12 ASTRONOMY • MAY 2020
Our local arm of the Milky Way
Galaxy is doing the wave. Harvard
University astronomers recently uncovered
a strange, wave-shaped structure within
500 light-years of the Sun, made up of sev-
eral interconnected stellar nurseries. Taken
as a whole, the wave is 400 light-years
wide and 9,000 light-years long, snaking
500 light-years above and below the plane
of the galaxy. Findings regarding the wave
— dubbed the Radcliffe Wave in honor of
the Radcliffe Institute for Advanced Study
at Harvard, where the discovery was made
— were published January 7 in Nature.
Researchers found the Radcliffe Wave
while analyzing data from the European
Space Agency’s Gaia mission, which
is tasked with charting the location
and motion of stars within our galaxy.
Combining that information with other
observations as well as simulations and
data visualization, they discovered the
undulating structure in the spiral arm of
the Milky Way closest to our solar system.
According to the team, the wave looks
long and straight from above, but from
the side, it appears to bob up and down
out of the plane of the galaxy. “The wave’s
very existence is forcing us to rethink our
understanding of the Milky Way’s 3D
structure,” said study co-author Alyssa
Goodman, co-director of the Science
Program at the Radcliffe Institute for
Advanced Study, in a press release.In particular, the wave’s discovery
disproves the existence of a feature
known as “Gould’s Belt,” which astrono-
mers believed was a ring-shaped structure
of star-forming regions circling the Sun.
Many of the regions previously thought to
reside in parts of this belt are instead part
of the Radcliffe Wave, the team found,
while other regions associated with the
belt now appear to be unrelated to either
the wave or any ring structure. “For a
long time, people have been trying to
figure out if these molecular clouds actu-
ally form a ring in 3D,” said lead author
João Alves of the University of Vienna.
“Instead, what we’ve observed is the
largest coherent gas structure we know
of in the galaxy, organized not in a ring
but in a massive, undulating filament. ...
It’s been right in front of our eyes all the
time, but we couldn’t see it until now.”
Now, thanks to the team’s efforts, we
can see it, and the information will allow
astronomers to better understand the
properties of the star-forming clouds of
gas and dust that are part of the wave.
That’s because calculating the size and
mass of such clouds relies on knowing
their distance, which until now has been
difficult to measure. Additionally, the
data combination techniques used by
the team can set the stage for even more,
perhaps also surprising, findings about
the structure of the Milky Way. — A.K.Giant wave of star-forming
clouds lies near the Sun
SHOCKING
SCENE
Galaxies rarely live alone.
Pulled together by gravity,
they congregate into groups
that, over time, can merge
into larger clusters. That’s
what’s happening in NGC
6338, where two galaxy
groups are rushing toward
each other at about 4 million
mph (6.4 million km/h).
This composite image shows
the scene of the impending
smashup in X-rays and
optical light. The hottest
gas, with temperatures over
36 million degrees Fahrenheit
(20 million degrees Celsius)
is colored red. Cooler gas in
the cores of the two groups
appears blue, while stars and
galaxies shining in optical
light are white. The thin strip
of hot gas between the cores,
as well as the gas around
them, has been heated by
shocks generated during the
collision, similar to the sonic
booms made by supersonic
aircraft. Although this
pattern of shocked gas has
been predicted by simulations
in the past, NGC 6338 is the
first galaxy group merger that
shows clearly this effect. — A.K.QUA NTUM GRAVITY
DO THE WAVE. The
Radcliffe Wave is a
recently discovered
9,000-light-year-long
structure composed of
star-forming regions,
colored red in this
artist’s rendition, which
also shows the Milky
Way Galaxy and the Sun
(yellow). Although
astronomers aren’t sure
what caused the ripple-
like structure, they do
know it has interacted
with the Sun in the past.
ALYSSA GOODMAN/HARVARD
UNIVERSITYX-RAY: CHANDRA: NASA/CXC/SAO/E. O'SULLIVAN. XMM: ESA/XMM/E. O'SULLIVAN. OPTICAL: SDSS