The Sun is observed through a telescope with an H-alpha filter. Credit: Marshall Space Flight Center, NASA.

“In 1926 ... [t]here were no national observatories (except the Naval Observatory), very little chance for guest observing elsewhere, no radio astronomy, no X-ray astronomy, no satellite astronomy, and very little infrared or even red astronomy!”[1] Bold added.

## Astronomy

With respect to the color red in astronomy, there are studies of the red shift, which may be considered an entity, sources of red radiation, and the redness of objects.

In wavelengths, red astronomy covers 620 - 750 nm.

Infrared or red radiation from a common household radiator or electric heater is an example of thermal radiation, as is the heat emitted by an operating incandescent light bulb. Thermal radiation is generated when energy from the movement of charged particles within atoms is converted to electromagnetic radiation.

Infrared (IR) light is electromagnetic radiation with longer wavelengths than those of visible light, extending from the nominal red edge of the visible spectrum at 700 nanometres (nm) to 1 mm. This range of wavelengths corresponds to a frequency range of approximately 430 THz down to 300 GHz,[2] and includes most of the thermal radiation emitted by objects near room temperature. Infrared light is emitted or absorbed by molecules when they change their rotational-vibrational movements.

Far-red light is light at the extreme red end of the visible spectrum, between red and infra-red light. Usually regarded as the region between 710 and 850 nm wavelength, it is dimly visible to some human eyes.

## Planetary sciences

"Far-red light is largely reflected or transmitted by plants on Earth because of the absorbance spectrum of chlorophyll, and it is perceived by the plant photoreceptor phytochrome. However, some organisms can use it as a source of energy in photosynthesis.[3][4] Far-red light also is used for vision by certain organisms such as some species of deep-sea fishes.[5][6][7]

The name Pomo, or Pomo people, originally meant "those who live at red earth hole" and was once the name of a village in southern Potter Valley near the present-day community of Pomo.[8] It may have referred to local deposits of the red mineral magnesite, used for red beads, or to the reddish earth and clay such as hematite mined in the area.[9]

Some argue that cosmetic body art was the earliest form of ritual in human culture, dating over 100,000 years ago from the African Middle Stone Age. The evidence for this comes in the form of utilised red mineral pigments (red ochre) including crayons associated with the emergence of Homo sapiens in Africa.[10][11][12][13]

A variant of ochre containing a large amount of hematite, or dehydrated iron oxide, has a reddish tint known as "red ochre". Red ochre, Fe2O3, takes its reddish color from the mineral hematite, which is a dehydrated iron oxide.

## Red minerals

Rhodolite is the rose-pink to red mineral, a type of garnet, in this magnesium iron aluminum silicate mineral. Credit: Dave Dyet.
This is a specimen of Breithauptite on calcite from the Samson Mine, St Andreasberg, Harz Mountains, Lower Saxony, Germany. Credit: Leon Hupperichs.
Cinnabar is a naturally occurring cochineal-red, towards brownish red and lead-gray, mercury-sulfide mineral. Credit: H. Zell.
This Crocoite specimen is from the Red Lead Mine, Tasmania, Australia. Credit: Eric Hunt.
Eudialyte is a somewhat rare, red silicate mineral. Credit: Baryonyx.
Hematite is a blood colored ore. Credit: DanielCD~commonswiki.
This is a close-up of hematitic banded iron formation specimen from Upper Michigan. Scale bar is 5.0 mm. Credit: Wilson44691.

Rhodolite is a varietal name for rose-pink to red mineral pyrope, a species in the garnet group.

Chemically, rhodolite is an iron-magnesium-aluminium silicate, [(Mg,Fe)3Al2(SiO4)3,] part of the pyrope-almandine solid-solution series, with an approximate garnet composition of Py70Al30.

Breithauptite is a nickel antimonide mineral with the simple formula NiSb. Breithauptite is a metallic opaque copper-red mineral crystallizing in the hexagonal - dihexagonal dipyramidal crystal system. It is typically massive to reniform in habit, but is observed as tabular crystals. It has a Mohs hardness of 3.5 to 4 and a specific gravity of 8.23.

It occurs in hydrothermal calcite veins associated with cobalt–nickel–silver ores.

Cinnabar or cinnabarite (red mercury(II) sulfide (HgS), native vermilion), is the common ore of mercury. Its color is cochineal-red, towards brownish red and lead-gray. Cinnabar may be found in a massive, granular or earthy form and is bright scarlet to brick-red in color.[14] Generally cinnabar occurs as a vein-filling mineral associated with recent volcanic activity and alkaline hot springs. Cinnabar is deposited by epithermal ascending aqueous solutions (those near surface and not too hot) far removed from their igneous source.

Crocoite is a mineral consisting of lead chromate, PbCrO4. Crystals are of a bright hyacinth-red color. Relative rarity of crocoite is connected with specific conditions required for its formation: an oxidation zone of lead ore bed and presence of ultramafic rocks serving as the source of chromium (in chromite).

Eudialyte is a somewhat rare, red silicate mineral, which forms in alkaline igneous rocks, such as nepheline syenites.

Hematite is the mineral form of iron(III) oxide (Fe2O3), one of several iron oxides. Hematite is colored black to steel or silver-gray, brown to reddish brown, or red. Huge deposits of hematite are found in banded iron formations.

## Theoretical red radiation astronomy

"[S]ince the majority of carbon and S giants do not possess nearly so much lithium [as has been detected in some red-giant stars], it is necessary to postulate that the production process involves some unusual events."[15]

"[H]elium-burning shell flashes in advanced stages of stellar evolution [may] occasionally induce complete convection of the outer envelope down to the helium-burning shell. If the hydrogen mixing is relatively small for the first 107 seconds, the result may be the production of large amounts of heavy elements by the s-process. When complete mixing commences, the 3He in the envelope will be converted to 7Be, and the subsequent delayed electron capture to form 7Li may allow enough lithium to remain near the surface to account for the very large lithium abundances in some S and carbon red-giant stars. ... the 7Li/6Li ratio in these stars should be quite large (> 100)."[15]

## Entities

"[G]alaxies R8 and R10 lie at z = 1.290 - 1.298 while the other 3 are at z = 1.317 - 1.320, i.e., there is a "gap" pf 2500 km s-1 in the mean rest frame, though there is no clear segregation on the sky. These may be two separate physical entities (e.g., filaments/sheets/subclusters), but this speculation lies beyond the available data."[16]

## Sources

"Most of the sources are resolved in [Hubble Space Telescope] HST F814W imaging so they are certainly galaxies and not M stars."[16]

## Objects

"The nature of extremely red objects (EROs) remains an open question in understanding the faint galaxy population at z > 1."[16]

## Continua

"The other 3 red galaxies are devoid of strong emission lines, but they do show continuum breaks identifiable as the rest-frame mid-UV breaks at 2640 Å and 2900 Å (Figure 2)."[16]

## Absorptions

Balmer lines can appear as absorption or emission lines in a spectrum, depending on the nature of the object observed. In stars, the Balmer lines are usually seen in absorption, and they are "strongest" in stars with a surface temperature of about 10,000 kelvin (spectral type A). In the spectra of most spiral and irregular galaxies, [active galaxy] AGNs, H II regions and planetary nebulae, the Balmer lines are emission lines.

## Emissions

The Hubble Space Telescope [Advanced Camera for Surveys] ACS image has H-alpha emission of the Red Rectangle shown in blue. Credit: ESA/Hubble and NASA.

"[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."[17]

"The ERE was first recognized clearly in the peculiar reflection nebula called the Red Rectangle by Schmidt, Cohen, & Margon (1980)."[17]

The Red Rectangle Nebula, so called because of its red color and unique rectangular shape, is a protoplanetary nebula in the Monoceros constellation. Also known as HD 44179, the nebula was discovered in 1973 during a rocket flight associated with the AFCRL Infrared Sky Survey called Hi Star.

## Bands

The Red Rectangle is a proto-planetary nebula. Here is the Hubble Space Telescope Advanced Camera for Surveys (ACS) image. Broadband red light is shown in red. Credit: JPL/NASA.

The "ERE manifests itself through a broad, featureless emission band of 60 < FWHM < 100 nm, with a peak appearing in the general wavelength range 610 < λp < 820 nm."[17]

In the Red Rectangle Nebula, diffraction-limited speckle images of it in visible and near infrared light reveal a highly symmetric, compact bipolar nebula with X-shaped spikes which imply toroidal dispersion of the circumstellar material. The central binary system is completely obscured, providing no direct light.[18]

Using various flames such as from a Bunsen burner, "[s]trontium yields two red bands and one orange band."[19]

## Backgrounds

"[T]he instrumental response, sky background, and seeing conspire to provide the best images. All the available bands are used to define colors with which we separate cluster members from the background in order to minimize dilution of the weak-lensing signal by unlensed objects."[20]

"We select red galaxies with colors redder than the color-magnitude sequence of cluster E/SO galaxies. The sequence forms a well-defined line due to the richness and relatively low redshifts of our clusters. These red galaxies are expected to lie in the background by virtue of k-corrections which are greater than for the red cluster sequence galaxies".[20]

"Typically the proportion of blue galaxies used is around 50% of the red background."[20]

"[T]he gravitational shear field [is derived] by locally averaging the corrected distortions of color-selected background galaxies of each cluster. ... Maps of the surface number-density distribution of color-selected cluster member galaxies [have been produced], with the gravitational shear of background galaxies overlaid ... Profiles of background red galaxy counts, whose intrinsic slope is relatively shallow [have been observed] ... the utility of the background red galaxies for measuring magnification [has been established]."[20]

"The distortion profiles measured are among the most accurate constructed to date and the great depth of the Subaru imaging permits magnification profiles to be established with unprecedented detail using the background red galaxy counts."[20]

## Meteors

"[T]he ionization energy of oxygen atoms in a meteor trail can be converted into red-orange emission of atmospheric-system molecular oxygen bands, resulting in a red afterglow in meteor trains associated with fireballs brighter than magnitude -8."[21]

"The red meteors definitely predominated among those with any marked colour."[22]

"Red meteors were rare in all cases, and the shower results fell within the sporadic annual spread of 0.396—0.8% of the total meteors seen. They were generally recorded as being fainter than expected, in line with the scotopic eye's poor red appreciation."[23]

## Cosmic rays

"The 22Ne/20Ne ratio measured in solar CRs (0.13) agrees with the Ne-A ratio in meteorites (0.12) and is taken here as standard solar system value. The disagreement between solar wind (0.07) and solar flare (0.13) measurements is not understood, but the close similarity between isotopic ratios of C, N, O, and Mg measured in normal solar flares and meteoritic values testifies that, at least, there is no isotopic fractionation in these flares (Mewaldt et al. 1981; Dietrich and Simpson 1981)."[24]

"Potential 22Ne sources are: (1) young supernova remnants (SNRs) still bearing the print of explosive hydrogen burning, (2) nova outbursts, (3) pulsing red giants, and (4) carbon-rich Wolf-Rayet stars and more generally H-deprived stars."[24]

"Pulsing red giants are thought to synthesize 22Ne in significant quantities in their helium-burning shells and to eject the processed material brought to the surface via stellar wind and, subsequently, via envelope ejection in the planetary nebula (PN) phase (Iben and Truran 1978). However, the 22Ne excess relative to the solar system abundance in the ejected material is (even before dilution) less than the 22Ne excess infered at the CR sources.2 The weakest point of the red giant scenario is it requires, in an ad hoc fashion, evolved stars near 4 Mʘ, and 4 Mʘ only, to emit most CRs, since virtually no dilution with CRs of any other origin is allowed."[24]

"[T]he ability of red giants to accelerate [cosmic rays] CRs is questionable. In the framework of acceleration by stellar winds (Cassé and Paul 1980), they are energetically unfavorable, compared to younger mass-losing objects: mass loss rate ~ 10-8 - 10-9 yr-1 and terminal velocity of the wind νw ~ 10 km s-1 against > 10-5 Mʘ yr-1 and νw ~ 2000 km s-1 for Wolf-Rayet (W-R) stars."[24]

## Neutrons

"Seven percent of a normal solar mass contains nearly 2 X 1051 iron atoms. The total number of admixed protons is of the order of 4 X 1050. Almost all of these are converted to neutrons by the 12C(p,γ)13N(e+ν)13C and 13C(α,n)16O reactions. Since significant synthesis of heavy elements requires the production of 10-102 neutrons per iron nucleus (Clayton et al. 1961; Seeger, Fowler, and Clayton 1965; Seeger and Fowler 1966), it may be seen that significant s-process production of heavy elements [such as lithium] would occur only if the metal content of the star is less than solar by two orders of magnitude."[15]

"A ratio Rb/Sr ≃ 0.05 [may be derived] for the s-processed material from the He-burning shell ... [involving] the branch in the s-process path at 85Kr [that] may be used to determine the neutron density at the time of s-processing. The derived ratio is consistent with predicted neutron densities for operation of the s-process during the interpulse intervals in low-mass asymptotic giant branch (AGB) stars but clearly inconsistent with much higher neutron densities predicted for the running of the s-process in the He-shell thermal pulses of intermediate mass AGB stars and probably also of low-mass AGB stars."[25]

"The absence of 96Zr sets an upper limit on the neutron density at the s-process site which is higher than and, therefore, consistent with the limit set by the Rb abundances in related stars."[25]

## Protons

"The total number of admixed protons in [seven percent of a normal solar mass] is of the order of 4 X 1050."[15]

"Diamond nanocrystals (size 100 nm) emit bright luminescence at 600–800 nm when exposed to green and yellow photons. The photoluminescence, arising from excitation of the nitrogen-vacancy defect centers created by proton-beam irradiation and thermal annealing, closely resembles the extended red emission (ERE) bands observed in reflection nebulae and planetary nebulae. The central wavelength of the emission is 700 nm".[26]

## Electrons

This is the first color image of a sprite, taken from an aircraft. Credit: Eastview.
A sprite is seen and photographed from International Space Station. Credit: Image Science and Analysis Laboratory, NASA-Johnson Space Center. "The Gateway to Astronaut Photography of Earth.".

"The appearance of extensive red emission wings typifies emission lines formed in spherical expanding atmospheres with electron scattering present."[27]

"[E]lectron energization in the lower ionosphere [may be] due to lightning induced transient electromagnetic pulses."[28]

Sprites are large-scale electrical discharges that occur high above thunderstorm clouds, or cumulonimbus, giving rise to a quite varied range of visual shapes flickering in the night sky. They are triggered by the discharges of positive lightning between an underlying thundercloud and the ground.

Sprites appear as luminous reddish-orange flashes. They often occur in clusters within the altitude range 50–90 km above the Earth's surface. Sporadic visual reports of sprites go back at least to 1886, but they were first photographed on July 6, 1989 by scientists from the University of Minnesota and have subsequently been captured in video recordings many thousands of times.

## Positrons

"The observation of a red supergiant at the site of a peculiar SN I outburst would be a direct means of confirming or refuting the hypothesis that a peculiar SN I explodes in a binary system with a red supergiant."[29]

"The main source of emission of the secondary ejecta in a sufficiently late stage (≥100d) will be the energy of the radioactive Ni56 → Co56 → Fe56 in the surrounding material of the supernova."[29]

"The energy of Co56 decay is distributed between γ-ray quanta with an energy of the order of 1 MeV (96%) and positrons (4%)."[29]

## Neutrinos

"[N]on-standard neutrino losses [may have an] impact on the red giant branch (RGB)".[30]

## Gamma rays

A gamma-ray burst (GRB) may have an afterglow at longer wavelengths. Specifically, GRB 000418 has a "very red afterglow [which is] further evidence for dust extinction ... [where] the GRB was associated with a dusty star-forming region."[31]

## X-rays

"The combination of X-ray absorption, red near-IR continuum, polarized optical continuum, and broad lines in the majority of 2MASS [active galactic nucleus] AGNs suggests that they are viewed at an intermediate line of sight with respect to dusty, nuclear material (e.g., torus/disk/wind), as has been proposed for the similarly polarized [broad absorption line] BAL [quasi-stellar object] QSOs."[32]

## Superluminals

"[M]ultiwavelength observations of the superluminal X-ray transient GRO J1655-40 [have been performed] during and following the prominent hard X-ray outburst of 1995 March-April."[33]

"The red color of the optical counterpart with E(B - V) = +1.3 ± 0.2, as recently determined by deep 200 nm Hubble Space Telescope (HST) observations in 1995 May (Horne et al. 1996), indicates significant absorption in the direction of GRO J1655-40 (see also B95a). Spectroscopic CTIO observations of GRO J1655-40 carried out in early 1995 May revealed Doppler-shifted high-excitation emission lines superposed on an F-type or early G-type stellar absorption spectrum (Bailyn et al. 1995b, hereafter B95b)."[33]

## Plasma objects

The Hessdalen Light HL is an unexplained light usually seen in the Hessdalen valley in the municipality of Holtålen in Sør-Trøndelag county, Norway.

"HL [may be] formed by a cluster of macroscopic Coulomb crystals in a plasma produced by the ionization of air and dust by alpha particles during radon decay in the dusty atmosphere."[34]

"HL are characterized mostly by white color and sometimes by red color. It occurs mostly at night, more often in the winter season and with a peak around midnight."[34]

## Gaseous objects

With respect to "gaseous objects with mass below the H-burning limit. ... the hydrogen-rich gaseous objects with mass below the minimum main sequence mass of ~ 0.08 Msun. ... the minimum mass of a gaseous fragment may be as low as 0.001 Msun."[35]

"The first stage is the initial gravitational collapse phase during which the object moves more or less vertically downward in the H-R diagram. The color of the object is red (or very red) during this stage."[35]

"During [the second evolutionary] stage the object, still luminous, starts sliding down on its cooling curve, and its color gets redder and redder as the surface temperature decreases with time."[35]

## Liquid objects

"When solid or liquid objects formed in the early Solar System, either by condensation from the vapor phase or by melting and crystallization of preexisting material, each of these isotopic chronometers is expected to have been reset."[36]

"The majority of near-Earth asteroids (NEAs) whose taxonomic types are known belong to Tholen's S-class (Tholen, 1984). The reflectance spectra of S-class asteroids show absorption bands due to pyroxene or olivine or both, and the continuum slopes are moderately red."[37]

"The continuum slope of smaller NEAs is much redder than that of meteorite spectra, indicating either a compositional difference, or some space weathering effect."[37]

## Rocky objects

"The rocky objects have rather smooth, red spectra (the Mars spectrum has some incompletely-removed terrestrial features). The gas giants have depressed red albedoes. Cloud-covered Venus has a depressed blue albedo."[38]

## Hydrogens

The spectral series of hydrogen is displayed on a logarithmic scale. Credit: OrangeDog.
Electron transitions and their resulting wavelengths for hydrogen. Energy levels are not to scale. Credit: Szdori and OrangeDog.
The spectrum shows the lines in the visible due to emission from elemental hydrogen. Credit:Teravolt.
Milky Way is viewed by H-Alpha Sky Survey. Credit: David Brown and Douglas Finkbeiner.

The emission spectrum of atomic hydrogen is divided into a number of spectral series, with wavelengths given by the Rydberg formula. These observed spectral lines are due to electrons moving between energy levels in the atom. The spectral series are important in astronomy for detecting the presence of hydrogen and calculating red shifts. The spectral lines of hydrogen correspond to particular jumps of the electron between energy levels. The simplest model of the hydrogen atom is given by the Bohr model. When an electron jumps from a higher energy to a lower, a photon of a specific wavelength is emitted.

The spectral lines are grouped into series according to n. Lines are named sequentially starting from the longest wavelength/lowest frequency of the series, using Greek letters within each series. For example, the 2 → 1 line is called "Lyman-alpha" (Ly-α), while the 7 → 3 line is called "Paschen-delta" (Pa-δ). Some hydrogen spectral lines fall outside these series, such as the 21 cm line; these correspond to much rarer atomic events such as hyperfine transitions.[39] The fine structure also results in single spectral lines appearing as two or more closely grouped thinner lines, due to relativistic corrections.[40]

The energy differences between levels in the Bohr model, and hence the wavelengths of emitted/absorbed photons, is given by the Rydberg formula[41]:

${\displaystyle {1 \over \lambda }=R\left({1 \over (n^{\prime })^{2}}-{1 \over n^{2}}\right)\qquad \left(R=1.097373\times 10^{7}\ \mathrm {m} ^{-1}\right)}$

where n is the initial energy level, n′ is the final energy level, and R is the Rydberg constant. Meaningful values are returned only when n is greater than n′ and the limit of one over infinity is taken to be zero.

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

A hydrogen-alpha filter is an optical filter designed to transmit a narrow bandwidth of light generally centered on the H-alpha wavelength. They are characterized by a bandpass width that measures the width of the wavelength band that is transmitted.[42] These filters are manufactured by multiple (~50) layers of vacuum-deposited layers. These layers are selected to produce interference effects that filter out any wavelengths except at the requisite band.[43] Alternatively, an etalon may be used as the narrow band filter (in conjunction with a "blocking filter" or energy rejection filter) to pass only a narrow (<0.1 nm) range of wavelengths of light centred around the H-alpha emission line. The physics of the etalon and the dichroic interference filters are essentially the same (relying on constructive/destructive interference of light reflecting between surfaces), but the implementation is different (an interference filter relies on the interference of internal reflections). Due to the high velocities sometimes associated with features visible in H-alpha light (such as fast moving prominences and ejections), solar H-alpha etalons can often be tuned (by tilting or changing the temperature) to cope with the associated Doppler effect.

## Heliums

The spectrum shows the lines in the visible due to emission from elemental helium. Credit:Teravolt.

Helium has at least one weak line in the red.

## Lithiums

This spectrograph shows the visual spectral lines of lithium. Credit: T c951.

"[T]he standard solar models have enjoyed tremendous success recently in terms of agreement between the predicted outer structure and the results from helioseismology[, but] some observed properties of the Sun still defy explanation, such as the degree of Li depletion" [the "solar Li abundance is roughly a factor of 200 below the meteoritic abundance"].[44]

In some 824 red giant stars, the Li I 670.78 nm line was detected in several stars, "but only the five objects ... presented a strong line."[45]

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

"The lithium content of red-giant stars is highly variable (Wallerstein and Conti 1969). The largest amounts of lithium are found in three carbon stars, WZ Cas, WX Cyg, and T Ara, being of the order of 10-2 of calcium. ... Boesgaard (1970) has found a similar high lithium abundance in the S star T Sgr. This is a higher ratio of lithium to calcium than is found in T Tauri stars or in meteorites."[15]

## Berylliums

This spectrograph shows the visual spectral lines of beryllium. Credit: Penyulap.

Beryllium has at least six emission/absorption lines across the red.

## Borons

This spectrograph shows the visual spectral lines of beryllium. Credit: Penyulap.

Boron has a red line near the orange portion of the visual spectrum.

## Carbons

The spectrum shows the lines in the visible due to emission from elemental carbon. Credit:Teravolt.

Carbon has one strong line in the red.

## Nitrogens

The spectrum shows the lines in the visible due to emission from elemental nitrogen. Credit:Kurgus.
The red light depicts nitrogen emission ([N II] 658.4 nm); green, hydrogen (H-alpha, 6563A); and blue, oxygen (5007A). These are "cometary knots" in the Helix nebula. Credit: NASA Robert O Dell Kerry P. Handron Rice University, Houston Texas.
This NASA Hubble Space Telescope image shows one of the most complex planetary nebulae ever seen, NGC 6543, nicknamed the "Cat's Eye Nebula." Credit: NASA J.P.Harrington and K.J.Borkowski University of Maryland.

Nitrogen has an emission line at 658.4 nm.

At right is an image gaseous objects ("cometary knots") discovered in the thousands. These knots are imaged with the Hubble Space Telescope while exploring the Helix nebula, the closest planetary nebula to Earth at 450 light-years away in the constellation Aquarius. Although ground-based telescopes have revealed such objects, astronomers have never seen so many of them. The most visible knots all lie along the inner edge of the doomed star's ring, trillions of miles away from the star's nucleus. Although these gaseous knots appear small, they're actually huge. Each gaseous head is at least twice the size of our solar system; each tail stretches for 100 billion miles, about 1,000 times the distance between the Earth and the Sun. The image was taken in August 1994 with Hubble's Wide Field Planetary Camera 2. The red light depicts nitrogen emission ([NII] 658.4 nm).

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

## Oxygens

The spectrum shows the lines in the visible due to emission from elemental oxygen. Credit:Teravolt.
This is a spectrum of Ring Nebula (M57) in range 450.0 — 672.0 nm. Credit: Minami Himemiya.

Oxygen (O I) has two red lines at 630.0 and 636.4 nm. In the red there are the atomic oxygen transitions of the "forbidden oxygen red doublet at 6300.304 and 6363.776 Å (1D - 3P)"[46]. Atmospheric O2 has a red line at 686.72 nm.

"The oxygen abundance [may be determined] using the oxygen forbidden line at 630nm"[47]. "[R]atios [of] O/Fe ... are in agreement with the ratios found in the metal-poor red giants, suggesting that no real difference exists between dwarfs and giants."[47]

"The forbidden oxygen line (λ 630.03nm) is weak in dwarf stars"[47]

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).

## Fluorines

This diagram contains the emission and absorption lines for the element fluorine. Credit: Alex Petty.

"Fluorine abundances for red giants of type K, Ba, M, MS, S, SC,N, and J [may be] obtained from the [infrared] rotation-vibration lines of the molecule HF. There appears to be a clear correlation between [F/O] and 12C/16O since N stars display F abundances up to 30 times the solar system value. This correlation points toward the He-burning shell as the site of F synthesis. The nuclear chain 14N(α,γ)18F(β+)18O(p,α)15N(α,γ)19F (where protons come from 13C(α,n)16O followed by 14N(n,p)14C) operating at the very beginning of He-burning is the most likely for 19F production in thermal pulses."[48]

## Neons

This is a visual spectrum of neon showing its many emission lines. Credit: McZusatz.

Neon has many lines across the red.

## Irons

Iron has two emission lines occurring in the solar corona at 637.451 nm from Fe X and 705.962 nm from Fe XV.[49]

## Nickels

Nickel has an emission line occurring in the solar corona at 670.183 nm from Ni XV.[49]

## Rubidiums

This image shows the flame test for Rubidium. Credit: Didaktische.Medien.

Rubidium occurs in the mineral lepidolite and is found through the use of a spectroscope. Because of the bright red lines in its emission spectrum.

## Zirconiums

"Zirconium isotopic abundances [may be] determined from ZrO bandheads near 6925 Å via synthetic spectra for a sample of S stars."[25]

## Alloys

"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."[50]

"The PL efficiencies measured for HAC and Si-HAC alloys are consistent with dust estimates for reflection nebulae and planetary nebulae, but exhibit substantial photoluminescence below 540 nm which is not observed in astrophysical environments."[50]

"Optical constants measured at normal incidence for iron (Bolotin et al., 1969) and for iron-nickel alloys (Sasovskaya and Noskov, 1974) also predict a red-sloped spectrum."[51]

## Atmospheres

"Na D and Hα stellar line profiles for a sample of 19 stars in M55 and M13, [may provide] spectral features indicative of mass motions in the atmospheres of red giants."[52]

"[O]nly stars with a luminosity above log(L/L)≈2.8 show Hα emission."[52]

## Materials

A "series of amorphous and crystalline materials: natural coal, amorphous hydrogenated carbon, amorphous hydrogenated silicon carbide, porous silicon, and crystalline silicon nanoparticles [have been investigated to determine the carrier of the Extended Red Emission (ERE)]."[53]

## Meteorites

"The Raman spectra of some [interplanetary dust particle] IDPs also show red photoluminescence that is similar to the excess red emission seen in some astronomical objects and that has also been attributed to [polycyclic aromatic hydrocarbons] PAH s and hydrogenated amorphous carbon. Moreover, a part of the carbonaceous phase in IDPs and meteorites contains deuterium to hydrogen ratios that are greater than those for terrestrial samples."[54]

## Sun

This is a red image of the Sun taken through a solar telescope. Credit: Totallyhaywire2.

As shown in the image at the top of the resource, the Sun is red when imaged in Hα. The additional image at right here is a red image taken through a solar telescope.

"An earlier flare classification is based on spectral observations. The scheme uses both the intensity and emitting surface. The classification in intensity is qualitative, referring to the flares as: (f)aint, (n)ormal or (b)rilliant. The emitting surface is measured in terms of millionths of the hemisphere and is described below. (The total hemisphere area AH = 6.2 × 1012 km2.)

Classification Corrected Area
[millionths of hemisphere]
S < 100
1 100 - 250
2 250 - 600
3 600 - 1200
4 > 1200

A flare then is classified taking S or a number that represents its size and a letter that represents its peak intensity, e.g.: Sn is a normal subflare.[55]

## Mercury

This is Mercury in real colors, processed from clear and blue filtered Mariner 10 images. Credit: Images processed by Ricardo Nunes.

"A higher-reflectance, relatively red material occurs as a distinct class of smooth plains that were likely emplaced volcanically; a lower-reflectance material with a lesser spectral slope may represent a distinct crustal component enriched in opaque minerals, possibly more common at depth."[56]

"The distinctively red smooth plains (HRP) appear to be large-scale volcanic deposits stratigraphically equivalent to the lunar maria (20), and their spectral properties (steeper spectral slope) are consistent with magma depleted in opaque materials. The large areal extent (>106 km2) of the Caloris HRP is inconsistent with the hypothesis that volcanism was probably shallow and local (10); rather, such volcanism was likely a product of extensive partial melting of the upper mantle."[56]

"Despite the dearth of ferrous iron in silicates, Mercury's surface nonetheless darkens and reddens with time like that of the Moon. This darkening and reddening has been interpreted to be the result of production of nanophase iron (e.g., Pieters et al., 2000; Hapke, 2001), which could be derived from an opaque phase in the crustal material or from delivery by micrometeorite impacts (Noble and Pieters, 2003). On the Moon, deposits that are brighter and redder than the average Moon spectrum appear to be lower in iron (e.g., highland material); deposits that are darker and redder than average are higher in iron (e.g., low-Ti mare material) (Lucey et al., 1995)."[57]

## Venus

This WAC image was taken through a narrow-band filter centered at 630 nanometers. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

"During the MESSENGER mission's second flyby of Venus, the Wide Angle Camera (WAC) of the Mercury Dual Imaging System (MDIS) acquired images through all of its 11 narrow-band color filters of the approaching planet. The surface of Venus is shrouded in clouds, and the WAC images returned from the encounter show this cloud covered view, as seen in a previously released image. However, by processing the WAC images and "stretching" the gray scale used to display the images, subtle differences in the clouds of Venus are revealed, as seen in the image here. This WAC image was taken through a narrow-band filter centered at 630 nanometers, and in this stretched image, global circulation patterns can be seen in the atmosphere of Venus."[58]

## Earth

The Belt of Venus is imaged at 42,000 feet. Credit: George7378.
The Belt of Venus is shown at sunrise, over a horizon where the sea meets the sky, looking west from Twin Peaks, San Francisco. Credit: Brocken Inaglory.
The Belt of Venus is shown at sunset, looking east from the Marin Headlands just north of San Francisco. (Note: there is a thin greyish cloud layer partially obscuring the horizon in this image.) Credit: Brocken Inaglory.
The Full moon is rising, as seen through the Belt of Venus. A very small part of the Earth's shadow (dark blue) is also visible in this image, but the horizon here is too high to see more of the Earth's shadow. Credit: Fir0002.

The Belt of Venus or Venus's Girdle is the Victorian-era name for an atmospheric phenomenon seen at sunrise and sunset. Shortly after sunset or shortly before sunrise, the observer is, or is very nearly, surrounded by a pinkish glow (or anti-twilight arch) that extends roughly 10°–20° above the horizon. Often, the glow is separated from the horizon by a dark layer, the Earth's shadow or "dark segment". The Arch's light rose (pink) color is due to backscattering of reddened light from the rising or setting Sun.

The first image at left shows the Belt of Venus from 42,000 feet altitude locally above the Earth's surface.

In the image at right is the Belt of Venus, a pink band that is visible above the dark blue of the Earth's shadow, in the same part of the sky. No defined line divides the Earth's shadow and the Belt of Venus; one colored band blends into the other in the sky.

In the image at second left, the Belt of Venus is shown at sunset, looking east from the Marin Headlands just north of San Francisco. There is a thin greyish cloud layer partially obscuring the horizon in this image.

The second image at right shows the full Moon rising as seen through the Belt of Venus.

In the right viewing conditions, a pink (or orange or purple) band is visible in the twilight sky just above the dark blue band of the Earth's shadow. This pink band is called the "anti-twilight arch" or "Belt of Venus". The name "Belt of Venus" is not connected with the planet Venus; the Belt of Venus is part of Earth's upper atmosphere which is illuminated by the setting or rising sun. It is visible either after the sun ceases to be visible (at sunset) or before the sun becomes visible (at sunrise).[59][60]

When the sun is near the horizon at sunset or sunrise, the light from the sun is red; this is because the light is reaching the observer through an especially thick layer of the atmosphere, which works as a filter, scattering all but the red light.

From the viewpoint of the observer, the red sunlight directly illuminates small particles in the lower atmosphere on the other side of the sky from the sun. The red light is backscattered to the observer, and that is why the Belt of Venus appears pink.

The lower the sunset sun descends, the less clearly distinguished the boundary between the Earth's shadow and Belt of Venus becomes. This is because now the setting sun illuminates a thinner part of the upper atmosphere. The red light is not scattered there because there are fewer particles, and the eye only sees the "normal" (usual) blue sky, which is due to Rayleigh scattering from air molecules. Eventually, both the Earth's shadow and the Belt of Venus dissolve into the darkness of the night sky.[60]

## Moon

A total lunar eclipse on February 9, 2009, shows the reddish light falling on the moon's surface. Credit: John Buonomo. {{free media}}
This view of the Moon was taken through a narrowband red filter (H-alpha). Credit: ESO and Andy Strappazzon.

During a lunar eclipse, a very small amount of light from the sun does however still reach the Moon, even when the lunar eclipse is total; this is light which has been refracted or bent as it passes through the Earth's atmosphere. This sunlight has been scattered by the dust in the Earth's atmosphere, and thus that light is red, in the same way that sunset and sunrise light is red. This weak red illumination is what causes the eclipsed Moon to be dimly reddish or copper-colored in appearance.[61]

"This image of the crescent Moon [at left] shows sunlight skimming across the heavily pocked surface, filling its craters with shadows. This is a fairly flat region of the Moon, but elsewhere, high mountains can be found, with some peaks reaching about 5000 metres. When backlit by the Sun, these mountains cast long shadows on the lunar surface."[62]

"This view of the Moon was taken through a narrowband red filter (H-alpha). The height of the image is about 30 arcminutes."[62]

"The MPG/ESO 2.2-metre telescope at La Silla in Chile is a powerful instrument that can capture distant celestial objects, but it has been used here to image a heavenly body that is much closer to home: the Moon."[62]

## Mars

The tenuous atmosphere of Mars is visible on the horizon in this low-orbit photo. Credit: NASA.
Mars is imaged from Hubble Space Telescope on October 28, 2005, with dust storm visible. Credit: NASA, ESA, The Hubble Heritage Team (STScI/AURA), J. Bell (Cornell University) and M. Wolff (Space Science Institute).

Mars is the fourth planet from the Sun in the Solar System. Named after the Roman god of war, Mars, it is often described as the "Red Planet" as the iron oxide prevalent on its surface gives it a reddish appearance.[63] The red-orange appearance of the Martian surface is caused by iron(III) oxide, more commonly known as hematite, or rust.[64] Much of the surface is deeply covered by finely grained iron(III) oxide dust.[65][66]

## Ceres

"Over five hours of observations 15 spectra of asteroid [Ceres] with step 2.46 nm in the region 336 - 746 nm were obtained to see whether the spectral characteristics of the Ceres surface changed with the rotation phase. On processing the observational material, it turned out that the spectra of the asteroid are unusually red (i.e., a remarkable rise of relative reflection coefficients with wave length growth occurs)."[67]

## Jupiter

An infrared image of GRS (top) shows its warm center, taken by the ground based Very Large Telescope. An image made by the Hubble Space Telescope (bottom) is shown for comparison. Credit: NASA/JPL/ESO and NASA/ESA/GSFC.

The Great Red Spot (GRS) is a persistent anticyclonic storm, 22° south of Jupiter's equator, which has lasted for at least 191 years and possibly longer than 356 years.[68][69] The storm is large enough to be visible through Earth-based telescopes. Its dimensions are 24–40,000 km west–to–east and 12–14,000 km south–to–north. The spot is large enough to contain two or three planets the size of Earth. At the start of 2004, the Great Red Spot had approximately half the longitudinal extent it had a century ago, when it was 40,000 km in diameter. The Great Red Spot's latitude has been stable for the duration of good observational records, typically varying by about a degree.

It is not known exactly what causes the Great Red Spot's reddish color. Theories supported by laboratory experiments suppose that the color may be caused by complex organic molecules, red phosphorus, or yet another sulfur compound. The Great Red Spot (GRS) varies greatly in hue, from almost brick-red to pale salmon, or even white. The reddest central region is slightly warmer than the surroundings, which is the first evidence that the Spot's color is affected by environmental factors.[70] The spot occasionally disappears from the visible spectrum, becoming evident only through the Red Spot Hollow, which is its niche in the South Equatorial Belt. The visibility of GRS is apparently coupled to the appearance of the SEB; when the belt is bright white, the spot tends to be dark, and when it is dark, the spot is usually light. The periods when the spot is dark or light occur at irregular intervals; as of 1997, during the preceding 50 years, the spot was darkest in the periods 1961–66, 1968–75, 1989–90, and 1992–93.[71]

## Comets

Armada of "mini-comets" is left behind by Comet LINEAR as observed by the Hubble Space Telescope and the 2.2-meter telescope in Hawaii. Credit: NASA, University of Hawaii, H. Weaver (John Hopkins University).
Visual photograph of Comet West in early March 1976 shows red gases coming off the comet's head. Credit: Peter Stättmayer (Munich Public Observatory) and ESO.

"My first thought was Hubble Space Telescope does it again! We caught the fish! This is amazing, very exciting, very neat."[72]

"Actually, I would have been more amazed if Hubble saw no pieces ... They just had to be there. The amount of heat available from sunlight just isn't enough to boil away something the size of a mountain in so short a time".[73]

"On July 27th, ground-based observers had lost sight of the bright core of the comet and were suggesting that the nucleus had totally disintegrated into a pile of dust. ... On Weaver's screen was at least a half dozen "mini-comets" with tails, resembling the shower of glowing fireballs from an aerial firework. They are clustered in the lance-head tip of an elongated stream of dust seen from a ground-based telescope."[74]

At left is an image of Comet West. "Comet West was a stunning sight in the predawn sky of March, 1976, bright with a tall and broad dust tail. ... [T]he comet [was] discovered on photographs taken in August 1975 by Richard West of the European Southern Observatory ... Comet West passed perihelion on February 25, 1976, at a distance of 0.20 a.u. [and] had reached about magnitude -3 at perihelion. Several observers saw it telescopically in daylight, and John Bortle observed it with the naked eye shortly before sunset. ... The following morning, March 7, ... It was brilliant, with a head as bright as Vega (which was nearly overhead) and a huge tail, about 20 degrees tall, straight near the bottom and bending to the left in its upper reaches. The comet quickly faded during March".[75]

"The λλ6300, 6363 Auroral red doublet of [OI] has been measured on digital sky-subtracted spectra of nine cometary nuclei ... The cometary oxygen lines are confined to their nuclear source, so that small apertures include much of the oxygen emission, particularly for small comets with Δ ≳ 1.0 AU."[76]

## Sirius

Around 150 AD, the Hellenistic astronomer Claudius Ptolemy described Sirius as reddish, along with five other stars, Betelgeuse, Antares, Aldebaran, Arcturus and Pollux, all of which are clearly of orange or red hue.[77] The discrepancy was first noted by amateur astronomer Thomas Barker, ... who prepared a paper and spoke at a meeting of the Royal Society in London in 1760.[78] The existence of other stars changing in brightness gave credence to the idea that some may change in colour too; Sir John Herschel noted this in 1839, possibly influenced by witnessing Eta Carinae two years earlier.[77] Thomas Jefferson Jackson See resurrected discussion on red Sirius with the publication of several papers in 1892, and a final summary in 1926.[77] He cited not only Ptolemy but also the poet Aratus, the orator Cicero, and general Germanicus as colouring the star red, though acknowledging that none of the latter three authors were astronomers, the last two merely translating Aratus' poem Phaenomena.[77] Seneca, too, had described Sirius as being of a deeper red colour than Mars.[79] However, not all ancient observers saw Sirius as red. The 1st century AD poet Marcus Manilius described it as "sea-blue", as did the 4th century Avienus.[77] It is the standard star for the color white in ancient China, and multiple records from the 2nd century BC up to the 7th century AD all describe Sirius as white in hue.[80][81]

In 1985, German astronomers Wolfhard Schlosser and Werner Bergmann published an account of an 8th century Lombardic manuscript, which contains De cursu stellarum ratio by St. Gregory of Tours. The Latin text taught readers how to determine the times of nighttime prayers from positions of the stars, and Sirius is described within as rubeola — "reddish". The authors proposed this was further evidence Sirius B had been a red giant at the time.[82]

## Luminous red novas

V838 Monocerotis in this real visual image from the Hubble Space Telescope is a prototypic luminous red nova. Credit: NASA, ESA and H.E. Bond (STScI).

A luminous red nova (abbr. LRN, pl. luminous red novae, pl.abbr. LRNe) is a stellar explosion thought to be caused by the merger of two stars. They are characterised by a distinct red colour, and a light curve that lingers with resurgent brightness in the infrared. Luminous red novae are not to be confused with standard novae, explosions that occur on the surface of white dwarf stars. The visible light lasts for weeks or months, and is distinctively red in colour, becoming dimmer and redder over time. As the visible light dims, the infrared light grows and also lasts for an extended period of time, usually dimming and brightening a number of times. Some astronomers believe it to be premature to declare a new class of stellar explosions based on such a limited number of observations. For instance, Pastorello et al. 2007[83] explained that the event may be due to a type II-p supernova and Todd et al. 2008[84] pointed out that supernovae undergoing a high level of extinction will naturally be both red and of low luminosity.

## Red dwarfs

This is a real visual image of AZ Cancri. Credit: SDSS Data Release 6.

A red dwarf is a small and relatively cool star on the main sequence, either late K or M spectral type. Red dwarfs are by far the most common type of star in the Galaxy, at least in the neighborhood of the Sun. Proxima Centauri, the nearest star to the Sun, is a red dwarf. Due to their low luminosity, individual red dwarfs cannot easily be observed. From Earth, none are visible to the naked eye.[85]

Typical characteristics[86]
Stellar
Class
Mass
(Mʘ)
(Rʘ)
Luminosity
(Lʘ)
Teff
(K)
M0V 60% 62% 7.2% 3,800
M1V 49% 49% 3.5% 3,600
M2V 44% 44% 2.3% 3,400
M3V 36% 39% 1.5% 3,250
M4V 20% 26% 0.55% 3,100
M5V 14% 20% 0.22% 2,800
M6V 10% 15% 0.09% 2,600
M7V 9% 12% 0.05% 2,500
M8V 8% 11% 0.03% 2,400
M9V 7.5% 8% 0.015% 2,300

The red dwarf AZ Cancri is shown in the visual image at right.

"[O]ut [of] a sample of 3,897 red dwarfs ... [the Kepler Space Telescope ]has identified 95 exoplanet candidates circling them. Three of these candidates are roughly Earth-size and orbit within their stars' "Goldilocks zone," where liquid water (and possibly life as we know it) can exist."[87]

## Red giants

This is a real visual image of the red giant Mira by the Hubble Space Telescope. Credit: Margarita Karovska (Harvard-Smithsonian Center for Astrophysics) and NASA.

A red giant is a luminous giant star The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower. The appearance of the red giant is from yellow orange to red, including the spectral types K and M, but also class S stars and most carbon stars. The most common red giants are the so-called red giant branch stars (RGB stars). Another case of red giants are the asymptotic giant branch stars (AGB). To the AGB stars belong the carbon stars of type C-N and late C-R. The stellar limb of a red giant is not sharply-defined, as depicted in many illustrations. Instead, due to the very low mass density of the envelope, such stars lack a well-defined photosphere. The body of the star gradually transitions into a 'corona' with increasing radii.[88]

## Red supergiants

"Red supergiants (RSGs) are supergiant stars (luminosity class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive. Betelgeuse and Antares are the best known examples of a red supergiant. These stars have very cool surface temperatures (3500–4500 K), and enormous radii. The five largest known red supergiants in the Galaxy are VY Canis Majoris, VV Cephei A, V354 Cephei, RW Cephei and KW Sagittarii, which all have radii about 1500 times that of the [S]un (about 7 astronomical units, or 7 times as far as the Earth is from the [S]un). The radius of most red giants is between 200 and 800 times that of the Sun. Absolute luminosities may reach -10 magnitude compared to +5 for our Sun.

## Red clumps

This Hertzsprung-Russell diagram shows the evolution of stars of different masses. The red clump is marked RC on the green line showing the evolution of a star of 2 solar masses. Credit: Jesusmaiz and Rursus.

The red clump is a feature in the Hertzsprung-Russell diagram of stars. The red clump is considered the metal-rich counterpart to the horizontal branch. Stars in this part of the Hertzsprung-Russell diagram are sometimes called clump giants. These stars are more luminous than main sequence stars of the same surface temperature (or colder than main sequence stars of comparable luminosity), or above and to the right of the main sequence on the Hertzsprung-Russell diagram.

## Tip of the red giant branch

Tip of the red-giant branch (TRGB) is a primary distance indicator used in astronomy. It uses the luminosity of the brightest red giant branch stars in a galaxy to gauge the distance to that galaxy. It has been used in conjunction with observations from the Hubble Space Telescope to determine the relative motions of the Local Cluster of galaxies within the Local Supercluster. ... [There] is a sharp discontinuity in the evolutionary track of the star on the HR diagram.[89] This discontinuity is called the tip of the red giant branch. When distant stars at the TRGB are measured in the I-band, their magnitude is somewhat insensitive to their composition of elements with more mass than helium (metallicity) and their mass. This makes the technique especially useful as a distance indicator. The TRGB indicator uses stars in the old stellar populations (Population II).[90]

## Interstellar reddening

In interstellar astronomy, visible spectra can appear redder due to scattering processes in a phenomenon referred to as interstellar reddening[91] — 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.

## Star-forming regions

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

"The gas in the clouds of NGC 6559, mainly hydrogen, is the raw material for star formation. When a region inside this nebula gathers enough matter, it starts to collapse under its own gravity. The center of the cloud grows ever denser and hotter, until thermonuclear fusion begins and a star is born. The hydrogen atoms combine to form helium atoms, releasing energy that makes the star shine. In regions where it is very dense, the dust completely blocks the light behind it, as is the case for the dark isolated patches and sinuous lanes to the bottom left-hand side and right-hand side of the image".[92]

"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."[93]

"The two colors of the cloud represent a pair of nebulas. Once the young stars are born, they "energize" the hydrogen surrounding them, ESO officials said. The gas then creates the red wispy cloud — known to astronomers as an "emission nebula" — in the center of the image."[94]

"These young stars are usually of spectral type O and B, with temperatures between 10 000 and 60 000 K, which radiate huge amounts of high energy ultraviolet light that ionises the hydrogen atoms."[95]

"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."[94]

NGC 6559 is planetary nebula located at a distance of about 5000 light-years from Earth, in the constellation of Sagittarius.

"NGC 6559 is a cloud of gas and dust located at a distance of about 5000 light-years from Earth, in the constellation of Sagittarius (The Archer). The glowing region is a relatively small object, just a few light-years across, in contrast to the one hundred light-years and more spanned by its famous neighbour, the Lagoon Nebula (Messier 8, eso0936). Although it is usually overlooked in favour of its distinguished companion, NGC 6559 has the leading role in this new picture."[95]

"The Milky Way fills the background of the image with countless yellowish older stars. Some of them appear fainter and redder because of the dust in NGC 6559."[95]

"This eye-catching image of star formation was captured by the Danish Faint Object Spectrograph and Camera (DFOSC)".[95]

## BL Lacertae objects

This is an image of H 0323+022 using the red (R) filter. Credit: Renato Falomo, ESO NTT.

QSO B0323+022 is a BL Lacertae object. The image at right is taken with the ESO NTT using the R filter.

## Starburst galaxy

This mosaic image taken by the Hubble Space Telescope of Messier 82 combines exposures taken with four colored filters that capture starlight from visible and infrared wavelengths as well as the light from the glowing hydrogen filaments. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).

"The presence of ERE has been established spectroscopically in ... the starburst galaxy M82 (Perrin, Darbon, & Sivan 1995)."[17]

"This mosaic image [at right] is the sharpest wide-angle view ever obtained of M82. The galaxy is remarkable for its bright blue disk, webs of shredded clouds, and fiery-looking plumes of glowing hydrogen blasting out of its central regions."[96]

"Throughout the galaxy's center, young stars are being born 10 times faster than they are inside our entire Milky Way Galaxy. The resulting huge concentration of young stars carved into the gas and dust at the galaxy's center. The fierce galactic superwind generated from these stars compresses enough gas to make millions of more stars."[96]

"In M82, young stars are crammed into tiny but massive star clusters. These, in turn, congregate by the dozens to make the bright patches, or "starburst clumps," in the central parts of M82. The clusters in the clumps can only be distinguished in the sharp Hubble images. Most of the pale, white objects sprinkled around the body of M82 that look like fuzzy stars are actually individual star clusters about 20 light-years across and contain up to a million stars."[96]

"The rapid rate of star formation in this galaxy eventually will be self-limiting. When star formation becomes too vigorous, it will consume or destroy the material needed to make more stars. The starburst then will subside, probably in a few tens of millions of years."[96]

"Located 12 million light-years away, M82 appears high in the northern spring sky in the direction of the constellation Ursa Major, the Great Bear. It is also called the "Cigar Galaxy" because of the elliptical shape produced by the oblique tilt of its starry disk relative to our line of sight."[96]

"The observation was made in March 2006, with the Advanced Camera for Surveys' Wide Field Channel. Astronomers assembled this six-image composite mosaic by combining exposures taken with four colored filters that capture starlight from visible and infrared wavelengths as well as the light from the glowing hydrogen filaments."[96]

## Red shifts

Absorption lines in the visible spectrum of a supercluster of distant galaxies (right), are compared to absorption lines in the visible spectrum of the Sun (left). Arrows indicate redshift. Wavelength increases up towards the red and beyond (frequency decreases). Credit: Dr. Schorsch and Kes47.

"Ideally all intrinsic colours should be found from unreddened stars. This is possible for dwarf and giant stars later than about A0 (Johnson, 1964) ... However, it cannot be used for stars of other spectral classes since they are all relatively infrequent in space, and generally reddened."[97]

Redshift happens when light seen coming from an object that is moving away is proportionally increased in wavelength, or shifted to the red end of the visible spectrum. More generally, where an observer detects electromagnetic radiation outside the visible spectrum, "redder" amounts to a technical shorthand for "increase in electromagnetic wavelength" — which also implies lower frequency and photon energy in accord with, respectively, the wave and quantum theories of light. Redshifts are attributable to the Doppler effect, familiar in the changes in the apparent pitches of sirens and frequency of the sound waves emitted by speeding vehicles; an observed redshift due to the Doppler effect occurs whenever a light source moves away from an observer.

## Cosmological redshifts

Cosmological redshift is seen due to the expansion of the universe, and sufficiently distant light sources (generally more than a few million light years away) show redshift corresponding to the rate of increase of their distance from Earth.

## Gravitational redshifts

Gravitational redshifts are a relativistic effect observed in electromagnetic radiation moving out of gravitational fields.

## Blue shifts

A decrease in wavelength is called blueshift and is generally seen when a light-emitting object moves toward an observer or when electromagnetic radiation moves into a gravitational field.

## Astrochemistry

"Electron microprobe analyses indicate that the Apollo 15 red glasses with 13.8 wt.% TiO2 were produced in a volcanic fire-fountain. They are composed of three chemical groups connected by a pronounced trend. The liquidus phase relations are compatible with the view that the most Mg-rich group (A) was produced by partial melting of Ti-rich cumulates at a depth of about 480 km. The other red glasses (B and C) may have been derived from the group A magma by fractional crystallization at pressures less than 5 kbar."[98]

## Geography

"Providing very high geometrical resolution and position accuracy and with four multispectral bands, HRSC-AX data delivered even better biotope mapping results than the analysis of aerial photographs. According to Ehlers etal.(2003), the red band, near infrared band and particularly the NDVI ((nIRR)/(nIR–R)) were useful to distinguish vegetation from non-vegetation and to differentiate vegetation types."[99]

"The HRSC sensor has an enormous potential in vegetation mapping at small scales, particularly the HRSC-AX with its red band introduced for terrestrial observations. ... The HRSC-AX camera is recommended for applications in biogeography."[99]

The "Hasselo stadial [is] at approximately 40-38,500 14C years B.P. (Van Huissteden, 1990)."[100]

The "Hasselo Stadial [is a glacial advance] (44–39 ka ago)".[101]

The "earliest known astronomy anywhere in the world [is] that of the Australian Aborigines, whose culture has existed for some 40,000 years".[102]

"The Aranda tribes of Central Australia, for example, distinguish red stars from white, blue and yellow stars."[102]

## Astrophysics

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.

## Catalogs

In 1866, after the new observatory had been completed, Schjellerup assembled a catalog of red stars.

The Perkins Observatory catalog includes "the full subclasses used are the following: G0, G5, G8, K0, K1, K2, K3, K4, K5, M0, M1, M2, M3, M4, M5, M6, M7, and M8."[103]

## Technology

The High Resolution Stereo Camera HRSC-AX150 has the red band of 635-685 nm, and the HRSC-AX047 has 570-680 nm.[99]

## Hypotheses

1. The color astronomies have special values unique to each.

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