We often think of the moon as a place, but in fact it is a hundred million places, an archipelago of solitude. You could go from100 degrees below zero to 100 degrees above with a small step. You could yell in your friend's ear and he would never hear you. Without an atmosphere to transmit heat or sound, each patch of the moon is an island in an unnavigable sea.

The atmosphere of a planet is what binds its surface into a unified whole. It lets conditions such as temperature vary smoothly. More dramatically, events such as the impact of an asteroid, the eruption of a volcano and the emission of gas from a factory's chimney can have effects that reach far beyond the spots where they took place. Local phenomena can have global consequences. This characteristic of atmospheres has begun to capture the interest of astronomers who study the Milky Way galaxy.

For many years, we have known that an extremely thin atmosphere called the interstellar medium envelops our galaxy and threads the space between its billions of stars. Until fairly recently, the medium seemed a cold, static reservoir of gas quietly waiting to condense into stars. You barely even notice it when looking up into the starry sky. Now we recognize the medium as a tempestuous mixture with an extreme diversity of density, temperature and ionization. Supernova explosions blow giant bubbles; fountains and chimneys may arch above the spiral disk; and clouds could be falling in from beyond the disk. These and other processes interconnect far-flung reaches of our galaxy much as atmospheric phenomena convey disturbances from one side of Earth to the other.

This article was originally published with the title The Gas between the Stars.

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