My first Magazine Sky & Telescope - 04.2019 | Seite 20

Galactic Sunset
Meanwhile, spiral galaxies possess an additional reservoir
of fuel: atomic gas. Made mostly of individual hydrogen and
helium atoms, this gas is less dense than the molecular vari-
ety and rarely forms stars. The Milky Way has 8 billion solar
masses of the stuff; if it can convert this gas into fuel, then
the gas will be enough to support its current star-formation
rate for another 4 billion years.
“Bringing it in is the problem,” Elmegreen says. Whereas
most of the molecular gas is closer to the galactic center than
we are, most of the atomic gas is more remote, forming a disk
that extends far beyond the stellar one in which we reside.
But just as a star that skirts by the solar system should gravi-
tationally fl ing comets from the distant regions toward the
Sun, so a passing galaxy should nudge the atomic gas in the
Milky Way’s outer disk inward, making the gas dense enough
LMC
p HEAVENLY WRECKAGE This composite radio (pink) and optical image shows the Magellanic Stream, a vast swath of gas stretched out behind the
Large and Small Magellanic Clouds (white dots at head of stream). The gas will eventually become part of the Milky Way.
to become molecular and spark new stars. Conveniently, the
Andromeda Galaxy will swing by in about 5 billion years,
although the latest measurements of its motion suggest it will
score less of a direct hit than thought a few years ago. But,
Elmegreen says, “it doesn’t take much of an interaction to
bring that gas in.”
Support Your Local Galaxy
When it comes to future sources of gas, there’s more nearby
than just an upcoming push from our spiral neighbor in
Andromeda. Like other giants, the Milky Way has plenty of
lesser galaxies that revolve around it. Most of these satellite
galaxies have lost their gas, presumably to our galaxy, which
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A PR I L 2 019 • SK Y & TELESCOPE
means that stars in other galaxies forged some of the atoms
in your body. But unlike most giant spirals, the Milky Way is
especially lucky, for it also boasts a pair of satellites that are
close, bright, and still full of gas: the Large and Small Magel-
lanic Clouds, 160,000 and 200,000 light-years away.
For billions of years, the Magellanic Clouds have probably
been dancing around each other, their gravity tearing out each
other’s gas. This lost gas now stretches more than half a mil-
lion light-years through our galaxy’s outer halo. The stream
could refuel the Milky Way’s disk and thereby sustain its spiral.
Moreover, new work has quadrupled the Magellanic
Stream’s estimated gas mass. Astronomers have long known
of the neutral atomic hydrogen gas there, because it emits
AO
OBSERVATORY,
DIGITA
SMC