Two astronomers from Greece have managed to model the
three-dimensional structure of an interstellar gas cloud, and
found that it’s on the order of 10 times more spacious than
it originally appeared.
The shape and structure of Musca, described in the journal
Science, could help scientists probe the mysterious
origins and evolution of stars—and by extension, the planets
that surround them.
Finding the 3-D structure of such clouds “has been a ‘holy
grail’ in studies of the interstellar medium for many years
now,” said senior author Konstantinos Tassis, an astrophysicist
at the University of Crete.
Interstellar clouds serve as the celestial cradles for nascent
stars, which condense out of these enormous conglomerations of
gas and dust. These cold, dusty, magnetized clouds can reach a
million times the mass of the sun. But because they’re filled
with molecular hydrogen that blocks the light of background
stars, they typically appear as holes in an otherwise bright
night sky. They’re more easily studied using infrared light.
But even in infrared light, these clouds are difficult to study
because we can see them only as flat structures, even though
they’re actually three-dimensional. We know very little about
how dense they are, what shape they are and how they’re
“All sorts of different physical and chemical processes take
place in their interior, and as a result, the process of
star formation is poorly understood,”
Tassis said in an email. “How does a giant cloud of a million
solar masses break up into smaller pieces, and how do these
fragments condense into stars similar to our sun? What makes a
cloud form a lot of small stars or a few larger ones?”
“These problems, although they are directly related to the
question of the origin of our sun, our planet, and, ultimately,
ourselves, are still very much a mystery,” he added.
About a decade ago, astrophysicist Paul Goldsmith of the Jet
Propulsion Laboratory in La Canada Flintridge and his
colleagues discovered strange hair-like wisps surrounding such
gas clouds, rather like the cilia of a bacterium. Amid the
chaos of a gas cloud, these ordered structures drew
astronomers’ attention. How did they form, and why?
“Understanding how you make new stars is really a critical
challenge for modern astrophysics,” Goldsmith, who was not
involved in the new paper, said in an interview. “These
molecular clouds are where new stars are formed, and so
understanding the structure of these clouds, and how deep they
are, what their three-dimensional structure is, is
obviously critical for understanding the whole picture.”
While completing his doctoral work at the University of Crete,
lead author Aris Tritsis (now a postdoctoral fellow at
Australian National University) concluded that these striations
were actually caused by magnetic waves leaving their imprint on
the cloud’s gas.
“It was then that we realized that these striations might
encode a global vibration if the cloud is isolated, a ‘song,’ a
pattern of frequencies that could reveal the true, 3-D shape of
the cloud,” Tassis said.
To try and use those magnetosonic waves to understand the shape
of an interstellar cloud, they pulled data from the European
Space Agency’s infrared Herschel Space Observatory, which can
see into the infrared. They focused on Musca, which lies in the
Southern Hemisphere roughly 500 light-years from Earth.
Musca, a filamentary cloud that’s long and thin, made an ideal
target because it was relatively isolated. This meant that its
striations were unlikely to have been warped by “noise” coming
from nearby structures, Tassis said.
Because the waves are basically trapped within the interstellar
cloud, the wavelength will actually hold information about its
dimensions. After using the striations to determine the
wavelength of this “global vibration,” the scientists were able
to determine the true shape of this gas cloud.
From our vantage point, Musca looks like a needle. But the
magnetosonic waves revealed that the gas cloud actually was
shaped like a pancake—one we were viewing edge-on. All in all,
the cloud seems to measure roughly 24 light-years wide by 18
light-years across and one light-year thick.
“In much the same way that a piccolo flute makes a much
different sound than a tuba (the air vibrates with different
frequencies in the two cases because the shape and size of the
instruments are very different), a pancake-shaped cloud
vibrates in a tune that is very different than that of a
needle-shaped cloud,” Tassis said. “Musca very clearly vibrates
like a pancake, not a needle. It is not a subtle effect, it is
This meant that the gas cloud was far more voluminous than
previously thought—roughly on the order of 10 times larger,
Tassis said. And because the same amount of gas filled that
bigger-than-expected space, it meant the cloud was much less
dense than scientists had expected.
“It was a huge surprise to us,” Tassis said.
Goldsmith, whose team originally identified the existence of
striations, praised the work.
“This is great. This is exciting,” the astrophysicist said.
“Now we have to figure out if we can confirm that by some other
kind of measurement.”
The discovery that Musca is a pancake and not a prototypical
needle-like filament totally changes scientists’ understanding
of the balance of forces that shaped this gas cloud and
influenced its star-forming process, Tassis added.
For one thing, a less dense gas cloud would have a much lower
rate of star formation. On top of that, the molecular
demographics of sparser clouds are different from denser ones.
Dense clouds, for example, are more likely to have
nitrogen-based molecules such as ammonia.
The shape of such a cloud can be very telling too: Magnetic
forces make pancake-like clouds, turbulence forms needle-like
clouds and thermal forces result in roundish, blobby clouds,
Tassis said. If scientists can now start to render more of
these clouds in three dimensions, they won’t mistake a
pancake-shaped cloud for a needle-shaped one. That means
they’ll start to have a much better sense of the forces at
“Now that we know Musca is a pancake, we know that at least for
this particular cloud, magnetic forces must play a key role in
the star-formation process taking place in its interior,”
Armed with knowledge of Musca’s three-dimensional structure,
other scientists can now draw out more information about the
chemical and physical properties of this interstellar gas
“With its 3-D structure revealed, Musca will now act as a
prototype laboratory to study star formation in greater detail
than ever before,” Tassis said. “The Musca star-formation saga
is only now beginning, and this is a very exciting development
that goes beyond this particular discovery.”
More information: Aris Tritsis et al. Magnetic
seismology of interstellar gas clouds: Unveiling a hidden
dimension, Science (2018). DOI: 10.1126/science.aao1185