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The Discovery of Cosmic Radio NoiseNatural radio emission from our Galaxy was detected accidentally in 1932 by Karl GutheJansky, a physicist working as a radio engineer for Bell Telephone Laboratories. Why didn't"real" astronomers of the time pursue radio astronomy and make this discovery first? In part,because they knew too much. They knew that stars are nearly blackbody radiators at visiblewavelengths. The spectral brightness B at frequency · of an ideal blackbody radiator is givenby Planck's Law

B (T ) ;

whereB is the power emitted per unit area per unit bandwidth per steradian of solid angle by a blackbody,

erg s Joule s = Planck's constant,· frequency in cycles per second, or hertz (so Hz = s ),

erg K Joule K Boltzmann's constant, cm s m s the speed of light, and

T is the absolute temperature (K) of the black body.

In the low-frequency radio limit, the dimensionless quantity for mostastronomical sources. For example, the photosphere (the visible surface) of the Sun hastemperature K. At · GHz, which was near the high-frequency limit of radiotechnology in 1932,

0

Replacing the exponential term in Planck's equation by its Taylor-series approximation

yields the simple Rayleigh-Jeans approximation

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

·

· =c2

2h·3 1

exp( )h·kT

À 1

·

h :63 0 Ù 6 Â 1 À27 = :63 0 6 Â 1 À34

= À1

k :38 0 Ù 1 Â 1 À16 :38 0 À1 = 1 Â 1 À23 À1 =c :00 0 Ù 3 Â 1 10 :00 0 À1 = 3 Â 1 8 À1 =

h·=(kT ) Ü 1

T 800 Ù 5 = 1

h·

kTÙ 6:63 0 erg s 0 HzÂ 1 À27 Â 1 9

1:38 0 erg K 800 KÂ 1 À16 À1 Â 5Ù 8Â 1 À6

exp :: Òh·

kT

ÓÀ 1 Ù 1 +

h·

kT+ : À 1 Ù h·

kT

B (T ) · Ùc2

2h·3

h·

kT=

c22kT·2

=Õ22kT

The Discovery of Cosmic Radio Noise http://www.cv.nrao.edu/course/astr534/Discovery.html

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more-or-less correct; in fact, even the most sensitive modern radio telescopes could notdetect the 1 GHz blackbody emission from the photosphere of a star like the Sun at thedistance pc of the nearest star.

Example: What is the flux density S at · GHz of a T 800 K blackbody the size of theSun (radius cm) at the distance of the nearest star, about 1 parsec

( cm)?

The flux density S of a source is defined as the power per unit area received in a unitbandwidth (Á· Hz) at frequency · (so the mks units of S are W m Hz ). The fluxdensity received from a compact source having brightness B and subtending a solid angle

sr is simply

S Ê

B

B

Since Hz = s and sr is dimensionless,

Ê :71 0 sr

The flux densities of astronomical sources are so small in mks units that radio astronomersnormally express flux densities in Jy (for Jansky) defined as , or mJy (Jy), or ÖJy ( Jy). Thus

d Ù 1

· = 1 = 5R 0 Ì Ù 7Â 1 10

d 0 Ù 3Â 1 18

·

= 1 · À2 À1

·

Ê Ü 1

· = B·

· =c2

2kT· 2

· Ù(3:00 0 cm s )Â 1 10 À1 2

2 :38 0 erg K 800 K 10 Hz)Â 1 Â 1 À16 À1 Â 5 Â ( 9 2

À1

B :78 0 erg cm sr · Ù 1 Â 1 À15 À2 À1

=d2

ÙR2Ì Ù

(3 0 cm)Â 1 18 2

Ù(7 0 cm)Â 1 10 2

Ù 1 Â 1 À15

S :0 0 erg cm · Ù 3 Â 1 À30 À2

S :0 0 J m :0 0 W m Hz · Ù 3 Â 1 À33 À2 Ù 3 Â 1 À33 À2 À1

10 W m Hz À26 À2 À1 10 À3

10 À6


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This is too faint even for modern radio telescopes, which can detect continuum sources asfaint as Jy.

In the 1920s, Bell Telephone offered transatlantic telephone service based on "shortwave"( m) radio transmissions. Natural radio static caused serious interference with thesetransmissions, 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 produced a fanbeam near the horizon and could be rotated in azimuth (the angle measured from north toeast along the horizon). Jansky discovered that most of the static is caused by numeroustropical thunderstorms. In addition he found a steady "hiss" whose strength rose and fellalmost daily, with a period of 23 hours and 56 minutes. He recognized that this is length ofthe sidereal day (the time it takes the Earth to rotate once in the reference frame of the fixedstars), deduced that the hiss originated outside the solar system, and identified the direction ofthe Galactic center as the source of the strongest emission. He published his results in thepaper "Electrical Disturbances of Apparently Extraterrestrial Origin" (Jansky, K. J. 1933, Proc.IRE, 21, 1387).

Karl Jansky and the antenna that discovered cosmic radio static. It rotated in azimuth on fourwheels scavenged from a Ford Model T. An accurate replica of this antenna is located at theNRAO in Green Bank, WV. Image credit

S :3 ÖJy · Ù 0

S 00 Ö Ø 1

Õ 5 Ø 1


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

Jansky's discovery appeared on the front page of the New York Times, but Bell Telephone hadno practical interest in the cosmic component of radio static and reassigned Karl Jansky toother projects. Jansky himself believed that the cosmic noise was thermal emission because itproduced a steady hiss in headphones that sounded like the hiss produced by vacuum-tubeamplifiers. Skeptical astronomers couldn't understand how such strong (equivalent to theemission from a K blackbody covering most of the inner Galaxy) radio noise wasproduced and ignored it.

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

"My interest in radio astronomy began after reading the orginal articles by Karl Jansky. Forsome 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 sixtycountries and making WAC [Worked All Continents, an amateur-radio award], there didnot appear to be any more worlds to conquer."

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

T 0 Ø 2Â 1 5


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

Since Reber also expected , the low-frequency spectrum of a black body, he startedobserving at · 300 MHz, the highest technically feasible frequency in 1937. When he failedto see anything, he concluded that the radio spectrum of the Galaxy was not Planckian. Nexthe tried 910 MHz, still with no luck, but "since I am a rather stubborn Dutchman, this had theeffect of whetting my appetite for more." In 1938 he finally succeeded in detecting andmapping (with about angular resolution) the Galaxy at 160 MHz, thereby confirmingJansky's discovery and demonstrating that the radio emission has a distinctly nonthermalspectrum. He observed only at night because automotive ignition interference in Wheaton, ILwas 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).

B · / ·2

= 3

10 Î


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Grote Reber with an early radio receiver. Image credit

Then World War II intervened, hindering astronomical research but stimulating enormousprogress in radio and radar technology. The same engineers and physicists who developedand used this technology during the war led the rapid scientific development of radioastronomy immediately afterward.

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


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