This is an artist's rendering of the Interstellar Boundary Explorer (IBEX) satellite. Credit: NASA/Goddard Space Flight Center Conceptual Image Lab.

Neutrals astronomy is the astronomy of observing neutral atoms or molecules, their sources and apparent entities or objects of origin.

## Strong forces

A "new type of neutron star model (Q stars) [is such that] high-density, electrically neutral baryonic matter is a coherent classical solution to an effective field theory of strong forces and is bound in the absence of gravity. [...] allows massive compact objects, [...] and has no macroscopic minimum mass."[1]

"Compact objects in astronomy are usually analyzed in terms of theoretical characteristics of neutron stars or black holes that are based upon calculations of equations of state for matter at very high densities. At such high densities, the effects of strong forces cannot be neglected. There are several conventional approaches to describing nuclear forces, all of which find that for a baryon number greater than ~250, a nucleus will become energetically unbound. High-density hadronic matter is not stable in these theories until there are enough baryons for gravitational binding to form a neutron star, typically with a minimum mass ≳ 0.1 M and maximum mass ≲ 3 M."[1]

"Another possibility [called "baryon matter"] is that in the absence of gravity high-density baryonic matter is bound by purely strong forces. [...] nongravitationally bound bulk hadronic matter is consistent with nuclear physics data [...] and low-energy strong interaction data [...] The effective field theory approach has many successes in nuclear physics [...] suggesting that bulk hadronic matter is just as likely to be a correct description of matter at high densities as conventional, unbound hadronic matter."[1]

"The idea behind baryon matter is that a macroscopic state may exist in which a smaller effective baryon mass inside some region makes the state energetically favored over free particles. [...] This state will appear in the limit of large baryon number as an electrically neutral coherent bound state of neutrons, protons, and electrons in β-decay equilibrium."[1]

## Electroweak interactions

Weak neutral current interactions are one of the ways in which subatomic particles can interact by means of the weak force. These interactions are mediated by the Z boson. The discovery of weak neutral currents was a significant step toward the unification of electromagnetism and the weak force into the electroweak force, and led to the discovery of the W and Z bosons.

## Neutrons

The image is a schematic view of the Mount Norikura solar neutron telescope. Credit: Y. Muraki, K. Murakami, M. Miyazaki, K. Mitsui. S. Shibata, S. Sakakibara, T. Sakai, T. Takahashi, T. Yamada, and K. Yamaguchi.

A "new detector to observe solar neutrons [has been in operation] since 1990 October 17 [...] at the Mount Norikura Cosmic Ray Laboratory (CRL) of [the] Institute for cosmic Ray Research, the University of Tokyo."[2]

"The solar neutron telescope [image at right] consists of 10 blocks of scintillator [...] and several lead plates which are used to place kinetic energies Tn of incoming particles into three bands (50-360 MeV, 280-500 MeV, and ≥ 390 MeV)."[2] The telescope is inclined to the direction of the Sun by 15°.[2] The plane area of the detector is 1.0 m2 and protected by lead plates (Pb) to eliminate gamma-ray and muon background from the side of the detector.[2] The anti-coincident counter (A) is used to reject the muons and gamma rays, coming from the side of the detector and the top scintillators.[2] (P) and (G) are used to identify the proton events and gamma rays.[2] The central scintillator blocks are optically separated into 10 units.[2]

"The horizontal scintillator just above the 10 vertical scintillators distinguishes neutral particles (neutrons) from the charged particles (mainly muons, protons and electrons)."[2]

The Neutron Monitor aboard Ulysses was used to measure cosmic rays as well as neutrons.

## Protons

A "new type of neutron star model (Q stars) [is such that] high-density, electrically neutral baryonic matter is a coherent classical solution to an effective field theory of strong forces and is bound in the absence of gravity. [...] allows massive compact objects, [...] and has no macroscopic minimum mass."[1]

"Compact objects in astronomy are usually analyzed in terms of theoretical characteristics of neutron stars or black holes that are based upon calculations of equations of state for matter at very high densities. At such high densities, the effects of strong forces cannot be neglected. There are several conventional approaches to describing nuclear forces, all of which find that for a baryon number greater than ~250, a nucleus will become energetically unbound. High-density hadronic matter is not stable in these theories until there are enough baryons for gravitational binding to form a neutron star, typically with a minimum mass ≳ 0.1 M and maximum mass ≲ 3 M."[1]

"Times for accumulation of chemically significant dosages on icy surfaces of Centaur, Kuiper Belt, and Oort Cloud objects from plasma and energetic ions depend on irradiation position within or outside the heliosphere. Principal irradiation components include solar wind plasma ions, pickup ions from solar UV ionization of interstellar neutral gas, energetic ions accelerated by solar and interplanetary shocks, including the putative solar wind termination shock, and galactic cosmic ray ions from the Local Interstellar Medium (LISM)."[3]

Flux spectra have been derived "from spacecraft data and models for eV to GeV protons at 40 AU, a termination shock position at 85 AU, and in the LISM."[3]

"The ‘bubble’ of solar wind plasma and frozen-in magnetic fields expanding out from the solar corona, within a few radii of the Sun, to boundaries with the local interstellar gas and plasma near about 100 AU is called the heliosphere. Dependent on points of origin at the Sun, and on time phase during the eleven year cycle of solar activity, the solar wind plasma expands radially outward at speeds of 300–800 km/s. Neutral atoms flowing into the heliosphere from the Very Local Interstellar Medium (VLISM) can be ionized by solar UV, and by charge exchange with solar wind ions, then picked up by magnetic fields in the outward plasma flow. Due to inverse-square fall-off of solar wind ion density with distance from the Sun, these interstellar pickup ions increasingly contribute to the plasma pressure and become the dominant component beyond the orbit of Saturn (Burlaga et al., 1996; Whang et al., 1996). Further out near 90–100 AU (Stone, 2001; Stone and Cummings, 2001; Whang and Burlaga, 2002) the outflowing plasma is expected to encounter the solar wind termination shock where flow speeds abruptly transition to sub-sonic values ∼100 km/s. The shock position is dependent in part on the plasma and neutral gas density in the Local Interstellar Medium (LISM) and could move into the giant planet region, or even nearer to the Earth’s orbit, if the Sun passed through a region of much higher LISM density (Zank and Frisch, 1999; Frisch, 2000). Further out at 120 AU or more should be the heliopause, the contact boundary between the diverted solar wind plasma flows and the in-flowing interstellar plasma. The intervening region between the termination shock and the heliopause is called the heliosheath. In this latter region the previously radial flow of the solar wind is diverted into a direction downstream from the ∼26 km/s flow of the interstellar gas to form a huge teardrop-shaped structure called the heliotail which extends hundreds to perhaps thousands of AU from the Sun into the VLISM."[3]

"Within the heliosphere the interplanetary environment of solar wind plasma, solar (SEP) and interplanetary energetic particles, and galactic cosmic rays (GCR) has long been surveyed in-situ beyond Neptune’s orbit at 30 AU, since 1983 and 1990 by the Pioneer 10 and 11 spacecraft, and since 1987 and 1989 by Voyager 1 and 2. Of these, the Pioneers are no longer transmitting data and the Voyagers are now respectively at 89 and 71 AU, far beyond the 48 AU semi-major axis (a) cutoff of the Classical KBO population but within the range of aphelia 48 < Q < 103 AU for known Centaurs (perihelia at 5 < q < 35 AU) and Scattered KBOs (q > 35 AU). Voyager 1 is expected to cross the termination shock, later followed by Voyager 2, within the next several years and possibly to exit the heliosphere across the heliopause within its remaining ∼17 + years of operational lifetime. Both spacecraft will have been silent for millennia before reaching the Oort Cloud region at 104 to 105 AU. Within the next quarter century NASA may launch an interstellar probe (e.g., Mewaldt et al., 2001a) moving outward at 10 AU/year with the ultimate goal of surveying the VLISM environment out to several hundred AU. Until then, the next mission to the outer solar system is planned to be New Horizons (Stern and Spencer, 2003), which will fly by the Pluto/Charon system in 2015 and thereafter attempt several flybys of accessible KBOs. Enroute to Pluto this mission may attempt at least one Centaur flyby after swinging by Jupiter in 2007."[3]

"The initial solar wind conditions at the inner boundary at 1 AU are radial outward speed V = 441 km/s, solar wind proton density N = 7.0/cc and temperature T = 9.8 × 104 K, and interplanetary magnetic field = 7.0 × 10−5 Gauss. The interstellar hydrogen atoms at the solar wind termination shock are taken to have speed 20 km/s and temperature 1 × 104 K, while H0 density, and the energy partition ratio for ions, are varied to give good fits to radial speed and temperature profiles measured by the operational plasma spectrometer on Voyager 2. Good fits are obtained for a neutral density of 0.09/cc and a partition ratio of 0.05, which means that five percent of the total energy from the pickup process goes into solar wind protons. For the LISM plasma ions, which are not included in the Wang and Richardson model, we compute convecting maxwellian (Vasyliunas, 1971) distributions for the LISM parameters T ∼ 7000 K, u ∼ 26 km/s, and N ∼ 0.1/cc of interstellar protons as derived from Wood and Linsky (1997)."[3]

"For the present work we define ‘cosmic ray’ protons as being those with energies above 0.1 MeV from sources within and outside the heliosphere. Sources include solar energetic particle (SEP) events, acceleration by interplanetary shocks and the solar wind termination shock, and inward diffusion through the heliosheath of galactic cosmic rays thought mostly to be accelerated by interstellar shocks from supernova explosions. Protons and heavier ions accelerated at the termination shock, after pickup from photo-ionization of interstellar gas neutrals, are called anomalous cosmic rays (ACR)."[3]

"Near solar minimum the ACR ions, including protons, are dominant components of radiation dosage outward from ∼40 AU to the outer heliosphere, while these ions largely disappear at solar maximum. There is a 22-year cycle in the polarity of the solar dipole magnetic field, which is frozen into the solar wind plasma within several radii of the Sun and thereby carried outward into the heliosphere. Due to sign-dependent transport effects, the ACR ions accelerated at the termination shock have larger fluxes, and more positive radial gradients, at 40 to 85 AU near the Ecliptic when the solar dipole moment is directed southward (qA < 0 polarity) than when it is northward (qA > 0 polarity)."[3]

"For protons the primary radiation dosage process is deposition of energy within the volume of material as a function of depth. This deposition occurs either by electronic ionization of target atoms or by direct collisions with nuclei within the atoms. Nuclear collisions are purely elastic, as for billiard balls, up to some threshold energy for inelastic collisions, which can also excite or break up the struck nucleus with increasing effect at higher energies."[3]

"For the 85-AU termination shock location the times at 0.1-μm depth drop to 107 to 108 years, while in the LISM the electronic time scale even at 1 cm is below the 109-year limit. Flux and dosage rates increase by orders of magnitude in this depth range from 40 AU out into the LISM. From 40 AU to the termination shock this trend reflects the positive radial intensity gradient for ACR protons diffusing inward from the shock acceleration source."[3]

"Oort Cloud comets, and possibly Scattered KBOs with aphelia near the heliosheath and VLISM, are maximally irradiated, while Classical KBOs near 40 AU are minimally irradiated. Radial intensity gradients ≾􏰀 +10%/AU of ACR ions might account for spatial variations in color within this latter population, e.g., redder objects with increasing perihelia in the 32 < q < 45 AU range as reported by Doressoundiram et al. (2002) and at this conference by Doressoundiram (2003)."[3]

## Mesons

Notation: let the symbol GZK represent Greisen-Zatsepin-Kuzmin.

Single π0 production occurs "in neutral current neutrino interactions with water by a 1.3 GeV wide band neutrino beam."[4]

"The Gamma-Ray Spectrometer (GRS) on [Solar Maximum Mission] SMM has detected [...] at least two of the flares have spectral properties >40 MeV that require gamma rays from the decay of neutral pions. [Pion] production can occur early in the impulsive phase as defined by hard X-rays near 100 keV."[5]

Gamma-ray "emission matches remarkably well both the position and shape of the inner [supernova remnant] SNR shocked plasma. Furthermore, the gamma-ray spectrum shows a prominent peak near 1 GeV with a clear decrement at energies below a few hundreds of MeV as expected from neutral pion decay."[6]

"Neutral current single π0 production induced by neutrinos with a mean energy of 1.3GeV is measured at a 1000 ton water Cherenkov detector as a near detector of the K2K long baseline neutrino experiment."[4]

"The single π0 production rate by atmospheric neutrinos could be usable to distinguish between the νµ ↔ ντ and νµ ↔ νs oscillation hypotheses. The NC rate is attenuated in the case of transitions of νµ’s into sterile neutrinos, while it does not change in the νµ ↔ ντ scenario."[4]

Based on interactions between cosmic rays and the photons of the cosmic microwave background radiation (CMB) ... cosmic rays with energies over the threshold energy of 5x1019 eV ... interact with cosmic microwave background photons ${\displaystyle \gamma _{\rm {CMB}}}$ to produce pions via the ${\displaystyle \Delta }$ resonance,

${\displaystyle \gamma _{\rm {CMB}}+p\rightarrow \Delta ^{+}\rightarrow p+\pi ^{0},}$

or

${\displaystyle \gamma _{\rm {CMB}}+p\rightarrow \Delta ^{+}\rightarrow n+\pi ^{+}.}$

Pions produced in this manner proceed to decay in the standard pion channels—ultimately to photons for neutral pions, and photons, positrons, and various neutrinos for positive pions. Neutrons decay also to similar products, so that ultimately the energy of any cosmic ray proton is drained off by production of high energy photons plus (in some cases) high energy electron/positron pairs and neutrino pairs.

The pion production process begins at a higher energy than ordinary electron-positron pair production (lepton production) from protons impacting the CMB, which starts at cosmic ray proton energies of only about 1017eV. However, pion production events drain 20% of the energy of a cosmic ray proton as compared with only 0.1% of its energy for electron positron pair production. This factor of 200 is from two sources: the pion has only about ~130 times the mass of the leptons, but the extra energy appears as different kinetic energies of the pion or leptons, and results in relatively more kinetic energy transferred to a heavier product pion, in order to conserve momentum. The much larger total energy losses from pion production result in the pion production process becoming the limiting one to high energy cosmic ray travel, rather than the lower-energy light-lepton production process.

The pion production process continues until the cosmic ray energy falls below the pion production threshold. Due to the mean path associated with this interaction, extragalactic cosmic rays traveling over distances larger than 50 Mpc (163 Mly) and with energies greater than this threshold should never be observed on Earth. This distance is also known as GZK horizon.

## Neutrinos

The Sudbury Neutrino Observatory is a 12-meter sphere filled with heavy water surrounded by light detectors located 2000 meters below the ground in Sudbury, Ontario, Canada. Credit: A. B. McDonald (Queen's University) et al., The Sudbury Neutrino Observatory Institute.
In this photograph is recorded the first use of a hydrogen bubble chamber to detect neutrinos, on November 13, 1970. A neutrino hit a proton in a hydrogen atom. The collision occurred at the point where three tracks emanate on the right of the photograph. Credit: Argonne National Laboratory.
Flux (Φ) of 8B solar neutrinos which are μ or τ flavor vs the flux of electron neutrinos (Φe) deduced from the three neutrino reactions in the Sudbury Neutrino Observatory (SNO). Credit: Ahmad et al..

The "neutrino fluxes [may be] predicted by such scenarios [as the standard model or grand unification] if consistency with the observed cosmic ray flux and the universal γ-ray background at 1 − 10 GeV is required. Flux levels detectable by proposed km3 scale neutrino observatories are allowed by these constraints. Bounds on or detection of a neutrino flux above ~ 1 EeV would allow neutrino astronomy to probe grand unification scale physics."[7]

"The shapes of the [ultra-high energy] UHE nucleon and γ-ray spectra predicted within ["top-down"] TD models are “universal” in the sense that they depend only on the physics of [a supermassive elementary "X" particle associated with some grand unified theory (GUT)] X particle decay."[7]

"In contrast to the universality of UHE spectral shapes, the predicted γ-ray flux below ∼ 1014 eV (the threshold for pair production of photons on the [cosmic microwave background] CMB) and the predicted neutrino flux depend on the total energy release integrated over redshift and thus on the specific TD model."[7]

"Observational data on the universal γ-ray background in the 1 − 10 GeV region [27], to which the generic cascade spectrum would contribute directly, turn out to provide an important constraint. Since the UHE γ-ray flux is especially sensitive to certain astrophysical parameters such as the extragalactic magnetic field (EGMF), a reliable calculation of the predicted spectral shapes requires numerical methods."[7]

"The calculations take into account all the relevant interactions with the (redshift dependent) universal low energy photon background in the radio, microwave and optical/infrared regime."[7]

"Above ≃ 100 EeV the corresponding fluxes would dominate all present model predictions for AGN neutrino fluxes [14] as well as the flux of “cosmogenic” neutrinos produced by interactions of UHE [cosmic rays] CRs with the universal photon background [37,38,31]."[7]

The "constraint imposed by requiring that TD scenarios do not overproduce the measured universal γ-ray background at 1 − 10 GeV implies an upper limit on these neutrino fluxes which only depends on the ratio r of energy injected into the neutrino versus [electromagnetic] EM channel, and not on any specific TD scenario or even a possible connection to UHE CRs."[7]

A neutrino is an electrically neutral, weakly interacting elementary subatomic particle[8] with half-integer spin. ... Neutrinos do not carry electric charge, which means that they are not affected by the electromagnetic forces that act on charged particles such as electrons and protons. Neutrinos are affected only by the weak sub-atomic force, of much shorter range than electromagnetism, and gravity, which is relatively weak on the subatomic scale. They are therefore able to travel great distances through matter without being affected by it.

"If neutrinos have negligible rest mass, the present density expected for relic neutrinos from the big bang is nν = 110 (Tγ/2.7 K)3 cm–3 for each two-component species. This is of order the photon density nγ, differing just by a factor 3/11 (i.e. a factor 3/4 because neutrinos are fermions rather than bosons, multiplied by 4/11, the factor by which the neutrinos are diluted when e+–e annihilation boosts the photon density). This conclusion holds for non-zero masses, provided that mvc2 is far below the thermal energy (~ 5 MeV) at which neutrinos decoupled from other species and that the neutrinos are stable for the Hubble time. Comparison with the baryon density, related to Ω via nb = 1.5 x 10–5 Ωb h2 cm–3, shows that neutrinos outnumber baryons by such a big factor that they can be dynamically dominant over baryons even if their masses are only a few electron volts. In fact, a single species of neutrino would yield a contribution to Ω of Ωv = 0.01 h–2 (mv)eV, so if h = 0.5, only 25 eV is sufficient to provide the critical density."[9]

"Neutrinos of nonzero mass would be dynamically important not only for the expanding universe as a whole but also for large bound systems such as clusters of galaxies. This is because they would now be moving slowly: if the universe had cooled homogeneously, primordial neutrinos would now be moving at around 200 (mv)-1eV km s–1. They would be influenced even by the weak (~ 10–5 c2) gravitational potential fluctuations of galaxies and clusters. If the three (or more) types of neutrinos have different masses, then the heaviest will obviously be gravitationally dominant, since the numbers of each species should be the same."[9]

"Using the neutral current [NC], elastic scattering [ES], and charged current [CC] reactions and assuming the standard 8B shape, the νe component of the 8B solar flux is Φe = 1.76±0.05 ([statistical uncertainty] stat.)±0.09 ([systematic uncertainty]syst.) x 106 cm-2s-1 for a kinetic threshold of 5 MeV. The non-νe component is Φµτ = 3.41±0.45 (stat.) +0.48 or -0.45 (syst.) x 106 cm-2s-1, 5.3σ greater than zero, providing strong evidence for solar νe flavor transformation."[10]

"The Sudbury Neutrino Observatory (SNO) detects 8B solar neutrinos through the reactions:"[10]

${\displaystyle \nu _{e}+d\rightarrow p^{+}+p^{+}+e^{-}(CC),}$
${\displaystyle \nu _{x}+d\rightarrow p^{+}+n^{0}+\nu _{x}(NC),}$
${\displaystyle \nu _{x}+e^{-}\rightarrow \nu _{x}+e^{-}(ES).}$

"The charged current reaction (CC) is sensitive exclusively to electron-type neutrinos, while the neutral current reaction (NC) is equally sensitive to all active neutrino flavors (x = e, μ, τ). The elastic scattering reaction (ES) is sensitive to all flavors as well, but with reduced sensitivity to νμ and ντ."[10]

"The bands intersect [in the figure at right] at the fit values for Φe and Φµτ, indicating that the combined flux results are consistent with neutrino flavor transformation assuming no distortion in the 8B neutrino energy spectrum."[10]

## Electrons

A "PeV energy photon cannot deliver information from a source at the edge of our own galaxy because it will annihilate into an electron [positron] pair in an encounter with a 2.7 Kelvin microwave photon before reaching our telescope."[11]

"In general, energetic photons above a threshold E given by

${\displaystyle 4E\epsilon \sim (2m_{e})^{2},}$

where E and ε are the energy of the high-energy and background photon, respectively. [This] implies that TeV-photons are absorbed on infrared light, PeV photons on the cosmic microwave background and EeV photons on radio-waves".[11]

"Each [optical module] OM contains a 10 inch [photo-multiplier tube] PMT that detects individual photons of Cerenkov light generated in the optically clear ice by muons and electrons moving with velocities near the speed of light."[11]

"Radio Cerenkov experiments detect the Giga-Hertz pulse radiated by shower electrons produced in the interaction of neutrinos in ice."[11]

"Above a threshold of ≃ 1PeV, the large number of low energy(≃ MeV ) photons in a shower will produce an excess of electrons over positrons by removing electrons from atoms by Compton scattering. These are the sources of coherent radiation at radio frequencies, i.e. above ∼ 100MHz."[11]

## Positrons

"The positrons can annihilate in flight before being slowed to thermal energies, annihilate directly with electrons when both are at thermal energies, or form positronium at thermal energies (or at greater than thermal energies if positronium formation occurs via charge exchange with neutrals)."[12]

"Positrons entering a gaseous medium at [0.6 to 4.5 MeV] are quickly slowed by ionizing collisions with neutral atoms and by long-range Coulomb interactions with any ionized component."[12]

## Gamma rays

"The important conclusion is that, independently of the specific blueprint of the source, it takes a kilometer-scale neutrino observatory to detect the neutrino beam associated with the highest energy cosmic rays and gamma rays."[11]

"As with supernovae, [gamma-ray burst] GRB are expected to radiate the vast majority of their initial energy as thermal [MeV] neutrinos."[11]

"Protons [shocked protons: TeV - EeV neutrinos] accelerated in GRB can interact with fireball gamma rays and produce pions that decay into neutrinos."[11]

"In a GRB fireball, neutrons can decouple from protons in the expanding fireball. If their relative velocity is sufficiently high, their interactions will be the source of pions and, therefore, neutrinos [GeV]. Typical energies of the neutrinos produced are much lower than those resulting from interactions with gamma rays."[11]

## X-rays

This ROSAT image is an Aitoff-Hammer equal-area map in galactic coordinates with the Galactic center in the middle of the 0.25 keV diffuse X-ray background. Credit: The Max Planck Institute for Extraterrestrial Physics, Snowden et al. 1995, ApJ, 454, 643; Imagine the Universe! is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

By comparing the soft X-ray background with the distribution of neutral hydrogen, it is generally agreed that within the Milky Way disk, super soft X-rays are absorbed by this neutral hydrogen.

The ROSAT image at the right is an Aitoff-Hammer equal-area map in galactic coordinates with the Galactic center in the middle of the 0.25 keV diffuse X-ray background.

## Ultraviolets

"Massive neutrinos are expected to decay into lighter neutrinos and uv photons, with lifetimes long on the Hubble scale."[13]

## Blues

Supernova SN 1987A is one of the brightest stellar explosions since the invention of the telescope more than 400 years ago.[14] Credit: ESA/Hubble & NASA.

"On February 23.316 UT, 1987, [blue] light and neutrinos from the brightest supernova in 383 years arrived at Earth ... it has been observed ... at all wavelengths from radio through gamma rays, SN 1987A is the only object besides the Sun to have been detected in neutrinos."[15]

At left is an image of supernova SN 1987A, one of the brightest stellar explosions since the invention of the telescope more than 400 years ago.[14]

Four days after the event was recorded, the progenitor star was tentatively identified as Sanduleak -69° 202, a blue supergiant.[16] This was an unexpected identification, because at the time a blue supergiant was not considered a possibility for a supernova event in existing models of high mass stellar evolution. Many models of the progenitor have attributed the color to its chemical composition, particularly the low levels of heavy elements, among other factors.[15]

## Cyans

The Necklace Nebula glows brightly in this Nasa Hubble Space Telescope image. Credit: NASA.

"A giant cosmic necklace glows brightly in this Nasa Hubble Space Telescope image."[17]

"The object, aptly named the Necklace Nebula, is a recently discovered planetary nebula, the glowing remains of an ordinary, sun-like star."[17]

"The nebula consists of a bright ring, measuring 12trillion miles wide, dotted with dense, bright knots of gas that resemble diamonds in a necklace."[17]

"Newly discovered: The Necklace Nebula glows brightly in this composite image taken by the Hubble Space Telescope last month. The glow of hydrogen, oxygen, and nitrogen are shown by the colours blue, green and red respectively".[17]

"It is located 15,000 light-years away in the constellation Sagitta."[17]

"A pair of stars orbiting close together produced the nebula, also called PN G054.2-03.4."[17]

"About 10,000 years ago, one of the ageing stars ballooned to the point where it engulfed its companion star. The smaller star continued orbiting inside its larger companion, increasing the giant’s rotation rate. The bloated companion star spun so fast that a large part of its gaseous envelope expanded into space. Due to centrifugal force, most of the gas escaped along the star’s equator, producing a ring. The embedded bright knots are dense gas clumps in the ring. The pair is so close, only a few million miles apart, that they appear as one bright dot in the centre. The stars are furiously whirling around each other, completing an orbit in a little more than a day."[17]

## Reds

This region of sky includes glowing red clouds of mostly hydrogen gas. Credit: ESO.

"[T]he extended red emission (ERE) [is] observed in many dusty astronomical environments, in particular, the diffuse interstellar medium of the Galaxy. ... silicon nanoparticles provide the best match to the spectrum and the efficiency requirement of the ERE."[18]

"The broad, 60 < FWHM < 100 nm, featureless luminescence band known as extended red emission (ERE) is seen in such diverse dusty astrophysical environments as reflection nebulae17, planetary nebulae3, HII regions (Orion)12, a Nova11, Galactic cirrus14, a dark nebula7, Galaxies8,6 and the diffuse interstellar medium (ISM)4. The band is confined between 540-950 nm, but the wavelength of peak emission varies from environment to environment, even within a given object. ... the wavelength of peak emission is longer and the efficiency of the luminescence is lower, the harder and denser the illuminating radiation field is13. These general characteristics of ERE constrain the photoluminescence (PL) band and efficiency for laboratory analysis of dust analog materials."[19]

In interstellar astronomy, visible spectra can appear redder due to scattering processes in a phenomenon referred to as interstellar reddening[20] — similarly Rayleigh scattering causes the atmospheric reddening of the Sun seen in the sunrise or sunset and causes the rest of the sky to have a blue color. This phenomenon is distinct from redshifting because the spectroscopic lines are not shifted to other wavelengths in reddened objects and there is an additional dimming and distortion associated with the phenomenon due to photons being scattered in and out of the line-of-sight.

"The Danish 1.54-metre telescope located at ESO’s La Silla Observatory in Chile has captured a striking image of NGC 6559, an object that showcases the anarchy that reigns when stars form inside an interstellar cloud. This region of sky includes glowing red clouds of mostly hydrogen gas, blue regions where starlight is being reflected from tiny particles of dust and also dark regions where the dust is thick and opaque."[21]

"The blue section of the photo — representing a "reflection nebula" — shows light from the newly formed stars in the cosmic nursery being reflected in all directions by the particles of dust made of iron, carbon, silicon and other elements in the interstellar cloud."[22]

Massive astrophysical compact halo object, or MACHO, is a general name for any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body composed of normal baryonic matter, which emits little or no radiation and drifts through interstellar space unassociated with any planetary system. Since MACHOs would not emit any light of their own, they would be very hard to detect. MACHOs may sometimes be black holes or neutron stars as well as brown dwarfs or unassociated planets. White dwarfs and very faint red dwarfs have also been proposed as candidate MACHOs.

## Superluminals

"Because neutrinos are electrically neutral, conventional Cherenkov radiation of superluminal neutrinos does not arise or is otherwise weakened. However neutrinos do carry electroweak charge and ... may emit Cherenkov-like radiation via weak interactions when traveling at superluminal speeds."[23]

"[S]uperluminal neutrinos may lose energy rapidly via the bremsstrahlung [Cherenkov radiation] of electron-positron pairs ${\displaystyle (\nu \rightarrow \nu +e^{-}+e^{+}).}$"[24]

Assumption:

"muon neutrinos with energies of order tens of GeV travel at superluminal velocity."[24]

For "all cases of superluminal propagation, certain otherwise forbidden processes are kinematically permitted, even in vacuum."[24]

Consider

${\displaystyle \nu _{\mu }\rightarrow {\begin{bmatrix}{\nu _{\mu }+\gamma }&(a)\\{\nu _{\mu }+\nu _{e}+{\overline {\nu }}_{e}}&(b)\\{\nu _{\mu }+e^{+}+e^{-}}&(c)\end{bmatrix}}}$[24]

"These processes cause superluminal neutrinos to lose energy as they propagate and ... process (c) places a severe constraint upon potentially superluminal neutrino velocities. ... Process (c), pair bremsstrahlung, proceeds through the neutral current weak interaction."[24]

"Throughout the shower development, the electrons and positrons which travel faster than the speed of light in the air emit Cherenkov radiation."[25]

"High energy processes such as Compton, Bhabha, and Moller scattering, along with positron annihilation rapidly lead to a ~20% negative charge asymmetry in the electron-photon part of a cascade ... initiated by a ... 100 PeV neutrino"[26].

## Plasma objects

A hot plasma ion 'steals' charge from a cold neutral atom to become an Energetic Neutral Atom (ENA).[27] Credit Mike Gruntman.
The ENA leaves the charge exchange in a straight line with the velocity of the original plasma ion.[27] Credit: Mike Gruntman.

"In 1951, prior to the Space Age, the existence of energetic neutral hydrogen atoms (as high as 70 keV in energy) in space plasma was discovered."[28]

"ENA imaging permits study of the ways in which our entire plasma environment -- including the magnetopause, ring current, plasmasphere, auroral zones, plasma sheet, and the ionosphere -- reacts to the changing conditions of the solar wind (Williams, 1990)."[28]

## Hydrogens

Proton–hydrogen charge-exchange collisions [such as those shown at right] are often the most important process in space plasma because hydrogen is the most abundant constituent of both plasmas and background gases and hydrogen charge-exchange occurs at very high velocities involving little exchange of momentum.

The "efforts to study the 21 cm hydrogen line from the northern and southern hemispheres in 1954 and 1959 [...] were combined and provided the first full-galaxy radio map of neutral hydrogen in the Milky Way".[29]

A hydrogen atom is about 0.11 nm in diameter.

The "relative motion in a hydrogen atom in crossed electric and magnetic fields leads to peculiar quasi-ionized states with an electron localized very far from a proton."[30]

The Solar Wind Anisotropies (SWAN) aboard SOHO "is the only remote sensing instrument on SOHO that does not look at the Sun. It watches the rest of the sky, measuring hydrogen that is ‘blowing’ into the Solar System from interstellar space. By studying the interaction between the solar wind and this hydrogen gas, SWAN determines how the solar wind is distributed. As such, it can be qualified as SOHO’s solar wind ’mapper’."[31]

## Sun

As stars are defined as luminous balls of plasma, the Sun may not qualify as its photosphere has a plasma concentration of approximately 10-4. The rest is composed of neutral atoms at about 5800 K.

## Mercury

"Measurements by instruments on MESSENGER during the spacecraft's three Mercury flybys have led to discoveries of previously undetected neutral (Mg) and ionized (Ca+) species in Mercury's neutral and ionized exosphere and mapped these and previously known constituents (Na, Ca) on the anti-sunward side of the planet and over the poles. [...] Some ions and neutrals can be released directly from mineral surfaces by electron-stimulated desorption (ESD). Because cross sections of neutrals can be higher than photon-stimulated desorption (PSD) cross sections and because active electron precipitation on both the day and night side of Mercury can produce ESD of ions, at least part of the ionized exosphere is produced directly from surface materials by ESD."[32]

## Earth

"Energetic neutral atoms (ENA), emitted from the magnetosphere with energies of ∼50 keV, have been measured with solid-state detectors on the IMP 7/8 and ISEE 1 spacecraft. The ENA are produced when singly charged trapped ions collide with the exospheric neutral hydrogen geocorona and the energetic ions are neutralized by charge exchange."[33]

"The IMAGE mission ... High Energy Neutral Atom imager (HENA) ... images [ENAs] at energies between 10 and 60 keV/nucleon [to] reveal the distribution and the evolution of energetic [ions, including protons] as they are injected into the ring current during geomagnetic storms, drift about the Earth on both open and closed drift paths, and decay through charge exchange to pre‐storm levels."[34]

## Moon

This is a computer generated model of the Lunar Atmosphere and Dust Environment Explorer (LADEE). Credit: NASA.

"The Lunar Atmosphere and Dust Environment Explorer (LADEE, at left) launched 07 September 2013 at 03:27 UT (06 September 11:27 EDT) on a Minotaur-V from Wallops Flight Facility. LADEE is designed to characterize the tenuous lunar atmosphere and dust environment from orbit. The scientific objectives of the mission are:(1) determine the global density, composition, and time variability of the fragile lunar atmosphere; and, (2) determine the size, charge, and spatial distribution of electrostatically transported dust grains and assess their likely effects on lunar exploration and lunar-based astronomy. Further objectives are to determine if the Apollo astronaut sightings of diffuse emission at 10s of km above the surface were Na glow or dust and document the dust impactor environment (size-frequency) to help guide design engineering for outpost and future robotic missions."[35]

"The orbiter will carry a Neutral Mass Spectrometer (NMS), an Ultraviolet/Visible Spectrometer (UVS), and a Lunar Dust Experiment (LDEX)."[35]

"The NMS is a quadrupole mass spectrometer designed ot detect species up to 150 amu and will look for CH4, S, O, Si, Kr, Xe, Fe, Al, Ti, Mg, OH, and H2O. The UVS will detect Al, Ca, Fe, K, Li, Na, Si, T, Ba, Mg, H2O, and O and will monitor the dust composition. The LDEX is an impact ionization dust detector designed to measure particles down to 0.3 microns at the spacecraft altitude. The LLCD is a test of a high data-rate optical (laser) communications system."[35]

## Mars

"The major atmospheric gases on Earth, Venus, and Mars were probably CO2, H2O, and N2. [The ions from the upper parts of an atmosphere] are often suprathermal, and their interactions can produce suprathermal neutral atoms as well [The] ionopause [...] separates the bound ionosphere from an outer region in which the solar wind is diverted and flows around and past the planet. This region still contains some neutral gas, and if such atoms are ionized by solar photons or electron impact, they are swept up in the flow."[36]

"There are strong reasons to believe that Mars once had much more atmospheric CO2 and H2O than it now has ... (Impacts, which may have eroded even larger amounts, operated at an earlier period.) ... The visible polar caps are thought to contain relatively small quantities. [...] More recently it has been proposed (35) that Mars may have had several episodes of high atmospheric pressure, warm conditions, and substantial precipitation of rain and snow, with a north polar ocean and southern glaciers."[36]

## Comets

The comet Hale–Bopp in the night sky. Credit: Philipp Salzgeber.

Def. a "celestial body consisting mainly of ice, dust and gas in a (usually very eccentric) orbit around the Sun and having a "tail" of melted matter blown away [back][37] from it by the solar wind when [as][37] it is close to [approaches][37] the Sun"[38] is called a comet.

Def. a "comet which orbits the Sun and which returns to the innermost point of its orbit at known, regular intervals"[39] is called a periodic comet.

Def. "any periodic comet with an orbital period of less than 200 years"[40] is called a short-period comet.

Most of the comets lay at the distant reaches of our system in a hypothesized Oort cloud. At the very edge of the solar system, these comets orbit in very large loops around the distant reaches of our solar system. The passing of nearby stars, or other objects can alter their orbit, sending them speeding towards the inner reaches of our solar system. These comets typically retain very large orbits such that they will not return (once seen in the inner solar system) for many thousands of years.

Cosmic "ray protons at energies up to 10 GeV [may be] able to build-up large amount of organic refractory material at depth of several meters in a comet during [its] long life in the Oort cloud (~4.6 x 107 yr). Ion bombardment might also lead to the formation of a substantial stable crust (Johnson et al., 1987)."[41]

## Long-period comets

Orbits of Comet Kohoutek (red) and the Earth (blue), illustrating the high orbital eccentricity of its orbit and its rapid motion when close to the Sun. Credit: NASA.

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[42] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[43]

Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories[42] when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition comets have a hyperbolic or parabolic osculating orbit which allows them to permanently exit the Solar System after a single pass of the Sun.[44] The Sun's Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).[45] Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion[46] that using a heliocentric unperturbed two-body curve fitting, best-fit suggests they may escape the Solar System.

As of 2018, 1I/ʻOumuamua is the only object with an eccentricity significantly greater than one that has been detected, indicating an origin outside the Solar System. While ʻOumuamua showed no optical signs of cometary activity during its passage through the inner Solar System in October 2017, changes to its trajectory—which suggests outgassing—indicate that it is probably a comet.[47] Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet.[48]

If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). If such objects entered the Solar System, they would have positive specific orbital energy and would be observed to have genuinely hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of magnitude.[49]

## Comet Halley

“During the Halley Monitoring Program at La Silla from Feb.17 to Apr.17,1986 ... In the light of the neutral CN-radical a continuous formation and expansion of [cyan] gas-shells could be observed.”[50] “The gas-expansion velocity decreases with increasing heliocentric distance from 1 km/s in early March to 0.8 km/s in April.”[50]

## Heliospheres

The heliosphere is a bubble in space "blown" into the interstellar medium (the hydrogen and helium gas that permeates the galaxy) by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself.

## Interstellars

Def. between the stars or among the stars is called interstellar.

Def. the dimming of light from the stars due to absorption and scattering from dust in the interstellar medium is called an interstellar extinction.

Def. the nature of the surrounding interstellar environment is called the interstellar medium.

The ISM consists of about 0.1 to 1 particles per cm3 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,[51] where star formation takes place.[52]

## Interstellar medium

This image is an all-sky map of neutral atoms streaming in from the interstellar boundary. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio.

"In 2009, NASA's Interstellar Boundary Explorer (IBEX) mission science team constructed the first-ever all-sky map [at right] of the interactions occurring at the edge of the solar system, where the sun's influence diminishes and interacts with the interstellar medium. A 2013 paper provides a new explanation for a giant ribbon of energetic neutral atoms – shown here in light green and blue -- streaming in from that boundary."[53]

"[T]he boundary at the edge of our heliosphere where material streaming out from the sun interacts with the galactic material ... emits no light and no conventional telescope can see it. However, particles from inside the solar system bounce off this boundary and neutral atoms from that collision stream inward. Those particles can be observed by instruments on NASA’s Interstellar Boundary Explorer (IBEX). Since those atoms act as fingerprints for the boundary from which they came, IBEX can map that boundary in a way never before done. In 2009, IBEX saw something in that map that no one could explain: a vast ribbon dancing across this boundary that produced many more energetic neutral atoms than the surrounding areas."[53]

""What we are learning with IBEX is that the interaction between the sun's magnetic fields and the galactic magnetic field is much more complicated than we previously thought," says Eric Christian, the mission scientist for IBEX at NASA's Goddard Space Flight Center in Greenbelt, Md. "By modifying an earlier model, this paper provides the best explanation so far for the ribbon IBEX is seeing.""[53]

### H I regions

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.

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

These regions are non-luminous, save for emission of the 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.

The degree of ionization in an H I region is very small at around 10−4 (i.e. one particle in 10,000). The temperature of an H I region is about 100 K,[54] and it is usually considered as isothermal, except near an expanding H II region.[55]

For hydrogen, complete ionization "obviously reduces its cross section to zero, but ... the net effect of partial ionization of hydrogen on calculated absorption depends on whether or not observations of hydrogen [are] used to estimate the total gas. ... [A]t least 20 % of interstellar hydrogen at high galactic latitudes seems to be ionized".[56]

"When detection of neutral hydrogen (HI) absorption of the pulsar signal is possible, an estimate, or at least a limit on the distance may be obtained using a Galactic rotation model".[57]

"There is strong evidence for an elongated cavity in the neutral component of the [local insterstellar medium] LISM. This cavity surrounds the Sun and extends several hundred parsecs into quadrant 3 (Lucke 1978). The cavity appears as a region of low reddening extending 500 pc between ℓ = 210° and 255° and 1.5 kpc toward ℓ = 240°. Running counter to this is very heavy obscuration beyond ~100 pc in the first quadrant. Similarly, HI column densities derived from ultraviolet observations show a marked paucity in HI along LOSs directed towards ℓ = 230° (Frisch & York 1983; Paresce 1984). A similar morphology for this cavity is gleaned from NaI absorption measurements".[57]

"To further characterize the distribution of electrons in the LISM it is useful to relate their location to other interstellar features, such as bubbles, superbubbles, and clouds of neutral gas. There is strong evidence for an elongated cavity in the neutral component of the LISM. [...] There are several features of interest within this cavity. One of these is the local hot bubble (LHB): a volume encompassing the Sun distinguished by low neutral gas densities and a 106 K, soft X-ray emitting gas"[57]

The "neutral hydrogen column density [has] a level of N(HI)= 5 × 1019 cm−2"[57]

"Distance estimates now exist for a few hundreds of pulsars, resulting from three basic techniques: neutral hydrogen absorption (in combination with the Galactic rotation curve), trigonometric parallax and from associations with objects of known distance".[58]

### Cold neutral mediums

H I regions of the ISM contain the cold neutral medium (CNM). The CNM constitutes 1-5 % by volume of the ISM, ranges in size from 100-300 pc, has a temperature between 50 and 100 K, with an atom density of 20-50 atoms/cm3.[59] The CNM has hydrogen in the neutral atomic state and emits the 21 cm line.

### Warm neutral mediums

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.[59]

The "peak emissivity is enhanced by about 23% for the WIM [and only 11 % for the warm neutral medium (WNM)], although the peak frequency remains unchanged."[60]

## Planetary nebulas

NASA's Hubble Space Telescope has captured the sharpest view yet of the most famous of all planetary nebulae: the Ring Nebula (M57). Credit: The Hubble Heritage Team (AURA/STScI/NASA).
This is a spectrum of Ring Nebula (M57) in range 450.0 — 672.0 nm. Credit: Minami Himemiya.

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).

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).

## Molecular clouds

This cloud of gas and dust is being deleted. Credit: Hubble Heritage Team (STScI/AURA), N. Walborn (STScI) & R. Barbß (La Plata Obs.), NASA.

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."[61]

A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2).

Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H2 is CO (carbon monoxide). The ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies.[62]

Such clouds make up < 1% of the ISM, have temperatures of 10-20 K and high densities of 102 - 106 atoms/cm3. These clouds are astronomical radio and infrared sources with radio and infrared molecular emission and absorption lines.

## Giant molecular clouds

A vast assemblage of molecular gas with a mass of approximately 103–107 times the mass of the Sun[63] is called a giant molecular cloud (GMC). GMCs are ≈15–600 light-years in diameter (5–200 parsecs).[63] Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average density of a GMC is 102–103 particles per cubic centimetre. Although the Sun is much denser than a GMC, the volume of a GMC is so great that it contains much more mass than the Sun. The substructure of a GMC is a complex pattern of filaments, sheets, bubbles, and irregular clumps.[64]

The densest parts of the filaments and clumps are called "molecular cores", whilst the densest molecular cores are, unsurprisingly, called "dense molecular cores" and have densities in excess of 104–106 particles per cubic centimeter. Observationally molecular cores are traced with carbon monoxide and dense cores are traced with ammonia. The concentration of dust within molecular cores is normally sufficient to block light from background stars so that they appear in silhouette as dark nebulae.[65]

GMCs are so large that "local" ones can cover a significant fraction of a constellation; thus they are often referred to by the name of that constellation, e.g. the Orion Molecular Cloud (OMC) or the Taurus Molecular Cloud (TMC). These local GMCs are arrayed in a ring in the neighborhood of the Sun coinciding with the Gould Belt.[66] The most massive collection of molecular clouds in the galaxy forms an asymmetrical ring around the galactic center at a radius of 120 parsecs; the largest component of this ring is the Sagittarius B2 complex. The Sagittarius region is chemically rich and is often used as an exemplar by astronomers searching for new molecules in interstellar space.[67]

## Exocomets

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way.[68] The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987.[69][70] A total of 10 such exocomet systems have been identified as of 2013, using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.[68][69]

## Supernova remnants

This is an image of NGC 2080, the Ghost Head Nebula. Credit: NASA, ESA and Mohammad Heydari-Malayeri (Observatoire de Paris, France).
The Crab Nebula is a remnant of an exploded star. This image shows the Crab Nebula in various energy bands, including a hard X-ray image from the HEFT data taken during its 2005 observation run. Each image is 6′ wide. Credit: .

"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.[71] The 847 keV and 1238 keV gamma-ray lines from 56Co decay have been detected.[71]

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."[72]

On July 21, 1964, the Crab Nebula supernova remnant was discovered to be a hard X-ray (15 – 60 keV) source by a scintillation counter flown on a balloon launched from Palestine, Texas, USA. This was likely the first balloon-based detection of X-rays from a discrete cosmic X-ray source.[73]

"The high-energy focusing telescope (HEFT) is a balloon-borne experiment to image astrophysical sources in the hard X-ray (20–100 keV) band.[74] Its maiden flight took place in May 2005 from Fort Sumner, New Mexico, USA. The angular resolution of HEFT is ~1.5'. Rather than using a grazing-angle X-ray telescope, HEFT makes use of a novel tungsten-silicon multilayer coatings to extend the reflectivity of nested grazing-incidence mirrors beyond 10 keV. HEFT has an energy resolution of 1.0 keV full width at half maximum at 60 keV. HEFT was launched for a 25-hour balloon flight in May 2005. The instrument performed within specification and observed Tau X-1, the Crab Nebula."[75]

## Wolf-Rayet stars

"At the low density given by the spherically symmetric wind model (see Table 1), the dominant species in the gas are atomic ions while as the gas number density increases, the recombination of ions takes place and the gas composition is governed by neutral-phase chemistry, that is, the dominant species are neutral atoms and molecules although electrons and some ions are still present in relatively large amounts (for example, C+, O+ and He+)."[76]

## Technology

This image shows the IBEX (photo cells forward) being surrounded by its protective nose cone. Credit: NASA (John F. Kennedy Space Center).

"The sensors on the IBEX spacecraft are able to detect energetic neutral atoms (ENAs) at a variety of energy levels."[77]

The satellite's payload consists of two energetic neutral atom (ENA) imagers, IBEX-Hi and IBEX-Lo. Each of these sensors consists of a collimator that limits their fields-of-view, a conversion surface to convert neutral hydrogen and oxygen into ions, an electrostatic analyzer (ESA) to suppress ultraviolet light and to select ions of a specific energy range, and a detector to count particles and identify the type of each ion.

"IBEX–Lo can detect particles with energies ranging from 10 electron–volts to 2,000 electron–volts (0.01 keV to 2 keV) in 8 separate energy bands. IBEX–Hi can detect particles with energies ranging from 300 electron–volts to 6,000 electron–volts (.3 keV to 6 keV) in 6 separate energy bands. ... Looking across the entire sky, interactions occurring at the edge of our Solar System produce ENAs at different energy levels and in different amounts, depending on the process."[77]

“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).”[78]

## Spacecraft

Clouds of material are along the paths of the Voyager 1 and Voyager 2 spacecraft through interstellar space. Credit: NASA, ESA, and Z. Levay (STScI).

The Voyager 1 spacecraft is a 722 kg (1,592 lb) space probe launched by NASA on September 5, 1977 to study the outer Solar System and interstellar medium.

The Cosmic Ray System (CRS) determines the origin and acceleration process, life history, and dynamic contribution of interstellar cosmic rays, the nucleosynthesis of elements in cosmic-ray sources, the behavior of cosmic rays in the interplanetary medium, and the trapped planetary energetic-particle environment.

Measurements from the spacecraft revealed a steady rise since May in collisions with high energy particles (above 70 MeV), which are believed to be cosmic rays emanating from supernova explosions far beyond the Solar System, with a sharp increase in these collisions in late August. At the same time, in late August, there was a dramatic drop in collisions with low-energy particles, which are thought to originate from the Sun.[79]

"It's important for us to be aware of what kinds of objects are present beyond our solar system, since we are now beginning to think about potential interstellar space missions, such as Breakthrough Starshot."[80]

At "least two interstellar clouds [have been discovered] along Voyager 2's path, and one or two interstellar clouds along Voyager 1's path. They were also able to measure the density of electrons in the clouds along Voyager 2's path, and found that one had a greater electron density than the other."[81]

"We think the difference in electron density perhaps indicates a difference in composition of overall density of the clouds."[80]

A "broad range of elements [were detected]] in the interstellar medium, such as electrically charged ions of magnesium, iron, carbon and manganese [and] neutrally charged oxygen, nitrogen and hydrogen."[81]

## References

1. Safi Bahcall; Bryan W. Lynn; Stephen B. Selipsky (October 10, 1990). "New Models for Neutron Stars". The Astrophysical Journal 362 (10): 251-5. doi:10.1086/169261. Retrieved 2014-01-11.
2. Y. Muraki; K. Murakami; M. Miyazaki; K. Mitsui. S. Shibata; S. Sakakibara; T. Sakai; T. Takahashi; T. Yamada et al. (December 1, 1992). "Observation of solar neutrons associated with the large flare on 1991 June 4". The Astrophysical Journal 400 (2): L75-8. Retrieved 2013-12-07.
3. John F. Cooper; Eric R. Christian; John D. Richardson; Chi Wang (2004). Davies J.K., Barrera L.H.. ed. Proton irradiation of Centaur, Kuiper Belt, and Oort Cloud objects at plasma to cosmic ray energy, In: The First Decadal Review of the Edgeworth-Kuiper Belt. 92. Dordrecht: Springer. pp. 261-277. doi:10.1007/978-94-017-3321-2_24. Retrieved 19 June 2019.
4. S. Nakayama; C. Mauger; M.H. Ahn; S. Aoki; Y. Ashie; H. Bhang; S. Boyd; D. Casper et al. (July 2005). "Measurement of single π0 production in neutral current neutrino interactions with water by a 1.3 GeV wide band muon neutrino beam". Physics Letters B 619 (3-4): 255-62. Retrieved 2014-03-22.
5. Forrest D. J.; Vestrand W. T.; Chupp E. L.; Rieger E.; Cooper J. F.; Share G. H. (August 1985). Neutral Pion Production in Solar Flares, In: 19th International Cosmic Ray Conference. 4. NASA. pp. 146-9. Bibcode: 1985ICRC....4..146F. Retrieved 2014-10-01.
6. A. Giuliani; M. Cardillo; M. Tavani; Y. Fukui; S. Yoshiike; K. Torii; G. Dubner; G. Castelletti et al. (1 December 2011). "Neutral Pion Emission from Accelerated Protons in the Supernova Remnant W44". The Astrophysical Journal Letters 742 (2): L30. doi:10.1088/2041-8205/742/2/L30. Retrieved 2014-10-02.
7. Günter Sigl; Sangjin Lee; David N. Schramm (January 1997). "Cosmological Neutrino Signatures for Grand Unification Scale Physics". Physics Letters B 392 (1-2): 129-34. Retrieved 2014-02-07.
8. "Neutrino, In: Glossary for the Research Perspectives of the Max Planck Society". Max Planck Gesellschaft. Retrieved 2012-03-27.
9. Martin J. Rees (December 1984). "Is the Universe flat?". Journal of Astrophysics and Astronomy 5 (4): 331-48. Retrieved 2013-12-18.
10. Q.R. Ahmad; R.C. Allen; T.C. Andersen; J.D. Anglin; J.C. Barton; E.W. Beier; M. Bercovitch; J. Bigu et al. (2002). "Direct evidence for neutrino flavor transformation from neutral-current interactions in the Sudbury Neutrino Observatory". Physical Review Letters 89 (1): e011301. Retrieved 2014-02-07.
11. Francis Halzen; Dan Hooper (June 12, 2002). "High-energy neutrino astronomy: the cosmic ray connection". Reports on Progress in Physics 65 (7): 1025-107. doi:10.1088/0034-4885/65/7/201. Retrieved 2014-02-08.
12. M. D. Leising; D. D. Clayton (December 1, 1987). "Positron annihilation gamma rays from novae". The Astrophysical Journal 323 (1): 159-69. doi:10.1086/165816. Retrieved 2014-02-01.
13. A De Rujula, SL Glashow (September 15, 1980). "Galactic neutrinos and UV astronomy". Physical Review Letters 45 (09): 942-4. doi:10.1103/PhysRevLett.45.942. Retrieved 2014-02-08.
14. "Hubble Revisits an Old Friend, In: Picture of the Week". ESA/Hubble. Retrieved 17 October 2011.
15. W. David Arnett; John N. Bahcall; Robert P. Kirshner; Stanford E. Woosley (1989). "Supernova 1987A". Annual Review of Astronomy and Astrophysics 27: 629-700. doi:10.1146/annurev.aa.27.090189.003213. Retrieved 2013-05-31.
16. G. Sonneborn (1987). Minas Kafatos, Andreas Gerasimos Michalitsianos. ed. The Progenitor of SN1987A, In: Supernova 1987a in the Large Magellanic Cloud. Cambridge University Press. ISBN 0-521-35575-3.
17. DAILY MAIL REPORTER (August 12, 2011). Giant Necklace Nebula brightly glows with dense knots of blue, green and red gases. United Kingdom: Daily Mail. Retrieved 2014-02-24.
18. Adolf N. Witt; Karl D. Gordon; Douglas G. Furton (July 1, 1998). "Silicon Nanoparticles: Source of Extended Red Emission?". The Astrophysical Journal Letters 501 (1): L111-5. doi:10.1086/311453. Retrieved 2013-07-30.
19. T. L. Smith; A. N. Witt (December 1999). "The Photoluminescence Efficiency of Extended Red Emission as a Constraint for Interstellar Dust". Bulletin of the American Astronomical Society 31: 1479. Retrieved 2013-08-02.
20. See Binney and Merrifeld (1998), Carroll and Ostlie (1996), Kutner (2003) for applications in astronomy.
21. eso1320a (May 2, 2013). The star formation region NGC 6559. La Silla Observatory, Chile: European Southern Observatory. Retrieved 2013-05-02.
22. M. Antonello; P. Aprili; B. Baibussinov; M. Baldo Ceolin; P. Benetti; E. Calligarich; N. Canci; F. Carbonara et al. (May 15, 2012). "A search for the analogue to Cherenkov radiation by high energy neutrinos at superluminal speeds in ICARUS". Physics Letters B 711 (3-4): 270-5. Retrieved 2012-07-28.
23. Andrew G. Glashow; Sheldon L. Glashow (October 2011). "Pair Creation Constrains Superluminal Neutrino Propagation". Physical Review Letters 107 (18): 181803. doi:10.1103/PhysRevLett.107.181803. Retrieved 2013-08-16.
24. A. Moralejo for the MAGIC collaboration (2004). "The MAGIC telescope for gamma-ray astronomy above 30 GeV". Memorie della Societa Astronomica Italiana 75: 232-9. Retrieved 2012-07-28.
25. P. W. Gorham; S. W. Barwick; J. J. Beatty; D. Z.Besson; W. R. Binns; C. Chen; P. Chen; J. M. Clem et al. (October 25, 2007). "Observations of the Askaryan Effect in Ice". Physical Review Letters 99 (17): 5. doi:10.1103/PhysRevLett.99.171101. Retrieved 2012-07-28.
26. Mike Gruntman. Charge Exchange Diagrams. Retrieved 2009-10-27.
27. K. C. Hsieh; C. C. Curtis (1998). Imaging Space Plasma With Energetic Neutral Atoms Without Ionization, In: Measurement Techniques in Space Plasmas: Fields. Geophysical Monograph 103. American Geophysical Union. pp. 235-49. Retrieved 2014-10-02.
28. Whitham D. Reeve (1973). Book Review. Anchorage, Alaska USA: Whitham D. Reeve. Retrieved 2014-01-11.
29. I. Dzyaloshinskii (May 1992). "Effects of the finite proton mass in a hydrogen atom in crossed magnetic and electric fields: a state with a giant electric dipole moment". Physics Letters A 165 (1): 69-71. doi:10.1016/0375-9601(92)91056-W. Retrieved 2014-02-13.
30. E. Quémerais (30 June 2003). SOHO Fact Sheet. Greenbelt, MD, USA: NASA/GSFC. Retrieved 2016-03-27.
31. Sprague, Ann L.; Vervack, R. J.; Killen, R. M.; McClintock, W. E.; Starr, R. D.; Schriver, D.; Trávnícek, P.; Orlando, T. M. et al. (2010). "MESSENGER: Insights Regarding the Relationship between Mercury's Surface and Its Neutral and Ionized Exosphere". Bulletin of the American Astronomical Society 42 (21.01): 985. Retrieved 2015-06-21.
32. E. C. Roelof; D. G. Mitchell; D. J. Williams (1985). "Energetic neutral atoms (E ∼ 50 keV) from the ring current: IMP 7/8 and ISEE 1". Journal of Geophysical Research 90 (A11): 10,991-11,008. doi:10.1029/JA090iA11p10991. Retrieved 2012-08-12.
33. D. G. Mitchell; K. C. Hsieh; C. C. Curtis; D. C. Hamilton; H. D. Voes; E. C. Roelof (2001). "Imaging two geomagnetic storms in energetic neutral atoms". Geophysical Research Letters 28 (6): 1151-4. doi:10.1029/2000GL012395. Retrieved 2012-08-12.
34. Richard C. Elphic; Sarah Noble; P. Butler Hine III (September 7, 2013). Lunar Atmosphere and Dust Environment Explorer (LADEE). Washington, DC USA: National Space Science Data Center, NASA. Retrieved 2014-01-07.
35. Donald M. Hunten (February 12, 1993). "Atmospheric Evolution of the Terrestrial Planets". Science 259 (5097): 915-20. Retrieved 2014-09-21.
36. Stephen G. Brown (5 November 2005). "comet". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 19 June 2019.
37. Paul G (25 February 2004). "comet". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 19 June 2019.
38. WikiPedant (4 November 2007). "periodic comet". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 19 June 2019.
39. AryamanA (11 February 2016). "short-period comet". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 19 June 2019.
40. G. Andronico; G. A. Baratta; F. Spinella; G. Strazzulla (October 1987). "Optical evolution of laboratory-produced organics - applications to Phoebe, Iapetus, outer belt asteroids and cometary nuclei". Astronomy and Astrophysics 184 (1-2): 333-6. Retrieved 2013-09-25.
41. "Small Bodies: Profile". NASA/JPL. 29 October 2008. Retrieved 11 August 2013.
42. Elenin, Leonid (7 March 2011). "Influence of giant planets on the orbit of comet C/2010 X1". Retrieved 11 August 2013.
43. Joardar, S; Bhattacharya, A. B; Bhattacharya, R (2008). Astronomy and Astrophysics. p. 21. ISBN 978-0-7637-7786-9.
44. Chebotarev, G. A. (1964). "Gravitational Spheres of the Major Planets, Moon and Sun". Soviet Astronomy 7: 618.
45. "JPL Small-Body Database Search Engine: e > 1". JPL. Retrieved 13 August 2013.
46. Gohd, Chelsea (27 June 2018). "Interstellar Visitor 'Oumuamua Is a Comet After All". Space.com. Retrieved 27 September 2018.
47. "C/1980 E1 (Bowell)". JPL Small-Body Database (1986-12-02 last obs). Retrieved 13 August 2013.
48. McGlynn, Thomas A.; Chapman, Robert D. (1989). "On the nondetection of extrasolar comets". The Astrophysical Journal 346: L105. doi:10.1086/185590.
49. Wolfhard Schlosser; Rita Schulz; Paul Koczet (1986). The cyan shells of Comet P/Halley, In: Proceedings of the 20th ESLAB Symposium on the Exploration of Halley's Comet. 3. European Space Agency. pp. 495-8. Bibcode: 1986ESASP.250c.495S.
50. O'Dell, C. R.. Nebula. World Book, Inc.. Retrieved 2009-05-18.
51. Dina Prialnik (2000). An Introduction to the Theory of Stellar Structure and Evolution. Cambridge University Press. pp. 195–212. ISBN 0-521-65065-8.
52. Karen C. Fox (February 5, 2013). A Major Step Forward in Explaining the Ribbon in Space Discovered by NASA’s IBEX Mission. Greenbelt, MD USA: NASA's Goddard Space Flight Center. Retrieved 2013-02-06.
53. L. Spitzer, M. P. Savedoff (1950). "The Temperature of Interstellar Matter. III". The Astrophysical Journal 111: 593. doi:10.1086/145303.
54. Savedoff MP, Greene J (November 1955). "Expanding H II region". Astrophysical Journal 122 (11): 477–87. doi:10.1086/146109.
55. Robert Morrison; Dan McCammon (July 1983). "Interstellar photoelectric absorption cross sections, 0.03-10 keV". The Astrophysical Journal 270 (7): 119-22.
56. M. Toscano; M. C. Britton; R. N. Manchester; M. Bailes; J. S. Sandhu; S. R. Kulkarni; S. B. Anderson (October 1, 1999). "Parallax of PSR J1744–1134 and the local interstellar medium". The Astrophysical Journal 523 (2): L171. doi:10.1086/312276. Retrieved 2014-04-19.
57. R. Stepanov; P. Frick; A. Shukurov; D. Sokoloff (August 2002). "Wavelet tomography of the Galactic magnetic field I. The method". Astronomy & Astrophysics 391 (08): 361-8. doi:10.1051/0004-6361:20020552. Retrieved 2014-04-20.
58. K. Ferriere (2001). "The Interstellar Environment of our Galaxy". Reviews of Modern Physics 73 (4): 1031–66. doi:10.1103/RevModPhys.73.1031.
59. Yacine Ali-Haïmoud (2013). "Spinning dust radiation: a review of the theory". Advances in Astronomy 2013 (462697). doi:10.1155/2013/462697. Retrieved 2014-10-19.
60. Robert Nemiroff (MTU); Jerry Bonnell (USRA) (June 30, 2003). Disappearing Clouds in Carina. Goddard Space Flight Center, Greenbelt, Maryland, USA: NASA. Retrieved 2012-09-05.
61. Craig Kulesa. Overview: Molecular Astrophysics and Star Formation. Retrieved September 7, 2005.
62. See, e.g., Table 1 and the Appendix of Murray, N. (2011). "Star Formation Efficiencies and Lifetimes of Giant Molecular Clouds in the Milky Way". The Astrophysical Journal 729 (2): 133. doi:10.1088/0004-637X/729/2/133.
63. J. P. Williams; L. Blitz; C. F. McKee (2000). The Structure and Evolution of Molecular Clouds: from Clumps to Cores to the IMF, In: Protostars and Planets IV. Tucson: University of Arizona Press. pp. 97.
64. Sagittarius B2 and its Line of Sight
65. Sanders, Robert (7 January 2013). "Exocomets may be as common as exoplanets". UC Berkeley. Retrieved 30 July 2013.
66. "'Exocomets' Common Across Milky Way Galaxy". Space.com. 7 January 2013. Archived from the original on 16 September 2014. Retrieved 8 January 2013.
67. Beust, H.; Lagrange-Henri, A.M.; Vidal-Madjar, A.; Ferlet, R. (1990). "The Beta Pictoris circumstellar disk. X – Numerical simulations of infalling evaporating bodies". Astronomy and Astrophysics 236: 202–216. ISSN 0004-6361.
68. Figueiredo N; Villela T; Jayanthi UB; Wuensche CA; Neri JACF; Cesta RC (1990). "Gamma-ray observations of SN1987A". Rev Mex Astron Astrofis. 21: 459–62.
69. News Release Number: STScI-2001-34 (December 19, 2001). Wallpaper: The Ghost-Head Nebula (NGC 2080). NASA and the Hubble Space Telescope. Retrieved 2012-07-21.
70. F. A. Harrison; Steven Boggs; Aleksey E. Bolotnikov; Finn E. Christensen; Walter R. Cook III; William W. Craig; Charles J. Hailey; Mario A. Jimenez-Garate et al. (2000). Joachim E. Truemper, Bernd Aschenbach. ed. [proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=900102 "Development of the High-Energy Focusing Telescope (HEFT) balloon experiment"]. Proc SPIE. X-Ray Optics, Instruments, and Missions III 4012: 693. doi:10.1117/12.391608. proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=900102.
71. Marshallsumter (April 15, 2013). X-ray astronomy. San Francisco, California: Wikimedia Foundation, Inc. Retrieved 2013-05-11.
72. I. Cherchneff; Y.H. Le Teuff; P.M. Williams; A.G.G.M. Tielens (May 2000). "Dust formation in carbon-rich Wolf-Rayet stars. I. Chemistry of small carbon clusters and silicon species". Astronomy and Astrophysics 357 (5): 572-80. Retrieved 2011-12-05.
73. Dave McComas; Lindsay Bartolone (May 10, 2012). IBEX: Interstellar Boundary Explorer. San Antonio, Texas USA: NASA Southwest Research Institute. Retrieved 2012-08-11.
74. "Submillimetre astronomy". Wikipedia (San Francisco, California: Wikimedia Foundation, Inc). June 2, 2012. Retrieved 2012-06-08.
75. http://www.lifeslittlemysteries.com/2984-voyager-spacecraft-solar-system.html
76. Julia Zachary (9 January 2017). How New Hubble Telescope Views Could Aid Interstellar Travel. Space.com. Retrieved 2017-01-11.
77. Charles Q. Choi (9 January 2017). How New Hubble Telescope Views Could Aid Interstellar Travel. Space.com. Retrieved 2017-01-11.