Given the distance to the majority of astronomical objects of interest, the only practical way of obtaining information about these objects is by an analysis of radiation. In this context radiation may refer to electromagnetic waves, gravitational waves, or so-called "cosmic rays," which are actually high energy particles. Gravitational radiation and cosmic rays will be considered later.
There are two mechanisms by which energy is transferred from one point to another. One is an energy transfer due to the net motion of matter between the two points, as for example when a ball is thrown from one person to another. The other mechanism is that of wave motion, which does not involve any net motion of matter, but rather the propagation of a disturbance of some sort.
A wave consists of a disturbance of some sort which travels in a material medium (or through space in the case of electromagnetic waves). While many sorts of wave are possible, the most common situation involves a periodic wave in which the waveform is repeated indefinitely. Waves are further classified as longitudinal or transverse, with longitudinal waves having a disturbance which is parallel to the direction of wave motion, and transverse waves having a distrubance which is perpendicular to the direction of wave motion.
A periodic wave is characterized by its amplitude, and wavelength. In addition, a wave of a given type travels through a given medium with a characteristic wave speed. The wave speed and wavelength are related to each other by the wave equation.
velocity = (frequency) X (wavelength)
When two or more waves are present simultaneously in the same medium interference occurs. At the two extremes, the interference may be constructive or destructive. More generally, the interference of waves is intermediate - neither completely constructive or destructive.
Waves may be caused to interfere for any number of reasons.
The waves passing through different parts of an opening are found to interfere with each other in a way that produces a phenomenon known as diffraction. The degree of diffraction depends on the relative sizes of the wavelength of the wave and the size of the opening through which the waves move.
When a wave encounters a boundary between two different media two things (in general) happen. Part of the incident wave will be reflected back into the incident medium and part will be refracted into the medium being encountered. Note that both of these phenomena occur due to a difference in the wave speed in the two media. The law of reflection states that the reflected wave will leave the boundary at the same angle as the angle of incidence. The law of refraction (or Snell's law) states that the angle of refraction and the angle of incidence are related by the indices of refraction of the two media (or equivalently the wave speeds in the two media).
Electromagnetic waves exhibit all of the behaviors described above (under proper circumstances), but differ in one fundamental way - EM waves require no material medium for their propagation. This is possible because an EM wave is not a disturbance of a material from its equilibrium position, but rather a disturbance in the values of the electric and magnetic fields in some region of space (whether or not a material is present).
Modern physical theory postulates four fundamental forces which are responsible for the various interactions between fundamental particles. These four forces are the gravitational, electromagnetic, strong nuclear, and weak nuclear. The electromagnetic force, in turn, is often further separated into electric and magnetic forces, although they are actually two facets of the single electromagnetic force. These four forces are pictured as being mediated by various fields.
Many of the fundamental particles such as the electron and proton possess an electrical charge, while others such as the neutron are uncharged. Under most circumstances macroscopic objects are uncharged, or nearly so, due to the fact that they are composed of very nearly equal numbers of positively charged and negatively charged particles. When a particle (or a larger object) does possess a net charge it produces an electric field in the surrounding space.
Fields of this sort are produced by electric charges at rest. If the charges are in motion, in addition to the electric field there exists a magnetic field. Thus, electric currents are the sources of magnetic field. As the net charge of most macroscopic objects is zero or nearly zero, so also the electric currents (in the form of electrons orbiting nuclei in atoms) in most macroscopic objects cancel in such a way that the magnetic fields of most materials are often zero or small.
Electromagnetism
Another result of elecromagnetic theory is that when a magnetic field in some region of space changes there is an electric field produced (Faraday's law). Conversely, a changing electric field in some region of space produces a magnetic field (Ampere's law).
Electromagnetic Waves
As a result of the interlinking of electric and magnetic fields which vary with time, accelerated electric charges are found to produce electric and magnetic fields which propagate through space. As with any wave there is a very specific propagation speed - in this case the speed of light which is c = 2.99 X 108 m/s.
One familiar example in which accelerating charges produce EM waves is the antenna.
Sources and Detectors for Various Spectral Ranges
Absorption of Radiation
Wien's Law
l max = 0.29/T
Stefan's Law
P = e
s A T4
(Click on the car's engine to hear doppler effect.)