That large regions of the galaxy can be influenced by the formation of
massive stars in a few localized regions requires that star formation somehow be
coordinated over long periods of time. The process commences with a single O-
type star or a cluster of such stars in a giant molecular cloud, around which
（5) stellar radiation, winds and explosions carve a modest cavity from of the
surrounding interstellar medium, concomitantly destroying the progenitor
cloud. Perchance this disturbance triggers star formation in a nearby cloud, and
so on, until the interstellar medium in this corner of the galaxy resembles Swiss
cheese, composed of what theorists have recently begun to refer to as bubbles,
（10) or superbubbles, one of the most exciting discoveries in astrophysics in years.
Within a hot bubble, the sun has revealed itself in x-rays emitted by highly
ionized trace ions such as oxygen and at some point in the process of its
formation, nearby bubbles commence to overlap, coalescing into a superbubble.
The energy from more and more O-type stars feeds this expanding superbubble
（15) until its natural buoyancy stretches it from the midplane up toward the halo,
forming a chimney—the superbubble thus becomes a pathway for hot interior
gas to spread into the upper reaches of the galactic atmosphere, producing a
widespread corona. Far from its source of energy, the coronal gas slowly starts
to cool and condense into clouds. Over eons, these clouds fall back to the
（20) galaxy's mid-plane, completing the fountain-like cycle and replenishing the
galactic disk with cool clouds from which star formation begins anew.
Star formation often occurs in sporadic but intense bursts, but 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, except in some galaxies where
（25) positive feedback has gained the upper hand. 20 million to 50 million years ago,
star formation in the central parts of M82 began running out of control. Our
galaxy, too, may have had sporadic bursts, and the way 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.
（30) Progress will be made in this complicated subject as astronomers continue
to study how the medium is cycled through stars, through the different phases
of the medium, and between the disk and the halo. Observations of other
galaxies give astronomers a bird's-eye view of the interstellar goings-on, and
answer such a critical question as whether stars are really the main source of
（35) power for the interstellar medium. The loop above the Cassiopeia superbubble1
looks uncomfortably similar to the prominences that arch above the surface of]
the sun, prominences which owe much to the magnetic field in the solar
atmosphere, and scientists have begun to wonder if magnetic activity dominates
our galaxy's atmosphere.
The primary function of the passage as a whole is to
A．indicate the direction that research into a particular physical phenomenon should take
B．illustrate the limitations of applying mathematics to complicated astrophysical phenomena
C．clarify the differences between an old explanation of a physical phenomenon and a new model of it
D．introduce a new explanation of a physical phenomenon
E．explain the difference between two related physical phenomena
