My first Magazine Sky & Telescope - 04.2019 | Page 18

Galactic Sunset
for making new stars; it’s also essential for spiral
structure. If a star explodes or a dwarf galaxy skirts
by, it stirs up the disk. Stars can’t dissipate this
turbulence, because they’re so small they simply dart
past one another and never settle into more sedate
trajectories. As a result, a disk made solely of stars
becomes thick and loses its spiral.
In contrast, gas clouds do dissipate turbulence,
because they are large. When they get stirred up,
they smack into one another, lose their random
motions, and fall back into the disk. So the disk stays thin
and the spiral pattern survives. Thus, to preserve its shape
and beauty, every spiral galaxy needs gas.
Running on Empty
For those who hope that the Milky Way’s beautiful spiral will
endure forever, the numbers on the galactic gas gauge are
bleak. “We’re simply burning through our fuel too
quickly,” says Andrew Fox (Space Telescope Science
Institute). Stars form primarily in molecular gas,
the type that is so cold and dense it can collapse and
give birth to new suns. The Milky Way possesses 1
to 2 billion solar masses of molecular gas. Each year
our galaxy converts about 2 solar masses of this gas
into stars, most of them red dwarfs less massive
than the Sun. Divide the molecular gas supply by
the star-formation rate and you discover that our
galaxy will consume its molecular gas in only about a billion
years. Stars do return some gas when they die, but long-lived
stars lock up most of it.
Nor is the Milky Way’s predicament unique. Other spiral
galaxies also face a looming gas shortage. “They’ll run out
of gas in fairly short times compared to their lifetimes,” Fox
says. Without gas, the spiral patterns will shrivel. Indeed,
q REFUELING Giant spiral galaxies such as the Milky Way acquire gas several ways, either from outside themselves (red arrows) or by reusing their
own material (white arrows). Some gas comes in pristine form from the cosmic web, some is taken from other galaxies, and some is recycled from the
galaxy’s own disk. Hot halo gas also condenses, sometimes cooled by recycled gas to form galactic precipitation.
Condensing halo gas or clouds
from intergalactic space
alo
ic h
t
c
la
Ga
Recycled gas from
supernovae
Disk
Small galaxy
being consumed
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A PR I L 2 019 • SK Y & TELESCOPE
Pristine gas from
cosmic web