Sky & Telescope - December 2020 - 30

Interstellar Dust

are few cosmic investigations that interstellar dust doesn't
impact in at least one way.
"If you want to understand everything from the space
between stars, the envelopes of stars, [to] the circumstellar
disks that form planets around stars, you need to know what
the dust is doing," says astronomer Sarah Sadavoy (Queen's
University, Canada). "You need to know what it is. You need
to know how it's changing. And by doing that, you'll be able
to better understand the universe and everything that you're
using to probe it."

The
Milky Way's disk,
by mass (baryonic)

91%

0.09%

Stars

Gas

Dust

Fine Galactic Soot
In early evenings, during late summer, the plane of the Milky
Way arcs overhead, divided by a dark lane known as the Great
Rift: a band made of clouds of solid particles in space that
block distant starlight.
Long ago, people saw stories in those dark patches. To the
Quechua people of Peru, these dark cloud constellations represented a menagerie of animals that recirculated Earth's water.
The Milky Way was a great celestial river, and when it rose
above the horizon, the animals ferried water into the sky and
released it as rain.
Quechuan storytellers weren't far from the truth: The
particles in those clouds do circulate elements through the
cosmos and are even a breeding ground for molecules of
water. Today, astronomers know the clouds are loaded with
solid grains, which appear not only in the Great Rift but in
many cosmic environments, including the exhaust from

dying stars, the majestic wakes of supernovae, and nascent
planetary systems.
Interstellar dust grains are microscopic, ranging roughly
in size from several to a few hundred nanometers, giving
them the consistency of ﬁne soot. Some grains are made of
silicates, minerals composed of mostly silicon and oxygen.
Carbonaceous, or carbon-laden, dust comes in an assortment
of types, ranging from organic macromolecules to nuggets of
graphite or diamond. Some dust grains are also coated with a
veneer of frozen water and other types of ice.
Astronomers learned this largely by observing how clouds
of dust ﬁlter starlight. The size, composition, and shape of
dust particles affect how they absorb and scatter particular
wavelengths of light. Dust also offers up some secrets via
thermal emission of infrared light or, for the coldest dust just
30 or so degrees above absolute zero, emission at submillime-

THE LIFE CYCLE OF INTERSTELLAR DUST
Diffuse clouds
form
Dust grains
coagulate,
cool gas clouds

Dust grains
mix with
interstellar
medium

Stars age and die,
release dust grains

Stars and
planetary
systems

DECEMBER 2 02 0 * SK Y & TELESCOPE

Radiation, supernovae
modify or destroy
dust grains

Grains build up
into planetesimals

Protostars form

DUST TE X T URE: SERGIO34 / SHU T TERSTOCK.COM;
DUST LIFE CYCLE: G REGG DINDER M A N / S&T

Molecular clouds
form, cores
collapse

http://www.SHUTTERSTOCK.COM

Sky & Telescope - December 2020

Table of Contents for the Digital Edition of Sky & Telescope - December 2020