If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see?a. Gas that changes only in very slow and steady ways, so that the movie would in fact be quite boring.b. The movie would alternate back and forth between being very bright when there is a lot of gas and very dark when there is very little gas.c. Gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles.d. The entire disk of the Milky Way would pulsate in and out as it contracts to form stars and then blows out in supernovae and then contracts to form stars again and so on.

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