Book contents
- Frontmatter
- Contents
- List of figures
- List of tables
- Preface
- Acknowledgments
- 1 Astronomy through the centuries
- 2 Electromagnetic radiation
- 3 Coordinate systems and charts
- 4 Gravity, celestial motions, and time
- 5 Telescopes
- 6 Detectors and statistics
- 7 Multiple telescope interferometry
- 8 Point-like and extended sources
- 9 Properties and distances of celestial objects
- 10 Absorption and scattering of photons
- 11 Spectra of electromagnetic radiation
- 12 Astronomy beyond photons
- Credits, further reading, and references
- Appendix: Units, symbols, and values
- Index
2 - Electromagnetic radiation
Published online by Cambridge University Press: 05 June 2012
- Frontmatter
- Contents
- List of figures
- List of tables
- Preface
- Acknowledgments
- 1 Astronomy through the centuries
- 2 Electromagnetic radiation
- 3 Coordinate systems and charts
- 4 Gravity, celestial motions, and time
- 5 Telescopes
- 6 Detectors and statistics
- 7 Multiple telescope interferometry
- 8 Point-like and extended sources
- 9 Properties and distances of celestial objects
- 10 Absorption and scattering of photons
- 11 Spectra of electromagnetic radiation
- 12 Astronomy beyond photons
- Credits, further reading, and references
- Appendix: Units, symbols, and values
- Index
Summary
What we learn in this chapter
Astronomers learn about the cosmos through the study of signals arriving at the earth in the form of electromagnetic radiation or as neutrinos, cosmic rays, meteorites, and, hopefully in the near future, gravitational waves. Electromagnetic radiation travels at speed c and can behave either as a wave or as a flux of photons each of energy E=hν. One can convert between wavelength, frequency and photon energy through algebraic or numerical relations. The bands of electromagnetic radiation extend from radio waves at the lowest frequencies to gamma rays at the highest. The average photon energy, or frequency, of radiation from an object is an indicator of the temperature of the emitting source if the radiation is thermal. Absorption of photons in the earth's atmosphere is frequency dependent so observations of some bands must be carried out from high altitude balloons or space vehicles. Similarly, absorption in the interstellar medium by dust and atoms renders the cosmos more or less transparent, depending upon the frequency band (see also Chapter 10).
Introduction
Electromagnetic radiation is the primary source of our knowledge of the cosmos. Its characteristics (e.g., speed and frequency) are briefly summarized in this chapter. At some frequencies, the radiation can penetrate the atmosphere and ground-based observations are feasible; at other frequencies the atmosphere is opaque and observations must be carried out from space. Particulate matter (e.g., cosmic ray protons and meteorites) also brings us information about the solar system and the Galaxy.
- Type
- Chapter
- Information
- Astronomy MethodsA Physical Approach to Astronomical Observations, pp. 22 - 33Publisher: Cambridge University PressPrint publication year: 2003