Introduction to Radio Astronomy

Download Introduction to Radio Astronomy

Post on 23-Sep-2016

215 views

Category:

Documents

1 download

Embed Size (px)

TRANSCRIPT

  • PROCEEDINGS OF THE IRE

    Introduction to Radio Astronomy *F. T. HADDOCKt, MtEMBER, IRE

    Summary-A general description of the nature of radio astron-omy, its differences from optical astronomy, a review of the earliestbeginnings of galactic and solar radio astronomy, and a listing ofother important observational discoveries is given. A nearly completebibliography of these early publications and of the principal reviewbooks and papers on Radio Astronomy is given.

    Some practical aspects and instrumental developments of possibleinterest to radio engineers are pointed out. The papers in this issue ofthe PROCEEDINGS on Radio Astronomy are discussed generally andindividually. A brief description of some new results on solar-burstspectra obtained at the Radio Astronomy Observatory at the Uni-versity of Michigan is presented.

    INTRODUCTIONUR CONCEPT of the Universe has been appreciablyenlarged by radio observations in recent years. It hasbeen found that radio waves are emitted from the

    Moon, Venus, Mars, Jupiter, and Saturn, from the Sun'sgaseous atmosphere, from the debris of exploded stars, fromneighboring galaxies of stars, from clusters of distant galaxies,from clouds of gas in the spiral arms of our Milky Way galaxy,from very distant Island Universes in collision, and from hun-dreds of invisible and unsuspected heavenly objects, popularlymiscalled radio stars. Radio studies have taken the lead inmapping the structure of our own galaxy, and have revealedthe fact that the Milky Way is a rather tightly wound spiralgalaxy. The solar system is traveling along the inner edge ofa spiral arm at a distance of 30,000 light years from the centerof rotation of this disk of stars and gas. Many astronomersbelieve that some of the fainter radio sources are at distancesbeyond the range of present optical telescopes and that withthe recent attainment of Doppler shift measurements on theradio hydrogen-line radiation from distant galaxies, cosmolo-gists may soon be given a fresh and penetrating view into thedepths of time and space.

    Although cosmic ray and meteor studies have made valu-able contributions, most of our knowledge of the Universehas been gained from electromagnetic light waves, falling onthe earth through a window in the atmosphere about fiveoctaves wide. Radio astronomy is now exploiting the existenceof a second window through the atmosphere 12 octaves wide.Radio waves shorter than a few millimeters are absorbed byatmospheric oxygen and water vapor; whereas the ionosphericlayers turn back radio waves longer than several decameters[11]. Thus radio astronomy observations are made at frequen-cies used in television, fm radio, microwave relay links, rocketand satellite telemetering and control, and radar.

    Light and radio waves are fundamentally the same exceptfor their length. Radio waves are thousands of times longerthan light waves; this large difference has two consequences ofgreat importance to astronomy. First, the interaction of radiowaves with interstellar gas, solar and planetary atmospheres,

    * Original manuscript received by the IRE, December 10, 1957.t University of Michigan, Ann Arbor, Mich.

    and cosmic dust clouds complements that of light waves.Large regions of the Universe are forever inaccessible to opti-cal study because of heavy obscuration by interstellar dustwhich, however, is transparent to radio waves. On the otherhand, tenuous ionized gas-regions such as exist around the Sunas its corona, and surrounding super-giant blue stars as brightgaseous nebulas, and even enveloping all the stars in spiralgalaxies, are transparent to light waves, but reflect, absorb, oremit radio waves. Thus radio methods offer new and powerfulmeans of studying these gaseous regions and, at the same time,penetrating the obscuration due to cosmic dust without theleast detectable effect. Second, the value of radio to astronomyis limited somewhat by the difficulty of obtaining high angularresolving power. Wave diffraction sets this instrumental limitas it does for optical telescopes, and since its blurring effect isproportional to wavelength, radio waves are very much harderto "focus sharply." For example, the most precise radio tele-scopes of today, operating on microwaves, are capable of pro-ducing angular resolution which is not quite as good as theunaided human eye with light waves. To compare with anoptical telescope in resolution a radio antenna would need to behundreds of miles in extent. However, position location is notthe same as resolving power; if a radio source is much brighterthan its surrounding background emission and its shape isassumed to be known, then its position can be located to amuch smaller angular uncertainty than is indicated by theresolving power of the antenna.With the largest optical telescopes it is practically impossi-

    dle to obtain the ultimate instrumental resolution given bywave diffraction theory because of the inhomogeneities in therefractive index of the earth's atmosphere. When the atmos-phere is very stable at the proper heights and stellar imagesare steady and sharp, the astronomer says that the "seeing" isgood. Pronounced twinkling, or scintillation, and "dancing" ofstellar images is poor "seeing." This "seeing" problem alsoexists in radio astronomy, with the additional complication,at the longer wavelengths, of ionospheric effects, including theFaraday rotation of the radio wave polarization, and occa-sional large absorption at decameter wavelengths. Undoubt-edly, "radio seeing" will also set a practical limit to the angularresolution obtainable at all wavelengths used in terrestrialradio astronomy.

    For the above reasons and for the fact that stars are rela-tively poor radio emitters, the radio telescope presents to us adustless and starless universe, populated largely by turbulentgas clouds.

    THE BEGINNINGSIt is very likely that the first attempt to measure the emis-

    sion of radio waves from an extraterrestrial source was that ofSir Oliver Lodge [2], well-known radio pioneer, who in a lec-ture before the Royal Institution of Great Britain on June 1,1894, only six years after Hertz discovered radio waves,stated, "I hope to try for long-wave radiation from the sun,filtering out the ordinary well-known waves by a blackboard

    In lieu of the usual "Scanning the Issue" page, the organizer of this special issuepresents below, for the benefit of the general reader, a discussion of the contents of thisissue. He has preceded the discussion by an excellent introduction which will orient thereader and will interpret for him the importance of radio astronomy to both astronomersand radio engineers.-The Editor

    - =1958 3

  • PROCEEDINGS OF THE IRE

    or other sufficiently opaque substance." He apparently did trysometime during 1897 to 1900 because he inserted into thethird edition of this lecture the following note: "I did not suc-ceed in this, for a sensitive coherer in an outside shed unpro-tected by the thick walls of a substantial building cannot bekept quiet for long. I found its (galvanometer) spot of lightliable to frequent weak and occasionally violent excursions,and I could not trace any of these to the influence of the sun.There were evidently too many terrestrial sources of disturb-ance in a city like Liverpool to make the experiment feasible.I don't know that it might not possibly be successful in someisolated country place; but clearly the arrangement must behighly sensitive in order to succeed."Inasmuch as the lecture then proceeded to discuss "a small

    complete detector . .. which is quite portable and easily setup" consisting of a battery, galvanometer, and coherer de-signed for the detection of centimeter wavelength radiationarising from spark excited spheres, it is presumed that it isthis wavelength region that Lodge had in mind for "long-waveradiation from the sun." Over forty years later, in 1942,Southworth, at the Bell Telephone Laboratories in NewJersey, first detected and measured the waves that Lodgeapparently had in mind.The first discovery of radio waves of extraterrestrial origin

    was made by Jansky [3] in 1932, also at the Bell TelephoneLaboratories, while studying the horizontal direction of ar-rival of atmospheric static at 20 mc (X = 14.6). Fig. 1 is aphotograph of his antenna. He noticed a very steady weakhissing static of unknown origin whose direction of arrivalchanged gradually around the compass in about 24 hours.Further study revealed that the periodicity was stellar ratherthan solar as first suspected; in fact, the maximum intensityoccurred when the center of the Milky Way was in the an-tenna beam. Secondary intensity maximum occurred when thebeam passed through the plane of the Milky Way at positionsfurther removed from the center of our galaxy.Two suggestions put forth by Jansky relating to the origin

    of this interstellar radio emission consistent with his observa-tions are of interest today. First, he stated that the mostfascinating explanation would be a disk-like distribution ofradio sources around the Earth like the stars. In a later paperhe mentioned that since he could not detect radio emissionfrom the Sun he supposed that some other class of heavenlybodies in the Milky Way must radiate a much greater ratiothan the Sun of energy in radio waves to that in the form oflight and heat. Second, he was immediately struck by thesimilarity between the sound produced by the received galac-tic radiation to that produced by the thermal agitation ofelectric charge in a resistor. Therefore he speculated that theradiation might be caused by some sort of thermal agitation

    Fig. 1-Karl G. Jansky and his 14.6 meterrotatable directional antenna.

    Fig. 2-Grote Reber and his 31-foot reflector after it was acquired bythe National Bureau of Standards in 1947 and erected on a turn-table at their field station near Sterling, Va.

    of charged particles such as found not only in stars, but alsoin the very considerable amount of interstellar matter dis-tributed throughout the Milky Way with an effective tem-perature of 15,000C. Later observations have largely sub-stantiated these speculations.

    Later Reber [5] in Illinois, measured the angular distributionof this cosmic emission at the much higher frequency of 160mc with a sharper antenna beam. Fig. 2 is a photograph of hisantenna. He found several subsidiary maxima, one in the con-stellation of Cygnus. The intensity at this higher frequencywas much less than Jansky found at 20 mc. Reber suggestedthat the radiation was generated thermally by collisions be-tween free electrons and positive ions (called free-free transi-tions) in interstellar matter ionized by star light. The theoryfor this type radiation was first given by Kramers [6] to ex-plain continuous X-ray spectra, and is physically equivalentto the Lorentz absorption theory used in ionospheric propa-gation.

    Early in 1946, Hey, Parsons, and Phillips [7] in England,found marked short-period irregular fluctuations in intensityat 64 mc (X=5 m) in the direction of the same subsidiarymaximum in Cygnus. Their observations indicated that thefluctuating source subtended an angle less than 20. It appearedto them that such fluctuations could only originate from asmall number of discrete sources and not from widely dis-tributed interstellar matter. Although they were incorrect inattributing the fluctuations to the source itself (turbulent in-homogeneities in the Earth's ionosphere were responsible)they were correct in pointing out the importance of their dis-covery. It was the first clue of the existence of a discrete radiosource, which was established the following year by Boltonand Stanley [8] by observing that the size of the source wasless than 8 minutes of arc. Among the score or so which havebeen identified optically [9, 10], there are objects of highlyunusual types, new to astronomy, as well as well-known ce-lestial bodies. Jansky's supposition that there exist heavenlybodies whose radio energy output relative to that of their heatand light output is much greater than the Sun has been dra-matically borne out. The source in Cygnus on the above ratiobasis is a better radio emitter than the Sun by a factor of 1018(a billion-billion times). It is now believed that a large part ofradio emission from the Milky Way is due to discrete radiosources in a disk-like distribution like the stars [11], and that a

    4 January

  • Haddock: Introduction to Radio Astronomy

    certain fraction, which is relatively more intense in the micro-wave region, arises thermally from free electron collisions withpositive ions in interstellar gas clouds [12, 13].The first documented recognition of the reception of radio

    waves from the Sun was made in 1942 by Hey [14] in Englandand independently by Southworth [15] in New Jersey. Bothwrote reports which had restricted circulation because of thewar and were not published until later. Reber [16] in Wheaton,Ill., independently discovered and identified solar radiation at160 mc in the following year and was the first to publish theevidence. He devoted only a few lines to the Sun in this paperon galactic radiation.Southworth used radar receivers and paraboloids in a de-

    liberate attempt to detect radio emission from the Sun. Hewas successful on June 29, at 10,000 mc and at 3000 mc aweek or two later. He correctly attributed this emission tothermally generated radiation. He has recently given a gooddetailed account of this work [17] and of his early associationwith Jansky at the Bell Telephone Laboratories in NewJersey. These measurements initiated the study of the radioundisturbed or "quiet" Sun. Now we will describe the dis-covery of radio bursts, the second main class of solar radioemission.

    Undoubtedly there have been many unpublished observa-tions of high hissing noise levels arising before or after radiofadeouts, but the first publication was in 1936 when Arakawa[18] noted that, "High noise accompanied these sudden fadings(4-20-mc band), especially on local circuits. The effect mustbe related to solar activities, since they occur only on daylightroutes." And two years later Heightman [19] wrote, "At suchtimes (when fadeouts occur) the writer has often observed thereception of a peculiar radiation, mostly on frequencies over20 mc, which on a receiver takes the form of a smooth thoughloud hissing sound. This is presumably caused by the arrivalof charged particles from the Sun on the aerial."

    During this period scientists [20] were studying the associa-tion of short-wave radio fadeouts with solar flares (chromo-spheric eruptions), terrestrial magnetic pulses, enhanced at-mospheric static, and sudden increases in noise level men-tioned above. It appears that the question of the origin of thisenhanced noise attracted the least attention. In 1939 onepaper appeared in Japan by Nakagami and Miya [21] whichdescribed an experiment made near 15 mc to measure theangle of arrival of noise "like that from a grinder" attaining alevel of 40 db above the background just before and shortlyafter a fadeout. They foun...