Baby, It's Busy Up There!

"The Gas between the Stars" by Ronald J. Reynolds, in Scientific American (Jan. 2002), 415 Madison Ave., New York, N.Y. 10017–1111.

A new and startling picture of the vast interstellar regions of the Milky Way has emerged over the past several decades. Astronomers long conceived of the "interstellar medium" as a static reservoir of very thin gases, little more than a nuisance that got in the way of their efforts to observe the stars. The medium was thought to be much like the atmosphere of the moon, which is to say no atmosphere at all—a medium that conducted neither sound nor heat.

"Now we recognize the medium as a tempestuous mixture with an extreme diversity of density, temperature, and ionization," reports Reynolds, an astronomer at the University of Wisconsin, Madison. (The medium is about 90 percent hydrogen in various forms and 10 percent helium, with trace amounts of other elements.)

"Supernova explosions blow giant bubbles"; there are "fountains," "chimneys," and "clouds." Conceptually, the interstellar medium increasingly looks like Earth’s atmosphere, binding together the galaxy and ensuring that an event in one place will have an impact in another. This new view is revolutionizing the way scientists comprehend the galaxy.

For example, it now appears that supernovas (exploding stars) create vast "hot bubbles," along with cosmic rays that "raise the pressure of the interstellar medium; higher pressures, in turn, compress the dense molecular clouds and increase the chance they will collapse [and form] stars." Oversized bubbles may extend all the way to the halo of the galaxy, each forming a kind of cosmic chimney that transports hot gases from its supernova to the outer reaches of the Milky Way, where the gases cool and rain back on the galaxy.

Stars thus seem to be the "main source of power for the interstellar medium." But it’s not a certainty. Reynolds says that the loop above one huge bubble "looks uncomfortably similar" to certain features of our own sun that are created by the sun’s magnetic field. It may be that magnetic activity dominates the galaxy’s atmosphere, just as it does that of the planets and stars. That would make the analogy between the interstellar atmosphere and our own earthly one "even more apt than we think."