There is little doubt that most of the disc of the Galaxy is filled with cosmic rays. What is not certain is whether or not they are also found in large numbers hi the deeper reaches of extragalactic space, apart from such special places as quasars and radio galaxies. At present this question is controversial, […]
One of the few methods of detecting cosmic rays in distant parts of the Galaxy is provided by the very new science of gamma-ray astronomy. When high-speed cosmic-ray protons collide with hydrogen atoms in the interstellar medium, nuclear reactions take place between them and a number of 7i°-mesons are created. These ?0 -mesons, or pions, […]
The interstellar medium is a very strong source of radio waves. Besides individual bright objects such as the supernova remnant Cassiopeia A and the H+ region Orion A, there is diffuse emission visible from all directions in the sky. This radio emission is the result of the interaction between cosmic-ray electrons and the Galactic magnetic […]
Almost all our knowledge of the Universe comes from the study of photons, whether they be of X-ray, visible, infrared or radio wave¬length. There is, however, an increasingly important branch of astronomy that is concerned with quite different particles, COSMIC RAYS. The term cosmic ray is applied to elementary particles, usually protons, electrons or the […]
The first interstellar molecule to be discovered at radio wave¬lengths, the OH RADICAL, is still the most difficult to understand. Although in some dark clouds the OH molecule behaves in a simple fashion, there are many locations in the Galaxy where the OH emission shows characteristics quite unlike those of a normal inter¬stellar molecule. The […]
Molecules are fragile objects; they cannot survive long in normal interstellar space because they are usually rapidly destroyed in bombardment by ultraviolet starlight. It follows, therefore, that the molecules we detect were formed only recently and that the dense clouds where we see them are the chemical ‘factories’ where they are produced. For a chemical […]
The first molecule to be discovered in space at radio wavelengths was the hydroxyl radical (OH) in 1963. Starting in 1968, there then came a rush of new molecule discoveries. Most of these were made at wavelengths of less than one centimeter by groups of astronomers using the 11 –metre radio telescope of the US […]
Molecules in space Like atoms, molecules absorb and emit radiation only at specific wavelengths, as they make transitions from one energy state to another. A molecule is a much more complicated object than an atom, however, and consequently has many more transitions available to it, especially at infrared and microwave wavelengths. This richness not only […]
At wavelengths shorter than 91 nm it is not only the dust grains in the interstellar medium which absorb starlight. At these extreme ultraviolet (EUV) wavelengths the hydrogen atoms themselves are very strong absorbers of radiation by virtue of their ability to be¬come ionized by such energetic photons. It is in the wavelength range just […]
The spectrum of a reddened star generally contains a number of interstellar absorption lines which cannot be attributed to any simple atom, ion, or molecule. These so-called DIFFUSE INTERSTELLAR LINES, or BANDS, are quite different in appearance from the atomic absorption lines in that they are much broader. Whereas an atomic line is typically less […]