Radio astronomy developed in several European countries on the initiative andwith the expertise of scientists who had been involved in radar researchduring World War II. This paper traces the influence of various radartechniques on the design of early radio telescopes, covering the period 1945–1955, after which time radio had become accepted as a natural disciplinewithin astronomy.

Radio astronomy is a subject full of serendipity and coincidence. This paper recounts the strange sequence of incidental events and oversights that led to the establishment of Jodrell Bank as one of radio astronomy's founding institutions. Particular emphasis is given to the purpose for which it was originally intended; the study of high-energy cosmic rays.


Radio Astronomy in the U.S. went through two distinct phases, thepioneering phase before the Second World War, when Karl Jansky andGrote Reber were the only two radio astronomers in the world, and thepost-WWII era, when the subject blossomed throughout the world,especially in England and Australia, while U.S. radio astronomylanguished at a relatively low level, until its resurgence a decadelater. Here in Budapest we are celebrating the 70th anniversary ofJansky's discovery, and I shall start with a review of his work, andthe subsequent work of Grote Reber, who died, regrettably, in December2002. Two historical references will be cited frequently: Serendipitous Discoveries in Radio Astronomy (Kellermann & Sheets,eds. 1983, referred to as Paper 1) and the two historicalcollections edited by W.T. Sullivan, Classics in RadioAstronomy (1982, referred to as Paper 2) and The Early Yearsof Radio Astronomy (1984, Paper 3).

Nearly all discrete radio sources are extragalactic and lie atcosmological distances. Most sources stronger than ~1 mJy at1.4 GHz are powered by active galactic nuclei (AGNs), while themajority of fainter sources are in starburst galaxies. Recent all-skysurveys have detected most of this AGN population, and smaller surveysare now sensitive enough to probe the evolution of star formation inselected areas. Sensitive new radio surveys will complementfar-infrared, ultraviolet, and X-ray surveys now being made in “cosmicwindows” having exptionally low interstellar extinction. They willhelp define the evolution of star-formation and dust-shrouded AGNsover cosmological timescales.

I present an simple, introductory review text, focusing on recent deep field studies using radio telescopes, including high-resolution, wide-field VLBI observations. The nature of the faint radio source population is discussed, taking into account complimentary data and results that are now available via the sub-mm, IR, optical and X-ray wave-bands. New developments regarding the increased sensitivity of VLBI and an expansion in the instruments field-of-view are also presented. VLBI may be an important tool in recognising distant, obscured AGN from the “contaminant” star forming galaxies that now dominate lower-resolution, sub-arcsecond and arcsecond radio observations.

The idea that the radio jets in AGN contain materialin relativistic motion is supported by many lines of observationalevidence, including morphology, brightness temperature estimatedwith interferometers and with intrinsic variations, interstellarscintillations, X-rays, and superluminal motion. These are largelyindependent, and taken together make an irrefutable case for relativisticmotion.


The observational properties of the relativistic jets of AGN arereviewed together with their implications for our understanding ofthe jets, from the subparsec scales probed by the highest-frequencyground VLBI observations and space VLBI observations at centimeterwavelengths to the kiloparsec scales observed with connected-elementinterferometers such as the Very Large Array. Special attention ispaid to the information gleaned from polarization observations,with the focus on the most recent new observational informationavailable and how it may affect our view of AGN jets.

Reference systems and frames are crucial for high precision absolute astrometric work, and their foundations must bewell-defined. The current frame, the International CelestialReference Frame, will be discussed: its history, the use of the groupdelay as the measured quantity, the positional accuracy of 0.3 mas,and possible future improvements. On the other hand, for thedetermination of the motion of celestial objects, accuraciesapproaching 0.01 mas can be obtained by measuring the differentialposition between the target object and nearby stationary sources.This relative astrometric technique uses phase referencing, andthe current techniques and limitations are discussed, usingthe resultsfrom four experiments. Brief comments are included on theinterpretation of the Jupiter gravity deflection experiment ofSeptember 2002.


We review some of the major achievements of observational studies ofextragalactic radio variability since its discovery in themid-1960s. In the second half of the paper we focus particularattention on developments in our understanding of short-termvariability at centimetre wavelengths which have taken place in thelast decade.

Magnetic fields are found everywhere in the cosmic Universe. Theyhave been known to exist on the Earth for centuries. They weredetected in the Sun, measured in situ in the planets, observed instars, galaxies and clusters of galaxies. Magnetic fields areresponsible for many observed effects like radio continuum emission,jets, beamed emission in pulsars, star formation, etc. Possibly thewhole universe is intimately governed by the presence of magneticfields. This review will summarize the observational results anddiscuss the present interpretation for the origin of the magneticfields.

Radio emission has been detected from all the stages of stellar evolutionacross the HR diagram. Its presence reveals both astrophysical phenomena andstellar activity which, otherwise, would not be detectable by other means. Thedevelopment of large, sensitive interferometers has allowed us to resolve theradio structure of several stellar systems, providing insights into the masstransfer process in close binary systems. I review the main characteristics ofthe radio emission from several kinds of stars, paying special attention tothose cases where such an emission originates in relativistic jets.


Only a handful of supernovae can be studied with milliarcsecondresolution using the VLBI technique. However, such rare studieshave permitted in various cases to estimate expansiondecelerations and to obtain information on supernova ejecta andcircumstellar media, as well as on the structure of the emittingregion. We review results on high resolution observations of thenearby radio supernovae: SN 1979C, SN 1986J, SN 1987A, andSN 1993J. For completeness, we also review the results on thesupernova remnants in M 82.


One of the most fundamental discoveries in physics and astrophysicswas made with the discovery of pulsars. Ever since, pulsars – andlater in particular binary and millisecond pulsars – have beenstudied as extreme end-states of stellar evolution, as super-denseobjects, as radio and high-energy sources, as probes for theinterstellar medium, and as super-precise clocks for fundamentalexperiments in gravitational physics. This article reviews some of theexciting applications of pulsar physics and the amazing prospects thatare promised by the advent of the Square-Kilometre-Array (SKA).

Since its prediction and first detection just over 50 years ago,the 21-cm line of neutral atomic hydrogen has been a key probe of thestructure and dynamics of the Milky Way Galaxy and of the astrophysics ofthe Galactic interstellar medium (ISM). Emission from Galactic atomichydrogen is seen in all directions and the structures revealed reflect theprocesses relevant to the evolution of the Galaxy and phase transitions ofthe ISM. As in other areas of astronomy, our knowledge of the neutral ISMfrom HI studies has progressed in lock step with radio telescope technology.Over the last several years interferometric surveys have providedarcminute-scale HI images that allow the atomic hydrogen component of theISM to be viewed in the context of the other major ISM components and revealdynamic structures in HI related to the interaction of ISM with the stellarcomponent of the Galaxy. The International Galactic Plane Survey, acollaboration to combined interferometric surveys from Canada, the US and Australia, is imaging over 90% of the stellar disk of the Milky Way in HI at arcminute resolution. When it is built in the post 2010 era, the prodigious survey speed and angular resolution of the Square Kilometre Array will affect another seminal advance in Galactic HI studies.


The 21 cm HI line was first detected in 1951, and two years laterit was detected in an extragalactic system. Since then, HIobservations have made important contributions to the field of extragalactic Astronomy. Among others, this review of a 50–year history of the field touches upon highlights that refeer to: the understanding of the structure of spiral galaxies, mapping the distribution of dark matter on galactic and cosmic scales, elucidating the dynamics and evolution of groups and clusters of galaxies, the discovery of the topology of the large–scale structure of the Universe, the measurement of cosmological parameters and the investigation of structures at high redshift.

Over the past forty years, investigations of radio recombinationlines have become an effective way of studying the interstellarmedium. The electron temperatures, densities, emission measures,kinematics parameters, elemental abundances and other physicalcharacteristics of emission processes in planetary nebulae,distributed ionized gas, stellar winds, the partially-ionizedinterstellar medium and external galaxies are determined with thelines of hydrogen, helium, carbon, etc. This astrophysicalphenomenon is unique because of the large (>1000) number ofpossible quantum transitions and because of the wide observablefrequency range (from ~150 GHz to ~10 MHz). In thisshort review the main results from recombination radio lineinvestigations are discussed. Special attention is given to theareas which have been given least attention in the literature suchas the radio recombination lines of very excited carbon atoms atthe extremely low-frequency (v < 30 MHz) end of the decameterwavelength range. This is important to stress in the 70-th jubileeof radio astronomy because cosmic radio emission was discovered byKarl Jansky at decameter wavelengths. Investigations of theextremely low-frequency radio recombination lines are also veryinteresting for the prospective LOFAR system.

Since the discovery of galactic masers in the mid 1960s, theirstudies have resulted in remarkable deepening of the understanding ofphysical processes in the interstellar medium and stars. This reviewpresents a brief historical retrospective and current status of radioastronomy studies of masing molecules that trace various stages ofstelar evolution.

Molecular emissions are powerful tracers of intense heating andstar-formation processes. In this paper we consider thecharacteristics of Megamaser emission that originate in some activenuclei that are powered by intense star-formation or accretion onto amassive compact object. The OH and possibly the H2CO Megamaseractivity occurs among the most luminous sources in the population of(Ultra-) Luminous Infrared Galaxies. In addition, we consider thesystematic behaviour of the line emission of high-density tracermolecules. An evolutionary scenario is presented for ULIRGs that mayexplain the molecular line ratios observed in the population of FIRgalaxies.

This paper contains a short overview of the chemistry of interstellar clouds based on radio astronomical measurements. First, an account of the properties of the Interstellar Medium (ISM) is followed by a historical review up to the time of the discovery of complex molecules. This is followed by a list of molecules and a short collection of molecular abundances, followed by an account of possible chemistries in molecular clouds, and finally, examples of typical galactic sources.