Although most of the visible light from H+ regions is in the form emission lines, the same is not true at longer wavelengths, where most of the energy is radiated in a continuum. There are, in fact two continua: bremsstrahlung or free-free emission from the ionized plasma, which causes the radio flux, and thermal radiation […]
The light from an H+ region is of a quite different character from that of a star. Whereas a normal star emits a continuous spectrum with dark absorption lines, an H+ region emits almost all its light in a few comparatively narrow emission lines .The most important of these emission lines are those of hydrogen, […]
H+ regions As its name implies an H+ REGION (or mi REGION as it is often written) is a part of space where the hydrogen in the interstellar medium is in an ionized rather than in a neutral state. The most common situation where this occurs is in the immediate vicinity of O-type stars, since […]
Because star formation takes a long time by human standards, the astronomer interested in this subject must take on the role of a cosmic archaeologist. He finds varied examples of collapsing clouds and young stars and attempts to put them in the correct evolutionary sequence. From measurements of the size, density, and temperature of an […]
The evolution of a protostar from being an isolated fragment of an interstellar cloud to becoming a main-sequence star takes hundreds of thousands or even millions of years; astronomers attempting to follow this process must therefore resort to computer calculations rather than direct observations. These calculations are very complicated since they involve estimating simultaneously the […]
The evolution of a cloud of interstellar gas depends on a balance between internal gravitational forces tending to make it contract and thermal pressure tending to make it expand. James Jeans, in 1926, first showed that a cloud of given temperature and density can collapse only if its mass is above a certain minimum value. […]
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 […]