Apart from the handful of remnants in our Galaxy which can be identified with particular explosive events, the ages of Supernova remnants must be determined by indirect means. In Some Cases, such as cas A, this may be; done by comparing the diameter of the remnant with its expansion velocity; in general, however, one must […]
X-rays have been detected from several supernova remnants by the use of satellite-borne telescopes. The case of the Crab Nebula has already been mentioned; here the X-rays are almost certainly due to synchrotron radiation. Most SNRs, however, do not contain a pulsar that can maintain the supply of the ultra-relativistic electrons which are necessary for […]
The CRAB NEBULA is almost certainly the result of a supernova explosion which was observed by Chinese astronomers on 4 July 1054. This supernova was so bright that it was risible in full daylight for three weeks. Strangely, however, there is no record of its having been seen in any of the European or Arabian […]
There are about a hundred known supernova remnants in Galaxy. Although visible, X-ray and gamma radiation have been detected from a few of these, most are seen only at radio wave¬lengths. The radio emission is caused by high-speed electrons, produced within the supernova, spiralling around magnetic field lines. This process, called synchrotron radiation, is also […]
The explosion of a supernova SUPERNOVA EXPLOSIONS are the most spectacular events that occur in our Galaxy. During the first week of its supernova phase, a star can reach an absolute magnitude of —20.5, which is 10 billion times more luminous than the Sun, and as bright as many complete galaxies. Supernova explosions are rare […]
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 […]
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 […]