More than twenty different elements have now been detected by their visible or, more commonly, by their ultraviolet interstellar absorption lines. Most of these elements exist both in their neutral and their ionized state; sulphur, for example, has been identified as S°, S+, S++, and S++,. Since the number of each kind of atom can […]
The original discovery of interstellar gas preceded the development of radio astronomy by several decades. In 1904 it was discovered I hat certain of the absorption lines in the spectrum of the binary star ? Orionis did not change in wavelength as the star moved around its orbit. It soon became clear that this particular […]
The hydrogen 21 -cm line Interstellar gas is more difficult to see than interstellar dust, despite the fact that there is much more of it. The reason for this is that while solid particles absorb and emit radiation over a wide range of wavelengths, gases emit and absorb only at certain dis¬crete wavelengths. In studying […]
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 […]
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 […]