Most celestial bodies emit light which is unpolarized. The few exceptions include certain white dwarf and peculiar A stars, sources of synchrotron radiation such as the Crab Nebula, and reflection nebulae. Nevertheless the light from many ordinary stars contains a measurable degree of linear polarization. The amount of polarization is greatest for stars which are […]
Two things can happen to light rays when they meet a dust par¬ticle ; they can be either absorbed into the grain or scattered back into space. The relative importance of these two processes depends on the albedo of the grain, which is defined thus: albedo = Light scattered/ light scattered + light absorbed The […]
Extinction and reddening The interstellar medium in nowhere completely transparent and nowhere completely opaque; a dusty region of space dims rather than blocks the- light passing through it. Thin dimming process is called INTERSTELLAR EXTINCTION. Interstellar extinction in front of a star can be expressed quantitatively, and is usually given the symbol Av whore Av […]
The relationship between the interstellar medium and the stars is a dynamic one; matter is exchanged between them all the time. Our Sun, for example, emits particles, in the form of a solar wind, and absorbs others in the form of cosmic rays. In this case the amount of mass exchanged is small compared with […]
The most noticeable manifestations of the interstellar medium are NEBULAE. The term ‘nebula’ is used fairly loosely in astronomy; the word is derived from the Latin for cloud, and can refer to almost any object that appears fuzzy or extended through a telescope. One of the early catalogues of nebulae was that of Messier, who […]
The Emptiness of space The briefest glance at the night sky reveals one of the most remarkable features about the Universe: its inhomogeneity. Most of the matter in the Universe is clumped together rather than spread out uniformly. This is true of our Solar System, which consists of planets with ‘space’ between them, of our […]
The origin of the planets constitutes one of the central problems of astronomy, perhaps the more so as it includes the origin of the Earth itself. The problem has remained a permanent challenge down the years, and has stimulated many attempts which even if not yet entirely successful have led to important new lines of […]
Comets have excited interest in men’s minds from time immemorial. There are two classes of comets. First, but less important, are the SHORT-PERIOD COMETS moving in eccentric orbits at planetary distances from the Sun, and of these about a hundred are known. Second, are the far more numerous LONG-PERIOD COMETS, of which almost 500 have […]
The dust which reaches the Earth , or is collected by spacecraft as micrometeorites makes its presence known in another way. The particles that make up this dust travel in elliptical orbits around the Sun and are concentrated in the plane of the ecliptic. They reflect the light of the Sun to give a luminous […]
If you view the sky on a clear moonless night you may see streaks of light several times per hour. These luminous streaks are called shooting stars or METEORS . The Solar System contains a large number of small particles moving in orbits around the Sun. In the vicinity of the Earth these particles typically […]