Radio astronomy developed in several European countries on the initiative and with the expertise of scientists who had been involved in radar research during World War II. This paper traces the influence of various radar techniques on the design of early radio telescopes, covering the period 1945–1955, after which time radio had become accepted as a natural discipline within 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, the pioneering phase before the Second World War, when Karl Jansky and Grote Reber were the only two radio astronomers in the world, and the post-WWII era, when the subject blossomed throughout the world, especially in England and Australia, while U.S. radio astronomy languished at a relatively low level, until its resurgence a decade later. Here in Budapest we are celebrating the 70th anniversary of Jansky's discovery, and I shall start with a review of his work, and the subsequent work of Grote Reber, who died, regrettably, in December 2002. Two historical references will be cited frequently: Serendipitous Discoveries in Radio Astronomy (Kellermann & Sheets, eds. 1983, referred to as Paper 1) and the two historical collections edited by W.T. Sullivan, Classics in Radio Astronomy (1982, referred to as Paper 2) and The Early Years of Radio Astronomy (1984, Paper 3).

Nearly all discrete radio sources are extragalactic and lie at cosmological distances. Most sources stronger than ~1 mJy at 1.4 GHz are powered by active galactic nuclei (AGNs), while the majority of fainter sources are in starburst galaxies. Recent all-sky surveys have detected most of this AGN population, and smaller surveys are now sensitive enough to probe the evolution of star formation in selected areas. Sensitive new radio surveys will complement far-infrared, ultraviolet, and X-ray surveys now being made in “cosmic windows” having exptionally low interstellar extinction. They will help define the evolution of star-formation and dust-shrouded AGNs over 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 material in relativistic motion is supported by many lines of observational evidence, including morphology, brightness temperature estimated with interferometers and with intrinsic variations, interstellar scintillations, X-rays, and superluminal motion. These are largely independent, and taken together make an irrefutable case for relativistic motion.


The observational properties of the relativistic jets of AGN are reviewed together with their implications for our understanding of the jets, from the subparsec scales probed by the highest-frequency ground VLBI observations and space VLBI observations at centimeter wavelengths to the kiloparsec scales observed with connected-element interferometers such as the Very Large Array. Special attention is paid to the information gleaned from polarization observations, with the focus on the most recent new observational information available 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 be well-defined. The current frame, the International Celestial Reference Frame, will be discussed: its history, the use of the group delay as the measured quantity, the positional accuracy of 0.3 mas, and possible future improvements. On the other hand, for the determination of the motion of celestial objects, accuracies approaching 0.01 mas can be obtained by measuring the differential position between the target object and nearby stationary sources. This relative astrometric technique uses phase referencing, and the current techniques and limitations are discussed, usingthe results from four experiments. Brief comments are included on the interpretation of the Jupiter gravity deflection experiment of September 2002.


We review some of the major achievements of observational studies of extragalactic radio variability since its discovery in the mid-1960s. In the second half of the paper we focus particular attention on developments in our understanding of short-term variability at centimetre wavelengths which have taken place in the last decade.

Magnetic fields are found everywhere in the cosmic Universe. They have been known to exist on the Earth for centuries. They were detected in the Sun, measured in situ in the planets, observed in stars, galaxies and clusters of galaxies. Magnetic fields are responsible for many observed effects like radio continuum emission, jets, beamed emission in pulsars, star formation, etc. Possibly the whole universe is intimately governed by the presence of magnetic fields. This review will summarize the observational results and discuss the present interpretation for the origin of the magnetic fields.

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


Only a handful of supernovae can be studied with milliarcsecond resolution using the VLBI technique. However, such rare studies have permitted in various cases to estimate expansion decelerations and to obtain information on supernova ejecta and circumstellar media, as well as on the structure of the emitting region. We review results on high resolution observations of the nearby radio supernovae: SN 1979C, SN 1986J, SN 1987A, and SN 1993J. For completeness, we also review the results on the supernova remnants in M 82.


One of the most fundamental discoveries in physics and astrophysics was made with the discovery of pulsars. Ever since, pulsars – and later in particular binary and millisecond pulsars – have been studied as extreme end-states of stellar evolution, as super-dense objects, as radio and high-energy sources, as probes for the interstellar medium, and as super-precise clocks for fundamental experiments in gravitational physics. This article reviews some of the exciting applications of pulsar physics and the amazing prospects that are 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 the structure and dynamics of the Milky Way Galaxy and of the astrophysics of the Galactic interstellar medium (ISM). Emission from Galactic atomic hydrogen is seen in all directions and the structures revealed reflect the processes relevant to the evolution of the Galaxy and phase transitions of the ISM. As in other areas of astronomy, our knowledge of the neutral ISM from HI studies has progressed in lock step with radio telescope technology. Over the last several years interferometric surveys have provided arcminute-scale HI images that allow the atomic hydrogen component of the ISM to be viewed in the context of the other major ISM components and reveal dynamic structures in HI related to the interaction of ISM with the stellar component of the Galaxy. The International Galactic Plane Survey, a collaboration 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 later it was detected in an extragalactic system. Since then, HI observations 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 recombination lines have become an effective way of studying the interstellar medium. The electron temperatures, densities, emission measures, kinematics parameters, elemental abundances and other physical characteristics of emission processes in planetary nebulae, distributed ionized gas, stellar winds, the partially-ionized interstellar medium and external galaxies are determined with the lines of hydrogen, helium, carbon, etc. This astrophysical phenomenon is unique because of the large (>1000) number of possible quantum transitions and because of the wide observable frequency range (from ~150 GHz to ~10 MHz). In this short review the main results from recombination radio line investigations are discussed. Special attention is given to the areas which have been given least attention in the literature such as the radio recombination lines of very excited carbon atoms at the extremely low-frequency (v < 30 MHz) end of the decameter wavelength range. This is important to stress in the 70-th jubilee of radio astronomy because cosmic radio emission was discovered by Karl Jansky at decameter wavelengths. Investigations of the extremely low-frequency radio recombination lines are also very interesting for the prospective LOFAR system.

Since the discovery of galactic masers in the mid 1960s, their studies have resulted in remarkable deepening of the understanding of physical processes in the interstellar medium and stars. This review presents a brief historical retrospective and current status of radio astronomy studies of masing molecules that trace various stages of stelar evolution.

Molecular emissions are powerful tracers of intense heating and star-formation processes. In this paper we consider the characteristics of Megamaser emission that originate in some active nuclei that are powered by intense star-formation or accretion onto a massive compact object. The OH and possibly the H2CO Megamaser activity occurs among the most luminous sources in the population of (Ultra-) Luminous Infrared Galaxies. In addition, we consider the systematic behaviour of the line emission of high-density tracer molecules. An evolutionary scenario is presented for ULIRGs that may explain the molecular line ratios observed in the population of FIR galaxies.

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.