The Intercloud Medium (Clouds ,Nebulae Star Births And Deaths)

Less is known about the intercloud medium than about the clouds. The intercloud medium appears to contain both atomic and ionized hydrogen, and there are several lines of evidence to suggest that the average concentration of free electrons in the intercloud medium is about 3 X 104 atoms m-3. Observations at 21-cm wavelength suggest that the intercloud medium is a warm, partially-ionized gas with a density of about 2 x 105 atoms m-3, a temperature of perhaps 8000 K, and with about one hydrogen atom in seven ionized by some means. Ultraviolet .observations, however, do not completely support this picture and suggest that the free electrons exist in low density H+ regions, separate from the atomic hydrogen. These H+ regions are ionized by distant 0 and B stars or, alternatively, by the X-rays from the occasional super-novae that explode in the Galaxy.

Ultraviolet studies have also demonstrated the existence of yet another form of interstellar gas, namely an ionized component with an extremely low density, but a temperature in excess of 100000K. These regions are thought to represent ‘holes’ in the interstellar medium caused by supernova explosions. These holes can reach a diameter of 50pc and, it is conjectured, may join up with each other and produce a network of tunnels through the inter¬stellar medium, rather like a Swiss cheese.

One of the most prominent effects of the free electrons in the interstellar medium is to delay radio waves as they pass through the Galaxy .The delay is most noticeable in the case of signals and the phenomenon is therefore often called PULSAR DISPERSION. The delay is greater for low-frequency radio signals than for high-frequency ones; a 300-MHz signal from a 300-pc distant pulsar would be delayed by about 0.5 sec by the interstellar medium, whereas a 150-MHz signal would be delayed by 2 sec. This 1.5 sec time difference in the arrival of the different frequency pulses is easily measured, and can sometimes be used to make an estimate of the distance to the pulsar.

Another effect attributable to free electrons is INTERSTELLAR SCATTERING. Irregularities,” such as clouds, in the density of the free electrons can lead to scattering of radio waves as they pass through the Galaxy. The effect is small and significant only at low frequencies; at 80MHz the scattering is typically a few tenths of a second of arc. The effect can nevertheless impede the measurement of the true sizes of compact radio sources such as quasars and OH masers.

Filed under: Clouds


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