Sky & Telescope - December 2020 - 31

IN TERSTELL A R REDDENING: G REGG DINDER M A N / S&T; DA RK NEBUL A: ESO (2)

ter and millimeter wavelengths, a part of the electromagnetic
spectrum only detectable with equipment akin to radio dishes
(S&T: Aug. 2020, p. 12).
"Our only way of understanding what interstellar dust
is will be through making inferences from what we see and
asking ourselves, what kind of physical stuff can make the
world look the way it does," says dust researcher Bruce Draine
(Princeton).
Interstellar dust clouds didn't get a modern astronomical treatment until the early 1900s. Astronomer E. E. Barnard, along with his contemporary Max Wolf, spent the ﬁrst
decades of the 20th century photographing some of these
dark regions and concluded that something in space was
blocking the light from more distant stars.
"I did not at ﬁrst believe in these dark obscuring masses,"
he wrote in the January 1919 Astrophysical Journal. "[My] own
photographs convinced me . . . that many of these markings
were not simply due to an actual want of stars, but were really
obscuring bodies nearer to us than the distant stars."
About a decade later, astronomer Robert Trumpler took
a stab at dissecting these obscuring bodies. "Most likely the
absorbing medium is made up of particles of various sizes,
ranging from free electrons and atoms, small solid dust particles, up to larger meteoritic bodies," he wrote in the October
1930 Publications of the Astronomical Society of the Paciﬁc.
Trumpler came to that conclusion after noting that stars
in distant clusters were dimmer than expected for their
distance and appeared redder than nearby stars of the same
spectral type. He attributed this to ﬁne particles permeating
the galaxy that, much like Earth's atmosphere, preferentially
scatter blue light and let red light pass. This reddening effect
is a cornerstone of dust research to this day.
While astronomers quibble over the details, most agree
that interstellar dust comes from dying stars, which spend
their lives forging progressively heavier elements in their
cores. At the end of their lives, stars release those elements
into space via stellar winds (in the case of most stars) or
powerful supernova explosions (for the stellar heavyweights).
Regardless of how these atoms
get into space, once there, and
far enough from a star's radiation, they ﬁnd each other and
start to link up, forming ever
larger conglomerates.
Dust contributes little to
the overall mass of a galaxy. By

p INTERSTELLAR REDDENING Dust suffusing the galaxy preferentially
scatters short wavelengths, making distant stars appear redder than they
actually are. The farther or dustier the distance, the redder the star looks.

mass, the Milky Way is composed mostly of stars and dark
matter, plus a relatively small amount of gas. The total mass
of the dust is just one percent that of the gas. And yet it plays
an outsized role in the formation of stars and planets.
"Without dust, it would be a very different galaxy and
universe," Gordon says.
Stars form in dense pockets of gas, which gravitationally
attract even more gas, building up to a critical mass, at which
point the gas collapses and, eventually, ignites nuclear fusion.
But to get under way, this birthing process needs a cloud of
mostly hydrogen molecules, and it needs that cloud to be
cold. If the gas is hot, the molecules whiz around too much
for gravity to take hold.
Dust takes care of both requirements. It provides a surface
upon which a hydrogen atom can alight, pair up with another
hydrogen atom, and then pop off as a full-ﬂedged molecule.
(That surface is also critical for interstellar chemistry: The
terrain provides a place where, for example, hydrogen atoms
can meet up with hydroxide ions to form water molecules.)
Dust is also an efﬁcient cooling agent. It shields molecular

u BARNARD 68 One of the dark
nebulae that E. E. Barnard cataloged, B68 is a Bok globule: a dark,
dusty cloud that blocks starlight at
visible and near-infrared wavelengths
(left). But when seen in longer infrared wavelengths (right), the cloud
largely disappears and background
stars shine through.
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Sky & Telescope - December 2020

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