$S_\nu \approx 0.3~\mu{\rm Jy}$$

This is undetectably faint, even for modern radio telescopes.

In the 1920s, Bell Telephone offered transatlantic telephone service based on "shortwave" ($\lambda \sim 15$ m) radio transmissions.  Natural radio static caused serious interference with these transmissions, so Bell asked their young electrical engineer Karl Jansky to determine its origin.  Jansky built the antenna shown below to monitor radio static at 20.5 MHz. It produces a fan beam near the horizon and can be rotated in azimuth.  Jansky discovered that most of the static is caused by numerous tropical thunderstorms.  However, he also found a steady "hiss" whose strength rose and fell almost daily, with a period of 23 hours and 56 minutes.  He recognized that this is length of the sidereal day, deduced that the hiss originated outside the solar system, and identified the direction of the galactic center as the source of the strongest emission.  Radio astronomy was born in 1933.

Jansky and his antenna

Karl Jansky and the antenna that discovered cosmic radio static.  An accurate replica of this antenna is located at the NRAO in Green Bank, WV.  Image credit


Karl Jansky at blackboard

Karl Jansky pointing out the region of the Galactic plane emitting the strong cosmic noise. Image credit


Although Jansky's discovery appeared on the front page of the New York Times, Bell Telephone had no practical interest in the cosmic component of radio static, so Karl Jansky was reassigned to other projects.  Jansky himself believed that the cosmic noise was thermal emission because it produced a steady hiss in headphones that sounded like the hiss produced by vacuum-tube amplifiers.  Astronomers couldn't understand how such strong (equivalent to a $T \sim 2 \times 10^5$ K blackbody covering most of the Galactic center) static was produced and ignored it. 

The only person who took a serious interest in Jansky's discovery was the amateur radio operator and professional radio engineer Grote Reber.  He later wrote:

"My interest in radio astronomy began after reading the orginal articles by Karl Jansky.  For some years previous I had been an ardent radio amateur and considerable of a DX [long distance communication] addict, holding the call sign W9GFZ.  After contacting over sixty countries and making WAC [Worked All Continents, an amateur radio award], there did not appear to be any more worlds to conquer." 

Radio astronomy became his obsession.  He devoted years of his life to building the world's first radio antenna with a parabolic reflector in his back yard in Wheaton, IL and mapping the Galaxy with it.

Reber telescope image

Grote Reber's backyard radio telescope in Wheaton, IL. The parabolic reflector is about 10 m in diameter.  The original telescope was dismantled and reassembled near the NRAO visitors science center in Green Bank, WV.  Image credit

Since Reber also expected $B_\nu \propto \nu^2$, the Rayleigh-Jeans spectrum of a black body, he started his observations at $\nu = 3300$ MHz, the highest technically feasible in the late 1930s. When he failed to see anything, he concluded that the radio spectrum of the Galaxy was not Planckian. Next he tried 910 MHz, still with no luck, but "since I am a rather stubborn Dutchman, this had the effect of whetting my appetite for more." He finally succeeded in detecting and mapping (with about $10^\circ$ angular resolution) the Galaxy at 160 MHz, thereby confirming Jansky's discovery and demonstrating that the radio emission has a nonthermal spectrum.  He worked only at night (automotive ignition interference in Wheaton, IL was too strong during the day), recording radiometer meter readings by hand once per minute. His results were published in the Astrophysical Journal (Reber, G. 1940, ApJ, 91, 621). 

Reber at receiver

Grote Reber with an early radio receiver. Image credit

Then World War 2 intervened, hindering astronomical research but stimulating an explosion of radio and radar technology.  The same engineers and physicists who developed and used this technology during the war led the rapid scientific development of radio astronomy immediately after the war.

If you are interested in learning more about the early history of radio astronomy, read the NRAO web pages on this subject.