Sky & Telescope - April 2024 - 15

J. ALVES ET AL. / NATURE 2020
Galactic latitude (°)
10
20
30
Cepheus-near
Cam
Bridge 142 L1355
Maddalena
-30
-20
-10
Rosette
CMa
Orion A
Mon R2
X-bones
NGC 2264 IC 443
Mon OB1
Gem OB1
L1400
W3-W4-W5
L1302
California
λ Ori
Orion B
Taurus
LBN917
220
200
180
160
140
Galactic longitude (°)
HIDDEN IN PLANE SIGHT Viewed in the plane of the sky, facing away from our galaxy's center, the Radcliffe Wave is invisible. It's only when astronomers
create a 3D map of the nearby stellar nurseries (red circles) that the wave reveals its shape. (Not all of the nurseries shown are in the Wave.)
Without precise distances, it is difficult to pinpoint even
the most basic properties of these interstellar gas clouds, such
as their size or mass - let alone their distribution in the galaxy.
With the goal of helping astronomers better understand
how stars form, I set out to chart the distances to the closest
clouds with more accuracy than ever before.
But these clouds are simply dense blobs in a continuous
distribution of mostly hydrogen and helium gas known as the
interstellar medium. How does one find distances to structures
with no finite boundaries or sharp edges? To do so, I needed
help from a space mission called Gaia.
A 3D Dust-Mapping Revolution
One of the leading goals of the Gaia mission, launched in
2013, is to reveal the 3D structure of our Milky Way by
providing accurate distances to more than 1 billion stars,
about 1% of the total number of stars in our galaxy. While an
early-1990s mission, Hipparcos, provided distances to more
than 100,000 stars out to a few hundred light-years from the
Sun, Gaia now measures distances to stars 200 times more
accurately and over a much larger volume of our galaxy
(S&T: Feb. 2023, p. 34).
Gaia provides distances to stars, though, and I wanted to
find distances to the stuff between the stars. Key to solving
that puzzle is the composition of the interstellar medium.
About 99% of the mass of the interstellar medium is hydrogen
and helium gas, but about 1% is dust: tiny, sooty particles
composed of heavier elements like silicon and carbon. Dust
has an outsize influence on how we view our night sky due to
an effect called reddening, in which dust absorbs and scatters
blue light more than red light. The effect makes stars appear
redder than they would if no dust lay between us and them.
(A similar effect causes sunsets to appear red.) How reddened
a star becomes depends on how much dust its light passes
through before reaching our telescopes.
Historically, astronomers considered dust a nuisance, an
obnoxious smoke-like substance that obscures our view of the
cosmos. For me, though, dust was not a nuisance but a boon,
because encoded in the color of every reddened star is information
on how much dust the light has passed through on its
way to Earth. And now thanks to Gaia, we know many stars'
distances. Each of these stars thus helps reveal the distance
of the dusty clouds, since the dust must lie closer than the
star does for us to observe this reddening effect. The idea that
the colors and distances of stars can tell us the distances of
interstellar clouds underpins a technique known as 3D dustmapping.
Imagine
we want to use 3D dust-mapping to find the
distance to the Orion Nebula - one of the most famous
nearby regions of massive star formation. Stars are scattered
throughout the dusty cloud in 3D, but as seen in a 2D image
of the nebula, everything is projected onto the flat plane of
the sky. Yet stars behind the cloud (at greater distances) will
on average have more reddened colors than stars in front of
the cloud (at lesser distances). If we combine the colors of
stars with their distances from Gaia in this patch of the sky,
we will find a distance where a jump in reddening occurs.
That jump must be the distance of the cloud.
In reality, the situation is more complicated, because not
every star is the same intrinsic color in the absence of dust.
However, thanks to the precision of large photometric surveys
(which provide constraints on stellar colors sampled at difsk
yand tele scope .o r g * APRIL 2024 15
120
100
80
Perseus
IC 5146
Lacerta
North America
L1333
L1340
Cepheus-far
L988
L977
Cyg-X

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Sky & Telescope - April 2024

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