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The dusty side of the Sword of Orion is illuminated in this striking
infrared image from ESA's Hershel Space Observatory. Within the inset
image, the emission from ionized carbon atoms (C+) is overlaid in
yellow. Credit: ESA/NASA/JPL-Caltech. › Full image and caption
Life exists in a myriad of wondrous forms, but if you break any
organism down to its most basic parts, it's all the same stuff: carbon
atoms connected to hydrogen, oxygen, nitrogen and other elements. But
how these fundamental substances are created in space has been a
Now, astronomers better understand how molecules form that are
necessary for building other chemicals essential for life. Thanks to
data from the European Space Agency's Herschel Space Observatory,
scientists have found that ultraviolet light from stars plays a key role
in creating these molecules, rather than "shock" events that create
turbulence, as was previously thought.
Scientists studied the ingredients of carbon chemistry in the Orion
Nebula, the closest star-forming region to Earth that forms massive
stars. They mapped the amount, temperature and motions of the
carbon-hydrogen molecule (CH, or "methylidyne" to chemists), the
carbon-hydrogen positive ion (CH+) and their parent: the carbon ion
(C+). An ion is an atom or molecule with an imbalance of protons and
electrons, resulting in a net charge.
"On Earth, the sun is the driving source of almost all the life on
Earth. Now, we have learned that starlight drives the formation of
chemicals that are precursors to chemicals that we need to make life,"
said Patrick Morris, first author of the paper and researcher at the
Infrared Processing and Analysis Center at Caltech in Pasadena.
In the early 1940s, CH and CH+ were two of the first three molecules
ever discovered in interstellar space. In examining molecular clouds --
assemblies of gas and dust -- in Orion with Herschel, scientists were
surprised to find that CH+ is emitting rather than absorbing light,
meaning it is warmer than the background gas. The CH+ molecule needs a
lot of energy to form and is extremely reactive, so it gets destroyed
when it interacts with the background hydrogen in the cloud. Its warm
temperature and high abundance are therefore quite mysterious.
Why, then, is there so much CH+ in molecular clouds such as the Orion
Nebula? Many studies have tried to answer this question before, but
their observations were limited because few background stars were
available for studying. Herschel probes an area of the electromagnetic
spectrum -- the far infrared, associated with cold objects -- that no
other space telescope has reached before, so it could take into account
the entire Orion Nebula instead of individual stars within. The
instrument they used to obtain their data, HIFI, is also extremely
sensitive to the motion of the gas clouds.
One of the leading theories about the origins of basic hydrocarbons
has been that they formed in "shocks," events that create a lot of
turbulence, such as exploding supernovae or young stars spitting out
material. Areas of molecular clouds that have a lot of turbulence
generally create shocks. Like a large wave hitting a boat, shock waves
cause vibrations in material they encounter. Those vibrations can knock
electrons off atoms, making them ions, which are more likely to combine.
But the new study found no correlation between these shocks and CH+ in
the Orion Nebula.
Herschel data show that these CH+ molecules were more likely created
by the ultraviolet emission of very young stars in the Orion Nebula,
which, compared to the sun, are hotter, far more massive and emit much
more ultraviolet light. When a molecule absorbs a photon of light, it
becomes "excited" and has more energy to react with other particles. In
the case of a hydrogen molecule, the hydrogen molecule vibrates, rotates
faster or both when hit by an ultraviolet photon.
It has long been known that the Orion Nebula has a lot of hydrogen
gas. When ultraviolet light from large stars heats up the surrounding
hydrogen molecules, this creates prime conditions for forming
hydrocarbons. As the interstellar hydrogen gets warmer, carbon ions that
originally formed in stars begin to react with the molecular hydrogen,
creating CH+. Eventually the CH+ captures an electron to form the
neutral CH molecule.
"This is the initiation of the whole carbon chemistry," said John
Pearson, researcher at NASA's Jet Propulsion Laboratory, Pasadena,
California, and study co-author. "If you want to form anything more
complicated, it goes through that pathway."
Scientists combined Herschel data with models of molecular formation
and found that ultraviolet light is the best explanation for how
hydrocarbons form in the Orion Nebula.
The findings have implications for the formation of basic
hydrocarbons in other galaxies as well. It is known that other galaxies
have shocks, but dense regions in which ultraviolet light dominates
heating and chemistry may play the key role in creating fundamental
hydrocarbon molecules there, too.
"It's still a mystery how certain molecules get excited in the cores
of galaxies," Pearson said. "Our study is a clue that ultraviolet light
from massive stars could be driving the excitation of molecules there,
Herschel is a European Space Agency mission, with science instruments
provided by consortia of European institutes and with important
participation by NASA. While the observatory stopped making science
observations in April 2013, after running out of liquid coolant as
expected, scientists continue to analyze its data. NASA's Herschel
Project Office is based at NASA's Jet Propulsion Laboratory, Pasadena,
California. JPL contributed mission-enabling technology for two of
Herschel's three science instruments. The NASA Herschel Science Center,
part of IPAC, supports the U.S. astronomical community. Caltech manages
JPL for NASA.