# Chemicals/Heliums

Helium was first detected as an unknown, yellow spectral line signature in sunlight during a solar eclipse in 1868.
" ... je vis immédiatement une série de neuf lignes brillantes qui ... me semblent devoir être assimilées aux lignes principales du spectre solaire, B, D, E, b, une ligne inconnue, F, et deux lignes du groupe G." ( ... I saw immediately a series of nine bright lines that ... seemed to me should be classed as the principal lines of the solar spectrum, B, D, E, b, an unknown line, F, and two lines of the group G.)
"C. FRIEDLÄNDER and H. Kayser have independently claimed to have found helium in the [Earth's] atmosphere. On examination of some photographs of the spectrum of neon I have identified six of the principal lines of helium, which thus establishes beyond question the presence of this gas in the air. The amount present in the neon it is, of course, impossible to estimate, but the green line (wave-length 5016 [501.6 nm]) is the brightest, as would be expected from the low pressure of the helium in the neon."

## Theoretical heliums

"Helium rain [has been] proposed to explain the excessive brightness of Saturn".
"Saturn’s helium rain was predicted because [...] Saturn is warmer than it should be, based on its age and predicted rate of cooling. The falling rain releases heat that accounts for the difference."
Helium "rain is the best way to explain the scarcity of neon in the outer layers of the planet [Jupiter], the solar system’s largest. Neon dissolves in the helium raindrops and falls towards the deeper interior where it re-dissolves, depleting the upper layers of both elements, consistent with observations."
"Helium condenses initially as a mist in the upper layer, like a cloud, and as the droplets get larger, they fall toward the deeper interior."
"Neon dissolves in the helium and falls with it. So our study links the observed missing neon in the atmosphere to another proposed process, helium rain."
"The helium droplets form about 10,000 to 13,000 kilometers (6,000-8,000 miles) below the tops of Jupiter’s hydrogen clouds, under pressures and temperatures so high that" "you can’t tell if hydrogen and helium are a gas or a liquid."
"They’re all fluids, so the rain is really droplets of fluid helium mixed with neon falling through a fluid of metallic hydrogen."
"As the helium and neon fall deeper into the planet, the remaining hydrogen-rich envelope is slowly depleted of both neon and helium."
"The measured concentrations of both elements agree quantitatively with our calculations."

## Emissions

Helium has green lines at 501 and 505 nm, while a line at 493 nm is in the cyan and 587 nm is in the yellow.
Note that helium does have emission lines in the blue.
As shown in the above spectrum, helium has at least one emission line in the violet.
The He I emission lines are at 414.3 nm and 447.1 nm.

## Clouds

"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."

## Cosmic rays

“About 89% of cosmic rays are simple protons or hydrogen nuclei, 10% are helium nuclei or alpha particles, and 1% are the nuclei of heavier elements. ... Solitary electrons ... constitute much of the remaining 1%.”
Def. a "nucleus of a helium-3 atom" is called a helion.

## Muons

"The Moon shadow was measured as a deficit of downgoing muon events, which are the majority of events detected by IceCube and are generated by the interaction of cosmic rays with the Earth’s atmosphere. Only muons reaching the detector when the Moon was at least 15 degrees above the horizon were selected for analysis. Over 300 million events passed this initial cut, with about 68% of the events estimated to be produced by proton cosmic rays and another 23% by helium cosmic rays."

## Ultraviolets

There is a He I (Is$^{21}$S-Is2p$^2$P) transition at 58.4 nm and another Is$^{21}$S-Is3p$^1$P) at 53.7 nm. The He II lines are at 25.6, 30.4, and 164.0 nm.
There is another He II line at 320.3 nm.

## Opticals

The spectral diagram above shows the emission spectrum of helium over the wavelength range of 200 to 1000 nm (near-ultraviolet through near-infrared). The upper plot has linearly scaled intensities, while the lower plot has logarithmically scaled intensities (to show fainter lines). The line wavelengths and intensities are from NIST.

## Visuals

The helium emission spectrum in the visual range above is with a nanometer scale below to estimate wavelengths.

## Violets

Helium has at least one emission line in the violet.
The He I emission lines are at 414.3 nm and 447.1 nm.

## Blues

"The Wide Field Camera 3 (WFC3), a new camera aboard NASA's Hubble Space Telescope, snapped this image of the planetary nebula, catalogued as NGC 6302, but more popularly called the Bug Nebula or the Butterfly Nebula. WFC3 was installed by NASA astronauts in May 2009, during the servicing mission to upgrade and repair the 19-year-old Hubble telescope."
"What resemble dainty butterfly wings are actually roiling cauldrons of gas heated to more than 36,000 degrees Fahrenheit. The gas is tearing across space at more than 600,000 miles an hour—fast enough to travel from Earth to the Moon in 24 minutes!"
"NGC 6302 lies within our Milky Way galaxy, roughly 3,800 light-years away in the constellation Scorpius. The glowing gas is the star's outer layers, expelled over about 2,200 years. The "butterfly" stretches for more than two light-years, which is about half the distance from the Sun to the nearest star, Alpha Centauri."
"The central star itself cannot be seen, because it is hidden within a doughnut-shaped ring of dust, which appears as a dark band pinching the nebula in the center. The thick dust belt constricts the star's outflow, creating the classic "bipolar" or hourglass shape displayed by some planetary nebulae."
"The star's surface temperature is estimated to be about 400,000 degrees Fahrenheit, making it one of the hottest known stars in our galaxy. Spectroscopic observations made with ground-based telescopes show that the gas is roughly 36,000 degrees Fahrenheit, which is unusually hot compared to a typical planetary nebula."
"The WFC3 image reveals a complex history of ejections from the star. The star first evolved into a huge red-giant star, with a diameter of about 1,000 times that of our Sun. It then lost its extended outer layers. Some of this gas was cast off from its equator at a relatively slow speed, perhaps as low as 20,000 miles an hour, creating the doughnut-shaped ring. Other gas was ejected perpendicular to the ring at higher speeds, producing the elongated "wings" of the butterfly-shaped structure. Later, as the central star heated up, a much faster stellar wind, a stream of charged particles traveling at more than 2 million miles an hour, plowed through the existing wing-shaped structure, further modifying its shape."
"The image also shows numerous finger-like projections pointing back to the star, which may mark denser blobs in the outflow that have resisted the pressure from the stellar wind."
"The nebula's reddish outer edges are largely due to light emitted by nitrogen, which marks the coolest gas visible in the picture. WFC3 is equipped with a wide variety of filters that isolate light emitted by various chemical elements, allowing astronomers to infer properties of the nebular gas, such as its temperature, density, and composition."
"The white-colored regions are areas where light is emitted by sulfur. These are regions where fast-moving gas overtakes and collides with slow-moving gas that left the star at an earlier time, producing shock waves in the gas (the bright white edges on the sides facing the central star). The white blob with the crisp edge at upper right is an example of one of those shock waves."
"NGC 6302 was imaged on July 27, 2009, with Hubble's Wide Field Camera 3 in ultraviolet and visible light. Filters that isolate emissions from oxygen, helium, hydrogen, nitrogen, and sulfur from the planetary nebula were used to create this composite image."
The filters used for this image are F373N ([O II], purple), F469N (He II, blue), F502N ([O III], cyan), F656N (Hα, brown), F658N ([N II], orange), and F673N ([S II], white).

## Cyans

Helium has a line at 493 nm in the cyan.

## Greens

Helium has green lines at 501 and 505 nm.

## Yellows

"The helium emission lines behave in a qualitatively similar way to the calcium triplet. The 5876 Å line (Fig. 1e) is the dominant line in all the spectra, although two other transitions (6678 and 7065) are also in emission in most of the stars and have nearly identical profiles." He I is 587.6 nm, a yellow emission line. The He I photospheric emission line "narrow component is present in emission ... with [chromospheric] veilings larger than 0.4, being conspicuous even in those heavily veiled stars". The chromospheric veiling apparently results in the emission broadening of the He I emission from the chromosphere which is partially added to the He I narrow emission from the photosphere.
"The radiative loss for both broad and narrow emission--i.e., the excess of emission over the external continuum expressed in percentages of the photospheric fluxes--is F$_{ph}$(1 + ν)EW$_{obs}$, where F$_{ph}$ is the nearby photospheric flux and EW$_{obs}$ is the equivalent width of the observed emission component." "[B]y assumption, [the dynamo] controls the narrow component fluxes."
In the spectrum at right the yellow He I emission line is detected and recorded at normalized intensities (to the oxygen III line) from the Ring Nebula.

## Reds

Helium has at least one weak line in the red.

## Infrareds

"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]."
There are additional infrared lines of emission from helium: 1012.0 , 2112, 2166, 2188, and 2346 nm.

## Nucleosynthesis

"The alpha process, also known as the alpha ladder is one of two classes of [nuclear] reactions [for converting] helium into heavier elements, the other being the triple-alpha process."
Possible reactions:
    $\mathrm {_{6}^{12}C} +\mathrm {_{2}^{4}He} \rightarrow \mathrm {_{8}^{16}O} +\gamma +Q$, Q = 7.16 MeV
    $\mathrm {_{8}^{16}O} +\mathrm {_{2}^{4}He} \rightarrow \mathrm {_{10}^{20}Ne} +\gamma +Q$, Q = 4.73 MeV
    $\mathrm {_{10}^{20}Ne} +\mathrm {_{2}^{4}He} \rightarrow \mathrm {_{12}^{24}Mg} +\gamma +Q$, Q = 9.31 MeV
    $\mathrm {_{12}^{24}Mg} +\mathrm {_{2}^{4}He} \rightarrow \mathrm {_{14}^{28}Si} +\gamma +Q$, Q = 9.98 MeV
    $\mathrm {_{14}^{28}Si} +\mathrm {_{2}^{4}He} \rightarrow \mathrm {_{16}^{32}S} +\gamma +Q$, Q = 6.95 MeV
    $\mathrm {_{16}^{32}S} +\mathrm {_{2}^{4}He} \rightarrow \mathrm {_{18}^{36}Ar} +\gamma$
    $\mathrm {_{18}^{36}Ar} +\mathrm {_{2}^{4}He} \rightarrow \mathrm {_{20}^{40}Ca} +\gamma$
    $\mathrm {_{20}^{40}Ca} +\mathrm {_{2}^{4}He} \rightarrow \mathrm {_{22}^{44}Ti} +\gamma$
    $\mathrm {_{22}^{44}Ti} +\mathrm {_{2}^{4}He} \rightarrow \mathrm {_{24}^{48}Cr} +\gamma$
    $\mathrm {_{24}^{48}Cr} +\mathrm {_{2}^{4}He} \rightarrow \mathrm {_{26}^{52}Fe} +\gamma$
    $\mathrm {_{26}^{52}Fe} +\mathrm {_{2}^{4}He} \rightarrow \mathrm {_{28}^{56}Ni} +\gamma$
    $\mathrm {_{28}^{56}Ni} +\mathrm {_{2}^{4}He} +\gamma \rightarrow \mathrm {_{30}^{60}Zn}$
"Alpha process elements (or alpha elements) are so-called since their most abundant isotopes are integer multiples of the ... alpha particle). Alpha elements are [atomic number] [atomic number] Z ≤ 22: (C, N), O, Ne, Mg, Si, S, Ar, Ca, Ti. They [may be] synthesized by alpha-capture ... Silicon and calcium are purely alpha process elements. Magnesium can be burned by proton capture reactions. ... Oxygen ... enhancement is well correlated with an enhancement of other alpha process elements. ... C and N are considered alpha process elements, [when] they are synthesized in nuclear alpha-capture reactions."
"The abundance of alpha elements in stars is usually expressed in a logarithmic manner:
    $[\alpha /Fe]=\log _{10}{\left({\frac {N_{\alpha }}{N_{Fe}}}\right)_{Star}}-\log _{10}{\left({\frac {N_{\alpha }}{N_{Fe}}}\right)_{Sun}}$,
Here $N_{\alpha }$ and $N_{Fe}$ are the number of alpha element atoms and Fe atoms per unit volume."
    $\mathrm {_{1}^{1}H} +\mathrm {_{1}^{1}H} \rightarrow \mathrm {_{2}^{2}He}$

## Gases

Helium is a "colorless and inert gas".
Helium is a noble gas.

## Liquids

Liquid helium as shown on the right is just above absolute zero in temperature. On the left, it is at the normal boiling point (4.2 K,1 atm), boiling slowly.
The second image on the left shows the beginning of the superfluid lambda helium state. When liquid helium is cooled down from 4.2 K to the lambda point at 2.2 K, the liquid for a brief moment boils up violently. Suddenly, boiling ceases. This is the transition from the normal phase to the superfluid phase.

## Solids

$^3$He has two superfluid phases "A" and "B". There is a solid phase at about 0.001 K due to a transition in spin structure from a disordered state to an ordered state.

## Rocks

"Helium, U, V, and Se make up the dominant group [in permafrost, Baker Lake Area, NWT], giving rise to highly anomalous He and U and weakly anomalous V and Se patterns."

## Astroglaciology

"We analyze helium (He) and neon (Ne) isotope data sets from the southeast Pacific sector of the Southern Ocean collected in 1992 and 1994 and describe a new method to estimate glacial meltwater fluxes independent of previous approaches."

## Sun

"In the 1920s, Payne [3] and Russell [4] reported that the Sun’s atmosphere consisted mostly of hydrogen (H) and helium (He), but Hoyle [5] notes that he and others "in the astronomical circles to which I was privy" (p. 153) continued until after the Second World War to believe that the Sun was made mostly of iron. Then Hoyle notes that "much to my surprise" (p. 154), the high-hydrogen, low-iron model was suddenly adopted without opposition."
For comparison with other stars, the Sun has the following properties:
* The effective temperature of the surface of the Sun's photosphere is 5,778 K.
* Metallicity, Z = 0.0122, "lowest seismic estimate of solar metallicity is Z = 0.0187 [to the] highest is Z = 0.0239, with uncertainties in the range of 12%-19%."
* Stellar companion: Jupiter at present, perhaps Uranus in some larger form previously
* Age: 4.57 billion years
* B-V = 0.656 ± 0.005
* Rotation rate: 7.189 x 10$^3$ km h$^{-1}$ (at the equator), equatorial circumference of 4,379,000 kilometres divided by sidereal rotation period of 609.12 hours
Solar eruption in the image on the right was captured by the Solar Dynamics Observatory using the extreme ultraviolet emission line at 30.4 nm of singly ionized Helium, or He II, and corresponds to a temperature of approximately 50,000 K.

## Metallicities

"The hydrogen mass fraction is generally expressed as $X\equiv {\frac {m_{\mathrm {H} }}{M}}$ where $M$ is the total mass of the system and $m_{\mathrm {H} }$ the mass of the hydrogen it contains."
"[T]he helium mass fraction is denoted as $Y\equiv {\frac {m_{\mathrm {He} }}{M}}$."
"[T]he metallicity—the mass fraction of elements heavier than helium—can be calculated as"
    $Z=\sum _{i>\mathrm {He} }{\frac {m_{i}}{M}}=1-X-Y.$
    $X=0.7393.$
    $Y=0.2485.$

## Earth

"This figure [on the right] depicts the oxygen, helium, and hydrogen ions that gush into space from regions near the Earth's poles. The faint yellow gas shown above the north pole represents [these gases] lost from Earth into space; the green gas is the aurora borealis - or plasma energy pouring back into the atmosphere."

## Saturn

The outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium. ... The proportion of helium is significantly deficient compared to the abundance of this element in the Sun.

## Uranus

The atmosphere is composed of mainly hydrogen and helium with around 2% methane.
The helium molar fraction, i.e. the number of helium atoms per molecule of gas, is 0.15 ± 0.03 in the upper troposphere, which corresponds to a mass fraction 0.26 ± 0.05.

## Neptune

At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium.

## Stars

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".
For the star HD 124448, "[s]pectrograms extending from Hα to about λ 3530, obtained at the McDonald Observatory, show no hydrogen lines either in absorption or in emission, although the helium lines are sharp and strong."
"The abundance of hydrogen appears to be very low in the atmosphere of this star. Besides the strong He I spectrum, lines of O II and C II are present."
"Two objects (SB 21 and TON-S 103) turn out to be extreme helium stars, with y ≈ 1.0."

## Interstellar medium

"The [ISM] consists of about 0.1 to 1 particles per cm$^3$ and is typically composed of roughly 70% hydrogen by mass, with most of the remaining gas consisting of helium. This medium has been chemically enriched by trace amounts of heavier elements that were ejected from stars as they passed beyond the end of their main sequence lifetime. Higher density regions of the interstellar medium form clouds, or diffuse nebulae, where star formation takes place."

## Planetary nebulas

"In this October 1998 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)."

## Wolf-Rayet stars

Notation: WN5 is a component of V444 Cygni, with its Wolf-Rayet (W) spectrum dominated by Nitrogen$_{III-V}$ and Helium$_{I-II}$ lines and WN2 to WN5 considered hotter or "early".
"The color temperature of the central part of the WN5 disk for λ < 7512 Å, where the main source of opacity is electron scattering, is T$_c$ = 80,000-100,000 K. This high temperature represents the electron temperature slightly below the surface of the WN5 core--the level at which the star becomes optically thick in electron scattering."

## White dwarfs

"Helium is unquestionably absent from the atmospheres of [white dwarf] DA stars, and [there is a] low metal abundance".
"The object [in the image on the right] is actually a small white dwarf star undergoing a helium flash — one of only a handful of examples of such an event ever witnessed by astronomers."
"Normally, the white dwarf stage is the last in the life cycle of a low-mass star. In some cases, however, the star reignites in a helium flash and expands to return to a red giant state, ejecting huge amounts of gas and dust in the process, before once again shrinking to become a white dwarf."
"It is a dramatic and short-lived series of events, and Sakurai’s Object has allowed astronomers a very rare opportunity to study the events in real time. The white dwarf emits sufficient ultraviolet radiation to illuminate the gas it has expelled, which can just be seen in this image as the ring of red material."

## Blue subdwarfs

"The dominant population" in the Palomar-Green Catalog of Ultraviolet Excess Stellar Objects "is that of the hot, hydrogen atmosphere subdwarfs, the sdB stars, which comprise nearly 40 percent of the sample." "The helium-rich sdO stars account for 13% of the total. The hot white dwarfs of spectral types DA, DB, and DO account for 21%, 2.8%, and 1.0% of the sample; cooler DC or DZ white dwarfs add another 1.2%"

## Red giants

It is "suggested that Li produced in the helium envelopes of red giants comes to the surface of the stars as the result of a strong convection."

## Stellar surface fusion

"Concerning the particles which interact at the Sun, evidence for accelerated $^3$He enrichment was obtained from the detection (Share & Murphy 1998) of a gamma-ray line at 0.937 MeV produced by the reaction $^{16}$O($^3$He,p)$^{18}$F$^∗$".
For "essentially all of [some 20] flares $^3$He/$^4$He can be as large as 0.1, while for some of them values as high as 1 are possible. In addition, [...] for the particles that interact and produce gamma rays, $^3$He enrichments are present for both impulsive and gradual flares."
"Plasma is the medium for magnetically or inertially-confined controlled thermonuclear fusion. A plasma of deuterium and tritium ions heated to a temperature of 10$^8$ degrees Kelvin undergoes thermonuclear burn, producing energetic helium ions and neutrons from fusion reactions."
On the right is a diagram illustrating deuterium-tritium fusion in three steps:
* the D and T accelerating towards each other at thermonuclear speeds,
* the creation of an unstable He-5 nucleus,
* the ejection of a neutron and repulsion of the He-4 nucleus.
The "energy" is released in the form of the velocities of the constituent parts being thrown apart, or radiated apart.

## Cosmogony

"The cosmic helium abundance can however be measured with sufficient precision to suggest that the primordial $^4$He is less than 26 per cent at the 3 σ level (Pagel 1982). This is compatible with Ω$_b$h$^2$) ≲ 0.1 but not with Ω$_b$h$^2$ = 1 (for ≥ 3 species of neutrinos)."

## Locations on Earth

The image on the right shows the geographic locations within the United States of helium-rich gas fields.

## Recent history

The recent history period dates from around 1,000 b2k to present.
Helium "was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with detecting the element along with Norman Lockyer during the solar eclipse of 1868, and Lockyer was the first to propose that the line was due to a new element, which he named."

## Technology

Estimated 2013 U.S. fractional helium use by category. Total use is 47 million cubic meters.

## Magnetometers

In the image on the right is the dual mode Vector/Scalar Helium Magnetometer (V/SHM) instrument, flown on the Cassini mission.
"The helium vector magnetometer [in the image on the left] measures the fine structure of the interplanetary field, maps the Jovian field, and provides field measurements to evaluate solar wind interaction with Jupiter. The magnetometer operates in any one of eight different ranges, the lowest of which covers magnetic fields from ±0.01 to ±4.0 gamma; the highest fields up to ±140,000 gamma; i.e., ±1.4 Gauss."
"The surface field of the Earth is approximately 0.5 Gauss."

## Balloons

“The Balloon-borne Large Aperture Submillimeter Telescope (BLAST) is a submillimeter telescope that hangs from a high altitude balloon. It has a 2 meter primary mirror that directs light into bolometer arrays operating at 250, 350, and 500 µm. ... BLAST's primary science goals are:
* Measure photometric redshifts, rest-frame FIR luminosities and star formation rates of high-redshift starburst galaxies, thereby constraining the evolutionary history of those galaxies that produce the FIR/submillimeter background.
* Measure cold pre-stellar sources associated with the earliest stages of star and planet formation.
* Make high-resolution maps of diffuse galactic emission over a wide range of galactic latitudes.”
“High-altitude balloons and aircraft ... can get above [much] of the atmosphere. The BLAST experiment and SOFIA are two examples, respectively, although SOFIA can also handle near infrared observations.”
At left above "NASA's balloon-carried BLAST sub-millimeter telescope is hoisted into launch position on Dec. 25, 2012, at McMurdo Station in Antarctica on a mission to peer into the cosmos." The giant helium-filled balloon is slowly drifting about 36 km above Antarctica. It was "[l]aunched on Tuesday (Dec. 25) from the National Science Foundation's Long Duration Balloon (LDB) facility ... This is the fifth and final mission for BLAST, short for the Balloon-borne Large-Aperture Submillimeter Telescope. ... "BLAST found lots of so-called dark cores in our own Milky Way — dense clouds of cold dust that are supposed to be stars-in-the-making. Based on the number of dark cores, you would expect our galaxy to spawn dozens of new stars each year on average. Yet, the galactic star formation rate is only some four solar masses per year." So why is the stellar birth rate in our Milky Way so low? Astronomers can think of two ways in which a dense cloud of dust is prevented from further contracting into a star: turbulence in the dust, or the collapse-impeding effects of magnetic fields. On its new mission, BLAST should find out which process is to blame. ... [The 1800-kilogram] stratospheric telescope will observe selected star-forming regions in the constellations Vela and Lupus."

## Satellites

The Energetic Particle, Composition and Thermal Plasma Suite (ECT) of the Van Allen probes includes the Helium Oxygen Proton Electron electrostatic analyzer (HOPE).

## Orbital rocketry

"The Extreme ultraviolet Imaging Telescope (EIT) is an instrument on the SOHO spacecraft used to obtain high-resolution images of the solar corona in the ultraviolet range. The EIT instrument is sensitive to light of four different wavelengths: 17.1, 19.5, 28.4, and 30.4 nm, corresponding to light produced by highly ionized iron (XI)/(X), (XII), (XV), and helium (II), respectively. EIT is built as a single telescope with a quadrant structure to the entrance mirrors: each quadrant reflects a different colour of EUV light, and the wavelength to be observed is selected by a shutter that blocks light from all but the desired quadrant of the main telescope."

## Earth-trailing astronomy

"The Spitzer Space Telescope (SST), formerly the Space Infrared Telescope Facility (SIRTF) is an infrared space observatory launched ... from Cape Canaveral Air Force Station, on a Delta II 7920H ELV rocket, Monday, 25 August 2003 at 13:35:39 UTC-5 (EDT)."
"Cryogenic satellites that require liquid helium (LHe, T ≈ 4 K) temperatures in near-Earth orbit are typically exposed to a large heat load from the Earth, and consequently entail large usage of LHe coolant, which then tends to dominate the total payload mass and limits mission life. Placing the satellite in solar orbit far from Earth allowed innovative passive cooling such as the sun shield, against the single remaining major heat source to drastically reduce the total mass of helium needed, resulting in an overall smaller lighter payload, with major cost savings. This orbit also simplifies telescope pointing, but does require the Deep Space Network for communications."

## Spacecraft

The Juno spacecraft has aboard the Scalar Helium Magnetometer (SHM).

## Explorers

The neutron spectrometer (NS) aboard the Lunar Prospector "consisted of two canisters each containing helium-3 and an energy counter. Any neutrons colliding with the helium atoms give an energy signature which can be detected and counted."

## Hypotheses

* Helium fusion can be accomplished using muons.

## Resources

* Argons
* Bromines
* Chlorines
* Fluorines
* Heliums
* Hydrogens
* Iodines
* Kryptons
* Materials
* Mineraloids
* Neons
* Nitrogens
* Oxidanes
* Oxidation numbers
* Oxygens
* Radons
* Reactions
* Rocks as chemicals
* Xenons