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
10 - Absorption and scattering of photons
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
Our knowledge of celestial objects must take into account absorption and scattering of photons as they travel to earth observers. These processes are highly frequency dependent and thus affect some bands more than others. Photon–electron interactions include Rayleigh, Thomson and Compton scattering which explain, respectively, the blue sky, light from the solar corona, and a distorted spectrum of 3-K background radiation in the direction of x-ray emitting clusters of galaxies (Sunyaev–Zeldovich effect). Photons of very high energy, ≳ 1015 eV, are absorbed through pair production interactions with photons of the cosmic microwave background. Photons with energies from 13.6 eV (ultraviolet) through ∼2 keV (“soft” x ray) are absorbed by atoms in interstellar space through the photoelectric effect. Optical light from stars in the plane of the Galaxy is absorbed (extinction), reddened (color excess), and polarized by interstellar grains (dust). The polarized starlight maps out interstellar magnetic fields. A useful correlation exists between the locations of dust and hydrogen in the Galaxy.
The beam intensity that survives passage through a uniform absorbing medium decreases exponentially with distance traveled. The rate of decrease depends upon the cross section (m2 per absorbing atom) or opacity (m2 per kg) of the absorbing medium. Photoelectric absorption in the interstellar medium (ISM) depends strongly on the composition of the interstellar gases (cosmic abundances) and is a strong function of photon energy. […]
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- Astronomy MethodsA Physical Approach to Astronomical Observations, pp. 298 - 332Publisher: Cambridge University PressPrint publication year: 2003