Géneral Properties of Nuclei in Astronomy

 Géneral Properties of Nuclei in Astronomy

Constituents of Nucleus and Their Intrinsic Properties Rutherford's experiment on the scatterii falpha particles by mětallic foils was aimed at estimating the distancé of ciosest approach of än alpha particle nearest to the inner core of the atom, i.e. the atomic nucleus: It.was possible to estimate the size of an atòrmic nücleus from the Coulomb repulsive force berween an alpha pårticle of positive charge approaching the atomic core of positive charge when these' are separated by the shortest distance.

 It-was conclusivèly proved that the centrál core of an atom consists of a nucleus which is approximately spherieal in shape with rádius varying between and 10-13 em having a net positive charge.

 Prior to the discovery of neutron, proton-electron hypothesis was proposed on the basis of which the atomic nucleus wàs considered to Be composed of protons and electrons. It was assumed that electrons and protons were present within thě nucleus żXA which will have A protons and Z electrons pre-existing within the nucleus. 

It çould account for mass and charge of the nuçleus and explain B emission by nuclei but could not explain other properties: The electron-proton hypothesis of nuclear structure was rejected because it was contradictory to. the experimentally observed nuclear propeities.

 It was after the discovery of neutron by Chadwick as a pårticle with mass of about 1.67 × 1027 kg and equal to that of a proton but without any charge, that Heisenberg proposed neutron-proton hypothesis of nuclear structure in 1932. It was then well established that the atomic nucleus consisted of the protons, each of mass 1.6726 x 10 27 of mass L.6749 x 10 kg without any charge.

Intrinsic Properties Rutherford's experiment

Intrinsic Properties Rutherford's experiment

Constituents of Nucleus and Their Intrinsic Properties Rutherford's experiinent on the scattérii falpha particles by métallic foils wàs aimed at estimating the distance of closest approach of an alpha particle nearest to the inner core of the atom, i.e. the atomic nucleus: It.was possible to estimate the. size of an atomic nucleus from the Coulomb repulsive force between an alpha particle of positive charge approačhing the atomic core of positive charge when these' are separated by the shortest distance.

 It was conclusivèly proved that the central core of an atom consists of a nucleirs which is approximately spherical in shape with rádiuš varying between To 12 and 10 1 cm having a net positive charge. Prior to the discovery of neutron, proton-electron hypothesis was proposed on the basis of which the atohmic nucleus was considered to be composed of protons and electrons. was assumed that electrons and protòns were present within the nưcleus XA which will have A protons and Z electrons pre-existing within the nucleus.

 It çould, account for mass and charge of the nucleus and explain B emission by nuelel but could not explain other properties: The electron-proton hypothesis of nuclear structure was rejected because it was contradictory to the experimentally observed nuclear properties. It was after the discovery of neutron by Chadwick as a párticlé with mass of about 1.67 × 1027 kg nd equal to that of a proton but without any charge.

The slowly varying component of the solar radio radiation

 The slowly varying component of the solar radio radiation

The longer wavelengths from Sun that we receive on Earth originate in the corona where the characteristic temperature is high. The temperature relevant to the measured intensity of meter wavelengths is - 10° K. The intensity of this radiation undergoes slow variation, corresponding to the variations in the coronal structure. The measured radiation actually correspotids to a temperature of 10° K and a density nearly equel to three times the normal coronal density.

 The slowly varying component of the solar radio radiation thus fits with the concept that-it is thermal radiation trom regions of coronal condensations. The mechanism which gives risc to this radiation is free-free transition of clectrons in the hot coronal gas. The burst component of solar radio emission is sporadic, intense and nonthermal in character. This component is associated with the activity of the Sun. Different types of bursts have been recognized and the physics behind them is quite complex. As the name implies, bursts are manifested as the enhancement of. the radiosemission by onders of magnitude within periods as short as a few seconds.

 If the intensitaesof tursts are interpretedas of thermal origin, the corresponding equivalent blackbody temperatures would be of the order of 1012 K, which is absurd. The bursts are of different nature and they have been broadly classified into types 1, II, III. IV and V. Of these, all but the Type I bursts are correlated with flares.

 Type I bursts are not associated with flares or centres of activity. Type I bursts originate in noise storms that are produced high up in the corona usually above larger spots, their polarization showing no correlation with that o the spots. These bursts usually have lite-times of 0.1 to 0.2 seconds and during this period the wavelength of radiation remains fairly constant. The radiation.

 The theory of Gold and Hoyle thus predicts the release of the right amount of energy within the right amount of time as are observed in flares, through the meehanism of the magnetic field annihilation. The theory therefore implies the operation of a mechanism by which the twisted magnetic field which develops on the selar surface may become untwisted through flare discharge: Thistrunstormation irom twisted to untwisted field may have important bearing on the equilibriam configuration of the Sun. erg may be associated with such a twisted Soon after the production of radio frequency waves in the laboratory by Heinrich Hertz in 1888.

 Some scientists started thinking that the Sun might be a source of radie waves. Valuable work was one in this line by Wilsing. Scheiner, Nordmann, Jansky and others during the first few decades of this century and the end of the last century. But the actual detection of Sun.

The Second World War Independently By Stanley Hey of Britain

The second world war independently by Stanley Hey of Britain

  The shorter radio waves of wavelength - I cm come from the lower chromosphere which corresponds to a radiation temperature of 10 K. Meter and decameter wavelengths which are generated at the lower chromosphere are absorbed by ionized gas in the upper chromosphere and the corona, and therefore, cannot reach the Earth.

 The radio source was missed primarily due to poor technological advancement in those years. The discovery was finally made during the second world war independently by Stanley Hey of Britain, and by G.C. Southworth and G. Reber of the U.S.A. The subsequent decades were marked by intensive as well as extensive studies of the solar radio emission by scientists all over the world.

 The study of the radio-spectrum of the Sun has revealed that the radio emission from the Sun can be classified into two well-defined components: (a) a slowly varying component with sufficiently lone characteristic times, and (b) the sporadie burst component having much shorter characteristic times. The first component is of thermal origin and is recognized as the characteristic of the quiet Sun, while the second component is nonthermal which is characteristic of the disturbed Sun.

 As a hot body the Sun must emit radiations in rauio wavelengths, but since the temperature of the Sun is not uniquely defined (being different at different levels of the atmosphere) the nature of the solar thermal radio emission is altitude-dependent.

 The highly ionized corona is completely opaque to longer radio waves just as the case with the Earth's ionosphere. Thus, observing the Sun at different radio wavelengths means observing at different levels of its ätmosphere. This also enables us to distinguish between different levels in chromosphere and corona from which various disturbances give rise to radio emission.

Severny's theory has not emerged as fuli proof in magnetic field

Severny's theory has not emerged as fuli proof in magnetic field

 

 high gradient of fields. In fact, substantial modifications in the magnetic field structure are actually observed in the flare region after the flare has died out. Moreover, large magnetic field gradient is capable of accelerating particles to high energies which can subsequently emit radiation in wavelengths ranging from X-rays to long radio waves, as are actually observed in flares. But there are other observations of magnetic fields which Severny's cannot explain, and so Severny's theory has not emerged as fuli proof.

 A plausible theory of the origin of flares has also been proposed by Gold and Hoyle. Their theory relies for the energy source on coalescing and cancellation of two bundles of twisted magnetic lines of force. They imagine a filament consisting of a bundle of lines of force which emerges from a point in the photosphere and re-enters ät another point thus forming an arch.

 This bunch of lines of force may be twisted due to highly  In order that this energy may be released abruptly, a pair of such fil ments with initial magnetic fields in opposite directions but the same sense of twisting is hypothesized. Two such filaments placed close to each other will attract each other, interpenetrate, and a part of the field will be annihilated in a time-scale of the order of 10 seconds.convective motion in the subphotospheric layer.

 The energy density of this convective mass motion is 10 erg em only a few per cent of which is associated with flarë phenomena. The twisting of the field can greatly increase the intensit, of the initial moderate field. Calculation shows that an amount of magnetic energy equal to - 100 filament. This is the right amount of energy that is involved with flare phenomena.