History 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
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.
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.
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.
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).
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 , 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 . 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 made 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
Herschel provided 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.
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 Did
Herschel provided new information about the development and distribution of chemicals in space. Its ability to track water and other "tracer" molecules also enabled scientists to derive new insights about the evolution of stars and galaxies, both now and in the early universe.
Spectral imaging with Herschel allowed, 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 tracked 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 was 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 was used to study the 12C/13C ratio across the Galaxy, which is connected with galactic chemical evolution.
And Herschel studied 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.
