Sky & Telescope - April 2024 - 19

pebble making waves in a pond's surface. However, origins
closer to home are also possible: A series of supernova explosions
might have pushed the disk's gas hundreds of light-years
beyond the midplane of the disk.
Thanks to Gaia's third data release in 2022, we are closer
to discriminating between such ideas. Unlike Gaia's second
data release, which mainly provided information on the 3D
positions of stars, the third release also includes constraints
on the 3D motions for millions of stars, including many that
are still forming within the wave. These young stars inherit
the motion of their natal clouds and act as proxies for the
gas's motion, which is much more difficult to measure.
Using the 3D motions of the wave's baby stars, new work
by Harvard PhD student Ralf Konietzka shows that the Radcliffe
Wave doesn't just look like a wave, but it also moves like
a wave. In other words, it is oscillating.
The oscillation is consistent with being what physicists call
a traveling wave: Like waves propagating over the open ocean,
the crests and troughs of the Radcliffe Wave shift position
with time, so the gas disk in this section is never completely
level. The baby stars forming in the wave act similar to buoys,
bobbing up and down and revealing the rippling motions of
the gaseous disk. Comparing the specifics of this oscillation
with predictions from computer simulations should ultimately
shed light on exactly how the wave formed.
One thing, however, that the wave has already taught us
is that star formation is much more dynamic than previously
thought. Nearby stars do not form in static blobs of gas that
evolve gently in isolation. As our Milky Way's disk is bombarded
by external disturbances from outside and by supernova
explosions from within, its gas is repeatedly shaken and
stirred; as a result, star-forming clouds likely disperse quickly.
Our Sun passed by the Radcliffe Wave roughly 13 million
years ago and now lies (by chance) at the Local Bubble's
center. The dynamic nature of star formation means that the
part of the galaxy we are leaving behind may appear unrecognizable
13 million years in the future. The artists' impressions
of today are not the artists' impressions of tomorrow.
From Distances to Shapes
The 3D dust-mapping revolution has accelerated even faster
than I could have predicted. Thanks to advances in datascience
techniques led by Torsten Enßlin's group at the Max
Planck Institute for Astrophysics, the field has shifted from
being able to determine distances to clouds to being able to
outline their shapes. We can now figure out not just where
clouds are, but what they look like.
Analyzing the shapes of dense clouds in and beyond the
Radcliffe Wave in 3D for the first time, we recently found
that they are not spherical (as traditionally assumed)
but rather shaped like filaments. We've found that these
stretched-out clouds in turn appear to be intimately related
to cavities of tenuous gas carved out by supernovae.
When we matched up the orientation of these filaments
with the walls of cavities around supernovae, we found that
the closest clouds are draped over these cavities' boundaries.
Like a snowplow that sweeps up and compacts snow on the
edge of its blade, these supernovae sweep up the ambient gas,
evacuating the cavities and leading to a pile-up of denser gas
along their surfaces.
Our Sun happens to be traveling through the center of one
of these supernova-swept cavities, called the Local Bubble.
We've found that this bubble is actually still expanding,
sweeping up and compacting the gas in the closest region of
the Radcliffe Wave.
In fact, such bubbles are everywhere - in the wave and
around it, with a huge range of sizes. Those inside the wave
carve out its gas, while those outside push it this way and
that. The entire wave looks like holey string cheese (made of
Swiss instead of the traditional mozzarella).
This means that nearby star-forming clouds do not just
lie in the Radcliffe Wave, but they also lie on the surfaces of
bubbles. Stellar death therefore likely plays a role in shaping
stellar birth - regardless of whether supernovae are ultimately
responsible for the wave's overall undulation.
The Hidden Milky Way
Despite these exciting discoveries made close to home, the
solar neighborhood ultimately represents a small suburb
within the sprawling metropolis that is the Milky Way. About
90% of our galaxy is yet to be fully resolved with 3D dustmapping,
including the entire half of the Milky Way located
on the other side of our galaxy's center. Such vast distances
are largely beyond the reach of current visible-light observatories,
even those as powerful as Gaia.
To unlock the hidden Milky Way, we must turn to the
next generation of observatories, including the Nancy Grace
Roman Space Telescope, set to launch between late 2026 and
early 2027. With a field of view about 100 times larger than
that of the Hubble Space Telescope and equipped with an
infrared camera to peer through the heart of our Milky Way's
dusty disk, Roman will open new doors for understanding
the structure and dynamics of our galaxy. I and many
other galactic astronomers are advocating for a survey of the
disk with Roman. Such a survey will constrain the colors of
tens of billions of stars, a subset of which would also have
accurate distances - the two key ingredients necessary for
building reliable 3D dust maps over the remaining 90% of
our Milky Way.
The Copernican principle tells us we are not privileged
observers of the universe, so we cannot happen to lie right
beside the only example of a bubbly, sinusoidal wave of stellar
nurseries in our galaxy. There must be more out there, waiting
to be discovered. With next-generation space telescopes
like Roman, we will be on the hunt to find them.
¢ CATHERINE ZUCKER is an astronomer based in Cambridge,
Massachusetts. She spends her days combining data science,
data visualization, and gigantic ground- and space-based surveys
to better understand the galaxy we call home.
sk yand tele scope .o r g * APRIL 2024 19
http://skyandtelescope.org
Sky & Telescope - April 2024

Table of Contents for the Digital Edition of Sky & Telescope - April 2024
