The relationship between the interstellar medium and the stars is a dynamic one; matter is exchanged between them all the time. Our Sun, for example, emits particles, in the form of a solar wind, and absorbs others in the form of cosmic rays. In this case the amount of mass exchanged is small compared with the total mass of the Sun. Under certain conditions, however, much more drastic exchanges of matter between the stars and the interstellar medium take place; these events involve the births and deaths of stars.

New stars may be formed when the density of an interstellar cloud exceeds a critical value. Its internal gravitational forces then compress the cloud until it is hot enough to become a star in its own right. This process was very common when the Galaxy was young, and is still happening now in certain regions. Old stars, on the other hand, tend’ to expel matter into space by one of several processes; red-giant stars blow it off gradually, whereas planetary nebulae and supernovae eject it in an explosive event . Matter therefore alternates between being part of a star (or its associated planets) and being part of the interstellar medium. By doing so it becomes steadily enriched in the heavy elements, as these are synthesized in the stars. These heavy elements in inter¬stellar space may exist in a gaseous state, like the hydrogen and helium, or may condense-to make dust grains.

As yet, we do not know to what extent the depletion of the inter-stellar medium by star births, and its regeneration from old stars balance each other. It is possible, for example, that the older our Galaxy becomes the less interstellar matter it will contain.

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