About 1000 planetary nebulae are known. The best-studied ones have angular diameters between about 10 arc sec and several arc minutes, but planetaries can range in size from giant objects such as NGC 7293. the Helix Nebula , which is about one degree in diameter, to objects so small that they appear starlike even on […]
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 […]
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 […]
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 […]
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 […]