Passage IV

NATURAL SCIENCE: This passage is adapted from the article “The Gas between the Stars” by Ronald J. Reynolds (©2001 by Scientific American, Inc.).

For many years, we have known that an extremely thin atmosphere called the interstellar medium envelops our galaxy, the Milky Way, 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. Now we recognize the medium as a tempestuous mixture with an extreme diversity of density, temperature and ionization.

In fact, telescopes on the ground and in the space show the galaxy’s atmosphere to be as complex as any planet’s. Held by the combined gravitational pull of the stars and other matter, permeated by starlight, energetic particles and a magnetic field, the interstellar medium is continuously stirred, heated, recycled and transformed. Like any atmosphere, it has its highest density and pressure at the “bottom,” in this case the plane that defines the middle of the galaxy, where the pressure must balance the weight of the medium from “above.” Dense concentrations of gas—clouds—from near the midplane, and from the densest subcondensations, stars precipitate.

When stars exhaust their nuclear fuel and die, those that are at least as massive as the sun expel much of their matter back into the interstellar medium. Thus, as the galaxy ages, each generation of stars pollutes the medium with heavy elements. As in the water cycle on Earth, precipitation is followed by “evaporation,” so that material can be recycled over and over again.

Thinking of the interstellar medium as a true atmosphere brings unity to some of the most pressing problems in astrophysics. First and foremost is star formation. Although astronomers have known the basic principles for decades, they still do not grasp exactly what determines when and at what rate stars precipitate from the interstellar medium. Theorists used to explain the creation of stars only in terms of the local conditions within an isolated gas cloud. Now they are considering conditions in the galaxy as a whole.

Not only do these conditions influence star formation, they are influenced by it. What one generation of stars does determines the environment in which subsequent generations are born, live and die. Understanding this feedback is another of the great challenges for researchers. Feedback can be both positive and negative. On the one hand, massive stars can heat and ionize the medium and cause it to bulge out from the midplane. This expansion increases the ambient pressure, compressing the clouds and perhaps triggering their collapse into a new generation of stars. On the other hand, the heating and ionization can also agitate clouds, inhibiting the birth of new stars. When the largest star blow up, they can even destroy the clouds that gave them birth. In fact, nagetive feedback could explain why the gravitational collapse of clouds into stars so inefficient. Typically only a few percent of a cloud’s mass becomes stars.

A third conundrum is that star formation often occurs in sporadic but intense bursts. In the Milky Way the competing feedback effects almost balance out, so that stars form at an unhurried pace—just 10 per year on average. In some galaxies, however, such as the “exploding galaxy” M82, positive feedback has gained the upper hand. Starting 20 million to 50 million years ago, star formation in the central parts of M82 began running out of control, proceeding 10 times faster than before. Our galaxy, too, may have had sporadic bursts. How these starbursts occur and what turns them off must be tied to the complex relation between stars and the tenuous atmosphere from which they precipitate.

Finally, astronomers debate how quickly the atmospheric activity is petering out. The majority of stars—those less massive than the sun, which live tens or even hundreds of billions of year—do not contribute to the feed back loops. More and more of the interstellar gas is being locked up into very long lived stars. Eventually all the spare gas in our Milky Way may be exhausted, leaving only stellar dregs behind. How soon this will happen depend on whether the Milky Way is a closed box. Recent observation suggest that the galaxy is still an open system, both gaining and losing mass to its cosmic surroundings. High velocity clouds of relatively unpolluted hydrogen appear to be raining down from intergalactic space, rejuvenating our galaxy. Meanwhile the galaxy may be shedding gas in the form of a high-speed wind from its outer atmosphere, much as the sun slowly sheds mass in the solar wind.