Passage 33
Until recently most astronomers believed that the
space between the galaxies in our universe was a near-
perfect vacuum. This orthodox view of the universe is
now being challenged by astronomers who believe that a
(5) heavy “rain” of gas is falling into many galaxies from
the supposedly empty space around them. The gas
apparently condenses into a collection of small stars,
each a little larger than the planet Jupiter. These stars
vastly outnumber the other stars in a given galaxy. The
(10) amount of “intergalactic rainfall” into some of these
galaxies has been enough to double their mass in the
time since they formed. Scientists have begun to suspect
that this intergalactic gas is probably a mixture of gases
left over from the “big bang” when the galaxies were
(15) formed and gas was forced out of galaxies by supernova
explosions.
It is well known that when gas is cooled at a constant
pressure its volume decreases. Thus, the physicist Fabian
reasoned that as intergalactic gas cools, the cooler gas
(20) shrinks inward toward the center of the galaxy. Mean-
while its place is taken by hotter intergalactic gas from
farther out on the edge of the galaxy, which cools as it is
compressed and flows into the galaxy. The net result is a
continuous flow of gas, starting as hot gases in inter-
(25) galactic space and ending as a drizzle of cool gas called a
“cooling flow,” falling into the central galaxy.
A fairly heretical idea in the 1970’s, the cooling-flow
theory gained support when Fabian observed a cluster