Photon and nonphoton astronomy

The several types of signals received from space are outlined here.

Photons (electromagnetic waves)

The astronomical "light" that arrives at the earth from a distant source is known as electromagnetic radiation. This radiation can be described in terms of waves or in terms of photons. Electromagnetic waves are propagating electric and magnetic fields whereas photons are discrete bundles of energy. These two descriptions of light are difficult to reconcile with one another intuitively. Both are correct; the radiation can behave as one or the other under different circumstances.

The refraction of light in a prism is usually described in terms of waves, while the ejection of electrons from an illuminated surface (photoelectric effect) may be understood in terms of momentum transfers from individual photons.The detection of radio sources with high angular resolution by means of a technique called inter-ferometry arises from the interference of waves detected with two or more separate telescopes. In gamma-ray astronomy, individual photons are detected one-by-one with scintillation crystals that emit a detectable pulse of light when a single gamma ray interacts with the atoms of the crystal.

The electromagnetic waves described by Maxwell's equations are encountered as radio waves, infrared radiation, optical light, ultraviolet radiation, x rays and gamma rays. These different names simply specify ranges of wavelengths or frequencies. The lowest frequencies (or longest wavelengths) are radio waves, and the highest frequencies (or shortest wavelengths) are gamma rays. In the discrete picture, the photons, or quanta, carry energy and momentum much as a mass-bearing particle does. A radio photon has very low energy while a gamma-ray photon has very high energy. In both pictures, the energy is propagated at the speedof light (3.0 x 108 m/s in a vacuum), and the signal has the following properties: intensity (number of photons), frequency ("color"), polarization, and direction of travel.

Astronomers usually refer to radio and optical radiation in terms of waves, characterizing them with a wavelength X or a frequency v, while they refer to x rays and gamma rays as photons, characterizing them with an energy E. The terminologies are completely interchangeable; presented below are the relations between frequency of a wave and the energy of a photon.

Photons are invaluable for astronomy because they travel in straight lines, for the most part. Photons thus appear to come directly from the spot on the sky whence they originated. Not all of these photons arrive at the earth undisturbed however. Low-frequency radio waves undergo refraction in ionized interstellar plasma; radiation from distant quasars is bent by intervening galaxies which act as gravitational lenses; optical photons can be scattered by tiny grains of graphite, silicates and ice (called dust); ultraviolet photons and x rays can be absorbed by neutral atoms. Nevertheless, under many circumstances, photons do travel along a straight path from the source to the earth and are not strongly absorbed in the interstellar medium. Such photons provide astronomers with a relatively clear view of the cosmos.

Cosmic rays and meteorites

Information about the cosmos can also be gleaned from the detection and measurement of particulate matter. Cosmic rays are bits of matter (protons and heavier atomic nuclei) that travel with high energies, arriving at the earth from distant celestial regions (e.g., the sun, a supernova, an active galactic nucleus). Since most cosmic ray particles are charged, the weak and irregular magnetic fields that lie between the stars will change their directions of travel through the action of the magnetic F = q (v X B) force. The particles from a given source will spiral around the magnetic fields in the Galaxy for millions of years, circulating in the company of particles from many other sources.

Most cosmic rays thus arrive at the earth from a direction that bears little if any relation to the direction of their point of origin. That is, cosmic rays from many different sources can arrive at the earth from the same apparent direction. It is as if you were very near-sighted, took off your glasses, and looked at the sky. The light from many stars would be mixed together on your retina, and you would not be able to study individual stars. However, even with this mixing, cosmic rays provide important information about the details of supernova explosions and the interstellar spaces through which they travel. (See Section 12.3.)

Macroscopic chunks of matter, known as meteoroids, arrive at the earth from the solar system. As they penetrate the earth's atmosphere, they heat up due to atmospheric friction and lose material by vaporization; in the night sky, they are easily visible as bright rapidly moving objects. In this stage, they are called meteors ("shooting stars"). Some burn up completely before reaching the earth's surface. The remains of others reach the earth's surface (meteorites). Analyses of their chemical composition provide valuable data on the quantities of the chemical elements in the solar system and on the ages of those elements. Meteorites are reminiscent of the moon rocks brought to earth by Apollo astronauts except that they are a lot less trouble to obtain; they arrive on their own and from much greater distances!

Neutrino and gravitational-wave astronomy

As noted in Chapter 1, the universe can be probed through observations of neutrinos and gravitational waves. These are relatively new branches of astronomy, and substantial effort is now being expended on their development. Neutrino observations of the sun have been in progress for a number of years and new more sensitive experiments are now in progress. A burst of neutrinos was detected from the implosion of Supernova 1987A in the Large Magellanic Cloud, giving us a surprising and dramatic interior view of the core of a star at the instant of its collapse to a neutron star. (See Section 12.2.)

Gravitational waves have not been detected directly. Large (several kilometer) sensitive detectors are now operating and attempting detections. One candidate emitter that should be detectable is the gravitational "chirp" expected in the last moments in the life of a binary star system consisting of two neutron stars that are gravitationally bound to one another. The large accelerations of the two stars lead to the emission of gravitational radiation. This causes the system to lose energy. The stars thereby decrease their separation and orbit each other at increasing speeds. In turn, this increases the rate of energy loss to gravitational radiation. The inward spiraling of the orbits thus increases at a faster and faster pace. In the last few seconds before they finally merge to become a black hole, they should emit a strong chirp of about 1-kHz gravitational radiation. (See Section 12.4.)

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