The theory of stellar structure and evolution gives a fairly good account of the observed properties of stars. Part of the theory is concerned with the generation of nuclear energy inside stars, and it appears that during nuclear fusion processes a substantial flux of NEUTRINOS is produced. Neutrinos have no electrical charge and scarcely any […]
When an electrical charge is accelerated in a straight line or moves around an arc of a circle, the charge radiates an electromagnetic wave, similarly, when a massive object is accelerated general relativity predicts that it will radiate a GRAVITATIONAL WAVE. Although there is no celestial object we can point to as one that does […]
Materials such as silicon and germanium are known as SEMI¬CONDUCTORS. The electrons in these materials are fairly tightly bound to the atoms of which they are composed. Because there are few free electrons in these materials, only a little current flows when an electric field is applied across a specimen of the material. However, photons […]
There is a group of substances that will absorb photons and subsequently emit the electrons excited by them. These materials are said to be PHOTOEMISSIVE and they are used in PHOTOELECTRIC DETECTORS. The emitted electrons are called PHOTOELECTRONS. Materials such as caesium antimonide and gallium arsenide are good photoemitters and can have an efficiency as […]
The most frequently used light detector is still the PHOTOGRAPHIC PLATE. It is easy to get the incorrect impression that the advent of the many modern detectors described below has made the photo¬graphic plate obsolete. The quality and simplicity of use of the modern photographic plate ensures that it will be a long time before […]
At optical wavelengths it is convenient to think of the radiation as consisting of a stream of photons. All optical detectors function by transferring some or all of the energy of an incident photon to one or more electrons. The excited electrons arc then detected in a wide variety of ways. A number of basic […]
The basic form of radio-wave detector is the simple dipole or antenna. It consists of two vertical metal rods, one above the other and a quarter of a wavelength long. If, in the vicinity of the dipole’ there is an electromagnetic wave which has its electric field parallel to the length of the dipole then […]
The bewildering variety of devices used as radiation detectors could easily give the impression that this is a difficult subject, but this is not so. Electromagnetic radiation can only interact with charged particles. At wavelengths longer than those of X-rays the energy of a single photon is so small that only the lightest charged particles […]
Before considering the variety of techniques used to detect the radiation collected by telescopes and analysed with spectrographs or polarimeters, we need to establish what we must do to detect radiation. Most astronomical objects are very faint; they are ob¬served in the presence of a considerable amount of unwanted radiation, such as that from the […]
Electromagnetic waves consist of rapidly alternating crossed electric and magnetic fields. The radiation from most astronomical objects is such that the direction of the electric or the magnetic fields in the waves varies in a random manner. We say that the radiation from these objects is unpolarized. Some objects, however, emit waves which have these […]