The order and regularity of the planetary motions provided the key to the structure of the Solar System and the discovery of the law of gravitational attraction. Likewise, the study of galactic dynamics has been, and is, essential to a determination of the structure of the Galaxy. Careful observations of stellar radial velocities (motion of […]
At optical wavelengths, interstellar matter has proved to be more a hindrance than a help for the determination of galactic structure. Happily, in the radio domain (this is the region of the electromagnetic spectrum where the wavelengths are longer than one millimeter), the converse is the case: firstly because the galactic atmosphere is almost everywhere […]
Two simple observations with the naked eye show that the Milky Way contains gas. Firstly, certain nebulae such as the orion Nebula have a fuzzy appearance . Secondly the presence of obscuring matter may be inferred from the existence f the dark patches in the Milky Way that were mentioned . This matter is much […]
A faint luminous band can be observed in the clear night sky spanning a great circle on the celestial hemisphere. This band varies in width around an average of some 20°. Dark patches are seen in it, but it is clearly brighter than the sky background and hence it was long ago named the MILKY […]
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 […]