Fundamentals of Radio Astronomy

Astrophysics

Ronald L. Snell, Stanley Kurtz, Jonathan Marr

• Editore: CRC Press
• Anno: 2019
• ISBN:
  9780429647130
• Edizione: 1
• Argomento: Naturwissenschaften
• Categoria: Astronomie & Astrophysik

As demonstrated by five Nobel Prizes in physics, radio astronomy has contributed greatly to our understanding of the Universe. Courses covering this subject are, therefore, very important in the education of the next generation of scientists who will continue to explore the Cosmos.

This textbook, the second of two volumes, presents an extensive introduction to the astrophysical processes that are studied in radio astronomy. Suitable for undergraduate courses on radio astronomy, it discusses the physical phenomena that give rise to radio emissions, presenting examples of astronomical objects, and illustrating how the relevant physical parameters of astronomical sources can be obtained from radio observations.

Unlike other radio astronomy textbooks, this book provides students with an understanding of the background and the underlying principles, with derivations available for most of the equations used in the textbook.

Features:

• Presents a clear and concise discussion of the important astronomical concepts and physical processes that give rise to both radio continuum and radio spectral line emission
• Discusses radio emissions from a variety of astronomical sources and shows how the observed emissions can be used to derive the physical properties of these sources

• Includes numerous examples using actual data from the literature
  
  

Chapter 1

Introductory Material

IN 1932 Karl Jansky detected radio signals serendipitously from an extraterrestrial source, marking the beginning of the field of radio astronomy. Although Jansky deduced that the radio signals originated from beyond the solar system, more than two decades passed before we understood that the origin of this emission was due largely to synchrotron emission produced by high energy cosmic ray electrons interacting with the magnetic field of the Milky Way. The field of radio astronomy has advanced significantly since those early days, and observations at radio wavelengths today play a vital role in the study of astronomical objects ranging from solar system bodies to distant quasars. In the first volume of the Fundamentals of Radio Astronomy, we addressed how radio telescopes function and the methods used to make observations. In this second volume, we discuss what we can learn about the astrophysics of the Universe from radio wavelength observations.

The full range of electromagnetic radiation is enormous, spanning wavelengths from the very shortest gamma-ray radiation, through the x-ray, ultraviolet, optical and infrared, and ending with the very long radio wavelengths. Where the infrared (or far-infrared) wavelengths end and radio wavelengths start is ill-defined. However for the discussion in this book, we place the division at a wavelength of about 0.3 mm. Even the radio portion of the electromagnetic spectrum is quite broad, spanning over 4 orders of magnitude in wavelength, from wavelengths as short as 0.3 mm to wavelengths longer than 20 m, corresponding to frequencies as high as 1 × 10¹² Hz (or 1,000 GHz) to lower than 1.5 × 10⁷ Hz (or 15 MHz). Remember that the basic unit of frequency is hertz, where 1 Hz = 1 cycle per second, therefore 1 MHz corresponds to 10⁶ Hz and 1 GHz corresponds to 10⁹ Hz. Jansky’s original observations were made at the relatively low frequency of 20.5 MHz (or a wavelength of about 15 m); however astronomers today with modern telescopes and detectors routinely obtain observations at any frequency in the full radio window.

At radio wavelengths, just as at other wavelengths, we can make both broadband and spectroscopic measurements. Broadband measurements are designed to detect the emission produced by continuum emission processes as they measure the average emission over a broad range of wavelengths or frequencies. At optical wavelengths this type of measurement is often called photometry and is accomplished by passing the light through filters that transmit a well-defined but broad band of wavelengths for detection. Spectroscopic measurement requires much higher wavelength resolution, sufficient to detect individual spectral lines. The techniques used for filtering, dispersing and detecting the light at optical and radio wavelengths can be quite different. The techniques used at radio wavelengths to make both photometric and spectroscopic observations are described in detail in the first volume of this book. The present volume concentrates on the study of astronomical sources at radio wavelengths using both photometric and spectroscopic observations.

Before we discuss radio observations of different types of astronomical sources, we review some basic physics and astronomy. In the following sections we cover units used by radio astronomers, measures of the radiation from astronomical sources, sky coordinates, the Doppler effect and the cosmological redshift. In Chapter 2, we discuss the propagation of radiation through a medium and in Chapters 3 and 4 we present the physical processes responsible for producing both broadband radio continuum emission and radio spectral line emission. In subsequent chapters we apply these physical principles to discuss how radio observations can provide important insights into the properties of astronomical sources. The scope of radio studies in astronomy is very broad, and it is impossible to cover all aspects; therefore we have selected a subset of topics for inclusion in this volume.

1.1Units and Nomenclature

Le...