Sky & Telescope - April 2024 - 17

CENTER FOR ASTROPHYSICS, HARVARD & SMITHSONIAN / LEAH HUSTAK (STSCI)
large range in distances that astronomers never predicted
they could be related. Containing around 3 million times
our Sun's mass, the wave also represents the largest coherent
gaseous structure known in our galaxy.
The Sun lies roughly 400 light-years away from the closest
point of this colossus - the wave has been right in front
of our galactic noses. While many regions in the wave are
visible in the sky with a backyard telescope (S&T: Jan. 2023,
p. 26), we only discovered it now thanks to the huge boost
in 3D dust-mapping's distance resolution that Gaia enabled.
With accurate distances, we could finally turn the 2D view of
our sky into a 3D view of star formation throughout the solar
neighborhood.
So, given that stellar nurseries are excellent tracers of
the Milky Way's pinwheel pattern, what's the relationship
between this wave-like structure and the nearby spiral arms
seen in artists' impressions of our Milky Way? It turns out
that - at least as far out as our 3D dust maps can see - the
Radcliffe Wave is the gaseous reservoir of the nearest spiral
arm to our Sun, dubbed the Local Arm.
Previous pictures, which relied on the stellar nurseries
sparsely scattered throughout the galaxy, had suggested that
the Local Arm lies nestled within the Milky Way's pancakedisk
when seen edge-on. Artist's impressions show it as having
an arc shape when seen from above the disk, kind of like
a chocolate swirl in the batter.
But our new view, using the galaxy's ubiquitous dust to
trace where gas lies, suggests a different, much more dynamic
picture. Rather than being flat when seen edge-on, the Radcliffe
Wave dips above and below the midpoint of the disk's
central plane, with an amplitude of about 500 light-years.
This amplitude is about three times larger than the thickness
that astronomers have traditionally assumed for the Milky
Way's dense gaseous disk, out of which stars form, based on
approaches that average over the entire galaxy. And rather
than curving when seen bird's-eye, the wave is straight,
stretching about 25 times as long as it is wide.
In fact, on large scales, all spiral arms may be made up of
a series of such linear sections. It's only when stuck together
that they make a curved shape, like a long train of boxcars
rounding a bend in the tracks (S&T: Mar. 2023, p. 14). Needless
to say, we now need a new artist's impression of our
galactic backyard.
Origin of an Undulation
The wave's peculiar shape also has profound implications
for how the youngest stars in our local Milky Way may have
formed. To understand the formation of these youngest stars,
though, we need to understand the formation of the wave.
One culprit could be an external source of disturbance - a
clump of dark matter or an infalling cloud of gas hitting the
disk of the Milky Way would have shaken the disk up, like a
Local Bubble
Ophiuchus
Pipe
Sun
Taurus
Musca
Chamaeleon
Corona Australis
Lupus
NEW LIFE As the Local Bubble expands outward, it sweeps up gas like a snowplow, compressing it until stars begin to form. 3D dust-mapping
has revealed that stellar nurseries line the edges of the Local Bubble.
sk yand tele scope .o r g * APRIL 2024 17

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Sky & Telescope - April 2024

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