The Interstellar Medium (Between The Stars)
The Emptiness of space
The briefest glance at the night sky reveals one of the most remarkable features about the Universe: its inhomogeneity. Most of the matter in the Universe is clumped together rather than spread out uniformly. This is true of our Solar System, which consists of planets with ‘space’ between them, of our Galaxy,, which consists of stars with ‘space’ between them, and of the Universe which consists of galaxies and clusters of galaxies with ‘space’ between them. These three types of space are usually described as inter¬planetary, interstellar and intergalactic, and the matter, if any, which fills them is referred to as a medium. The properties of the interplanetary, interstellar and intergalactic media are quite different. This chapter is purely concerned with the interstellar medium
The main constituents of the INTERSTELLAR MEDIUM are gases, mainly hydrogen and helium, and minute solid dust particles. As far as we know there are no liquids in interstellar space. Although interstellar space is nowhere truly empty, it is in most places far more sparsely filled than even the best laboratory-produced vacuum. For example, one cubic meter of air at sea level contains about 5 x 1025 atoms. At the height at which artificial satellites orbit the earth, say 300km, the density has fallen to 1015m-s. In interstellar space the density is often a billion times less than this, with only one atom in each cubic centimeter. A ray of light leaving the surface of the Earth and travelling in a straight line across our Galaxy would intercept more gas in the first ten kilometers of its path through the Earth’s atmosphere than on the whole of the rest of its journey. This would also be true of a ray of light passing through the intergalactic medium to distant quasars in the Uni¬verse. Nevertheless, the volume of interstellar space is so great that the total quantity of interstellar material is very considerable. Our own Galaxy contains about 1010M0 of it, comprising some 10 per cent of its total mass. Most of this interstellar matter lies in the spiral arms and the disc of our Galaxy, in a layer a few hundred parsecs thick.
The interstellar medium affects and often hinders many of our studies of distant objects. Although there is, on average, only about one dust grain in every 100000 cubic meters of space, there are enough of them along the line of sight to many stars to render these stars almost invisible. Radio waves are attenuated and delayed by free electrons; ultraviolet and X-rays are absorbed by hydrogen and oxygen atoms. The night sky is confused by scat¬tered light and by glowing gas clouds; interstellar magnetic fields deflect and jumble fast-moving cosmic-ray particles in their paths across the Galaxy.
On the other hand, the interstellar medium presents us with a cosmic laboratory on a scale which* could never be achieved on Earth. In it we can study the physics and chemistry of matter at very low densities. For example, whereas most gaseous elements on the Earth exist as molecules, such as H2, 02, or NH3, in the low density of interstellar space single atoms are more usual. Collisions between atoms are rare in space so spectral lines retain their sharpness and are in some cases enhanced to a degree unobtainable in a terrestrial laboratory. Chemicals that are too reactive to exist on earth for more than a few moments are sometimes abundant in space. A gas cloud may be so massive that its motions are governed by its own gravity rather than that of a nearby object; another may be ionized and supported by a magnetic field. The study of the interstellar medium is thus the study of matter in a completely different environment.