Molecules are fragile objects; they cannot survive long in normal interstellar space because they are usually rapidly destroyed in bombardment by ultraviolet starlight. It follows, therefore, that the molecules we detect were formed only recently and that the dense clouds where we see them are the chemical ‘factories’ where they are produced. For a chemical […]
The first molecule to be discovered in space at radio wavelengths was the hydroxyl radical (OH) in 1963. Starting in 1968, there then came a rush of new molecule discoveries. Most of these were made at wavelengths of less than one centimeter by groups of astronomers using the 11 –metre radio telescope of the US […]
Molecules in space Like atoms, molecules absorb and emit radiation only at specific wavelengths, as they make transitions from one energy state to another. A molecule is a much more complicated object than an atom, however, and consequently has many more transitions available to it, especially at infrared and microwave wavelengths. This richness not only […]
At wavelengths shorter than 91 nm it is not only the dust grains in the interstellar medium which absorb starlight. At these extreme ultraviolet (EUV) wavelengths the hydrogen atoms themselves are very strong absorbers of radiation by virtue of their ability to be¬come ionized by such energetic photons. It is in the wavelength range just […]
The spectrum of a reddened star generally contains a number of interstellar absorption lines which cannot be attributed to any simple atom, ion, or molecule. These so-called DIFFUSE INTERSTELLAR LINES, or BANDS, are quite different in appearance from the atomic absorption lines in that they are much broader. Whereas an atomic line is typically less […]
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 […]