Visible And Ultraviolet Absorption Lines (Between The Stars)
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 absorption was taking place not in the atmosphere of the star, but in a region of interstellar space uninfluenced by the motion of the star. Since that time interstellar absorption lines have been observed in the spectra of several hundred stars. The lines are usually much narrower than those formed in stellar atmospheres and sometimes have multiple Doppler-shifted components, corresponding to absorption in clouds with different radial velocities.
Only four atoms, sodium, potassium, calcium and iron, and only two ions, those of singly-ionized calcium and titanium can be seen by their visible absorption lines. Of these, the neutral sodium and ionized calcium are by far the strongest. A large number of atoms and ions, however, have absorption lines at ultraviolet wave¬lengths and can be studied by the use of satellite or rocket-borne telescopes. Of all the ultraviolet lines, the Lyman- ? transition of neutral hydrogen at 121.6 nm is by far the strongest.
Despite the fact that most of the hydrogen is neutral, some interstellar elements, including both calcium and sodium, exist primarily in an ionized state. Along the line of sight to the star £ Ophiuchi, for example, it is estimated that 90 per cent of the gaseous calcium is in the singly ionized form Ca+, 9 per cent in the doubly ionized form Ca++ and one per cent in the form Ca°. The ionization of calcium and other elements is produced by the general back¬ground of starlight and, to some extent, by collisions with cosmic-ray particles. An atom which has been ionized will remain so until it collides and recombines with a free electron. The low density of the interstellar medium ensures that such collisions are rare; the probability that an atom is in an ionized state at any particular time can therefore be quite large.
A few simple interstellar molecules have been detected by their absorption lines. CH, CH+ and ON are seen at visible wavelengths; CO and H2 at ultraviolet wavelengths.