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This is a cross section of HSL color space. Credit: MaxPower.

Each form of radiation that an astronomical entity can emit, absorb, fluoresce, transmit, or reflect has a spectrum in time, space, intensity, or variation.

A spectrum of variation shows its colors.



Astronomy in general consists of observing the types of spectra apparently originating with an astronomical entity and describing these observations empirically if necessary with theory.


Radiation in its spectrum of forms from meteors to superluminals shows variation, time, space, and intensity.


This image exhibits forty-seven minerals that fluoresce in the visible while being irradiated in the ultraviolet. Credit: Hannes Grobe Hgrobe.
Fluorescing fluorite is from Boltsburn Mine Weardale, North Pennines, County Durham, England, UK. Credit: Didier Descouens.
Calcite fluoresces pink under long wave ultraviolet light. Credit: Herbert Art Gallery and Museum, Coventry.
Calcite fluoresces blue under short wave ultraviolet light. Credit: Herbert Art Gallery and Museum, Coventry.




Ultraviolet lamps are also used in analyzing minerals and gems. Materials may look the same under visible light, but fluoresce to different degrees under ultraviolet light, or may fluoresce differently under short wave ultraviolet versus long wave ultraviolet.

Ultraviolet lamps may cause certain minerals to fluoresce, and is a key tool in prospecting for tungsten mineralisation.

Many samples of fluorite exhibit fluorescence under ultraviolet light, a property that takes its name from fluorite.[3] Many minerals, as well as other substances, fluoresce. Fluorescence involves the elevation of electron energy levels by quanta of ultraviolet light, followed by the progressive falling back of the electrons into their previous energy state, releasing quanta of visible light in the process. In fluorite, the visible light emitted is most commonly blue, but red, purple, yellow, green and white also occur. The fluorescence of fluorite may be due to mineral impurities such as yttrium, ytterbium, or organic matter in the crystal lattice. In particular, the blue fluorescence seen in fluorites from certain parts of Great Britain responsible for the naming of the phenomenon of fluorescence itself, has been attributed to the presence of inclusions of divalent europium in the crystal.[4]

"Between 190 and 1700 nm, the ordinary refractive index varies roughly between 1.9 and 1.5, while the extraordinary refractive index varies between 1.6 and 1.4.[5]



Mount Redoubt in Alaska erupted on April 21, 1990. The mushroom-shaped plume rose from avalanches of hot debris that cascaded down the north flank. Credit: R. Clucas, USGS.
The volcanic eruption from Mount Pinatubo deposits a snowlike blanket of tephra on June 15, 1991. Credit: R.P. Hoblitt, USGS.

A spectrum of meteors may range in size (space), speed (time), numbers (intensity), or state (variation, e.g. rocky, liquid, gaseous, or plasma).

The 1910 approach, which came into naked-eye view around 10 April[6] and came to perihelion on 20 April,[6] was notable for several reasons: it was the first approach of which photographs exist, and the first for which spectroscopic data were obtained.[7] Furthermore, the comet made a relatively close approach of 0.15AU,[6] making it a spectacular sight. Indeed, on 19 May, the Earth actually passed through the tail of the comet.[8][9] One of the substances discovered in the tail by spectroscopic analysis was the toxic gas cyanogen,[10] which led astronomer Camille Flammarion to claim that, when Earth passed through the tail, the gas "would impregnate the atmosphere and possibly snuff out all life on the planet."[11] His pronouncement led to panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.[12] In reality, as other astronomers were quick to point out, the gas is so diffuse that the world suffered no ill effects from the passage through the tail.[11]


A typical comet nucleus has an albedo of 0.04.[14]



Of the Local Group, “[i]ts two dominant galaxies, the Milky Way and Andromeda (M31), are separated by a distance of ~700 kpc and are moving toward each other with a radial velocity of about -117 km s-1 (Binney & Tremaine 1987, p. 605).”[16] making Andromeda one of the few blueshifted galaxies. The Andromeda Galaxy and the Milky Way are thus expected to collide in about 4.5 billion years, although the details are uncertain since Andromeda's tangential velocity with respect to the Milky Way is only known to within about a factor of two.[17] A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy.[18] Such events are frequent among the galaxies in galaxy groups. The fate of the Earth and the Solar System in the event of a collision are currently unknown. If the galaxies do not merge, there is a small chance that the Solar System could be ejected from the Milky Way or join Andromeda.[19]


Asteroids in the solar system are categorized by size and number. Credit: Marco Colombo, DensityDesign Research Lab.{{free media}}

The center image is a log-log plot of frequency vs. size for asteroids in the solar system.


An atmospheric river forms over Hawai'i then heads toward California 10-11 April 2017. Credit: UW-CIMSS.{{fairuse}}

"Several times a year an atmospheric river [shown in the image on the right forming over Hawai'i]—a long, narrow conveyor belt of storms that stream in relentlessly from the Pacific Ocean—drops inches of rain or feet of snow on the U.S. west coast. Such a system triggered floods and mudslides in central and southern California this past weekend [2-3 February 2019]."[20]

"Atmospheric rivers flow through the sky about a mile above the ocean surface, and may extend across a thousand miles of ocean to the coast. Some bring routine rain but the more intense systems can carry as much water as 15 Mississippi Rivers. The series of storms striking land can arrive for days or, occasionally, weeks on end. They hit west-facing coastlines worldwide, although the U.S. experiences more than most other national coasts."[20]

The “atmospheric river scale” "ranks severity and impacts, from category 1 (weak) to category 5 (exceptional)."[20]

"Without a scale, we really had no way to objectively communicate what would be a strong storm or a weak one."[21]

"Scientists, the media and the public viewed atmospheric rivers as primarily a hazard, but the weaker ARs are quite beneficial. Water managers made it clear to us that a rating scale would be helpful."[21]

"The scale, published Tuesday in the Bulletin of the American Meteorological Society, ranks atmospheric rivers on five levels:"[20]

  • Category 1: Weak—primarily beneficial
  • Category 2: Moderate—mostly beneficial, but also somewhat hazardous
  • Category 3: Strong—balance of beneficial and hazardous
  • Category 4: Extreme—mostly hazardous, but also beneficial (if persistent drought)
  • Category 5—Exceptional—primarily hazardous

Cosmic raysEdit

The flux of cosmic-ray particles is a function of their energy. Credit: Sven Lafebre, after Swordy.[22]

At right is an image indicating the range of cosmic-ray energies. The flux for the lowest energies (yellow zone) is mainly attributed to solar cosmic rays, intermediate energies (blue) to galactic cosmic rays, and highest energies (purple) to extragalactic cosmic rays.[22]


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

Def. cosmic rays that are created when primary cosmic rays interact with interstellar matter are called secondary cosmic rays.

Galactic cosmic raysEdit

Space weather conditions are associated with solar activity. Credit: Daniel Wilkinson.{{free media}}
Galactic cosmic rays (GCR) are displayed from 1951 to 2006. Credit: Jbo166.

The "effect of time-variations in galactic cosmic rays on the rate of production of neutrons in the atmosphere [was studied using] a series of balloon and airplane observations of the [fast neutron] flux and spectrum of 1-10 MeV neutrons, in flights at high geomagnetic latitude, during [quiet times as well as during Forbush decreases, which are rapid decreases in the observed galactic cosmic rays following a coronal mass ejection (CME), and solar particle events for] the period of increasing solar modulation, 1965-1969. It also included latitude surveys in 1964-1965 and in 1968."[23]

In the image on the right for Forbush decreases, data include GOES-15 X-rays, energetic particles, and magnetometer. Cosmic Rays from the Moscow station show a Forbush Decrease.


Notation: let the symbol Z stand for atomic number.

let the symbol PeV stand for 1015 electron volts.

"The most dominant group is the iron group (Z = 25 − 27), at energies around 70 PeV more than 50% of the all-particle flux consists of these elements."[24]

In the graph on the right, the black line is cosmic-ray data and the red line is temperature. Ulysses data is included.

Solar cosmic raysEdit

This image shows an overview of the space weather conditions over several solar cycles including the relationship between sunspot numbers and cosmic rays. Credit: Daniel Wilkinson.
Comparison shows the observed (solar irradiance and sunspot number, symbols) and modeled (solid line) total magnetic flux Credit: Luis Eduardo A. Vieira and Sami K. Solanki.{{fairuse}}

"A persistent problem of solar cosmic-ray research has been the lack of observations bearing on the timing and conditions in which protons that escape to the interplanetary medium are first accelerated in the corona."[25]

For solar cosmic-rays, the apparent lack of proton acceleration in the corona seems justified, in contrast to the electrons, proton bremsstrahlung and gyrosynchrotron emission are negligible. This suggests a transit time anomaly, ΔTA, defined as follows:

ΔTA = ΔTonset - 11 min,

where ΔTonset is the deduced Sun-Earth transit time for the first arriving relativistic protons and 11 min is the nominal transit time for a ~2 GeV proton traversing a 1.3 AU Archimedes spiral path."[25] "The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV.

ΔTA may have values from "7-19 min for a small sample of well-connected ... cosmic-ray flares."[25] The transit time anomaly may be explained by a rise time associated with the ground-level events (GLEs). "The average GLE rise time ... for well-connected ... events ... defined to be the time from event onset to maximum as measured by the neutron monitor station showing the largest increase and whose asymptotic cone of acceptance ... includes the nominal direction of the Archimedean spiral path, is 21.3 min."[25]

"Data from an extensive air shower detector of ultrahigh-energy cosmic rays shows shadowing of the cosmic-ray flux by the Moon and the Sun with significance of 4.9 standard deviations. This is the first observation of such shadowing."[26]

"The ... solar proton flare on 20 April 1998 at W 90° and S 43° (9:38 UT) was measured by the GOES-9-satellite (Solar Geophysical Data 1998), as well as by other experiments on WIND ... and GEOTAIL. Protons were accelerated up to energies > 110 MeV and are therefore able to hit the surface of Mercury."[27]

Anomalous cosmic raysEdit






The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term temperature is used, since hot, thermal and cold neutrons are moderated in a medium with a certain temperature. The neutron energy distribution is then adopted to the Maxwellian distribution known for thermal motion. Qualitatively, the higher the temperature, the higher the kinetic energy is of the free neutron. Kinetic energy, speed and wavelength of the neutron are related through the De Broglie relation.

Moderated and other, non-thermal neutron energy distributions or ranges are

  • Fast neutrons with kinetic energies greater than 1 eV, 0.1 MeV or approximately 1 MeV, depending on the definition.
  • Slow neutrons a kinetic energy less than or equal to 0.4 eV.
  • Epithermal neutrons an energy from 1 eV to 10 keV.
  • Hot neutrons an energy of about 0.2 eV.
  • Thermal neutrons an energy of about 0.025 eV.[29] This is the most probable energy, while the average energy is 0.038 eV.
  • Cold neutrons an energy from 5 × 10−5 eV to 0.025 eV.
  • Very cold neutrons an energy from 3 × 10−7 eV to 5 × 10−5 eV.
  • Ultra cold neutrons an energy less than 3 × 10−7 eV.
  • Continuum region neutrons an energy from 0.01 MeV to 25 MeV.
  • Resonance region neutrons an energy from 1 eV to 0.01 MeV.
  • Low energy region neutrons an energy less than 1 eV.


Notation: WN5 is a component of V444 Cygni, with its Wolf-Rayet (W) spectrum dominated by NitrogenIII-V and HeliumI-II lines and WN2 to WN5 considered hotter or "early".

"The color temperature of the central part of the WN5 disk for λ < 7512 Å, where the main source of opacity is electron scattering, is Tc = 80,000-100,000 K. This high temperature represents the electron temperature slightly below the surface of the WN5 core--the level at which the star becomes optically thick in electron scattering."[30]



The colors chosen for the image on the right relate to density of the rock.


Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvo[31] whereby a neutrino created with a specific lepton flavor (electron, muon or tau) can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates. Neutrino oscillation is of theoretical and experimental interest since observation of the phenomenon implies that the neutrino has a non-zero mass.

Hard gamma raysEdit

Emergence of IC 310 is captured in a series of images. Credit: A. Neronov et al. and NASA/DOE/LAT collaboration.

"Fermi's Large Area Telescope (LAT) scans the entire sky every three hours, continually deepening its portrait of the sky in gamma rays, the most energetic form of light. While the energy of visible light falls between about 2 and 3 electron volts, the LAT detects gamma rays with energies ranging from 20 million to more than 300 billion electron volts (GeV)."[32]

"At higher energies, gamma rays are rare. Above 10 GeV, even Fermi's LAT detects only one gamma ray every four months from some sources."[32]

"Any object producing gamma rays at these energies is undergoing extraordinary astrophysical processes. More than half of the 496 sources [the Fermi hard-source list] in the new census are active galaxies, where matter falling into a supermassive black hole powers jets that spray out particles at nearly the speed of light."[32]

"One example is the well-known radio galaxy NGC 1275 [above left], which is a bright, isolated source below 10 GeV. At higher energies it fades appreciably and another nearby source begins to appear. Above 100 GeV, NGC 1275 becomes undetectable by Fermi, while the new source, the radio galaxy IC 310, shines brightly."[32]

"The catalog serves as an important roadmap for ground-based facilities called Atmospheric Cherenkov Telescopes, which have amassed about 130 gamma-ray sources with energies above 100 GeV. They include the Major Atmospheric Gamma Imaging Cherenkov telescope (MAGIC) on La Palma in the Canary Islands, the Very Energetic Radiation Imaging Telescope Array System (VERITAS) in Arizona, and the High Energy Stereoscopic System (H.E.S.S.) in Namibia."[32]

Soft gamma raysEdit

A view of 4C 71.07 from observations by the Burst and Transient Source Experiment. Credit: Mike McCollough, USRA.

On the right is a "view of 4C 71.07 from observations by the Burst and Transient Source Experiment. This helped convince scientists that they were studying data from the quasar and not some other source in the neighborhood."[33]

"Angela [Malizia] has now discovered this quasar in soft gamma rays."[34]

"It is also known as QSO 0836+710, a quasar or quasi-stellar object that emits baffling amounts of radio energy. (The numbers actually designate the same place in the sky: 71.07 is its declination, and 0836+710 is right ascension and declination.)"[33]

"It's basically the nucleus of a galaxy that is showing extraordinary activity."[34]

"What BATSE has discovered is that it can be a soft gamma-ray source."[34]

"This makes it the faintest and most distant object to be observed in soft gamma rays. 4C 71.07 has already been observed in gamma rays by the Energetic Gamma Ray Telescope (EGRET) also aboard the Compton Gamma Ray Observatory."[33]

"In the case of 4C 71.07, it's the brightest AGN seen above 20,000 electron volts (20 keV). Its average flux (the amount of radiation reaching our telescopes) is about 13 milliCrabs, or 13/1,000ths as much as the Crab Nebula, a standard candle in astrophysics."[33]

Hard X-raysEdit

This is an XMM Newton image of the Gemini gamma-ray source. Credit: P.A. Caraveo (INAF/IASF), Milan and ESA.

From 0.1 nm to 0.01 nm (about 12 to 120 keV) [are] hard X-rays.

The gamma-ray source Geminga, shown at right in hard X-rays by the satellite XMM Newton, is first observed by the Second Small Astronomy Satellite (SAS-2).

Geminga may be a sort of neutron star: the decaying core of a massive star that exploded as a supernova about 300,000 years ago.[35]

This nearby explosion may be responsible for the low density of the interstellar medium in the immediate vicinity of the Solar System. This low-density area is known as the Local Bubble.[36] Possible evidence for this includes findings by the Arecibo Observatory that local micrometre-sized interstellar meteor particles appear to originate from its direction.[37]

"Geminga is a very weak neutron star and the pulsar next to us, which almost only emits extremely hard gamma-rays, but no radio waves. ... Some thousand years ago our Sun entered this [Local Bubble] several hundred light-years big area, which is nearly dust-free."[38]

Soft X-raysEdit

The Sun in the soft X-rays as seen by the Hinode X-ray Telescope (XRT) on October 15, 2009. Credit: Joseph B. Gurman, Facility Scientist, Solar Data Analysis Center, ISAS/JAXA and NASA.


On the right is a soft X-ray image in the titanium-polyimide ("Ti_poly") filter from the Hinode X-Ray Telescope (XRT) obtained at: 2009/10/15 18:03 UTC.

"The primary filter for the sigmoid observations was the “thin-aluminum/polyimide” (or “Al/poly”) filter, imaging plasmas with temperature of roughly 2–5 MK in the active region."[39]

Super soft X-raysEdit

ASCA SIS spectrum of the intermediate polar EX Hya shows the H- and He-like K-alpha lines of Mg, Si, S, Ar, and Fe. Credit: Nicholas E. White.
The first detection of Pluto in X-rays has been made using NASA's Chandra X-ray Observatory in conjunction with observations from NASA's New Horizons spacecraft. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Center/Chandra X-Ray Center.

Super soft X-ray source "SSXSs are in most cases only detected below 0.5 keV".[40]

There are three SSXSs with bolometric luminosity of ~1038 erg/s that are novae: GQ Mus (BB, MW), V1974 Cyg (WD, MW), and Nova LMC 1995 (WD).[41] "Apparently, as of 1999 the orbital period of Nova LMC 1995 if a binary was not known."[40]


"[S]uper-soft X-rays [are] between 0.12 and 2.0 keV."[42]

The image on the right contains counts from super soft X-rays.

"The first detection of Pluto in X-rays has been made using NASA's Chandra X-ray Observatory in conjunction with observations from NASA's New Horizons spacecraft."[43]


"Detecting X-rays from Pluto is a somewhat surprising result given that Pluto - a cold, rocky world without a magnetic field - has no natural mechanism for emitting X-rays. However, scientists knew from previous observations of comets that the interaction between the gases surrounding such planetary bodies and the solar wind - the constant streams of charged particles from the sun that speed throughout the solar system -- can create X-rays."[43]


Ultra soft X-raysEdit

Ultra-soft X-rays are also known as grenz-rays (GRs).[44]

"Ultra soft X-ray spectra [are] in the range 60-250 eV".[45]

Extreme ultravioletsEdit

A full-disk multiwavelength extreme ultraviolet image of the sun taken by SDO on March 30, 2010. Credit: NASA/Goddard/SDO AIA Team.

"A full-disk multiwavelength extreme ultraviolet image of the sun [was] taken by SDO on March 30, 2010. False colors trace different gas temperatures. Reds are relatively cool (about 60,000 Kelvin, or 107,540 F); blues and greens are hotter (greater than 1 million Kelvin, or 1,799,540 F)."[46]



Far ultravioletsEdit

This image shows how the Earth glows in the ultraviolet. Credit: John W. Young, Apollo 16 lunar landing mission, NASA.

"This unusual false-color image [at right] shows how the Earth glows in ultraviolet (UV) light. The Far UV Camera/Spectrograph deployed and left on the Moon by the crew of Apollo 16 captured this image. The part of the Earth facing the Sun reflects much UV light and bands of UV emission are also apparent on the side facing away from the Sun. These bands are the result of aurora caused by charged particles given off by the Sun. They spiral towards the Earth along Earth's magnetic field lines."[47]

"An artificially reproduced color enhancement [at right] of a ten-minute far-ultraviolet exposure of Earth, taken with a filter which blocks the glow caused by atomic hydrogen but which transmits the glow caused by atomic oxygen and molecular nitrogen. Note that airglow emission bands are visible on the night side of Earth, one roughly centered between the two polar auroral zones and one at an angle to this extending northward toward the sunlit side of Earth. The UV camera was operated by astronaut John W. Young on the Apollo 16 lunar landing mission."[48]

Middle ultravioletsEdit

"Spectra of Venus (~35 Å resolution) and Jupiter (~50 Å resolution) were obtain using objective grating spectrographs in the 2300-3700 Å wavelength range. The geometric reflectivity of Jupiter, as a function of wavelength, lies in the range 0.15 to 0.25; that of Venus, 0.08 to 0.40."[49]

Near ultravioletsEdit

This composite image shows Z Camelopardalis, or Z Cam, a double-star system featuring a collapsed, dead star, called a white dwarf, and a companion star, as well as a ghostly shell around the system. Credit: NASA/JPL-Caltech/M. Seibert(OCIW)/T. Pyle(SSC)/R. Hurt(SSC).





The "huge shell around Z Cam [...] it could only be explained as the remnant of a full-blown classical nova explosion. This finding provides the first evidence that some binary systems undergo both types of explosions. Previously, a link between the two types of novae had been predicted, but there was no evidence to support the theory."[50]

"The faint bluish streak in the bottom right corner of the image is ultraviolet light reflected by dust that may or may not be related to Z Cam."[50]



A Photometric system is a set of well-defined passbands (or filters), with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.

Filter Letter Effective Wavelength Midpoint λeff For Standard Filter[51] Full Width Half Maximum[51] Variant(s) Description
U 365nm 66nm u, u', u* "U" stands for ultraviolet.
B 445nm 94nm b "B" stands for blue.
V 551nm 88nm v, v' "V" stands for visual.
G g, g' "G" stands for green (visual).
R 658nm 138nm r, r', R', Rc, Re, Rj "R" stands for red.
I 806nm 149nm i, i', Ic, Ie, Ij "I" stands for infrared.



Grey or gray is an intermediate color between black and white, a neutral or achromatic color, meaning literally a color "without color." [52] It is the color of a cloud-covered sky, of ash and of lead.[53]


The first image at left shows the various shades of grey.


White is the color of fresh milk and snow.[53] "of the colour of fresh milk or snow." See also Webster's New World Dictionary of American English, Third College Edition, (1988): "Having the color of pure snow or milk." See also The Random House College Dictionary of the English Language, Revised Edition,(1980). It is the color the human eye sees when it looks at light which contains all the wavelengths of the visible spectrum, at full brightness and without absorption. It does not have any hue.[53]

"de la couleur de la neige, du lait. Lumiere resultant de la combinaison de toutes les couleurs du spectre solaire."[54] (of the color of snow, of milk. Light resulting from the combination of all the colors of the solar spectrum.)








Black is "[t]he darkest color".[55]

"Se dit de la couleur la plus foncée, due à l'absence ou à l'absorption totale des rayons lumineux."[56]("said of the very darkest color, due to the absence or complete absorption of all rays of light.")

Human color visionsEdit




In humans there are three different iodopsins (rhodopsin analogs) that form the protein-pigment complexes photopsin I, II, and III. They are called erythrolabe, chlorolabe, and cyanolabe, respectively.[57] These photopsins have absorption maxima for yellowish-green (photopsin I), green (photopsin II), and bluish-violet light (photopsin III).

Cyanolabe, or tritan, has an absorption range of 400 to 500 nm and a peak absorption wavelength of 420-440 nm.[58][59]

In humans the OPN1SW gene (GeneID: 611) encodes cyanolabe.[60][61][62]

Color Frequency Wavelength
violet 668–789 THz 380–450 nm
blue 631–668 THz 450–475 nm
cyan 606–630 THz 476–495 nm
green 526–606 THz 495–570 nm
yellow 508–526 THz 570–590 nm
orange 484–508 THz 590–620 nm
red 400–484 THz 620–750 nm


There are several cases of astronomers who claimed that following a cataract operation, they could see shorter wavelengths than other people, slightly into the ultraviolet.

Color indexesEdit

Sample calibration colors[64]
Class B–V V–R Teff (K)
O5V –0.33 –0.15 42,000
B0V –0.30 –0.13 30,000
A0V –0.02 0.02 9,790
F0V 0.30 0.30 7,300
G0V 0.58 0.50 5,940
K0V 0.81 0.64 5,150
M0V 1.40 1.28 3,840




Abney effectEdit








Various shades of violet are shown. Credit: Mizunoryu, Badseed, Jacobolus.
Variations of violet are shown. Credit: Badseed.
The diagram shows various shades of purple. Credit: Mizunoryu, Badseed, Jacobolus.

Def. a “bluish-purple colour"[70] is called violet.

Def. a “colour/color that is a dark blend of red and blue; dark magenta"[71] is called purple.

Purple is a range of hues of color occurring between red and blue.[72] The Oxford English Dictionary describes it as a deep, rich shade between crimson and violet.[73]


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.[74]

"De la couleur du ciel sans nuages, de l'azur"[75]


The color box shows some of the variations of cyan. Credit: .



Aero blue is a fluorescent cyan color.

The word cerulean is probably derived from the Latin word caeruleus, "dark blue, blue or blue-green", which in turn probably derives from caelulum, diminutive of caelum, "heaven, sky".[76]

Natural gas (methane) has a cyan colored flame when burned with a mixture of air.


These color squares show a variety of greens. Credit: FedericoMP.
The word green has the same Germanic root as the words for grass and grow and is a common color reflected by leaves on Earth. Credit: The cat.
Malachite is a mineral occurring on Earth, like many greens, is colored by the presence of copper, specifically by basic copper(II) carbonate.[77] Credit: Rob Lavinsky.

Green has a wavelength range of approximately 520–570 nm, a frequency range of ~575–525 THz, with color coordinates of (0, 255, 0) and a hexagonal triplet of #00FF00 from sRGB source of sRGB approximation to NCS S 2060-G.[78]

Def. the "colour of growing foliage, as well as other plant cells containing chlorophyll; the colour between yellow and blue in the visible spectrum; one of the primary additive colour for transmitted light; the colour obtained by subtracting red and blue from white light using cyan and yellow filters"[79] is called green.

" nature chiefly conspicuous as the colour of growing herbage and leaves..."[80]

"Green is the color of emeralds, jade, and growing grass.[80] In the continuum of colors of visible light it is located between yellow and blue. Green is the color most commonly associated with nature and the environmental movement, Islam, spring, hope and envy.[81]

Green is the color you see when you look at light with a wavelength of roughly 520–570 nanometers.

It is one of the three additive colors, along with red and blue, which are combined on computer screens and color televisions to make all other colors.

In the subtractive color system, used in printing, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan.

On the HSV color wheel, also known as the RGB color wheel, the complement of green is magenta; that is, a purple color corresponding to an equal mixture of red and blue light. On a color wheel based on traditional color theory (RYB), the complementary color to green is considered to be red.[82]




These are examples of the various colors of yellow. Credit: Badseed.
Complements of yellow have a dominant wavelength in the range 380 to 480 nm. The green lines show several possible pairs of complementary colors. Credit: .
The image is of a horse colored with yellow ochre. from Lascaux cave. Credit: Cro-Magnon peoples.
This shows a field of yellow rapeseed. Credit: Petr Kratochvil.

Def. "[t]he colour of gold or butter; the colour obtained by mixing green and red light, or by subtracting blue from white light"[85] is called yellow.

Def. "a bright yellow colour, resembling the metal gold"[86]

is called


Yellow, in the form of yellow ochre pigment made from clay, was one of the first colors used in prehistoric cave art. The cave of Lascaux has an image of a horse colored with yellow estimated to be 17,300 years old.

Shades of yellow contains a more diverse set of yellow or yellow-like colors.


The box shows nine variations of the color orange. Credit: .

Def. the "colour of a ripe orange (the fruit); a color midway between red and yellow"[87] is called orange.


In wavelengths, red astronomy covers 620 - 750 nm.

Far redsEdit

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.


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,[88] 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.


Infrared astronomy, especially from space, explores up a vast portion of the spectrum beyond the red end of visible light. Credit: IRAS / ISO / 2MASS / Spitzer.

The wavelength of infrared light ranges from 0.75 to 300 micrometers. Infrared falls in between visible radiation, which ranges from 380 to 750 nanometers, and submillimeter waves. Infrared rays can be emitted, fluoresced, or reflected by an astronomical object.

Def. electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation, having a wavelength between 700 nm and 1 mm is called infrared.

Def. [e]lectromagnetic radiation having a wavelength approximately between 1 micrometre and 1 millimetre; perceived as heat is called infrared radiation.

Astronomers typically divide the infrared spectrum as follows:[89]

Designation Abbreviation Wavelength
Near Infrared NIR (0.7–1) to 5 µm
Mid Infrared MIR 5 to (25–40) µm
Far Infrared FIR (25–40) to (200–350) µm.

These are the approximate ranges for photon energies of the infrared bands:

Division Name Wavelength Photon Energy
Near-infrared 0.75-1.4 µm 0.9-1.7 eV
Short-wavelength infrared 1.4-3 µm 0.4-0.9 eV
Mid-wavelength infrared 3-8 µm 150-400 meV
Long-wavelength infrared 8–15 µm 80-150 meV
Far infrared 15 - 1,000 µm 1.2-80 meV
Wavelength range
Astronomical bands Telescopes
0.65 to 1.0 R and I bands All major optical telescopes
1.1 to 1.4 J band Most major optical telescopes and most dedicated infrared telescopes
1.5 to 1.8 H band Most major optical telescopes and most dedicated infrared telescopes
2.0 to 2.4 K band Most major optical telescopes and most dedicated infrared telescopes
3.0 to 4.0 L band Most dedicated infrared telescopes and some optical telescopes
4.6 to 5.0 M band Most dedicated infrared telescopes and some optical telescopes
7.5 to 14.5 N band Most dedicated infrared telescopes and some optical telescopes
17 to 25 Q band Some dedicated infrared telescopes and some optical telescopes
28 to 40 Z band Some dedicated infrared telescopes and some optical telescopes
330 to 370 Some dedicated infrared telescopes and some optical telescopes
450 submillimeter Submillimeter telescopes

The infrared band may be divided up based on the response of various detectors:[90]

  • Short-wave infrared: 1.0 to 3  µm (from the cut off of silicon to that of the MWIR atmospheric window. InGaAs covers to about 1.8  µm; the less sensitive lead salts cover this region.
  • Mid-wave infrared: 3 to 5  µm (defined by the atmospheric window and covered by indium antimonide [InSb] and HgCdTe and partially by lead selenide [PbSe]).
  • Long-wave infrared: 8 to 12, or 7 to 14  µm: the atmospheric window (Covered by HgCdTe and microbolometers).
  • Very-long wave infrared (VLWIR): 12 to about 30  µm, covered by doped silicon.

A commonly used sub-division scheme is:[91]

Division Name Abbreviation Wavelength Characteristics
Near-infrared NIR, IR-A DIN 0.75-1.4 µm Defined by the water absorption, and commonly used in fiber optic telecommunication because of low attenuation losses in the SiO2 glass (silica) medium. Image intensifiers are sensitive to this area of the spectrum. Examples include night vision devices such as night vision goggles.
Short-wavelength infrared SWIR, IR-B DIN 1.4-3 µm Water absorption increases significantly at 1,450 nm. The 1,530 to 1,560 nm range is the dominant spectral region for long-distance telecommunications.
Mid-wavelength infrared MWIR, IR-C DIN. Also called intermediate infrared (IIR) 3-8 µm In guided missile technology the 3-5 µm portion of this band is the atmospheric window in which the homing heads of passive IR 'heat seeking' missiles are designed to work, homing on to the Infrared signature of the target aircraft, typically the jet engine exhaust plume
Long-wavelength infrared LWIR, IR-C DIN 8–15 µm This is the "thermal imaging" region, in which sensors can obtain a completely passive picture of the outside world based on thermal emissions only and requiring no external light or thermal source such as the sun, moon or infrared illuminator. Forward-looking infrared (FLIR) systems use this area of the spectrum. This region is also called the "thermal infrared."
Far infrared FIR 15 - 1,000 µm (see also far-infrared laser).

NIR and SWIR is sometimes called "reflected infrared" while MWIR and LWIR is sometimes referred to as "thermal infrared." Due to the nature of the blackbody radiation curves, typical 'hot' objects, such as exhaust pipes, often appear brighter in the MW compared to the same object viewed in the LW.


Submillimeter waves lie at the far end of the infrared band, just before the start of the microwave band. Credit: Tatoute.

[T]erahertz radiation refers to electromagnetic waves propagating at frequencies in the terahertz range. It is synonymously termed submillimeter radiation, terahertz waves, terahertz light, T-rays, T-waves, T-light, T-lux, THz. The term typically applies to electromagnetic radiation with frequencies between high-frequency edge of the microwave band, 300 gigahertz (3 x 1011 Hz),"[92] and the long-wavelength edge of far-infrared light, 3000 GHz (3 x 1012 Hz or 3 THz). In wavelengths, this range corresponds to 0.1 mm (or 100 µm) infrared to 1.0 mm microwave.





Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz.[94] This broad definition includes both [ultra high frequency] UHF and [extremely high frequency] EHF (millimeter waves), and various sources use different boundaries.[95] In all cases, microwave includes the entire [super high frequency] SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum, with [radio frequency] RF engineering often putting the lower boundary at 1 GHz (30 cm), and the upper around 100 GHz (3 mm).

Microwave frequency bands
Letter Designation Frequency range Wavelength range Typical uses
U band 40 to 60 GHz 5.0 mm to 7.5 mm


Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 Gigahertz GHz to as low as 3 Kilohertz kHz, and corresponding to wavelengths from 1 millimeter to 100 kilometers.

Radar frequency bands
Band name Frequency range Wavelength range Notes
P < 300 MHz > 1 m 'P' for 'previous', applied retrospectively to early radar systems; essentially HF + VHF
mm 40–300 GHz 1.0–7.5 mm Millimetre band, subdivided as below. The frequency ranges depend on waveguide size. Multiple letters are assigned to these bands by different groups. These are from Baytron, a now defunct company that made test equipment.


Aurorae are mostly caused by energetic electrons precipitating into the atmosphere.[96] Credit: Samuel Blanc[1].

"Electron temperatures are generally derived from the ratio of auroral to nebular lines in [O III] or [N II]."[97] "[B]ecause of the proximity of strong night-sky lines at λ4358 and λλ5770, 5791, the auroral lines of [O III] λ4363 and [N II] λ5755 are often contaminated."[97]

Coronal cloudsEdit

"The temperature of yellow coronal regions is ... about 2.5 [x] 106 [K]. ... although some ions Ca XV will exist at lower, as well as higher temperatures."[98]

"The AS prominences [AS in Menzel-Evans' classification [4];] move with velocities exceeding by far the velocities of other types of prominences [7], [8]. As short-living phenomena, they are condensed quickly and the temperature of the coronal gases should rise in the early stages of their condensation. Indeed, the AS prominences use to be allied with yellow line emission (λ 5694)."[98]

"The yellow line is namely due to the ion Ca XV, according to Edlen's and Waldmeier's identification. ... the line λ 5694 is emitted by 3P1 - 3P0 transition of Ca XV."[98]

"The solar corona is not in thermodynamical equilibrium. In particular, the photo-recombination is compensated with electron impact ionization, while the reverse processes viz. the photoionization and recombination by impact with two electrons are there negligible."[98]


"[B]roadband optical photometry of Centaurs and Kuiper Belt objects [KBOs] from the Keck 10 m, the University of Hawaii 2.2 m, and the Cerro Tololo InterAmerican (CTIO) 1.5 m telescopes [shows] a wide dispersion in the optical colors of the objects, indicating nonuniform surface properties. The color dispersion [may] be understood in the context of the expected steady reddening due to bombardment by the ubiquitous flux of cosmic rays."[99]

Kuiper beltsEdit

"These authors proposed that the whole-disk surface colors of KBOs could be the result of the competition between the effects of irradiation of surface organics by cosmic-rays and the global resurfacing due to impacts. [...] When these high-energy protons collide with an icy target, they penetrate very [deep] under the surface."[100]


  1. The orthogonality between electric fields and magnetic fields came before 3D space.
  2. Orthogonality between electric fields and magnetic fields creates the illusion of three-dimensional space.
  3. Interference interactions create attractions and repulsions between fields which in turn produces radiance, motion, and allows time to be calibrated.
  4. Interactions create segmentation yielding an apparent particle/wave duality.

See alsoEdit


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External linksEdit