Two simple observations with the naked eye show that the Milky Way contains gas. Firstly, certain nebulae such as the orion Nebula have a fuzzy appearance . Secondly the presence of obscuring matter may be inferred from the existence f the dark patches in the Milky Way that were mentioned . This matter is much less conspicuous than the stars, because it does not itself generate energy. If all the matter in stars near the Sun were distributed evenly through the same volume of space, its density would be 7 x 10-21kgm-3, hence at most 12 per cent of the total mass in the solar neighborhood consists of gas and dust. Although it is very tenuous, this dispersed matter is an important constituent of the Galaxy.

The dimming of light as it travels through interstellar matter presents a formidable obstacle to the determination of the structure of the Galaxy. Light passing through a typical part of the galactic plane is dimmed by two magnitudes for every kiloparsec it travels. Consequently, optical observations in the plane are limited to the nearest 3 kpc , so that only local information can be obtained by optical telescopes. Worse still, the degree of obscuration depends strongly on the direction in which the observations are made . All of the structure determinations done in the early years by the technique of star counts have been proven invalid because of the then unsuspected irregularity of interstellar extinction. The dimming of the light depends also on the angle which the line of sight makes with the galactic plane. From measurements of the change of absorption with galactic latitude, it has been deduced that the mean thickness of the layer of interstellar matter is 150 pc. This is only 100 times” the average distance between stars, so apparently the gas layer is extremely thin with respect to its extent in the plane.
The layer of gas forms a curious galactic atmosphere, pervaded by a magnetic field; but the flux density is only 50 nT (nanotesla), which is 100 000 times weaker than the field of Earth. It appears that this field is too weak to influence galactic structure on scales larger than about one hundred parsec. Likewise, other aspects of the ‘galactic weather’, such as the propagation of blast waves from exploding novae and supernovae, the local ionization by hot stars, and the diffusion of cosmic rays, do not influence large-scale structure, but merely generate clouds with sizes up to hundreds of parsecs.

These phenomena leave long-lived traces in the galactic atmosphere. For example, the blast waves from exploding stars disperse matter into interstellar space. Since the stars that generate these shells of debris have changed part of their hydrogen by nuclear fusion into heavier elements (especially carbon, nitrogen, oxygen and iron), the chemical composition of the interstellar medium is modified by the addition of these heavy elements. In supernovae, elements much heavier than iron are made in the blast wave itself. Composition changes in the interstellar medium are also caused by highly energetic cosmic rays which collide with interstellar atomic nuclei and smash them up. The splinters left over from the collision are nuclei of light elements, especially those lying in the range between helium and carbon. It is now known that processes like these are important for galactic structure: new stars form from some of this modified matter, and then change it further by their own nuclear processes. Thus a record of the galactic weather is kept by the composition of stars and the interstellar matter; the evolution of the Galaxy may be traced by analyzing spectral lines in order to deduce this composition. It is fairly certain that our Galaxy formed about 12-14 billion years ago, from a collapsing cloud of. hydrogen (about three quarters of the total mass) and Helium (one quarter). (For comparison, note that the age of the Universe is probably 13-20 billion years.) Between 80 and 90 per cent of this gas condensed into a first generation of stars within a few hundred million years. After, or perhaps due to, this initial burst of star formation, the Galaxy settled down into the form it still presents to us today. We will now look at that form as it appears in radio astronomy.

Comments are closed.