Almost all planetary nebulae have a remarkably symmetric structure, often in the shape of a ring but sometimes more like an hourglass . The differences in shape are presumably mainly due to variations in the way the gas shell was initially expelled from the star. The speed of rotation of the star, its radiation pressure […]
Similarities to H+ regions Like an H+ region, a PLANETARY NEBULA is a cloud of interstellar gas, mainly hydrogen, ionized by a hot star. The important difference between the two types of nebula is generic; while H+ regions are usually a by-product of the birth of highly luminous O stare, planetary nebulae are old objects, […]
The Orion Nebula is the nearest H+ region to the Earth and the one that has received the greatest attention from astronomers. It is visible as a 4th magnitude fuzzy patch of light in the Sword region of the Orion constellation. On long exposures the nebula appears to be about half a degree across and […]
Since an H+ region is at a much higher temperature than the neutral gas which surrounds it, it exerts an outward pressure at the ionization front. The main characteristic of the evolution of an H+ region is therefore one of expansion. The birth of an H+ region is generally a by-product of the birth of […]
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. […]