User:Marshallsumter/Radiation astronomy/Chemistry

Def. the study of the abundance of elements and isotope ratios in Solar System objects, such as meteorites is called cosmochemistry.

Def. the study of interstellar atoms and molecules and their interaction with radiation is called molecular astrophysics.

Def. the "study of the chemical composition of stars and outer space"^[1] is called astrochemistry.

"An important goal for theoretical astrochemistry is to elucidate which organics are of true interstellar origin, and to identify possible interstellar precursors and reaction pathways for those molecules which are the result of aqueous alterations."^[2]

"¹⁸²Hf (T_1/2 = 9 x 10⁶ y) is believed to be formed by pure r-process during a supernova explosion".^[3]

"In order to understand much of the chemistry that underpins astronomical phenomena (e.g. star and planet formation) it is essential to probe the physico-chemistry of ice surfaces under astronomical conditions. The physical properties and chemical reactivity of such icy surfaces depends upon its morphology. Thus it is necessary to explore how the morphology of astrochemical ices is influenced by their local environment (e.g. temperature and pressure) and the mechanisms by which they are processed."^[4]

Most "planetary and lunar surfaces in our own solar system (Venus and Mercury being the exceptions) contain large amounts of ice."^[4]

"Irradiation of these species plausibly results in the dark neutrally colored centaurs and Kuiper belt objects (KBOs)."^[5]

"The surfaces of KBOs range from those which are neutrally reflecting—and thus appear to have essentially solar colors—to some of the reddest objects known in the solar system. The full range of colors is mixed at what appears to be nearly random throughout the outer solar system (Morbidelli & Brown 2005)."^[5]

"Olivines are described by Mg_2yFe_2-2ySiO₄, with y in the range [0, 1]."^[6] Substituting values for y from 0 to 1 produce ideal compositions from forsterite Mg₂SiO₄ to fayalite Fe₂SiO₄.

"Laboratory studies of the evolution of a magnesium silicate smoke from an amorphous condensate to a crystalline mineral by annealing in vacuum provide a foundation for the development of a silicate evolution index (SEI)."^[7]

"Emission from aromatic hydrocarbons dominate the mid-infrared emission of many galaxies, including our own Milky Way galaxy. Only recently have aromatic hydrocarbons been observed in absorption in the interstellar medium, along lines of sight with high column densities of interstellar gas and dust. [...] laboratory and theoretical astrochemistry studies are needed to explain astrophysical observations, such as a possible absorption feature due to interstellar “diamonds”".^[8]

"The [interstellar infrared emission features] IEFs are believed to arise in either interstellar grains or interstellar molecules, and because their emission can account for up to 30 – 40% of the overall emission of galaxies, solving problems related to their composition, size, and origin are essential for understanding the overall energy budget of radiation from galaxies."^[8]

"Astrochemistry is the chemistry of astronomical entities in which molecular compounds exist."^[9]

"Dense clouds also have a number of sub-classes. Although these entities are treated separately, they can and do interact."^[10]

"Ammonia clouds [and] water clouds ... will be absent in 51 Peg B ... At an effective temperature of roughly 1250 K, the primary cloud-forming materials near the surface are magnesium silicates and other silicate compounds."^[11]

"The most dominant group is the iron group (Z = 25 - 27), at energies around 70 PeV more than 50% of the all-particle [cosmic-ray] flux consists of these elements."^[12]

"[P]referential absorption of sunlight by ozone over long horizon paths gives the zenith sky its blueness when the sun is near the horizon".^[13]

"Plasma is the fourth state of matter, consisting of electrons, ions and neutral atoms, usually at temperatures above 10⁴ degrees Kelvin."^[14]

The bluish color of water is a composite of several contributing agents. Prominent contributors include dissolved organic matter and chlorophyll.^[15]

The image at right is a detailed, photo-like view of Earth based largely on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite.

"Viewed from space, the most striking feature of our planet is the water. In both liquid and frozen form, it covers 75% of the Earth’s surface. It fills the sky with clouds. Water is practically everywhere on Earth, from inside the rocky crust to inside our cells."^[16]

Spectrometers are able to measure the abundance and distribution of about 20 primary elements of the periodic table, including silicon, oxygen, iron, magnesium, potassium, aluminum, calcium, sulfur, and carbon.

"[O]nce ionized, the gas is rapidly heated by Coulomb collisions to the coronal cloud temperature, but as this material peels off, a cooler hydrogen-emitting region is left."^[17]

A number of spectral lines produced by interstellar gas, notably the hydrogen spectral line at 21 cm, are observable at radio wavelengths.^[18][19]

The familiar red H-alpha (Hα) 656 nm spectral line of hydrogen gas, which is the transition from the shell n = 3 to the Balmer series shell n = 2, is one of the conspicuous colors of the universe. It contributes a bright red line to the spectra of emission or ionization nebula, like the Orion Nebula, which are often H II regions found in star forming regions. In true-color pictures, these nebula have a distinctly pink color from the combination of visible Balmer lines that hydrogen emits.

Other signatures of magnetic clouds in the solar wind are now used in addition to the one described above: among other, bidirectional superthermal electrons, unusual charge state or abundance of iron, helium, carbon and/or oxygen.

The spectral lines from the atmospheres of spectral type O and B stars "show a large number of isolated and overlapping He I lines, the strongest of which are the spectral lines at 447.1 and 492.2 nm"^[20].

In some 824 red giant stars, the Li I 670.78 nm line was detected in several stars, "but only the five objects ... presented a strong line. Indeed, the Li subordinate line at 6103.6 Å was detected in these stars only."^[21]

Other signatures of magnetic clouds [in the solar wind] are now used in addition to the one described above: among other, bidirectional superthermal electrons, unusual charge state or abundance of iron, helium, carbon and/or oxygen.

Nitrogen emission ([NII] occurs in the red at 658.4 nm. Gaseous "cometary knots" have heads that have NII emission and are at least twice the size of our solar system.

The image at right is a color picture, taken with the Wide Field Planetary Camera-2. It is a composite of three images taken at different wavelengths. (red, hydrogen-alpha; blue, neutral oxygen, 630.0 nm; green, ionized nitrogen, 658.4 nm). This NASA Hubble Space Telescope image shows one of the most complex planetary nebulae ever seen, NGC 6543, nicknamed the "Cat's Eye Nebula." The image was taken on September 18, 1994. NGC 6543 is 3,000 light-years away in the northern constellation Draco. The term planetary nebula is a misnomer; dying stars create these cocoons when they lose outer layers of gas.

There are faraway active galaxies that show a blueshift in their O III emission lines.

Sodium produces two spectral lines known as D1 and D2, or the "sodium doublet". Their average wavelength, 589.3 nm, is often just called "D".

During the limb flares of December 18, 1956, a coronal line at 569.4 nm, a yellow line, occurred at 1822 UTC, 1900 UTC, undiminished up to 20,000 km above the solar limb, and at 2226 UTC, is identified as Ca XV.^[22]

The orange system in orange astronomy is a number of emission lines very close together forming a band in the orange portion of the visible spectrum. These lines are usually associated with particular molecular species, including ScO, YO, and TiO.^[23]

The orange system in orange astronomy is a number of emission lines very close together forming a band in the orange portion of the visible spectrum. These lines are usually associated with particular molecular species, including ScO, YO, and TiO.^[23]

The Fe VII emission line at 608.7 nm, "frequently observed in the spectra of astrophysical plasmas", has been detected in planetary nebulae, Seyfert galaxies, and quasars.^[24]

Other signatures of magnetic clouds in the solar wind are now used in addition to the one described above: among other, bidirectional superthermal electrons, unusual charge state or abundance of iron, helium, carbon and/or oxygen.

The orange system in orange astronomy is a number of emission lines very close together forming a band in the orange portion of the visible spectrum. These lines are usually associated with particular molecular species, including ScO, YO, and TiO.^[23]

The chemical element thorium [is] mapped by a GRS, with higher concentrations shown in yellow/orange/red in the left-hand side image shown on the right.

For stars, the metallicity is often expressed as "[Fe/H]", which represents the logarithm of the ratio of a star's iron abundance compared to that of the Sun (iron is not the most abundant heavy element, but it is among the easiest to measure with spectral data in the visible spectrum). The formula for the logarithm is expressed thus:

where ${\displaystyle N_{\mathrm {Fe} }}$  and ${\displaystyle N_{\mathrm {H} }}$  are the number of iron and hydrogen atoms per unit of volume respectively. The unit often used for metallicity is the "dex" which is a (now-deprecated) contraction of decimal exponent.^[25] By this formulation, stars with a higher metallicity than the Sun have a positive logarithmic value, while those with a lower metallicity than the Sun have a negative value. The logarithm is based on powers of ten; stars with a value of +1 have ten times the metallicity of the Sun (10¹). Conversely, those with a value of -1 have one tenth (10 ⁻¹), while those with -2 have a hundredth (10⁻²), and so on.^[26] Young Population I stars have significantly higher iron-to-hydrogen ratios than older Population II stars. Primordial Population III stars are estimated to have a metallicity of less than −6.0, that is, less than a millionth of the abundance of iron which is found in the Sun.

Def. an "atom or group of atoms bearing an electrical charge such as the sodium and chlorine atoms in a salt solution"^[27] is called an ion.

About 89% of cosmic rays are simple protons or hydrogen nuclei, 10% are helium nuclei of alpha particles, and 1% are the nuclei of heavier elements. Solitary electrons constitute much of the remaining 1%.

Def. the "colour of growing foliage, as well as other plant cells containing chlorophyll; the colour between yellow and blue in the visible spectrum; one of the primary additive colour for transmitted light; the colour obtained by subtracting red and blue from white light using cyan and yellow filters"^[28] is called green, or green radiation.

Usually, Hibonite ((Ca,Ce)(Al,Ti,Mg)₁₂O₁₉) as shown at right is a brownish black mineral. It is rare, but is found in high-grade metamorphic rocks on Madagascar. Some presolar grains in primitive meteorites consist of hibonite. Hibonite also is a common mineral in the Ca-Al-rich inclusions (CAIs) found in some chondrite chondritic meteorites. Hibonite is closely related to hibonite-Fe (IMA 2009-027, ((Fe,Mg)Al₁₂O₁₉)) an alteration mineral from the Allende meteorite.^[29] Hibonite is blue perhaps like the image at left in meteorite occurrence.

"Grain size varies from 98 to 530 lm with an average of *150 lm. Minor [elements] oxidation [from an iron–nickel–chromium–cobalt–phosphorus alloy] is evidenced by the presence of a light brown and blue surface layer composed of very fine-grained (<1 lm) crystals on the surface."^[30] The oxidation of minor elements in metallic alloys in the early solar system is indicated to possess at instances a blue surface layer.^[30]

Def. a layer of gases that may surround a material body of sufficient mass,^[31] and that is held in place by the gravity of the body is called an atmosphere.

Upon reaching the top of the mesosphere, the temperature starts to rise, but air pressure continues to fall. This is the beginning of the ionosphere, a region dominated by chemical ions. Many of them are the same chemicals such as nitrogen and oxygen in the atmosphere below, but an ever increasing number are hydrogen ions (protons) and helium ions. These can be detected by an ion spectrometer. The process of ionization removes one or more electrons from a neutral atom to yield a variety of ions depending on the chemical element species and incidence of sufficient energy to remove the electrons.

Def. matter "which may be shaped or manipulated, particularly in making something"^[32] is called a material.

Def. a "metallic or stony object or body that is the remains of a meteor"^[33] is called a meteorite.

Widmanstätten patterns, also called Thomson structures, are unique figures of long nickel-iron crystals, found in the octahedrite iron meteorites and some pallasites. They consist of a fine interleaving of kamacite and taenite bands or ribbons called lamellæ. Commonly, in gaps between the lamellæ, a fine-grained mixture of kamacite and taenite called plessite can be found.

By comparing astronomical observations with laboratory measurements, astrochemists can infer the elemental abundances, chemical composition, and temperatures of stars and interstellar clouds. This is possible because ions, atoms, and molecules have characteristic spectra: that is, the absorption and emission of certain wavelengths (colors) of light, often not visible to the human eye. However, these measurements have limitations, with various types of radiation (radio, infrared, visible, ultraviolet etc.) able to detect only certain types of species, depending on the chemical properties of the molecules. Interstellar formaldehyde was the first polyatomic organic molecule detected in the interstellar medium.

Depending primarily upon gas temperature, the presence of gas may be used to determine the composition of the gas body observed, at least the outer layer. Early spectroscopy^[34] of the Sun using estimates of "the line intensities of several lines by eye [to derive] the abundances of ... elements ... [concluded] that the Sun [is] largely made of hydrogen."^[35]

Other signatures of magnetic clouds in the solar wind are now used in addition to the one described above: among other, bidirectional superthermal electrons, unusual charge state or abundance of iron, helium, carbon and/or oxygen.

Calcium (Ca), chromium (Cr), iron (Fe), and titanium (Ti) emission lines have been detected in solar limb faculae.^[36]

In the image at right the iron (Fe XIV) green line is followed by doppler imaging to show associated relative coronal plasma velocity towards (-7 km/s side) and away from (+7 km/s side) the large angle spectrometric coronagraph LASCO satellite camera.

"Carroll et al. (1976) detected a number of coincidences between laboratory lines of FeH and weak unidentified solar lines, again in the blue and green wavelength region, in addition to the infrared."^[37]

The visible light we see is produced as electrons react with hydrogen atoms to produce H⁻ ions.^[38][39]

"Optical reflectance studies of Mercury provide evidence for Mg silicates."^[40]

The MESSENGER X-ray spectrometer (XRS) maps mineral composition within the top millimeter of the surface on Mercury by detecting X-ray spectral lines from magnesium, aluminum, sulphur, calcium, titanium, and iron, in the 1-10 keV range.^[41][42]

When viewed using radio astronomy, the resulting radar image of Venus at right shows that just beneath the cloud layers is a rocky object.

Def. that "part of outer space between the planets of a solar system and its star"^[43] is called interplanetary space.

Def. the material which fills the solar system and through which all the larger solar system bodies such as planets, asteroids and comets move is called an interplanetary medium.

Chemical ions above the Earth's atmosphere, moving at very high speeds and at concentrations up to 100 particles per cm³ (centimeter cubed, a unit of volume) constitute the interplanetary medium.

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. Pure quartz, traditionally called rock crystal (sometimes called clear quartz), is colorless and transparent or translucent.

Iceland spar, formerly known as Iceland crystal, is a transparent variety of calcite, or crystallized calcium carbonate. It has been speculated that the sunstone (a different mineral than the gem-quality sunstone) mentioned in medieval Icelandic texts was Iceland spar and that Vikings used its light-polarizing property to tell the direction of the sun on cloudy days, for navigational purposes.^[44][45]

A chemically pure and structurally perfect diamond is perfectly transparent with no hue, or color. However, in reality almost no gem-sized natural diamonds are absolutely perfect.

Def. the "gases surrounding the Earth or any astronomical body"^[46] is called an atmosphere.

On July 1, 1957, following the intense auroral display of the previous night, the variation in hydrogen (H) Hβ emission shows quite clearly that the sudden transition from an auroral arc to rays coincides with a decrease in the intensity of the hydrogen emission and an inversion of the polarity of the magnetic disturbance.^[47]

The atmosphere of Earth is composed of small particles called molecules.

When cosmic rays enter the Earth’s atmosphere they collide with molecules, mainly oxygen and nitrogen, to produce a cascade of billions of lighter particles, a so-called air shower.

These molecules in many instances are in turn made up of atoms of chemical elements. At each geographical location, specified in latitude and longitude, this gaseous envelope extends upward. The atmosphere of Earth changes with altitude. At high enough altitude the composition changes significantly, as does the temperature and pressure.

Between the Earth’s surface and various altitudes there is an electric field. It changes with altitude from about 150 volts per meter to lower values at higher altitude. In fair weather, it is relatively constant, in turbulent weather it is accompanied by ions. At greater altitude these chemical species continue to increase in concentration. For travel upwards, eventually protective clothing and appropriate breathing apparati are needed. The air pressure is lowering as is the ambient temperature.

The Earth has an ionosphere, a region dominated by chemical ions. Many of them are the same chemicals such as nitrogen and oxygen in the atmosphere below, but an ever increasing number are hydrogen ions (protons) and helium ions. These can be detected by an ion spectrometer.

From the ground below, or with spectrometers on platforms at higher altitude, including satellites, ion species and concentrations are measured. Into the exosphere or outer space, temperature rises from around 1,500°C (centigrade) to upwards of 100,000 K (kelvin).

"The production and escape of hot ions (H⁺ and H⁺₂) and hot atomic hydrogen by stellar ultraviolet radiation is ... likely".^[11]

Airglow is caused by various processes in the upper atmosphere, such as the recombination of ions which were photoionized by the sun during the day, luminescence caused by cosmic rays striking the upper atmosphere, and chemiluminescence caused mainly by oxygen and nitrogen reacting with hydroxyl ions at heights of a few hundred kilometers. It is not noticeable during the daytime because of the scattered light from the Sun.

"Auroras result from emissions of photons in the Earth's upper atmosphere, above 80 km (50 mi), from ionized nitrogen atoms regaining an electron, and oxygen and nitrogen atoms returning from an excited state to ground state.^[48] They are ionized or excited by the collision of solar wind and magnetospheric particles being funneled down and accelerated along the Earth's magnetic field lines; excitation energy is lost by the emission of a photon, or by collision with another atom or molecule:

oxygen emissions

green or brownish-red, depending on the amount of energy absorbed

nitrogen emissions

blue or red; blue if the atom regains an electron after it has been ionized, red if returning to ground state from an excited state.

The Chandra X-ray Observatory has detected X-rays from oxygen, magnesium, aluminum and silicon atoms on the Moon.^[49]

Both Luna 24 and Apollo 12 soil samples are from mare soils that reflect primarily cyan that is likely due to the presence of TiO₂ in the soils.^[50]

Gamma-ray spectrometers have been widely used for the elemental and isotopic analysis of airless bodies in the Solar System, especially the Moon^[51] These surfaces are subjected to a continual bombardment of high-energy cosmic rays, which excite nuclei in them to emit characteristic gamma-rays which can be detected from orbit. Thus an orbiting instrument can in principle map the surface distribution of the elements for an entire planet. They are able to measure the abundance and distribution of about 20 primary elements of the periodic table, including silicon, oxygen, iron, magnesium, potassium, aluminum, calcium, sulfur, and carbon. The chemical element thorium is mapped by a GRS, with higher concentrations shown in yellow/orange/red in the left-hand side image shown on the right.

At right is the result of an all Moon survey by the Lunar Prospector using an onboard neutron spectrometer (NS). Cosmic rays impacting the lunar surface generate neutrons which in turn loose much of their energy in collisions with hydrogen atoms trapped within the Moon's surface.^[52] Some of these thermal neutrons collide with the helium atoms within the NS to yield an energy signature which is detected and counted.^[52] The NS aboard the Lunar Prospector has a surface resolution of 150 km.^[52]

Based on the ³He-flare flux from the Sun's surface and Surveyor 3 samples (containing ¹⁵N and ¹⁴C implanted in lunar material by solar radiation) from the surface of the Moon, the level of nuclear fusion occurring in the solar atmosphere is approximately at least two to three orders of magnitude greater than that estimated from solar flares such as those of August 1972.^[53]

Mars is often described as the "Red Planet" as the iron(III) iron oxide prevalent on its surface gives it a reddish appearance.^[54] The red-orange appearance of the Martian surface is caused by iron(III) oxide, more commonly known as hematite, or rust.^[55] Much of the surface is deeply covered by finely grained iron(III) oxide dust.^[56][57]

Methane is found in the Martian atmosphere, first image at right, by carefully observing the planet throughout several Mars years with NASA's Infrared Telescope Facility and the W.M. Keck telescope, both at Mauna Kea, Hawaii.

"The Dynamic Albedo of Neutrons (DAN) is an active/passive neutron spectrometer that measures the abundance and depth distribution of H- and OH-bearing materials (e.g., adsorbed water, hydrated minerals) in a shallow layer (~1 m) of Mars' subsurface along the path of the MSL rover. In active mode, DAN measures the time decay curve (the "dynamic albedo") of the neutron flux from the subsurface induced by its pulsing 14 MeV neutron source."^[58]

At right is an X-ray image of Mars. X-radiation from the Sun excites oxygen atoms in the Martian upper atmosphere, about 120 km above its surface, to emit X-ray fluorescence. A faint X-ray halo that extends out to 7,000 km above the surface of Mars has also been found.^[59]

"[A]bsorption features in the submillimeter spectrum of Mars ... are due to the H₂O (1₁₀-1₀₁) and ¹³CO (5-4) rotational transitions."^[60]

"The distribution of water in the Martian atmosphere matches a profile of constant, 100% saturation from 10 to 45 km altitude."^[60]

On October 7, 2009, the presence of water ice was confirmed on the surface of [24 Themis] using NASA’s Infrared Telescope Facility. The surface of the asteroid appears completely covered in ice. As this ice layer is sublimated, it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface.^{[61][62][63][64]}

Trace amounts of water would be continuously produced by high-energy solar protons impinging oxide minerals present at the surface of the asteroid. The hydroxyl surface groups (S–OH) formed by the collision of protons (H⁺) with oxygen atoms present at oxide surface (S=O) can further be converted in water molecules (H₂O) adsorbed onto the oxide minerals surface. The chemical rearrangement supposed at the oxide surface could be schematically written as follows:

2 S-OH → S=O + S + H₂O

or,

2 S-OH → S–O–S + H₂O

where S represents the oxide surface.^[65]

"Some species previously detected on Jupiter, including CH₃D, C₂H₂, and C₂H₆, have been observed again near the pole. Newly discovered species, not previously observed on Jupiter, include C₂H₄, C₃H₄, and C₆H₆. All of these species except CH₃D appear to have enhanced abundances at the north polar region with respect to midlatitudes."^[66]

The orange and brown coloration in the clouds of Jupiter are caused by upwelling compounds that change color when they are exposed to ultraviolet light from the Sun. The exact makeup remains uncertain, but the substances are believed to be phosphorus, sulfur or possibly hydrocarbons.^[67][68] These colorful compounds, known as chromophores, mix with the warmer, lower deck of clouds. The zones are formed when rising convection cells form crystallizing ammonia that masks out these lower clouds from view.^[69]

"[F]or wavelengths between 0.35 and 0.45 mm ... the radiances can be matched by models which include NH₃ ice particles which are between 30 and 100 µm in size, regardless of the scale height characterizing the cloud."^[70]

At right is an "eerie view of Jupiter's moon Io in eclipse ... acquired by NASA's Galileo spacecraft while the moon was in Jupiter's shadow. Gases above the satellite's surface produced a ghostly glow that could be seen at visible wavelengths (red, green, and violet). The vivid colors, caused by collisions between Io's atmospheric gases and energetic charged particles trapped in Jupiter's magnetic field, had not previously been observed. The green and red emissions are probably produced by mechanisms similar to those in Earth's polar regions that produce the aurora, or northern and southern lights. Bright blue glows mark the sites of dense plumes of volcanic vapor, and may be places where Io is electrically connected to Jupiter."^[71]

"[T]he PH₃ 1-0 rotational line (266.9 GHz) line [has been detected] in [the atmosphere of] Saturn".^[72]

Much as with Venus prior to the Space Age, the dense, opaque atmosphere prevented understanding of Titan's surface until new information accumulated with the arrival of the Cassini–Huygens mission in 2004, including the discovery of liquid hydrocarbon lakes in the polar regions.

The atmosphere of Titan is largely composed of nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog.

"Methane possesses prominent absorption bands in the visible and near-infrared (IR) making Uranus aquamarine or cyan in color.^[73]

In January 1986, the Voyager 2 spacecraft flew by Uranus at a minimal distance of 107,100 km^[74] providing the first close-up images and spectra of its atmosphere. They generally confirmed that the atmosphere was made of mainly hydrogen and helium with around 2% methane.^[75] The atmosphere appeared highly transparent and lacking thick stratospheric and tropospheric hazes. Only a limited number of discrete clouds were observed.^[76]

Infrared spectroscopy conducted from 2001 to 2005 revealed the presence of water ice as well as frozen carbon dioxide on the surface of Titania, which in turn suggested that the moon may possess a tenuous carbon dioxide atmosphere with a surface pressure of about one 10 trillionth of a bar.

A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue,^[77] although Neptune's vivid azure differs from Uranus's milder cyan. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.^[78]

One of the substances discovered in the tail by spectroscopic analysis was the toxic gas cyanogen.^[79]

"In the green, the polarization of the pure silicate composition qualitatively appears a better fit to the shape of the observed polarization curves".^[6] "[B]ut they are characterized by a high albedo."^[6] The silicates used to model the cometary coma dust are olivene (Mg-rich is green) and the pyroxene, enstatite.^[6]

Cyan blue is the color of several cyanide (CN) containing materials, including CN detected in comet haloes.

"Lulin's green color comes from the gases that make up its Jupiter-sized atmosphere. Jets spewing from the comet's nucleus contain cyanogen (CN: a poisonous gas found in many comets) and diatomic carbon (C₂). Both substances glow green when illuminated by sunlight".^[80]

Ultraviolet line spectrum measurements are used to discern the chemical composition, densities, and temperatures of the interstellar medium, and the temperature and composition of hot young stars.

"The [microwave] detection of interstellar formaldehyde provides important information about the chemical physics of our galaxy. We now know that polyatomic molecules containing at least two atoms other than hydrogen can form in the interstellar medium."^[81] "H₂CO is the first organic polyatomic molecule ever detected in the interstellar medium".^[81]

"Over the past 30 years, radioastronomy has revealed a rich variety of molecular species in the interstellar medium of our galaxy and even others."^[82]

There are 110 currently known interstellar molecules.

The cyanide radical CN^- has been identified in interstellar space.^[83] The cyanide radical (called cyanogen) is used to measure the temperature of interstellar gas clouds.^[84]

Radio astronomy has resulted in the detection of over a hundred interstellar species, including radicals and ions, and organic (i.e. carbon-based) compounds, such as alcohols, acids, aldehydes, and ketones. One of the most abundant interstellar molecules, and among the easiest to detect with radio waves (due to its strong electric dipole moment), is CO (carbon monoxide). In fact, CO is such a common interstellar molecule that it is used to map out molecular regions.^[85] The radio observation of perhaps greatest human interest is the claim of interstellar glycine,^[86] the simplest amino acid, but with considerable accompanying controversy.^[87] One of the reasons why this detection is controversial is that although radio (and some other methods like rotational spectroscopy) are good for the identification of simple species with large dipole moments, they are less sensitive to more complex molecules, even something relatively small like amino acids.

An H I region is an interstellar cloud composed of neutral atomic hydrogen (H I), in addition to the local abundance of helium and other elements.

The warm neutral medium (WNM) is 10-20 % of the ISM, ranges in size from 300-400 pc, temperature between 6000 and 10000 K, is composed of neutral atomic hydrogen, has a density of 0.2-0.5 atoms/cm³, and emits the hydrogen 21 cm line.^[88]

Also, within the H I regions is the warm ionized medium (WIM), constituting 20-50 % by volume of the ISM, with a size around 1000 pc, a temperature of 8000 K, an atom density of 0.2-0.5 atoms/cm³, of ionized hydrogen, emitting the hydrogen alpha line and exhibiting pulsar dispersion.^[88]

SIMBAD contains 6,010 entries of the astronomical object type 'HI' (H I region).

These regions are non-luminous, save for emission of the hydrogen line at 21 cm (1,420 MHz) region spectral line. Mapping H I emissions with a radio telescope is a technique used for determining the structure of spiral galaxies.

An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place.

In December 2006, seven papers were published in the scientific journal, Science, discussing initial details of the sample analysis. Among the findings are: a wide range of organic compounds, including two that contain biologically usable nitrogen; indigenous aliphatic hydrocarbons with longer chain lengths than those observed in the diffuse interstellar medium; abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene, proving consistency with the mixing of solar system and interstellar matter, previously deduced spectroscopically from ground observations;^[89] hydrous silicates and carbonate minerals were found to be absent, suggesting a lack of aqueous processing of the cometary dust; limited pure carbon (CHON) was also found in the samples returned; methylamine and ethylamine was found in the aerogel but was not associated with specific particles.

"In this October 1998 [Hubble Space Telescope] image, the telescope has looked down a barrel of gas cast off by a dying star thousands of years ago. This photo reveals elongated dark clumps of material embedded in the gas at the edge of the nebula; the dying central star floating in a blue haze of hot gas. The nebula is about a light-year in diameter and is located some 2000 light-years from Earth in the direction of the constellation Lyra. The colors are approximately true colors. The color image was assembled from three black-and-white photos taken through different color filters with the Hubble telescope's Wide Field Planetary Camera 2. Blue isolates emission from very hot helium, which is located primarily close to the hot central star. Green represents ionized oxygen, which is located farther from the star. Red shows ionized nitrogen, which is radiated from the coolest gas, located farthest from the star. The gradations of color illustrate how the gas glows because it is bathed in ultraviolet radiation from the remnant central star, whose surface temperature is a white-hot 120,000 degrees Celsius (216,000 degrees Fahrenheit)."^[90]

In the spectrum at right several red astronomy emission lines are detected and recorded at normalized intensities (to the oxygen III line) from the Ring Nebula. In the red are the two forbidden lines of oxygen ([O I], 630.0 and 636.4 nm), two forbidden lines of nitrogen ([N II], 654.8 nm and [N II], 658.4 nm), the hydrogen line (Hα, 656.3 nm) and a forbidden line of sulfur ([S II], 671.7 nm).

"The 1₁₁ → 1₁₀ rotational transition of formaldehyde (H₂CO) [occurs] in absorption in the direction of four dark nebulae. The radiation ... being absorbed appears to be the isotropic microwave background".^[91] One of the dark nebulae sampled, per SIMBAD is TGU H1211 P5.

In the image at right is a molecular cloud of gas and dust that is being reduced. "Likely, within a few million years, the intense light from bright stars will have boiled it away completely. The cloud has broken off of part of the Carina Nebula, a star forming region about 8000 light years away. Newly formed stars are visible nearby, their images reddened by blue light being preferentially scattered by the pervasive dust. This image spans about two light years and was taken by the orbiting Hubble Space Telescope in 1999."^[92]

The Submillimeter Wave Astronomy Satellite (SWAS) is in low Earth orbit to make targeted observations of giant molecular clouds and dark cloud cores. The focus of SWAS is five spectral lines: water (H₂O), isotopic water (H₂¹⁸O), isotopic carbon monoxide (¹³CO), molecular oxygen (O₂), and neutral carbon (C I).

Some of the incontrovertible brown dwarf substellar objects are identified by the presence of the 670.8 nm lithium [I] line. The most notable of these objects was Gliese 229B, which was found to have a temperature and luminosity well below the stellar range. Remarkably, its near-infrared spectrum clearly exhibited a methane absorption band at 2 micrometres, a feature that had previously only been observed in gas giant atmospheres and the atmosphere of Saturn's moon, Titan. Methane absorption is not expected at the temperatures of main-sequence stars. This discovery helped to establish yet another spectral class even cooler than L dwarfs known as "T dwarfs" for which Gl 229B is the prototype. Lithium is generally present in brown dwarfs and not in low-mass stars. The presence of the lithium line in a candidate brown dwarf's spectrum is a strong indicator that it is indeed substellar. The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the lithium test. Some brown dwarfs emit X-rays; and all "warm" dwarfs continue to glow tellingly in the red and infrared spectra until they cool to planetlike temperatures (under 1000 K).

The orange band from molecular CaCl is "observed in the spectra of many carbon stars."^[93] "[T]he concentration of CaCl is strongly temperature and pressure dependent, but almost independent of the C/O ratio at a fixed pressure."^[94]

"The aluminium abundance was derived from the resonance line at 394.4nm, and Al is underabundant by ∼ −0.7 dex with respect to iron."^[95]

The Spite plateau (or Spite lithium plateau) is a baseline in the abundance of lithium found in old stars orbiting the galactic halo.

"Spectra of the helium 2.06 µm and hydrogen 2.17 µm lines ... confirm the existence of an extended region of high-velocity redshifted line emission centered near [Sgr A^*/IRS 16]."^[96]

The supernova SN1987A in the Large Magellanic Cloud (LMC) was discovered on February 23, 1987, and its progenitor is a blue supergiant (Sk -69 202) with luminosity of 2-5 x 10³⁸ erg/s.^[97] The 847 keV and 1238 keV gamma-ray lines from ⁵⁶Co decay have been detected.^[97]

At right is a Hubble Space Telescope image of the Ghost Head Nebula. "This nebula is one of a chain of star-forming regions lying south of the 30 Doradus nebula in the Large Magellanic Cloud. The red and blue light comes from regions of hydrogen gas heated by nearby stars. The green light comes from glowing oxygen, illuminated by the energy of a stellar wind. The white center shows a core of hot, massive stars."^[98]

A semiconductor detector is a device that uses a semiconductor (usually silicon or germanium).

When barium fluoride BaF₂ is used, gamma rays typically excite the fast component, while alpha particles excite the slow component.

Def. an "object, usually made of glass, that focuses or defocuses the light [or an electron beam] that passes through it"^[99] is called a lens.

Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths, whereas silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths.

Lenses are typically made of glass or transparent plastic. This glass is usually approximately 75% silicon dioxide (SiO₂), sodium oxide (Na₂O) from sodium carbonate (Na₂CO₃), calcium oxide, also called lime (CaO), and several minor additives. Glass does not contain the internal subdivisions associated with grain boundaries in polycrystals and hence does not scatter light in the same manner as a polycrystalline material. The surface of a glass is often smooth since during glass formation the molecules of the supercooled liquid are not forced to dispose in rigid crystal geometries and can follow surface tension, which imposes a microscopically smooth surface.

Ultraviolet telescopes 10 nm - 400 nm resemble optical telescopes, but conventional aluminium-coated mirrors cannot be used and alternative coatings such as magnesium fluoride or lithium fluoride are used instead.

The domes of observatories, such as in the image at right, and the objects inside used to observe and control these observatories are made of chemicals.

Radiation shielding refers to a mass of absorbing material placed around a radioactive source or to reduce incoming radiation.

The effectiveness of a material as a biological shield is related to its cross-section for scattering and absorption, and to a first approximation is proportional to the total mass of material per unit area interposed along the line of sight between the radiation source and the region to be protected. Hence, shielding strength or "thickness" is conventionally measured in units of g/cm². The radiation that manages to get through falls exponentially with the thickness of the shield. In X-ray facilities, the plaster on the rooms with the x-ray generator contains barium sulfate and the operators stay behind a leaded glass screen and wear lead aprons. Almost any material can act as a shield from gamma or x-rays if used in sufficient amounts.

Practical radiation protection tends to be a job of juggling the three factors to identify the most cost effective solution.

For specific radiation: even very energetic alpha particles can be stopped by a single sheet of paper, and beta particles can be absorbed by a few millimeters of aluminum. High energy beta-particle shielding requires low density materials, e.g. plastic, wood, water or acrylic glass (Plexiglas, Lucite).

X-rays and gamma-rays are best absorbed by atoms with heavy nuclei. Barium sulfate is useful.

Ultraviolet radiation can be absorbed by sunscreens, clothing, and protective eyewear.

1. It may take the complete electromagnetic spectrum to identify all organic compounds in the interplanetary, interstellar, or intergalactic media.

"Clouds 11-13 and cloud 20 are Lynds (1962) dark clouds³ and sites of low-mass star formation (e.g., Sandell, Reipurth, and Gahm 1987) at high Galactic latitude and they are included in the sample as a control group."^[100]

• G. H. Herbig (March 1974). "VY Canis Majoris. IV. The emission bands of ScO". The Astrophysical Journal 188 (3): 533-8. doi:10.1086/152744. 

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