We cannot examine interstellar dust in a laboratory. Small inter¬planetary particles that reach the Earth or that are intercepted by spacecraft are characteristic only of our Solar System and are prob¬ably quite different from those found in interstellar space. What knowledge we have about the size, shape and composition of interstellar grains is based on […]
Since starlight carries energy, any light that is absorbed (as opposed to scattered) by a dust grain tends to increase the temperature of the dust grain. This absorption of energy is balanced by the energy that the grain reradiates into space. The wavelength of the radiation from the grain depends on its temperature. Normal interstellar […]
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 […]
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 […]
In a close binary system, the stars are near enough to transfer matter from one to the other. Mass-flux rates of up to about a millionth of a solar mass per year have been observed by optical astronomers. A compact object in a close binary system can, therefore, receive enough matter to power it as […]
X-rays are associated with high-energy phenomena. The Sun emits most of its radiation at frequencies corresponding to visible light and has an effective surface temperature of about 5800 K. X-ray photons are roughly a thousand times more energetic than the photons of ordinary light, and so to produce X-rays we need either temperatures of several […]
An important parameter to measure is the mass of a neutron star. This can best be determined in the same manner as for ordinary stars, by the observation of a binary system that contains a neutron star. To get the masses unambiguously we really need an eclipsing, double-lined binary, in which both components give us […]