Star formation at the present epoch The birth of a star is a rare, slow event; all but a very few of the stars visible to the naked eye have existed longer than mankind. It is therefore first necessary to consider the evidence that new stars are being formed in the Galaxy at the present […]
Less is known about the intercloud medium than about the clouds. The intercloud medium appears to contain both atomic and ionized hydrogen, and there are several lines of evidence to suggest that the average concentration of free electrons in the intercloud medium is about 3 X 104 atoms m-3. Observations at 21-cm wavelength suggest that […]
The thickest and densest interstellar clouds consist mainly molecular hydrogen. The reason for this is that the formation A preservation of molecules is facilitated by a high concentration f gas (and consequently a greater frequency of atomic collisions! and also a high concentration of dust grains which assist the formation of hydrogen and shield it […]
The most crucial factor on which the properties of an interstellar cloud depend is the state of its most prevalent constituent, namely hydrogen; the state of the helium and of the heavier elements is of secondary importance. Although the various different forms of hydrogen can coexist to some extent, it is a reasonable simplification to […]
The unevenness of the interstellar medium The previous chapter described the ways in which the interstellar medium can be observed, and summarized the major properties of the dust, the gas, and the cosmic rays which comprise it. This chapter is concerned with the way in which the interstellar medium is generally concentrated into clouds, particularly […]
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 […]