Neptune's south pole is photographed by Voyager 2. Credit: NASA.

Blue astronomy is focused on the wavelength range 450–475 nm. This may involve emission, absorption, transmission, and reflection.

Stars are often referred to by their predominant color. For example, blue stragglers are found among the galactic halo globular clusters.[1] Blue main sequence stars, that are metal poor, ([Fe/H] ≤ -1.0) are most likely not analogous to blue stragglers.[1]

"[G]round-based UV [and blue astronomy] is a powerful facility for [the] study of [the] chemical evolution of [the] early Galaxy."[2] The UV and B astronomy are over the wavelength range 355.0–500.0 nm.[2]

Also, a topic of blue astronomy is the blueshift change in wavelength.

## Astronomy

Main source: Astronomy

When any effort to acquire a system of laws or knowledge focusing on an astr, aster, or astro, that is, any natural body in the sky especially at night,[3] discovers an entity emitting, absorbing, transmitting, reflecting, or fluorescing blue, succeeds even in its smallest measurement, blue astronomy is the name of the effort and the result. Once an entity, source, or object has been detected as emitting, absorbing, transmitting, reflecting, or fluorescing blue, it may be necessary to determine what the mechanism is. Usually this information provides understanding of the same entity, source, or object.

Blue is a color that suggests the sense of sight. Most people associate astronomy with the sense of seeing, what can be termed visual astronomy. As telescope optics transmit blue well, blue astronomy is also a field within optical astronomy.

Blue is between violet and green in the spectrum of visible light. Credit: Gringer.

Def. of the higher-frequency region of the part of the electromagnetic spectrum which is relevant in the specific observation is called blue.

Def. the colour of the clear sky or the deep sea, between green and violet in the visible spectrum, and one of the primary additive colours for transmitted light; the colour obtained by subtracting red and green from white light using magenta and cyan filters; or any colour resembling this is called blue.

## Planetary sciences

The Earth's shadow at sunrise is seen over a horizon where the sea meets the sky, looking west from Twin Peaks, San Francisco. Note: the lowest blue-grey area is not the sky but the surface of the Pacific Ocean. Credit: .
Earth's shadow and Belt of Venus 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: .

The Earth's shadow or Earth shadow (also sometimes known as the dark segment) are names for the shadow that the Earth itself casts on its atmosphere. This shadow is often visible from the surface of the Earth, as a dark band in the sky near the horizon. This atmospheric phenomenon can sometimes be seen twice a day, around the times of sunset and sunrise.

The effect of the Earth's shadow on the atmosphere is quite often visible in the sky, and yet often goes unrecognized. This shadow is visible to observers as it falls on the atmosphere of the Earth during the twilight hours. When the weather conditions and the observer's viewing point permit a clear sight of the horizon, the shadow can be seen as a dark blue or greyish-blue band.

Assuming the sky is clear, the Earth's shadow is visible in the opposite half of the sky to the sunset or sunrise, and is seen right above the horizon as a dark blue band.

In the image at right, the Earth's shadow at sunrise is seen over a horizon where the sea meets the sky, looking west from Twin Peaks, San Francisco. The lowest blue-grey area is not the sky but the surface of the Pacific Ocean.

At left is the Earth's shadow 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 Earth's shadow (as it is cast onto the atmosphere) can be observed during the twilight hours, assuming the sky is clear and the horizon is relatively unobstructed. At sunset the Earth's shadow is visible opposite the sunset in the eastern sky, just above the horizon. The shadow shows as a dark blue band that stretches over 180° of the horizon.[4][5] It is most noticeable at the antisolar point, exactly opposite the sunset.

"At sunrise, the Earth's shadow is seen in a similar way, but in the western sky. The Earth's shadow is best observed when there is a low horizon (such as over the sea), and when the sky conditions are very clear. In addition, the higher up an observer is standing to view the horizon, the sharper the shadow appears.[4][5]

At sunrise, the Earth's shadow can be seen to set as the sun itself rises, and at sunset, the Earth's shadow rises as the sun sets.[4]

## Skies

Main sources: Astronomy/Skies and Skies
A spectrum is taken of blue sky clearly showing solar Fraunhofer lines and atmospheric water absorption band. Credit: .

"A solar tracker is a device that orients a payload toward the sun.

Sunlight has two components, the "direct beam" that carries about 90% of the solar energy, and the "diffuse sunlight" that carries the remainder - the diffuse portion is the blue sky on a clear day and increases proportionately on cloudy days. As the majority of the energy is in the direct beam, maximizing collection requires the sun to be visible to the panels as long as possible.

"Diffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the colour of the sky. Of the total light removed from the direct solar beam by scattering in the atmosphere (approximately 25% of the incident radiation when the sun is high in the sky, depending on the amount of dust and haze in the atmosphere), about two-thirds ultimately reaches the earth as diffuse sky radiation.

"[P]referential absorption of sunlight by ozone over long horizon paths gives the zenith sky its blueness when the sun is near the horizon".[6]

## Colors

These are various shades of blue. Credit: Booyabazooka.
Blue, green and red are additive colors. All the colors you see on your computer screen are made by mixing them in different intensities. Credit: Bb3cxv.

Blue is the colour of the clear sky and the deep sea.[7]

"De la couleur du ciel sans nuages, de l'azur"[8]

## Blue minerals

Main sources: Minerals/Blues and Blue minerals

Often a mineral appears blue due to the presence of copper or sulfur. Glaucophane is a blue silicate that owes its color to its characteristic formation.

### Sodalite

A sample of sodalite-carbonate pegmatite from Bolivia has a polished rock surface. Credit: .

Sodalite is a rich royal blue mineral massive sodalite samples are opaque, crystals are usually transparent to translucent. Occurring typically in massive form, sodalite is found as vein fillings in plutonic igneous rocks such as nepheline syenites.

### Covellite

This covellite specimen is from the Black Forest of Germany. Credit: .

Covellite has been found in veins at depths of 1,150 meters, as the primary mineral. Covellite formed as clusters in these veins reaching one meter across.

### Lazurite

Lazurite is a deep blue tectosilicate. Credit: .

Lazurite is a tectosilicate mineral with sulfate, sulfur and chloride with formula: (Na,Ca)8[(S,Cl,SO4,OH)2|(Al6Si6O24)]. It is a feldspathoid and a member of the sodalite group. The colour is due to the presence of S3- anions. Lazurite is a product of contact metamorphism of limestone.

### Blueschist facies

This blueschist example is from Ile de Groix, France. Credit: .
 Eclogite Blueschist Greenschist Prehnite- Pumpellyite Zeolite Granulite Amphibolite Hornfels Sanidinite P (kbar) T (°C) 0 100 200 300 400 500 600 700 800 900 1000 0 2 4 6 8 10 12 14 16 18 20 Diagram showing metamorphic facies in pressure-temperature space. The domain of the graph corresponds to circumstances within the Earth's crust and upper mantle.

A metamorphic facies is a set of metamorphic mineral assemblages that were formed under similar pressures and temperatures.[9] The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure (See diagram at right).[9] Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in geological history of the area.[9] The boundaries between facies (and corresponding areas on the temperature v. pressure graph), are wide, because they are gradational and approximate.[9] The area on the graph corresponding to rock formation at the lowest values of temperature and pressure, is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.[9]

Blueschist is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15 to 30 kilometers and 200 to ~500 degrees Celsius. The blue color of the rock comes from the presence of the mineral glaucophane. Blueschists are typically found within orogenic belts as terranes of lithology in faulted contact with greenschist or rarely eclogite facies rocks. ... Blueschist, as a rock type, is defined by the presence of the minerals glaucophane + ( lawsonite or epidote ) +/- jadeite +/- albite or chlorite +/- garnet +/- muscovite in a rock of roughly basaltic composition. Blueschist often has a lepidoblastic, nematoblastic or schistose rock microstructure defined primarily by chlorite, phengitic white mica, glaucophane, and other minerals with an elongate or platy shape. Grain size is rarely coarse, as mineral growth is retarded by the swiftness of the rock's metamorphic trajectory and perhaps more importantly, the low temperatures of metamorphism and in many cases the anhydrous state of the basalts. However, coarse varieties do occur. Blueschists may appear blue, black, gray, or blue-green in outcrop.

### Glaucophane

This is a specimen of glaucophane with fuchsite. Credit: .

Glaucophane is a mineral belonging to the amphibole group, chemical formula Na2Mg3Al2Si8O22(OH)2. The blue color is very diagnostic for this species. It, along with the closely related mineral riebeckite are the only common amphibole minerals that are typically blue. Glaucophane forms in metamorphic rocks that are either particularly rich in sodium or that have experienced low temperature-high pressure metamorphism such as would occur along a subduction zone. This material has undergone intense pressure and moderate heat as it was subducted downward toward the mantle. It is glaucophane's color that gives the blueschist facies its name. Glaucophane is also found in eclogites that have undergone retrograde metamorphism.[10]

### Hauyne

This is a specimen of Haüyne on augite from the Somma-Vesuvius Complex, Naples Province, Italy. Credit: .

Hauyne, haüyne or hauynite [occurs] in Vesuvian lavas in Monte Somma, Italy,[11] ... It is a tectosilicate mineral with sulfate, with endmember formula Na3Ca(Si3Al3)O12(SO4).[12] ... It is a feldspathoid and a member of the sodalite group.[13][14] Haüyne occurs in phonolites and related leucite- or nepheline-rich, silica-poor, igneous rocks; less commonly in nepheline-free extrusives[15][13][14][16] and metamorphic rocks (marble).[13]

### Water ice

This image shows the blue water ice, or blue ice, of a glacier. Credit: .

Blue ice occurs when snow falls on a glacier, is compressed, and becomes part of a glacier blue ice was observed in Tasman Glacier, New Zealand in January 2011.[17] Ice is blue for the same reason water is blue: it is a result of an overtone of an oxygen-hydrogen (O-H) bond stretch in water which absorbs light at the red end of the visible spectrum.[18]

## Theoretical blue-ray astronomy

"Geologic evidence suggests that large amounts of water have likely flowed on Earth for the past 3.8 billion years—most of its existence. [It is] [b]elieved to have initially arrived on the surface through the emissions of ancient volcanoes".[19]

## Entities

After the Amarna period, Amun was painted with blue skin, symbolizing his association with air and primeval creation. Credit: Jeff Dahl.
Vishnu, the supreme god of Hinduism, is often portrayed as being blue, or more precisely having skin the colour of rain-filled clouds. Credit: Leon Meerson.
This is a pre-Columbian image of Huitzilopochtli the patron god of the Mexica tribe. Credit: Giggette.
Bhagwan Ayodhyapati Siyavara Shri Ramachandra is shown. Credit: Redtigerxyz.

In Egypt, blue was associated with the sky and with divinity. The Egyptian god Amun could make his skin blue so that he could fly, invisible, across the sky. Blue could also protect against evil; many people around the Mediterranean still wear a blue amulet, representing the eye of God, to protect them from misfortune.[20]

Many of the gods in Hinduism are depicted as having blue-coloured skin, particularly those associated with Vishnu, who is said to be the Preserver of the world and thus intimately connected to water. Krishna and Ram, Vishnu's avatars, are usually blue. Shiva, the Destroyer, is also depicted in light blue tones and is called neela kantha, or blue-throated, for having swallowed poison in an attempt to turn the tide of a battle between the gods and demons in the gods' favour.

Huitzilopochtli [at second right] was the patron god of the Mexica tribe. Originally he was of little importance to the Nahuas, but after the rise of the Aztecs, Tlacaelel reformed their religion and put Huitzilopochtli at the same level as Quetzalcoatl, Tlaloc, and Tezcatlipoca, making him a solar god.

Over the South presides the Blue Tezcatlipoca, Huitzilopochtli, the god of war.

Rama imaged at left is the Daśāvatāra seventh avatar of the god Vishnu in Hinduism,[21] and a king of Ayodhya in Hindu scriptures. In a few Rama-centric sects, Rama is considered the Supreme Being, rather than an avatar. Rama was born in Suryavansha (Ikshvaku Vansham) later known as Raghuvansha after king Raghu. When depicted with his brother Lakshman and consort Sita, with Hanuman kneeling in a state of prayer, this form is called Ram Parivar, and is the typical fixture depicting Rama in Hindu mandirs, or temples. [22] The Hindi word parivar translates as "family." [23]

## Sources

This image shows a beam of accelerated ions (perhaps protons or deuterons) escaping the accelerator and ionizing the surrounding air causing a blue glow. Credit: Lawrence Berkely National Laboratory.
This image shows the blue glow generated by the synchrotron radiation from M87's Energetic Jet. Credit: Hubble Space Telescope.
The image shows the blue glow given off by the synchrotron beam from the National Synchrotron Light Source. Credit: NSLS, Brookhaven National Laboratory.

The image above shows a blue glow in the surrounding air from emitted cyclotron particulate radiation.

The image at right shows the blue light, towards the lower right, due to synchrotron radiation, of the jet emerging from the bright active galactic nucleus (AGN) core of Messier 87.

At left is an image that shows the blue glow resulting from a beam of relativistic electrons as they slow down. This deceleration produces synchrotron light out of the beam line of the National Synchrotron Light Source.

## Objects

The Edinburgh-Cape Blue Object Survey is an astronomical catalog included in the list of astronomical catalogues. These catalogs are lists or tabulations of astronomical objects. They are grouped together because they share a common type, morphology, origin, means of detection, or method of discovery.

## Continua

This is a Hubble Space Telescope image of the Crab Nebula showing the diffuse blue region. Credit: NASA, ESA, J. Hester and A. Loll (Arizona State University).

"[T]he diffuse blue region is predominantly produced by synchrotron radiation, which is radiation given off by the curving motion of electrons in a magnetic field. The radiation corresponded to electrons moving at speeds up to half the speed of light."[24]

A synchrotron model for the continuum spectrum of the Crab Nebula fits the radiation given off.[25]

## Backgrounds

"The light blue background is the dayglow emission (less than 1 kR) caused by the interaction between the photoelectrons generated by solar UV radiation and atmospheric molecules and atoms."[26] This background occurs when imaging an Earth aurora from space using ultraviolet astronomy at the VUV wavelengths (135.6 ± 1.5 nm and 149.3 ± 1.5 nm).

"Isophotes of [blue (395.0-485.0 nm)] background starlight brightness (integrated starlight, diffuse galactic light, cosmic light) have been constructed near the celestial [NCP and SCP], ecliptic [NEP and SEP], and galactic [NGP and SGP] poles from observations accumulated by the Pioneer 10 [and 11] imaging photopolarimeter from beyond the asteroid belt."[27]

"Diffuse galactic [blue] light refers to starlight scattered by interstellar grains. Integrated light from extragalactic sources is referred to as cosmic light."[27]

"Earth-based observers are limited in their perception of background starlight, due to the presence of airglow line and continuum emission, zodiacal light, and the effects of atmospheric extinction and scattering."[27]

Some of the blue background data were collected on September 30, 1973 (day/year 279/73).[27]

"To determine the brightnesses, isophotes were generated from six days of merged data, chosen to optimize sky coverage. The days used and their heliocentric distances in AU are: 354/72 (3.27), 149/73 (4.23), 237/73 (4.64), 279/73 (4.81), 21/74 (5.08), 68/74 (5.15)."[27]

Conversion of units over the Pioneer blue (B) bandpass is[27]

${\displaystyle 1S_{10}(V)_{B}=1.16x10^{-8}ergcm^{-2}s^{-1}sr^{-1}nm^{-1}.}$

For example,

NCP ${\displaystyle S_{10}(V)_{B}=56,}$ NEP ${\displaystyle S_{10}(V)_{B}=66,}$ and NGP ${\displaystyle S_{10}(V)_{B}=31.}$[27]

Whereas,

SCP ${\displaystyle S_{10}(V)_{B}=74,}$ SEP ${\displaystyle S_{10}(V)_{B}=128,}$ and SGP ${\displaystyle S_{10}(V)_{B}=26.}$[27]

## Bands

"Extreme flocculent galaxies (e.g., NGC 7793) have very little structure in their old stellar population disks. Most of the structure seen in blue [B passband, λeff ≈ 436.0 nm, using baked 103a-O emulsions behind GG 385 filters] phtographs of these galaxies is from star formation or from very weak stellar density ripples. ... Intermediate-type spiral galaxies (e.g. M33, M101) can have both prominent blue star-formation features ans strong stellar density arms at the same time. ... In the grand design and intermediate-type galaxies, the blueness of the arms is independent of the arm amplitude, indicating either that the rate of star formation saturates to a constant value in a stellar density wave or material arm, or that star formation is independent of these density variations."[28]

## Meteor showers

Of some 670 Perseids examined for colors from 1985, 1988, and 1989, 128 were blue meteors, 3 were multi-colored yellow-blue and one was blue-green [cyan].[29] The average pre-atmospheric velocity is 59.9 km/s.[29]

The Perseids are a prolific meteor shower associated with the comet Swift-Tuttle. The Perseids are so-called because the point from which they appear to come, called the radiant, lies in the constellation Perseus.

Of 225 Geminids observed in 1990 some 38 were blue in color, with one yellow-blue and one blue-green [cyan].[29] The average pre-atmospheric velocity is 36.2 km/s.[29]

## Cosmic rays

"The Broad LAteral Non-imaging Cherenkov Array (BLANCA) takes advantage of the CASA-MIA particle array installation by augmenting it with 144 angle-integrating Cherenkov detectors. Located in Dugway, Utah at an atmospheric depth of 870 g cm−2, BLANCA uses the CASA trigger to collect Cherenkov light and records the Cherenkov lateral distribution from cosmic ray events in the energy range of the knee. The CASA trigger threshold imposes an energy threshold of ∼ 100 TeV on the Cherenkov array. However, BLANCA analysis uses events with a 200 TeV minimum to avoid composition bias introduced from the CASA trigger."[30]

"Each BLANCA detector contains a large Winston cone [43] which concentrates the light striking an 880 cm2 entrance aperture onto a photomultiplier tube. The concentrator has a nominal half-angle of 12.5° and truncated length of 60 cm. The Winston cones were aligned vertically with ∼ 0.5° accuracy. A two-output preamplifier increases the dynamic range of the detector. The minimum detectable density of a typical BLANCA unit is approximately one blue photon per cm2."[30]

## Neutrons

"[A] high-resolution, high-signal-to-noise UV-blue spectrum of the extremely metal-poor red giant HD 88609 [is used] to determine the abundances of heavy elements. Nineteen neutron-capture elements are detected in the spectrum."[31]

"[T]his object has large excesses of light neutron-capture elements, while heavy neutron-capture elements are deficient. The abundance pattern shows a continuously decreasing trend as a function of atomic number, from Sr to Yb, which is quite different from those in stars with excesses of r-process elements."[31]

"[T]he abundance pattern found in the two stars could represent the pattern produced by the nucleosynthesis process that provided light neutron-capture elements in the very early Galaxy."[31]

## Protons

"Although the mean metal abundance M/H varies from a factor 12 to 250 relative to the Sun, the abundance of lithium is very constant in all these [halo and old disk] stars, but the coolest ones where the convection zone is deep enough to induce 7Li destruction by proton fusion."[32]

"[T]he lithium has a fragile nucleus which is destroyed by proton fusion when the temperature reaches 2 106 K."[32]

"[F]or the limit between "halo stars" and "old disk stars", [there is] a discontinuity in the relation δ(U - B) versus [Fe/H] at about [Fe/H] = -1.0 dex ... "halo stars" ... are metal deficient by a factor ranging from 12 to 250 relative to the Sun. ... "old disk stars" ... are metal deficient by a factor ranging from 2 to 10."[32]

These well-known metal deficient stars have "high dispersion spectra in the blue wavelength range".[32]

"In all the stars analysed ... it is very unlikely that the lithium observed in the atmosphere was produced, even in part, inside the stars themselves, and then transported up into the atmospheres."[32]

"[T]he lithium rich stars are not statistically younger than the lithium poor stars."[32]

## Electrons

"[H]igh-quality spectrophotometric observations of 10 low-metallicity blue compact galaxies (BCGs) with oxygen abundance ranging from 12 + log(O/H) = 7.37 to 8.04 [are used] to determine the primordial helium abundance."[33]

"The main physical mechanism changing the He I line intensities from their recombination values is collisional excitation. To correct for it, we calculate the electron number density in the He+ zone by a self-consistent procedure which constrains the He I λ5876/λ4471, λ6678/λ4471 and λ7065/λ4471 line ratios to have their recombination values, after correction for collisional enhancement."[33]

## Positrons

In "the spectrum of a middle-aged [pulsar] PSR B0656+14 [may be] two wide, red and blue, flux depressions whose frequency ratio is about 2 and which could be the 1st and 2nd harmonics of electron/positron cyclotron absorption formed at magnetic fields [of] ~108 G in [the] upper magnetosphere of the pulsar."[34]

## Neutrinos

Supernova SN 1987A is one of the brightest stellar explosions since the invention of the telescope more than 400 years ago.[35] 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."[36]

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.[37]

Four days after the event was recorded, the progenitor star was tentatively identified as Sanduleak -69° 202, a blue supergiant.[38] 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.[36]

## Gamma rays

"For the highest possible rotation rates (about 400 km s-1), a novel sort of evolution is encountered in which single stars mix completely on the main sequence, never becoming red giants. Such stars, essentially massive "blue stragglers," produce helium-oxygen cores that rotate unusually rapidly. Such stars might comprise roughly 1% of all stars above 10 Mʘ and can, under certain circumstances, retain enough angular momentum to make [gamma-ray bursts] GRBs. Because this possibility is very sensitive to mass loss, GRBs are much more probable in regions of low metallicity."[39]

## X-rays

This is a visual image of Abell 370. Credit: NASA, ESA, the Hubble SM4 ERO Team and ST-ECF.

"The far-infrared to soft X-ray continuum of radio-quiet and steep-spectrum radio-loud [Palomar-Green (PG) Bright Quasar Survey] PG quasars can be described in a model consisting of two broad peaks: an 'infrared bump' at about 2 microns to 1 mm and the well-known 'big blue bump' at about 10 nm to 0.3 micron."[40]

"[T]he distant X-ray emitter Abell cluster A 370 (z = 0.373) [contains] a very particular [blue] ring-like structure of galaxies with a diffuse component lying near one of the very luminous galaxies of the cluster core."[41]

## Superluminals

Cherenkov radiation glows in the core of the Advanced Test Reactor. Credit: Matt Howard.

The frequency spectrum of Cherenkov radiation by a particle is given by the Frank–Tamm formula. Unlike fluorescence or emission spectra that have characteristic spectral peaks, Cherenkov radiation is continuous. Around the visible spectrum, the relative intensity per unit frequency is approximately proportional to the frequency. That is, higher frequencies (shorter wavelengths) are more intense in Cherenkov radiation. This is why visible Cherenkov radiation is observed to be brilliant blue. In fact, most Cherenkov radiation is in the ultraviolet spectrum—it is only with sufficiently accelerated charges that it even becomes visible; the sensitivity of the human eye peaks at green, and is very low in the violet portion of the spectrum.

## Plasma objects

Representation of upper-atmospheric lightning and electrical-discharge phenomena are displayed. Credit: .
The surface of a MEMS device is cleaned with bright, blue oxygen plasma in a plasma etcher to rid it of carbon contaminants. (100mTorr, 50W RF) Credit: .
This image of the Northern Lights shows the very rare blue light. Credit: Varjisakka.

Blue jets differ from sprites in that they project from the top of the cumulonimbus above a thunderstorm, typically in a narrow cone, to the lowest levels of the ionosphere 40 to 50 km (25 to 30 miles) above the earth. In addition, whereas red sprites tend to be associated with significant lightning strikes, blue jets do not appear to be directly triggered by lightning (they do, however, appear to relate to strong hail activity in thunderstorms).[42] They are also brighter than sprites and, as implied by their name, are blue in color. The color is believed to be due to a set of blue and near-ultraviolet emission lines from neutral and ionized molecular nitrogen.

"Blue starters were discovered on video from a night time research flight around thunderstorms [43] and appear to be "an upward moving luminous phenomenon closely related to blue jets."[44] They appear to be shorter and brighter than blue jets, reaching altitudes of only up to 20 km.[45]

Blue starters appear to be blue jets that never quite make it".[46]

Plasma cleaning involves the removal of impurities and contaminants from surfaces through the use of an energetic plasma created from gaseous species. Gases such as argon and oxygen, as well as mixtures such as air and hydrogen/nitrogen are used. The plasma is created by using high frequency voltages (typically kHz to >MHz) to ionise the low pressure gas (typically around 1/1000 atmospheric pressure), although atmospheric pressure plasmas are now also common.

At left is an image of the Northern Lights on Earth showing the very rare blue lights.

## Gaseous objects

Praia da Ursa, Sintra, Portugal is shown as part of a blue hour seascape seen in wide angle. Credit: Rnbc.
The Colosseum is shown during the blue hour. Credit: Diliff.
Blue hour in Paris is shown around the Eiffel Tower. Credit: Getfunky Paris.
Brandenburg Gate is shown in Berlin during the blue hour. Credit: Ondřej Žváček (Ondřej Žváček).
The illuminated mining lamp memorial in Moers is shown during the blue hour. Credit: kaʁstn.

The blue hour is the period of twilight each morning and evening where there is neither full daylight nor complete darkness. The time is considered special because of the quality of the light.

At first right is a seascape after sunset at Praia da Ursa, Sintra, Portugal, during the blue hour.

The first image at left shows the Colosseum in Rome during this blue hour.

The second image at right shows the blue hour in Paris around the Eiffel Tower and Pont Alexandre III at night.

The second image at left is the Brandenburg Gate in Berlin during the blue hour.

The last image at left is the illuminated mining lamp memorial in Moers during the blue hour.

## Liquid objects

This is a detailed, photo-like view of Earth based largely on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. Credit: Robert Simmon and Marit Jentoft-Nilsen, NASA.

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

"Viewed from space, the most striking feature of our planet is the water. In both liquid and frozen form, it covers 75% of the Earth’s surface. It fills the sky with clouds. Water is practically everywhere on Earth, from inside the rocky crust to inside our cells."[47]

## Rocky objects

This Sin-Kamen (Blue Rock) near Lake Pleshcheyevo used to be a Meryan shrine Credit: Viktorianec.
This is a blue rock, probably various copper minerals, from the Berkeley hills near San Francisco, California. Credit: Looie496.
This is an approximately natural color picture of the asteroid 243 Ida on August 28, 1993. Credit: NASA/JPL.

Sin-Kamen (Синь-Камень, in Russian literally – Blue Stone, or Blue Rock) is a type of pagan sacred stones, widespread in Russia, in areas historically inhabited by both Eastern Slavic (Russian), and Uralic tribes (Merya, Muroma[48]).

While in the majority of cases, the stones belonging to the Blue Stones type, have a black, or dark gray color, this particular stone [in the image] does indeed look dark blue, when wet.[49]

"Several types of rock surface materials can be recognized at the two sites [Viking Lander 1 and Viking Lander 2]; dark, relatively 'blue' rock surfaces are probably minimally weathered igneous rock, whereas bright rock surfaces, with a green/(blue + red) ratio higher than that of any other surface material, are interpreted as a weathering product formed in situ on the rock."[50]

At second right is an approximately natural color image of the asteroid 243 Ida. "There are brighter areas, appearing bluish in the picture, around craters on the upper left end of Ida, around the small bright crater near the center of the asteroid, and near the upper right-hand edge (the limb). This is a combination of more reflected blue light and greater absorption of near infrared light, suggesting a difference in the abundance or composition of iron-bearing minerals in these areas."[51]

"The [Sloan Digital Sky Survey] SDSS “blue” asteroids are related to the C-type (carbonaceous) asteroids, but not all of them are C-type. They are a mixture of C-, E-, M-, and P-types."[52]

## Heliums

Main sources: Chemicals/Heliums and Heliums
The spectrum shows the lines in the visible due to emission from elemental helium. Credit:Teravolt.
This image of NGC 6302 lists the emission lines with the color code. Credit: K. Noll and H. Bond (STScI) and B. Balick (University of Washington), H. Bushouse, J. Anderson, and M. Mutchler (STScI), and Z. Levay and L. Frattare (STScI).

Helium II does have an emission line in the blue at 469 nm.

"The Wide Field Camera 3 (WFC3), a new camera aboard NASA's Hubble Space Telescope, snapped this image of the planetary nebula, catalogued as NGC 6302, but more popularly called the Bug Nebula or the Butterfly Nebula. WFC3 was installed by NASA astronauts in May 2009, during the servicing mission to upgrade and repair the 19-year-old Hubble telescope."[53]

"What resemble dainty butterfly wings are actually roiling cauldrons of gas heated to more than 36,000 degrees Fahrenheit. The gas is tearing across space at more than 600,000 miles an hour—fast enough to travel from Earth to the Moon in 24 minutes!"[53]

"NGC 6302 lies within our Milky Way galaxy, roughly 3,800 light-years away in the constellation Scorpius. The glowing gas is the star's outer layers, expelled over about 2,200 years. The "butterfly" stretches for more than two light-years, which is about half the distance from the Sun to the nearest star, Alpha Centauri."[53]

"The central star itself cannot be seen, because it is hidden within a doughnut-shaped ring of dust, which appears as a dark band pinching the nebula in the center. The thick dust belt constricts the star's outflow, creating the classic "bipolar" or hourglass shape displayed by some planetary nebulae."[53]

"The star's surface temperature is estimated to be about 400,000 degrees Fahrenheit, making it one of the hottest known stars in our galaxy. Spectroscopic observations made with ground-based telescopes show that the gas is roughly 36,000 degrees Fahrenheit, which is unusually hot compared to a typical planetary nebula."[53]

"The WFC3 image reveals a complex history of ejections from the star. The star first evolved into a huge red-giant star, with a diameter of about 1,000 times that of our Sun. It then lost its extended outer layers. Some of this gas was cast off from its equator at a relatively slow speed, perhaps as low as 20,000 miles an hour, creating the doughnut-shaped ring. Other gas was ejected perpendicular to the ring at higher speeds, producing the elongated "wings" of the butterfly-shaped structure. Later, as the central star heated up, a much faster stellar wind, a stream of charged particles traveling at more than 2 million miles an hour, plowed through the existing wing-shaped structure, further modifying its shape."[53]

"The image also shows numerous finger-like projections pointing back to the star, which may mark denser blobs in the outflow that have resisted the pressure from the stellar wind."[53]

"The nebula's reddish outer edges are largely due to light emitted by nitrogen, which marks the coolest gas visible in the picture. WFC3 is equipped with a wide variety of filters that isolate light emitted by various chemical elements, allowing astronomers to infer properties of the nebular gas, such as its temperature, density, and composition."[53]

"The white-colored regions are areas where light is emitted by sulfur. These are regions where fast-moving gas overtakes and collides with slow-moving gas that left the star at an earlier time, producing shock waves in the gas (the bright white edges on the sides facing the central star). The white blob with the crisp edge at upper right is an example of one of those shock waves."[53]

"NGC 6302 was imaged on July 27, 2009, with Hubble's Wide Field Camera 3 in ultraviolet and visible light. Filters that isolate emissions from oxygen, helium, hydrogen, nitrogen, and sulfur from the planetary nebula were used to create this composite image."[53]

The filters used for this image are F373N ([O II], purple), F469N (He II, blue), F502N ([O III], cyan), F656N (Hα, brown), F658N ([N II], orange), and F673N ([S II], white).[53]

## Lithiums

Main sources: Chemicals/Lithiums and Lithiums
This spectrograph shows the visual spectral lines of lithium. Credit: T c951.

## Berylliums

This image shows the emission lines for beryllium. Absorption occurs by subtraction of these lines from the continuum. Credit: Penyulap.

The emission and absorption spectra for beryllium contain lines in the blue.

## Borons

Main sources: Chemicals/Borons and Borons
This image shows the emission lines for boron and their approximate locations in the visible spectrum. Absorption lines occur at the same locations but with subtraction of light from the continuum. Credit: Penyulap.

The emission and absorption spectra for boron contain lines on the border between violet and blue.

## Carbons

Main sources: Chemicals/Carbons and Carbons
The spectrum shows the lines in the visible due to emission from elemental carbon. Credit:Teravolt.

Carbon has three emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas: 464.742, 465.025, and 465.147 nm from C III.[54]

## Nitrogens

Main sources: Chemicals/Nitrogens and Nitrogens
The spectrum shows the lines in the visible due to emission from elemental nitrogen. Credit:Kurgus.

Nitrogen has two emission lines that occur in plasmas at 455.368 and 455.545 nm from N VII.[54]

There is an "(0,2) vibrational component of the B-x electronic transition of N2(+) at 470.9 nm."[55]

## Oxygens

Main sources: Chemicals/Oxygens and Oxygens
The spectrum shows the lines in the visible due to emission from elemental oxygen. Credit:Teravolt.

The spectrum above shows the lines in the visible due to emission from elemental oxygen.

## Fluorines

Main sources: Chemicals/Fluorines and Fluorines
This diagram contains the emission and absorption lines for the element fluorine. Credit: Alex Petty.

The emission and absorption spectra of fluorine contains at least eight lines or bands from the cyan to the ultraviolet.[56]

## Neons

Main sources: Chemicals/Neons and Neons
This image shows the emission lines for atomic neon. Absorption lines occur at the same locations by subtraction of light from the continuum. Credit: Teravolt.

Like fluorine, neon has at least fourteen emission and absorption lines or bands from the cyan to the violet.[57]

## Argons

Main sources: Chemicals/Argons and Argons

Argon has an emission line that occurs in an electron cyclotron resonance (ECR) heated plasmas: 473.591 nm from Ar II.[54]

## Titaniums

Main sources: Chemicals/Titaniums and Titaniums

Titanium has two emission lines at 456.3757 and 457.1971 nm from Ti II.[58]

## Chromiums

Main sources: Chemicals/Chromiums and Chromiums

Chromium has two emission lines at 455.8650 and 458.8199 nm from Cr II.[58]

## Irons

Main sources: Chemicals/Irons and Irons

Iron has one emission line at 458.3837 nm from Fe II.[58]

## Compounds

Main sources: Chemicals/Compounds and Compounds
An example of common occurring brownish hibonite. Credit: .
This specimen from Madagascar has a bluish cast that may indicate a composition similar to those grains found in meteorites. Credit: Rock Currier.

Usually, Hibonite ((Ca,Ce)(Al,Ti,Mg)12O19) as shown at right is a brownish black mineral. It is rare, but is found in high-grade metamorphic rocks on Madagascar. Some presolar grains in primitive meteorites consist of hibonite. Hibonite also is a common mineral in the Ca-Al-rich inclusions (CAIs) found in some chondrite or chondritic meteorites. Hibonite is closely related to hibonite-Fe (IMA 2009-027, ((Fe,Mg)Al12O19)) an alteration mineral from the Allende meteorite.[59] [Hibonite] is blue [perhaps like the image at left] in meteorite occurrence.

## Alloys

Main sources: Chemicals/Alloys and Alloys

"Grain size varies from 98 to 530 lm with an average of *150 lm. Minor [elements] oxidation [from an iron–nickel–chromium–cobalt–phosphorus alloy] is evidenced by the presence of a light brown and blue surface layer composed of very fine-grained (<1 lm) crystals on the surface."[60] "[T]he oxidation of minor elements in metallic alloys in the early solar system" is indicated to possess at instances a blue surface layer.[60]

## Atmospheres

"If we know beforehand that a nebula is not of the emission type, observations of its polarization enable us to go a step farther. In general, if a particle scatters light in such a way that it does not hold the energy for any length of time but simply defects it without change of wave-length, then the polarization of the deflected light has the following characteristics: (a) its plane of polarization is usually perpendicular to or, more rarely, parallel to the plane formed by the incident and the scattered rays; and (b) the amount of polarization is inversely correlated with the size of the particles. When the scattering particles are of the order of size of a few hundred molecules or smaller we have what is usually known as Rayleigh scattering. Here the polarization reaches very large values, as is evidenced by our own blue atmosphere, which gives values up to 70 per cent."[61]

## Materials

Main sources: Chemicals/Materials and Materials

"The depth of the absorption bands and the continuum reflectance of [Kuiper Belt Object] 1996 TO66 suggest the presence of a black- to slightly blue-colored, spectrally featureless particulate material as a minority component mixed with the water ice."[62]

## Meteorites

Main sources: Rocks/Meteorites and Meteorites
This view of a rock called "Block Island" shows the largest meteorite yet found on Mars. Credit: NASA/JPL-Caltech/Cornell University.

"This [image at right] is a false-color, red-green-blue composite view generated from images taken through the Pancam's 750-nanometer, 530-nanometer and 430-nanometer filters. The exaggeraged color is used for enhancing the visibility of differences among the types of rock and soil materials."[63]

"Analysis of Block Island's composition using the rover's alpha particle X-ray spectrometer confirmed that it is rich in iron and nickel. The rock is about 60 centimeters (2 feet) across."[63]

## Sun

Main sources: Stars/Sun and Sun (star)

As an astronomical object sets or rises in relation to the horizon, the light it emits travels through Earth's atmosphere, which works as a prism separating the light into different colors. The color of the upper rim of an astronomical object could go from green to blue to violet depending on the decrease in concentration of pollutants, as they spread throughout an increasing volume of atmosphere.[64]

Very occasionally, the amount of blue light is sufficient to be visible as a "blue flash".[65]

## Mercury

Main sources: Wanderers/Mercury and Mercury
The color image shown here at right was generated by combining the mosaics taken through the MESSENGER WAC filters that transmit light at wavelengths of 1000 nanometers (infrared), 700 nanometers (far red), and 430 nanometers (violet). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

"MESSENGER's Wide Angle Camera (WAC), part of the Mercury Dual Imaging System (MDIS), is equipped with 11 narrow-band color filters. As the spacecraft receded from Mercury after making its closest approach on January 14, 2008, the WAC recorded a 3x3 mosaic covering part of the planet not previously seen by spacecraft. The color image shown here was generated by combining the mosaics taken through the WAC filters that transmit light at wavelengths of 1000 nanometers (infrared), 700 nanometers (far red), and 430 nanometers (violet). These three images were placed in the red, green, and blue channels, respectively, to create the visualization presented here. The human eye is sensitive only across the wavelength range from about 400 to 700 nanometers. Creating a false-color image in this way accentuates color differences on Mercury's surface that cannot be seen in black-and-white (single-color) images."[66]

"Color differences on Mercury are subtle, but they reveal important information about the nature of the planet's surface material. A number of bright spots with a bluish tinge are visible in this image. These are relatively recent impact craters. Some of the bright craters have bright streaks (called "rays" by planetary scientists) emanating from them. Bright features such as these are caused by the presence of freshly crushed rock material that was excavated and deposited during the highly energetic collision of a meteoroid with Mercury to form an impact crater. The large circular light-colored area in the upper right of the image is the interior of the Caloris basin. Mariner 10 viewed only the eastern (right) portion of this enormous impact basin, under lighting conditions that emphasized shadows and elevation differences rather than brightness and color differences. MESSENGER has revealed that Caloris is filled with smooth plains that are brighter than the surrounding terrain, hinting at a compositional contrast between these geologic units. The interior of Caloris also harbors several unusual dark-rimmed craters, which are visible in this image. The MESSENGER science team is working with the 11-color images in order to gain a better understanding of what minerals are present in these rocks of Mercury's crust."[66]

## Venus

Main sources: Wanderers/Venus and Venus
This real-color image of Venus is processed from imaging through the clear and blue filters on Mariner 10. Credit: NASA or Ricardo Nunes.

In the image at right the whiter cloud areas have a bluish tint. This is a real-color image of Venus processed through the clear and blue filters onboard Mariner 10.

## Earth

Main source: Earth
The Earth has a blue halo when seen from space. Credit: .
The Earth can have a blue sky and a blue ocean. Credit: Frokor.
This is the famous Blue Marble image of Earth taken by Apollo 17. Credit: NASA. Photo taken by either Harrison Schmitt or Ron Evans (of the Apollo 17 crew).

"Earth is a blue planet"[67].

Atmospheric gases scatter blue light more than other wavelengths, giving the Earth a blue halo when seen from space. This is shown in the image at right.

When light passes through our atmosphere, photons interact with it through scattering. If the light does not interact with the atmosphere, it is called direct radiation and is what you see if you were to look directly at the Sun. Indirect radiation is light that has been scattered in the atmosphere. For example, on an overcast day when you cannot see your shadow there is no direct radiation reaching you, it has all been scattered. As another example, due to a phenomenon called Rayleigh scattering, shorter (blue) wavelengths scatter more easily than longer (red) wavelengths. This is why the sky looks blue; you are seeing scattered blue light. This is also why sunsets are red. Because the Sun is close to the horizon, the Sun's rays pass through more atmosphere than normal to reach your eye. Much of the blue light has been scattered out, leaving the red light in a sunset.

The bluish color of water is a composite of several contributing agents. Prominent contributors include dissolved organic matter and chlorophyll.[68]

The third image at right is the famous Blue Marble image of Earth taken by Apollo 17. The image shows the eastern Southern Atlantic Ocean, the South African portion of the Southern Ocean, and the Western Indian Ocean. The land consists of most of Africa, Madagascar, Saudi Arabia, and portions of Iran, Irag, Turkey, and southern Greece. The gaseous portion consists of water vapor clouds over the southern portion of this hemisphere. Antarctica is completely covered in snow (a water ice rocky substance).

If this third image is the one chosen to decide whether the Earth is a dwarf gaseous object, a dwarf liquid object, or a dwarf rocky object, the decision becomes difficult. Here, the Earth is primarily a liquid body.

## Moon

Main sources: Wanderers/Moon and Moon

"Nine out of 10 well-characterized Apollo 17 breccia matrices fall into Group 2, and this includes both the blue-grey breccias which are the dominant rock type at this site"[69].

"A 1953 telescopic photograph of a flash on the Moon is the only unequivocal record of the rare crash of an asteroid-sized body onto the lunar surface. ... A search of images from the Clementine mission reveals an ∼1.5-km high-albedo, blue, fresh-appearing crater with an associated ejecta blanket at the location of the flash."[70]

In terms of reflectance from the lunar surface, "the very dark 'blue' maria [are] such as found in Mare Tranquillitatis."[71]

"[T]he slope of the reflectance spectrum in the blue and ultraviolet ... is directly related to the percent TiO2 in the [lunar] surface soil (Charette et al., 1974)."[72]

## Mars

Main sources: Wanderers/Mars and Mars
This Mars rock reveals a bluish-gray interior to Mars Science Laboratory. Credit: NASA/JPL-Caltech/MSSS/ASU.
This natural color image is from NASA's Curiosity rover before it aimed two different instruments to study the rock known as "Jake Matijevic". Credit: NASA/JPL-Caltech/MSSS.
This is a color image made from the first post-sunset sequence of calibrated color images, with the color balance set to approximate what the sunset color would have looked like to the human eye. Credit: NASA/JPL/Texas A&M/Cornell.
This is a natural color image by the HiRISE camera on the Mars Reconnaissance Orbiter. Credit: NASA/JPL/University of Arizona.
This image shows the eastern (west-facing) side of an impact crater in the mid-latitudes of the Northern hemisphere. Credit: NASA/JPL/University of Arizona.
This image shows part of the floor of Rabe Crater, a large (108 kilometre diameter) impact crater in the Southern highlands of Mars. Credit: NASA/JPL/University of Arizona.
Gullies, exposed bedrock and deposits in an impact crater on Mars are photographed by the Mars Reconnaissance Orbiter. Credit: NASA/JPL/University of Arizona.
Dunes of sand-sized materials have been trapped on the floors of many Martian craters. Credit: NASA/JPL-Caltech/University of Arizona.
This view of layered rocks on the floor of McLaughlin Crater shows sedimentary rocks that contain spectroscopic evidence for minerals formed through interaction with water. Credit: NASA/JPL-Caltech/Univ. of Arizona.
The HiRISE image does not show the entire crater. Credit: NASA/JPL/University of Arizona.

"The Mast Camera (Mastcam) on NASA's Mars rover Curiosity showed researchers interesting internal color in this rock called "Sutton_Inlier," which was broken by the rover driving over it. The Mastcam took this image during the 174th Martian day, or sol, of the rover's work on Mars (Jan. 31, 2013). The rock is about 5 inches (12 centimeters) wide at the end closest to the camera. This view is calibrated to estimated "natural" color, or approximately what the colors would look like if we were to view the scene ourselves on Mars. The inside of the rock, which is in the "Yellowknife Bay" area of Gale Crater, is much less red than typical Martian dust and rock surfaces, with a color verging on grayish to bluish."[73]

"[T]he Chemistry and Camera (ChemCam) instrument zapped [the rock known as "Jake Matijevic"] with its laser on Sept. 21, 2012, and Sept. 24, 2012, which were the 45th and 48th sol, or Martian day of operations. ... black and white images were taken by ChemCam to look for the pits produced by the laser. ... [Later] the Alpha Particle X-ray Spectrometer trained its view. ... This image was obtained by Curiosity's Mast Camera on Sept. 21, 2012 PDT (Sept. 22 UTC), or sol 46. [For natural color, processors] white-balanced the color in this view to increase the inherent differences visible within the rock."[74] Other bluish rocks can just be seen. Curiosity removed the Martian dust from "Jake Matijevic" before photographing.

"On Sol 20 of its journey, Mars Exploration Rover Opportunity woke up around 5:30 in the martian afternoon to watch the sunset. A series of five sets of three-color images from the rover's panoramic camera was acquired looking toward the southwest. Each set used an infrared, green and violet filter, rather than the human red-green-blue, so that the maximum panoramic camera wavelength range could be covered by the observations, enhancing the scientific value of the measurements."[75]

"A color image was made [at lower right] from the first post-sunset sequence of calibrated color images, with the color balance set to approximate what the sunset color would have looked like to the human eye. The color seen in this first post-sunset image was then used to colorize each image in the sequence. Approximately one-minute gaps between consecutive color images meant the Sun's position changed within each color set, so the images had to be manually shifted to compensate for this motion. In this fashion, the position and brightness of the Sun are taken from each individual image, but the color is taken from a single set of images. The images were then combined into a movie where one color set fades gracefully into the next. Analysis of the five color sets shows that there were only small color variations during the sunset, so most of the real variations are captured in the movie."[75]

"The rapid dimming of the Sun near the horizon is due to the dust in the sky. There is nearly twice as much dust as there was when the Mars Pathfinder spacecraft, which landed on Mars in 1997, imaged the sunset. This causes the Sun to be many times fainter. The sky above the Sun has the same blue tint observed by Pathfinder and also by Viking, which landed on Mars in 1976. This is because dust in the martian atmosphere scatters blue light forward toward the observer much more efficiently than it scatters red light forward. Therefore, a "halo" of blueish sky color is always observed close to the Sun. We're only seeing half of this halo in the movie, because the other half is below the horizon."[75]

At second left is an image in natural color by the HiRISE camera of Cerberus Fossae Graben showing exposed blue material.

The third image at right shows "the eastern (west-facing) side of an impact crater in the mid-latitudes of the Northern hemisphere."[76]

"Like many mid-latitude craters, this one has gullies along its walls that are composed of alcoves, channels, and debris aprons. The origins of these gullies have been the subject of much debate; they could have formed by flowing water, liquid carbon dioxide, or dry granular flows. The orientation of these gullies is of interest because many craters only contain gullies on certain walls, such as those that are pole-facing. This could be due to changes in orbital conditions and differences in solar heating along specific walls."[76]

"Many of the other features observed in and around this crater however are indicative of an ice-rich terrain, which may lend credence to the water formation hypothesis, at least for the gullies visible here. The most notable of these features is the "scalloped" terrain in and around the crater. This type of terrain has been interpreted as a sign of surface caving, perhaps due to sublimation of underlying ice. (Sublimation is the process of a solid changing directly to a gas.)"[76]

"Another sign of ice is the presence of parallel lineations and pitted material on the floor of the crater, similar to what is referred to as concentric crater fill. Parallel linear cracks are also observed along the crater wall over the gullies, which could be due to thermal contraction of ice-rich material."[76]

"All of these features taken together are evidence for ice-rich material having been deposited in this region during different climatic conditions that has subsequently begun to melt and/or sublimate under current conditions. More recently, aeolian deposits have accumulated around the crater as evidenced by the parallel ridges dominating the landscape. Dust devil streaks are also visible crossing the aeolian ridges."[76]

The image at third left "shows part of the floor of Rabe Crater, a large (108 kilometre diameter) impact crater in the Southern highlands".[77]

"Dark dunes—accumulations of wind blown sand—cover part of crater's floor, and contrast with the surrounding bright-colored outcrops. The extreme close-up view reveals a thumbprint-like texture of smaller ridges and troughs covering the surfaces of the larger dunes. These smaller ripples are also formed and shaped by blowing wind in the thin atmosphere of Mars."[77]

"One puzzling question is why the dunes are dark compared with the relative bright layered material contained within the crater. The probable answer is that the source of the dark sand is not local to this crater; rather, this topographic depression has acted as a sand trap that has collected material being transported by winds blowing across the plains outside the crater."[77]

The fourth image at right shows "pristine gullies, some with bright deposits, and perhaps very recent."[78]

"In addition, there is exposed bedrock, which at HiRISE resolution, we can pick out fine details. Observations like this can also help gully modeling."[78]

The fourth image at left shows an example of "[d]unes of sand-sized materials have been trapped on the floors of many Martian craters. This is one ... from a crater in Noachis Terra, west of the giant Hellas impact basin. The High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter captured this view on Dec. 28, 2009."[79]

"The dunes here are linear, thought to be due to shifting wind directions. In places, each dune is remarkably similar to adjacent dunes, including a reddish (or dust colored) band on northeast-facing slopes. Large angular boulders litter the floor between dunes."[79]

The fifth image at right shows layered rocks on the floor of McLaughlin Crater.[80]

These "sedimentary rocks ... contain spectroscopic evidence for minerals formed through interaction with water. The High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter recorded the image."[80]

"A combination of clues suggests this 1.4-mile-deep (2.2-kilometer-deep) crater once held a lake fed by groundwater. Part of the evidence is identification of clay and carbonate minerals within layers visible near the center of this image. The mineral identifications come from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), also on the Mars Reconnaissance Orbiter. The scene covers an area about one-third of a mile (about 550 meters) across, at 337.6 degrees east longitude, 21.9 degrees north latitude. North is up."[80]

The fifth left image is a partial image of a recent impact crater on Mars captured by HiRISE. Although the HiRISE image does not show the entire crater, note that below the top rock layers cut through by the meteor is a blue layer. Some portions of this layer have slide down the crater wall beneath the layer. This suggests that the blue material is a layer beneath an outer Martian crust.

## Asteroids

This is an ultraviolet image of Pallas showing its flattened shape taken by the Hubble Space Telescope. Credit: NASA.

Pallas, minor-planet designation 2 Pallas, is the second asteroid to have been discovered (after Ceres), and one of the largest in the Solar System. It is estimated to comprise 7% of the mass of the asteroid belt,[81] and its diameter of 544 kilometres (338 mi) is slightly larger than that of 4 Vesta. It is however 10–30% less massive than Vesta,[82] placing it third among the asteroids.

"Spectrally blue (B-type) asteroids are rare, with the second discovered asteroid, Pallas, being the largest and most famous example."[83]

"[T]he negative optical spectral slope of some B-type asteroids is due to the presence of a broad absorption band centered near 1.0 μm. The 1 μm band can be matched in position and shape using magnetite (Fe3O4), which is an important indicator of past aqueous alteration in the parent body. ... Observations of B-type asteroid (335) Roberta in the 3 μm region reveal an absorption feature centered at 2.9 μm, which is consistent with the absorption due to phyllosilicates (another hydration product) observed in CI chondrites. ... at least some B-type asteroids are likely to have incorporated significant amounts of water ice and to have experienced intensive aqueous alteration."[83]

## Comets

Main source: Comets
Comet Holmes (17P/Holmes) in 2007 shows a blue ion tail on the right. Credit: Ivan Eder.
Comet Lovejoy has a blue ion tail leading away off to the left. Credit: NASA/Dan Burbank.

In addition to the usually cyan color of the plasma around the comet nucleus is a blue ion tail leading away to the right.

## Jupiter

Main sources: Wanderers/Jupiter and Jupiter
Zones, belts and vortices on Jupiter are shown. Credit: NASA/JPL/University of Arizona.

The wide equatorial zone is visible in the center surrounded by two dark equatorial belts (SEB and NEB).

"The large grayish-blue [irregular] "hot spots" at the northern edge of the white Equatorial Zone change over the course of time as they march eastward across the planet."[84]

"The Great Red Spot shows its counterclockwise rotation, and the uneven distribution of its high haze is obvious. To the east (right) of the Red Spot, oval storms, like ball bearings, roll over and pass each other. Horizontal bands adjacent to each other move at different rates. Strings of small storms rotate around northern-hemisphere ovals."[84]

"Small, very bright features appear quickly and randomly in turbulent regions, candidates for lightning storms."[84]

"The smallest visible features at the equator are about 600 kilometers (about 370 miles) across."[84]

"The clip consists of 14 unevenly spaced timesteps, each a true color cylindrical projection of the complete circumference of Jupiter, from 60 degrees south to 60 degrees north. The maps are made by first assembling mosaics of six images taken by Cassini's narrow-angle camera in the same spectral filter over the course of one Jupiter rotation and, consequently, covering the whole planet. Three such global maps -- in red, green and blue filters -- are combined to make one color map showing Jupiter during one Jovian rotation. Fourteen such maps, spanning 24 Jovian rotations at uneven time intervals comprise the movie."[84]

The passage of time is accelerated by a factor of 600,000.

## Europa

Main source: Europa
This Galileo spacecraft image shows the approximate natural color appearance of Europa. Credit: NASA/JPL/DLR.

"This image [at right] shows ... the approximate natural color appearance of Europa. ... Dark brown areas represent rocky material derived from the interior, implanted by impact, or from a combination of interior and exterior sources. Bright plains in the polar areas (top and bottom) are shown in tones of blue to distinguish possibly coarse-grained ice (dark blue) from fine-grained ice (light blue). Long, dark lines are fractures in the crust, some of which are more than 3,000 kilometers (1,850 miles) long. The bright feature containing a central dark spot in the lower third of the image is a young impact crater some 50 kilometers (31 miles) in diameter. This crater has been provisionally named "Pwyll" for the Celtic god of the underworld."[85]

## Io

Main source: Io
During an eclipse of Jupiter's moon Io on January 1, 2001, NASA's Cassini spacecraft recorded glows from auroras and volcanoes on Io. Credit: NASA/JPL/University of Arizona.

"The camera on Cassini captured images of eclipsed Io in several colors ranging from the near-ultraviolet to the near-infrared. A black-and-white movie clip of 48 clear-filter frames spanning two hours during the eclipse was released on February 5 (PIA02882). Here, two colors have been added to show the type of evidence used by imaging scientists in determining the source of Io's auroral glows. The color pictures were taken at lower resolution -- 120 kilometers (75 miles) per pixel rather than 60 kilometers(37 miles) per pixel -- and less frequently than the clear-filter images. White dots near the equator are volcanoes, some of which are much brighter than the faint atmospheric glows. The brightest of them is the volcano Pele."[86]

"Emissions of light (at wavelengths of 595 to 645 nanometers) likely arise from a tenuous atmosphere of oxygen. These glows would appear red to the eye and are consequently colored red in the movie. Emissions in near-ultraviolet wavelengths (between 300 and 380 nanometers), corresponding wavelength to the bright blue visible glows one would expect from sulfur dioxide. They have been colored blue in the movie. The blue glows are restricted to areas deep down in the atmosphere near the surface of Io, while the red glows are much more extensive, reaching heights of up to 900 kilometers (560 miles). This would be expected if the blue glows are indeed produced by sulfur dioxide, since sulfur dioxide molecules are heavier than oxygen atoms, so are more closely bound to the surface by gravity. The prominent blue and red regions near the equator of Io dance across the moon with the changing orientation of Jupiter's magnetic field, illustrating the relationship between Io's auroras and the electric currents that excite them."[86]

"A faint blue emission is visible near the north pole of Io, possibly due to a volcanic plume erupting from the volcano Tvashtar at high northern latitude on the side of Io opposite Cassini. This eruption, observed by both Galileo and Cassini, left an enormous red ring around Tvashtar, visible in image PIA02588, released on March 29, 2001."[86]

## Saturn

Main sources: Wanderers/Saturn and Saturn
The image shows Saturn's northern hemisphere from the Cassini spacecraft with Mimas in front. Credit: NASA/JPL/Space Science Institute.

In the image at right, "Mimas drifts along in its orbit against the azure backdrop of Saturn's northern latitudes in this true color view from NASA's Cassini spacecraft. The long, dark lines on the atmosphere are shadows cast by the planet's rings."[87]

"Saturn's northern hemisphere is presently relatively cloud-free, and rays of sunlight take a long path through the atmosphere. This results in sunlight being scattered at shorter (bluer) wavelengths, thus giving the northernmost latitudes their bluish appearance at visible wavelengths."[87]

This is an enhanced color view of Enceladus. Credit: NASA/JPL/Space Science Institute.

"The south polar terrain is marked by a striking set of 'blue' fractures and encircled by a conspicuous and continuous chain of folds and ridges, testament to the forces within Enceladus that have yet to be silenced."[88]

"The mosaic was created from 21 false-color frames taken during the Cassini spacecraft's close approaches to Enceladus on March 9 and July 14, 2005. Images taken using filters sensitive to ultraviolet, visible and infrared light (spanning wavelengths from 338 to 930 nanometers) were combined to create the individual frames."[88]

"The mosaic is an orthographic projection centered at 46.8 degrees south latitude, 188 degrees west longitude, and has an image scale of 67 meters (220 feet) per pixel. The original images ranged in resolution from 67 meters per pixel to 350 meters (1,150 feet) per pixel and were taken at distances ranging from 11,100 to 61,300 kilometers (6,900 to miles) from Enceladus."[88]

## Tethys

This view from the Cassini orbital mission at Saturn shows the high-resolution color of the leading hemisphere of Tethys. Credit: NASA/JPL/Space Science Institute/Universities Space Research Association/Lunar & Planetary Institute.

At right is the first global high-resolution color image of Tethys.

"The color map shows the prominent dusky bluish band along the equator, first seen by Voyager in 1980, and shown ... to be due to the bombardment and alteration of the surface by high energy electrons traveling slower than the satellite's revolution period."[89]

## Titan

Main source: Titan
This image of the south polar region of Titan shows a depression in the blue and orange haze layers. Credit: Cassini Imaging Team, NASA/JPL-Caltech/Space Science Institute.

At right is an image of the blue haze layer near the south polar region of Titan. "The moon's high altitude haze layer appears blue here whereas the main atmospheric haze is orange. The difference in color could be due to particle size of the haze. The blue haze likely consists of smaller particles than the orange haze."[90]

"The depressed or attenuated layer appears in the transition area between the orange and blue hazes about a third of the way in from the left edge of the narrow-angle image. The moon's south pole is in the upper right of this image."[90]

"The southern pole of Titan is going into darkness as the sun advances towards the north with each passing day. The upper layer of Titan's hazes is still illuminated by sunlight."[90]

"Images taken using red, green and blue spectral filters were combined to create this natural color view. The images were obtained on Sept. 11, 2011 at a distance of approximately 83,000 miles (134,000 kilometers) from Titan. Image scale is 2,581 feet (787 meters) per pixel."[90]

## Neptune

Main sources: Wanderers/Neptune and Neptune
This picture from the Voyager 2 sequence shows two of the four cloud features which have been tracked by the Voyager cameras during the past two months. Credit: NASA.

A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue,[91] although Neptune's vivid azure differs from Uranus's milder cyan. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.[92]

## Blue halo stars

"The distribution of the high-latitude faint blue stars over Teff ... [shows] that the principal sequence [has] two gaps, at colors corresponding to log Teff ~ 4.11 (gap 1) and log Teff ~ 4.33 (gap 2). ... [T]he gaps [may be] a horizontal-branch phenomenon. ... [C]urrent theoretical concepts of the advanced evolution of Population II stars can explain the majority of blue halo stars.”[93]

## Blue stragglers

This Hubble Space Telescope image of NGC 6397 shows a number of bright blue stragglers present.[94] Credit: NASA.

Blue stragglers (BSS) are main sequence stars in open or globular clusters that are more luminous and bluer than stars at the main sequence turn-off point for the cluster. Standard theories of stellar evolution hold that the position of a star on the Hertzsprung–Russell diagram should be determined almost entirely by the initial mass of the star and its age. In a cluster, stars all formed at approximately the same time, and thus in an H–R diagram for a cluster, all stars should lie along a clearly defined curve set by the age of the cluster, with the positions of individual stars on that curve determined solely by their initial mass. With masses two to three times that of the rest of the main sequence cluster stars, blue stragglers seem to be exceptions to this rule.[95] The resolution of this problem is likely related to interactions between two or more stars in the dense confines of the clusters in which blue stragglers are found.

## Hypervelocity stars

"To date, all of the reported hypervelocity stars (HVSs), which are believed to be ejected from the Galactic center, are blue and therefore almost certainly young.”[96]

## Luminous blue variables

Luminous blue variables, also known as S Doradus variables, are very bright, blue, hypergiant variable stars named after S Doradus, the brightest star of the Large Magellanic Cloud. They exhibit long, slow changes in brightness, punctuated by occasional outbursts in brightness during substantial mass loss events (e.g. Eta Carinae, P Cygni). They are extraordinarily rare. The General Catalogue of Variable Stars only lists 20 objects as SDor.[97]

LBVs can shine millions of times brighter than the Sun and, with masses up to 150 times that of the Sun, approaching the theoretical upper limit for stellar mass, making them among the most luminous, hottest, and most energy-releasing stars in the universe. If they were any larger, their gravity would be insufficient to balance their radiation pressure and they would blow away the excess mass through stellar wind. As they are, they barely maintain hydrostatic equilibrium because their stellar wind constantly ejects matter, decreasing the mass of the star. For this reason, there are usually nebulae around such stars created by these outbursts; Eta Carinae is the nearest and best-studied example. Because of their large mass and high luminosity, their lifetime is very short — only a few million years.

## Blue outliers

There are faraway active galaxies that show a blueshift in their [O III] emission lines. One of the largest blueshifts is found in the narrow-line quasar, PG 1543+489, which has a relative velocity of -1150 km/s.[98] These types of galaxies are called "blue outliers".[98]

## Blue dwarfs

Main sources: tars/Blues/Dwarfs and Blue dwarfs

"The Algol-type binaries are close, semi-detached, interacting binary star systems which contain a cool F–K giant or sub-giant secondary star that fills its Roche lobe and is losing mass to a hot B–A main sequence primary star."[99]

"The Algols contain a hot blue dwarf star with a magnetically-active late-type companion. In the close Algols, the gas stream flows directly into the photosphere of the blue mass-gaining star because it does not have enough room to avoid impact with that star."[99]

## Blue giants

This is a schematic Hertzsprung-Russel diagram. Credit: Rursus.

A blue giant is a star with a spectral type of O or B (thus being noticeably blue in appearance) and a luminosity class of III (giant). In the standard Hertzsprung-Russell diagram, blue giants are found in the upper left corner, due to their high luminosity and early spectral type.

Def. a very hot and very luminous star that emits visible light in the blue portion of the spectrum is called a blue giant.

## Blue-horizontal-branch stars

Notation: let the symbol BHB stand for Blue horizontal-branch.

"BHB stars are excellent tracers of Galactic halo dynamics because they are luminous and have a nearly constant absolute magnitude within a restricted color range"[100]

## Blue subdwarfs

Blue “subdward B [stars] ... are difficult to fit into the evolutionary scheme.”[93]

Subdwarf B stars were discovered by Zwicky and Humason around 1947 when they found subluminous blue stars around the north galactic pole. In the Palomar-Green survey they were discovered to be the commonest kind of faint blue star with a magnitude over 18. During the 1960s spectroscopy discovered that many of the sdB stars are deficient in helium, with abundances below that predicted by the big bang theory. In the early 1970s Greenstein and Sargent measured temperatures and gravity strengths and were able to plot their correct position on the Hertzsprung–Russell diagram.[101]

## Blue supergiants

Blue supergiants (BSGs) are supergiant stars (luminosity class I) of spectral type O or B. They are extremely hot and bright, with surface temperatures of 30,000-50,000 K. They can have radii up to about 25 solar radii. These rare and enigmatic stars are amongst the hottest and brightest in the known Universe.[102]

Def. a very large, hot and luminous star; a large blue giant is called a blue supergiant.

## Galaxies

Main sources: Stars/Galaxies and Galaxies
The NASA/ESA Hubble Space Telescope has captured a beautiful galaxy that, with its reddish and yellow central area, looks rather like an explosion from a Hollywood movie. Credit: ESA/Hubble & NASA.

The galaxy, called NGC 5010, is in a period of transition. The aging galaxy is moving on from life as a spiral galaxy, like our Milky Way, to an older, less defined type called an elliptical galaxy. In this in-between phase, astronomers refer to NGC 5010 as a lenticular galaxy, which has features of both spirals and ellipticals.

"Schweizer [1978] defined five observational characteristics that unambiguously identified NGC 7252 as a late-stage merger ... One of these new characteristics is the blue portion of the optical spectrum of the main body."[103]

## Blue shifts

"A blueshift is any decrease in wavelength (increase in frequency); the opposite effect is referred to as redshift. In visible light, this shifts the colour from the red end of the spectrum to the blue end. The term also applies when photons outside the visible spectrum (e.g. x-rays and radio waves) are shifted toward shorter wavelengths, as well as to shifts in the de Broglie wavelength of particles. Blueshift is most commonly caused by relative motion toward the observer, described by the Doppler effect. An observer in a gravity well will also see infalling radiation gravitationally blueshifted, described by General Relativity in the same way as gravitational redshift. In a contracting universe, cosmological blueshift would be observed; the expanding universe gives a cosmological redshift, and the expansion is observed to be accelerating.

## Locations on Earth

VERITAS is located at the basecamp of the Smithsonian Astrophysics Observatory's Fred Lawrence Whipple Observatory (FLWO) in southern Arizona. Credit: VERITAS.

"VERITAS (Very Energetic Radiation Imaging Telescope Array System) is a ground-based gamma-ray instrument operating at the Fred Lawrence Whipple Observatory (FLWO) in southern Arizona, USA. It is an array of four 12m optical reflectors for gamma-ray astronomy in the GeV - TeV energy range. These imaging Cherenkov [a bluish light] telescopes are deployed such that they have the highest sensitivity in the VHE energy band (50 GeV - 50 TeV), with maximum sensitivity from 100 GeV to 10 TeV. This VHE observatory effectively complements the NASA Fermi mission."[104]

The Collaboration between Australia and Nippon for a Gamma Ray Observatory in the Outback, (CANGAROO) is for very high energy cosmic gamma ray observation by telescope detecting Cherenkov light. It is located on the Woomera Prohibited Area in South Australia.[105]

## Ancient history

The ancient history period dates from around 8,000 to 3,000 b2k.

"Beginning in about 2500 BC, the ancient Egyptians began to produce their own blue pigment known as Egyptian blue, made by grinding silica, lime, copper and alkali, and heating it to 800 or 900 degrees C. This is considered the first synthetic pigment.[106]

"In Egypt, blue was associated with the sky and with divinity. The Egyptian god Amun could make his skin blue so that he could fly, invisible, across the sky. Blue could also protect against evil; many people around the Mediterranean still wear a blue amulet, representing the eye of God, to protect them from misfortune.[107]

## Physics

Main source: Physics
${\displaystyle {\frac {c2}{\lambda T}}{\frac {1}{e^{\frac {c2}{\lambda T}}-1}}e^{\frac {c2}{\lambda T}}-5=0.}$

c2 = 1.438833 cm K may be used to approximate a pair such as (475 nm, 6300 K).

For a simple calculator, y=(1.48833/(0.0000475*6300))*exp(1.48833/(0.0000475*6300))/(exp(1.48833/(0.0000475*6300))-1)-5, followed by print y, yields a value close to zero (8.183732E-03). The closer to zero the more accurate the estimate.

Although Planck's equation is not an exact fit to a star's spectral radiance, it may be close enough to suggest if a star is an astronomical blue source using the above derivative.

From a Planckian spectrum peaked in the blue radiation band, the wavelength temperature pairs are approximately (450 nm, 6700 K) and (475 nm, 6300 K).

## Technology

Main source: Technology

"Blue sensitive photodetectors have been fabricated from thallium bromide (TlBr) crystals for scintillation detection. Lu2SiO5 (LSO)/TlBr detectors consisting of 3 mm × 4 mm TlBr photodetectors coupled to 2 mm ×2 mm × 4 mm long LSO scintillators have demonstrated energy resolutions of 34.5% and 25.5% full width at half maximum for 511 keV and 662 keV γ rays, respectively."[108]

"ZnSe appears as an attractive material to blue and near UV optoelectronics."[109]

## Hypotheses

Main source: Hypotheses
1. All blue stars are not transparent.

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