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
of galaxies in the constellation Perseus and found the
(30) central galaxy, NGC 1275, to be a strange-looking object
with irregular, thin strands of gas radiating from it.
According to previous speculation, these strands were
gases that had been blown out by an explosion in the
galaxy. Fabian, however, disagreed. Because the strands
(35) of gas radiating from NGC 1275 are visible in optical
photographs, Fabian suggested that such strands consisted
not of gas blown out of the galaxy but of cooling flows
of gas streaming inward. He noted that the wavelengths
of the radiation emitted by a gas would changes as the
(40) gas cooled, so that as the gas flowed into the galaxy and
became cooler, it would emit not x-rays, but visible light,
like that which was captured in the photographs. Fabian’s
hypothesis was supported by Canizares’ determination in
1982 that most of the gas in the Perseus cluster was at a
(45) temperature of 80 million degrees Kelvin, whereas the
gas immediately surrounding NGC 1275 (the subject of
the photographs) was at one-tenth this temperature.