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History/Origin of Chemicals
A few hundred thousand years after the Big Bang, protons and
electrons cooled down enough to settle into atoms of hydrogen and,
to a much lesser degree, helium and traces of other elements.
No other chemicals are thought to have existed in significant
amounts until the universe was millions of years old and stars began
to form. Stars are responsible for the creation of all the other
natural elements and many of the chemical compounds that exist
today. The very elements of which our planet and our bodies are made
were themselves made in a star.
Elements
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Elements up to the weight of iron are manufactured in stars.
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The process begins when a star forms from a collapsing hydrogen cloud.
The gravitational pressure at the star's core generates heat, which
ignites a thermonuclear fusion reaction that converts the core's
hydrogen into helium. This process, called "nucleosynthesis,"
continues until the core's hydrogen is exhausted. What happens next
depends on the star's mass.
Observations indicate that most stars are massive enough to enter
a second round of nucleosynthesis. The depleted core - now rich in
helium - contracts further, generating enough heat to start a
thermonuclear reaction in a shell of hydrogen surrounding it, which
fuses that hydrogen into helium. If the core's temperature gets hot
enough, it undergoes a second wave of thermonuclear fusion itself,
turning its helium into carbon and oxygen.
The more massive the star, the more generations of
nucleosynthesis it will experience. The most massive stars can have
several layers of fusion going on at the same time, with the
outermost converting hydrogen to helium, a shell beneath it turning
helium into carbon and oxygen, a shell beneath that producing
heavier elements, a shell beneath that creating even heavier
elements, and so on down to a core in which iron is produced.
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Elements heavier than iron are formed when a supernova explodes.
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Once a star forms an iron core, its days are numbered. Up to
that point, the nuclear fusion reactions produce energy, creating
an outward pressure that counterbalances the inward pressure of
gravity. But iron fusion uses up energy instead of producing it.
So the outward pressure stops and even reverses, gravity takes over,
and the star rapidly implodes until suddenly a vast number of
neutrinos blast out of the core, blowing the rest of the star to
bits in a supernova explosion that may be as bright as an entire
galaxy.
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These gaseous supernova remnants glow in colors determined by the
elements they contain. For example, the dark blue areas are
rich in oxygen and the red material is rich in sulfur.
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Since this happens only to very massive stars that have undergone
a full range of stellar evolution, the explosion releases a wide
variety of new elements into the interstellar medium (ISM), which
may ultimately incorporate them into new stars and continue the
nucleosynthesis process.
The violent supernova blast produces powerful shock waves which
create regions so dense and hot that they fuse some of the star's
heavy elements into still heavier elements. It is in these supernova
shocks that all natural elements heavier than iron are created,
including uranium, the heaviest natural element found on Earth.
Supernova shocks create even heavier elements (as do experiments
with supercolliders), but they decay much more quickly than uranium.
Chemical Compounds
The nuclear reactions that transform one element into another
require the enormous energies of a star. The energy needs of
chemical reactions, which combine elements to form compounds, are
much more modest. Many kinds of environments, from cold interstellar
clouds of atoms to toroids encircling
Active Galactic Nuclei
play host to chemical processes.
Spectroscopic observations at radio wavelengths have detected
more than 100 species of molecules in space. Most are organic (that
is, carbon-hydrogen compounds). Many of the molecules, such as
water, carbon monoxide, and formaldehyde, are found commonly on
Earth, but some are exotic species seen only in space (or, in some
cases, in advanced laboratory experiments). Species that would be
unstable on Earth can endure in areas of very low density and
temperature, where there is insufficient energy to trigger their
conversion to more stable varieties.
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The entire cycle of the birth and death of stars promotes
chemical reactions.
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Interstellar clouds are mostly hydrogen and some helium. But if
they've hosted a few generations of stars, they also may contain
oxygen, carbon, and heavier elements - the raw materials for
chemical processes. Complex molecules form in dense regions,
shielded from the disruptive ultraviolet radiation of nearby stars.
In diffuse regions, where incoming UV photons destroy larger
molecules faster than they can form, only small, simple molecules
can survive.
Molecular clouds host ion-molecule reactions leading to
unsaturated molecules, ions, and radicals. They are also where
deuterium fractionation (forming molecules that contain the hydrogen
isotope, deuterium) takes place.
As such a cloud forms a dense core that begins to collapse,
oxygen atoms stuck to the surfaces of dust grains combine with
hydrogen to form water ice mantles. As the collapse proceeds and
densities and temperatures rise, the ice sublimates, enriching the
molecular gas. And more chemical reactions take place, including
ones that lead to water.
As the protostar develops, strong bipolar outflows induce shock
waves in the surrounding molecular cloud which cause rapid
compression of the gas, briefly heating it to high temperatures that
induce chemical reactions, again including formation of water.
Before the developing star blows away the dense gas and dust
surrounding it, its growing heat drives endothermic reactions such
as the conversion of oxygen and hydrogen into water, and releases
frozen water molecules among other kinds. At the same time, intense
ultraviolet radiation forms a "photon dominated region" (PDR) near
the star, which strips dust grains of their icy mantles and breaks
gaseous molecules down into simpler chemicals or their constituent
atoms.
Throughout its active lifetime, the star produces a stellar wind
of protons, electrons, and ions in which complex molecules are
generated.
Further chemical processes take place at the end of the star's
life. When a star becomes a red giant, it swells to the point where
its outer surface becomes cool enough to allow condensation of some
of its heavy elements into solid particles (grains of dust). The
outer envelope becomes so distended that its surface gravity can't
hold the gases and dust that comprise its atmosphere, and they blow
away into the ISM through a powerful stellar wind.
Water
One chemical compound of particular interest is water. As icy
mantles on dust grains, water is a major reservoir of oxygen (since
each molecule of H2O includes one atom of oxygen).
Infrared spectra reveal water in the Orion nebula, a region of
intense star formation.
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Studies with
ISO
suggested that water is one of the most important
cooling agents in dense molecular clouds, enabling them to
contract enough to form stars. More recent studies, including
observations with
SWAS
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indicate that water does not play such a key role. Herschel will
contribute important evidence to further one side or the other.
Determining water's abundance has been an outstanding problem in
astrophysics. The spectra of space-borne water molecules are
inaccessible from the ground and even from airborne observatories
like
SOFIA
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But Herschel will pick up where ISO left off in the search for water
in space. It will provide detailed information about how much water
there is, where it is, how it is formed, and the role it plays in
interstellar chemistry and star formation.
Water's spectrum has many lines with intrinsic strengths that
vary over several orders of magnitude and at energy levels from
almost zero to several thousand Kelvins. And water's energy levels
are very sensitive to gas density and temperature and to the thermal
radiation of dust. So different water lines will be indicators of
vastly different environments. Water is likely to become the most
important tracer of star formation processes, including cloud core
infalls, shocks, hot cores, and related phenomena.
Herschel will make it possible for the first time to get a
complete inventory of the most important rotational lines of water,
enabling scientists to trace the evolution of water from formation
to dissociation.
Dust
Here on Earth, we think of dust as bits of debris that rub off of
larger objects. But in space - and in the history of matter in the
Universe - solid matter begins as dust.
Almost all of the ISM's iron, magnesium, and silicon is in the
form of dust, as is much of the carbon and some of the oxygen and
nitrogen.
Dust is important in chemical evolution because it provides
surfaces on which water and other chemicals can condense out of
their gaseous form. And it provides a platform on which chemicals
can come together and react, as when a mantle of water ice forms out
of hydrogen and oxygen atoms.
Other Interesting Compounds
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Much of the brightness in this infrared view of the Horsehead nebula
comes from PAH emissions.
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Herschel will provide a unique opportunity to obtain precise
information about the size and structure of such interesting
space-borne molecules as PAHs, fullerenes, carbon chains, and amino
acids.
Polycyclic Aromatic Hydrocarbons (PAHs) are organic molecules
common on Earth. They are also the most abundant complex molecules
observed in the ISM.
Fullerenes are a third form of carbon, along with graphite and
diamond. Configured in the pattern of the geodesic domes designed by
Buckminster Fuller, they are nearly spherical and extremely stable.
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The Cat's Eye Nebula has an AGB star at its center.
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Carbon chains are an important component of the organic inventory
in cold molecular clouds and in carbon-rich outflows from the poles
of AGB stars (stars in the latter stages of their evolution).
The fundamental building blocks of proteins, amino acids
represent an intermediate step in the prebiotic evolution of life. A
rich inventory of amino acids have been found in carbonaceous
meteorites, presenting the intriguing possibility that the
precursors of life on Earth could have come from space.
What Herschel Will Do
Herschel will provide new information about the development and
distribution of chemicals in space. Its ability to track water and
other "tracer" molecules will also enable scientists to derive new
insights about the evolution of stars and galaxies, both now and in
the early universe.
Spectral imaging with Herschel will allow, for the first time,
study of the distribution and excitation of key molecular species,
some of which can be detected only at the wavelengths that Herschel
covers.
In particular, Herschel will track the evolution of molecules
during the star formation process. It will survey protostellar
regions from the beginning of cloud collapse, through the shock
chemistry induced by a protostar's bipolar outflow, to the naked T
Tauri phase in which the new star has blown away its dusty envelope
but not yet ignited nuclear fusion.
Herschel will be able to observe Galactic nitrogen and carbon ion
fine structure emission, among the most important probes of the ISM
in the Milky Way. Its superb spectral resolution capability will be
used to study the 12C/13C ratio across the Galaxy, which is
connected with galactic chemical evolution.
And Herschel will study the synthesis of elements in the early
universe, especially the main phase of metal production at high
redshifts. Investigations of lines of [Si II], [S III], [O I], [N II],
and [C II] are particularly promising because they are expected
to be very luminous and detectable out to extremely large distances.
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