Many photons en route to the earth from distant objects do not arrive safely. They may encounter an electron, an atom, a molecule, a dust grain, a star or planet, or even gravitational fields and be absorbed or scattered into different directions of propagation. Scattered photons may appear to an observer as a faint background glow around an object, as a distorted or displaced image, or as a general glow from interstellar space; they no longer arrive directly from the object from which they emanated.

This removal of photons from the image of an object can dramatically affect its appearance, spectrum or temporal variability. An astrophysicist routinely takes these processes into account when studying celestial objects. In fact, the absorption of photons can be quite helpful; it allows detection of diffuse gases between the stars and galaxies. The bending of light beams by gravitational fields allows us to detect dark matter in galaxies and clusters of galaxies.

Interstellar or intergalactic space is not the only place where the loss of photons takes place. It turns out that photons from the interior of a star can not reach the surface; we observe only photons from the star's atmosphere. We infer the physical processes taking place within a star from the indirect evidence offered by its surface. In an attempt to overcome this problem in stars, physicists and astronomers in recent years have been studying neutrinos created in the center of the sun. Neutrinos are created in energy-generating nuclear reactions deep within the sun. They have such a low probability for absorption that they can travel freely through the sun and escape from its surface.

Absorption or scattering, in general, will make a distant object appear fainter, just as a distant street light appears dimmed on a foggy night. If the brightness of a star is the primary clue to its distance, a star so dimmed would appear to be more distant than it really is. Stars in the galactic plane are strongly affected in this way. These processes often depend strongly upon the frequency of the radiation being absorbed. For example infrared radiation can emerge from deep within star forming regions (e.g., the Orion nebula) whereas optical light can not.

The amount of absorption can also vary with angular position over an extended object (if the intervening matter is unevenly distributed) or with time (matter occasionally intervenes). An example of the latter is the periodic partial obscuration of x rays from a neutron star in an accreting binary system by gas in the accretion disk that comes into the line of sight once each orbit.

Fortunately, the physics of the absorbing and scattering processes is usually quite well known from laboratory experiments and theory. Thus it is often possible to reliably infer information about the distant emitting objects. Furthermore, since the absorbing medium is sometimes a part of the system being observed, absorption can tell us about the nature of the system itself.

This chapter will introduce some of the processes involved in absorption and scattering, specifically photon-electron and photon-photon interactions, the extinction of starlight, and the photoelectric effect. The concepts of cross section and opacity are also presented. The fate of photons of various wavelengths (radio, optical, x-ray, etc.) as they travel is a powerful probe of interstellar and intergalactic spaces. These processes apply equally well to the fate of photons as they traverse the earth's atmosphere (Section 2.4).

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