Radiation astronomy/Electromagnetics

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This is a colour composite image of RCW120. Credit: ESO/APEX/DSS2/ SuperCosmos/ Deharveng(LAM)/ Zavagno(LAM).{{free media}}

Radiation astronomy is often performed using electromagnetics. Electromagnetics are most familiar as light, or electromagnetic radiation.

The image at right is a colour "composite image of RCW120. It reveals how an expanding bubble of ionised gas about ten light-years across is causing the surrounding material to collapse into dense clumps where new stars are then formed. The 870-micron submillimetre-wavelength data were taken with the LABOCA camera on the 12-m Atacama Pathfinder Experiment (APEX) telescope. Here, the submillimetre emission is shown as the blue clouds surrounding the reddish glow of the ionised gas (shown with data from the SuperCosmos H-alpha survey). The image also contains data from the Second Generation Digitized Sky Survey (I-band shown in blue, R-band shown in red)."[1]

Electromagnetic radiation astronomy is a broader concept physics subject heading used by the American Physical Society (APS).[2]

Gamma rays[edit | edit source]

This is an image of quasar 3C 279 in gamma rays. Credit: NASA EGRET Compton observatory team.

Def. very high frequency (and therefore very high energy) electromagnetic radiation emitted as a consequence of radioactivity is called a gamma ray.

Def. electromagnetic radiation consisting of gamma rays is called gamma radiation.

Gamma rays typically have frequencies above 10 exahertz (or >1019 Hz), and therefore have energies above 100 keV and wavelengths less than 10 picometers (less than the diameter of an atom). However, this is not a hard and fast definition, but rather only a rule-of-thumb description for natural processes. Gamma rays from radioactive decay are defined as gamma rays no matter what their energy, so that there is no lower limit to gamma energy derived from radioactive decay. Gamma decay commonly produces energies of a few hundred keV, and almost always less than 10 MeV. In astronomy, gamma rays are defined by their energy, and no production process need be specified. The energies of gamma rays from astronomical sources range over 10 TeV, at a level far too large to result from radioactive decay. A notable example is extremely powerful bursts of high-energy radiation normally referred to as long duration gamma-ray bursts, which produce gamma rays by a mechanism not compatible with radioactive decay.

"The unusually wide span of the gamma-ray spectral window [covers] at least ten decades of photon energies (~105 - 1015 eV)".[3]

The Rosemary Hill Observatory (RHO) started observing 3C 279 in 1971,[4] and was further observed by the Compton Gamma Ray Observatory in 1991, when it was unexpectedly discovered to be one of the brightest gamma ray objects in the sky.[5] It is also one of the most bright and variable sources in the gamma ray sky monitored by the Fermi Space Telescope. Apparent superluminal motion was detected during observations first made in 1973 in a jet of material departing from the quasar, though it should be understood that this effect is an optical illusion caused by naive estimations of the speed, and no truly superluminal motion is occurring.[6]

Markarian (Mrk) 1501 is the first Seyfert I galaxy to have superluminal motion.[7] Mrk 1501 is an ultraviolet, X-ray, and gamma-ray source.

Hard gamma rays[edit | edit source]

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)."[8]

"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."[8]

"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."[8]

"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."[8]

"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."[8]

Soft gamma rays[edit | edit source]

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."[9]

"Angela [Malizia] has now discovered this quasar in soft gamma rays."[10]

"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.)"[9]

"It's basically the nucleus of a galaxy that is showing extraordinary activity."[10]

"What BATSE has discovered is that it can be a soft gamma-ray source."[10]

"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."[9]

"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."[9]

X-rays[edit | edit source]

This false-color image shows comet Tempel 1 as seen by Chandra X-ray Observatory on June 30, 2005, Universal Time. Credit: NASA/JPL-Caltech/UMD.

Def. an action or process of throwing or sending out a traveling X-ray in a line, beam, or stream of small cross section is called X-radiation.

X-rays span 3 decades in wavelength, frequency and energy.

Although the more energetic X-rays, photons with an energy greater than 30 keV (4,800 aJ) can penetrate the air at least for distances of a few meters (they would never have been detected and medical X-ray machines would not work if this was not the case) the Earth's atmosphere is thick enough that virtually none are able to penetrate from outer space all the way to the Earth's surface. X-rays in the 0.5 to 5 keV (80 to 800 aJ) range, where most celestial sources give off the bulk of their energy, can be stopped by a few sheets of paper; ninety percent of the photons in a beam of 3 keV (480 aJ) X-rays are absorbed by traveling through just 10 cm of air.

The X-rays observed from comets such as Tempel 1 on the right are caused by an interaction between highly charged oxygen in the solar wind and neutral gases from the comet.

Hard X-rays[edit | edit source]

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.[11]

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.[12] Possible evidence for this includes findings by the Arecibo Observatory that local micrometre-sized interstellar meteor particles appear to originate from its direction.[13]

"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."[14]

Soft X-rays[edit | edit source]

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.

From 10 to 0.1 nanometers (nm) (about 0.12 to 12 keV) they are classified as soft x-rays.

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."[15]

Super soft X-rays[edit | edit source]

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".[16]

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).[17] "Apparently, as of 1999 the orbital period of Nova LMC 1995 if a binary was not known."[16]

U Sco, a recurrent nova as of 1999 unobserved by ROSAT, is a WD (74-76 eV), Lbol ~ (8-60) x 1036 erg/s, with an orbital period of 1.2306 d.[17]

"[S]uper-soft X-rays [are] between 0.12 and 2.0 keV."[18]

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."[19]

"There is a significant difference in scale between the optical and X-ray images. New Horizons made a close flyby of Pluto but Chandra is located near the Earth, so the level of detail visible in the two images is very different. The Chandra image is 180,000 miles across at the distance of Pluto, but the planet is only 1,500 miles across. Pluto is detected in the X-ray image as a point source, showing the sharpest level of detail available for Chandra or any other X-ray observatory."[19]

"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."[19]

"The immediate mystery is that Chandra's readings on the brightness of the X-rays are much higher than expected from the solar wind interacting with Pluto's atmosphere. The Chandra detection is also surprising since New Horizons discovered Pluto's atmosphere was much more stable than the rapidly escaping, "comet-like" atmosphere that many scientists expected before the spacecraft flew past in July 2015. In fact, New Horizons found that Pluto's interaction with the solar wind is much more like the interaction of the solar wind with Mars, than with a comet. While Pluto is releasing enough gas from its atmosphere to make the observed X-rays, there isn't enough solar wind flowing directly at Pluto at its great distance from the Sun to make them according to certain theoretical models."[19]

Ultra soft X-rays[edit | edit source]

Ultra-soft X-rays are also known as grenz-rays (GRs).[20]

"Ultra soft X-ray spectra [are] in the range 60-250 eV".[21]

"Ultrasoft X-rays: When wavelength is greater than (10) A."[22] An Ångström is equal to 0.1 nm.

"The emitted X-ray radiation with wavelength between about 0.01 to 10 nm are important for studies of solar system nature and astrophysical applications because the composition of comets can be definitely specified which gives a fair picture about the solar system development in its early stages."[22]

Soft "or less penetrating x-rays, [...] have wavelengths from 3 to 20 Å, and "ultrasoft" x-rays with wavelengths in the 20 to 100 Å range."[23]

Ultraviolets[edit | edit source]

Def. "the spectral region bounded on the long wavelength side at about λ3000 by the onset of atmospheric ozone absorption and on the short wavelength side at λ912 by the photoionization of interstellar hydrogen" is called the ultraviolet.[24]

The draft ISO standard on determining solar irradiances (ISO-DIS-21348)5 describes the following ranges:

Name Abbreviation Wavelength range in nanometers Energy per photon
Before UV spectrum Visible light above 400 nm below 3.10 eV
Ultraviolet A, long wave, or black light UVA 400 nm–315 nm 3.10–3.94 eV
Near NUV 400 nm–300 nm 3.10–4.13 eV
Ultraviolet B or medium wave UVB 315 nm–280 nm 3.94–4.43 eV
Middle MUV 300 nm–200 nm 4.13–6.20 eV
Ultraviolet C, short wave, or germicidal UVC 280 nm–100 nm 4.43–12.4 eV
Far FUV 200 nm–122 nm 6.20–10.2 eV
Vacuum VUV 200 nm–100 nm 6.20–12.4 eV
Low LUV 100 nm–88 nm 12.4–14.1 eV
Super SUV 150 nm–10 nm 8.28–124 eV
Extreme EUV 121 nm–10 nm 10.2–124 eV
Beyond UV range X-rays below 10 nm above 124 eV

"Vacuum UV" is so named because it is absorbed strongly by air and is, therefore, used in a vacuum. In the long-wave limit of this region, roughly 150–200 nm, the principal absorber is the oxygen in air.

Extreme ultraviolets[edit | edit source]

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)."[25]

"Some of the images from the spacecraft show never-before-seen detail of material streaming outward and away from sunspots. Others show extreme close-ups of activity on the sun’s surface. The spacecraft also has made the first high-resolution measurements of solar flares in a broad range of extreme ultraviolet wavelengths."[25]

"The Extreme Ultraviolet Variability Experiment measures fluctuations in the sun’s radiant emissions. These emissions have a direct and powerful effect on Earth’s upper atmosphere -- heating it, puffing it up, and breaking apart atoms and molecules."[25]

Far ultraviolets[edit | edit source]

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."[26]

"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."[27]

Middle ultraviolets[edit | edit source]

"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."[28]

Near ultraviolets[edit | edit source]

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).

"This composite image [on the right] 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. The massive shell provides evidence of lingering material ejected during and swept up by a powerful classical nova explosion that occurred probably a few thousand years ago."[29]

"The image combines data gathered from the far-ultraviolet and near-ultraviolet detectors on NASA's Galaxy Evolution Explorer on Jan. 25, 2004. The orbiting observatory first began imaging Z Cam in 2003."[29]

"Z Cam is the largest white object in the image, located near the center. Parts of the shell are seen as a lobe-like, wispy, yellowish feature below and to the right of Z Cam, and as two large, whitish, perpendicular lines on the left."[29]

"Z Cam was one of the first known recurrent dwarf nova, meaning it erupts in a series of small, "hiccup-like" blasts, unlike classical novae, which undergo a massive explosion."[29]

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."[29]

"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."[29]

"The yellow objects are strong near-ultraviolet emitters; blue features have strong far-ultraviolet emission; and white objects have nearly equal amounts of near-ultraviolet and far-ultraviolet emission."[29]

Opticals[edit | edit source]

This is an optical image of U Camelopardalis from the Hubble Space Telescope. Credit: ESA/Hubble, NASA and H. Olofsson (Onsala Space Observatory).

Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it.[30] Optics usually describes the behavior of visible, ultraviolet, and infrared light. Geometric optics treats light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces. Physical optics is a more comprehensive model of light, which includes wave effects such as diffraction and interference that cannot be accounted for in geometric optics.

Although visible light itself extends from approximately 4000 Å to 7000 Å (400 nm to 700 nm),[31] the same equipment used at these wavelengths is also used to observe some near-ultraviolet and near-infrared radiation.

"A bright star [in the image at left] is surrounded by a tenuous shell of gas in this unusual image from the NASA/ESA Hubble Space Telescope. U Camelopardalis, or U Cam for short, is a star nearing the end of its life. As it begins to run low on fuel, it is becoming unstable. Every few thousand years, it coughs out a nearly spherical shell of gas as a layer of helium around its core begins to fuse. The gas ejected in the star’s latest eruption is clearly visible in this picture as a faint bubble of gas surrounding the star."[32]

"U Cam is an example of a carbon star. This is a rare type of star whose atmosphere contains more carbon than oxygen. Due to its low surface gravity, typically as much as half of the total mass of a carbon star may be lost by way of powerful stellar winds."[32]

"Located in the constellation of Camelopardalis (The Giraffe), near the North Celestial Pole, U Cam itself is actually much smaller than it appears in Hubble’s picture. In fact, the star would easily fit within a single pixel at the centre of the image. Its brightness, however, is enough to overwhelm the capability of Hubble’s Advanced Camera for Surveys making the star look much bigger than it really is. The shell of gas, which is both much larger and much fainter than its parent star, is visible in intricate detail in Hubble’s portrait. While phenomena that occur at the ends of stars’ lives are often quite irregular and unstable (see for example Hubble’s images of Eta Carinae, potw1208a), the shell of gas expelled from U Cam is almost perfectly spherical."[32]

"The image was produced with the High Resolution Channel of the Advanced Camera for Surveys [using the 606 nm and 814 nm filters]."[32]

Polars[edit | edit source]

Massive outbursts from the hypergiant star VY Canis Majoris are mapped with polarized light. Credit: NASA, ESA, and R. Humphreys (University of Minnesota).

Light given off by a star is un-polarized, i.e. the direction of oscillation of the light wave is random. However, when the light is reflected off the atmosphere of a planet, the light waves interact with the molecules in the atmosphere and they are polarized.[33]

"The outbursts [shown in polarized light in the image on the right] are from VY Canis Majoris, a red supergiant star that is also classified as a hypergiant because of its very high luminosity. The eruptions have formed loops, arcs, and knots of material moving at various speeds and in many different directions. The star has had many outbursts over the past 1,000 years as it nears the end of its life."[34]

"The polarized light shows how the dust is distributed."[34]

"VY Canis Majoris is ejecting large amounts of gas at a prodigious rate".[35]

"With these observations, we have a complete picture of the motions and directions of the outflows, and their spatial distribution, which confirms their origin from eruptions at different times from separate regions on the star."[35]

"The outermost material was ejected about 1,000 years ago, while a knot near the star may have been ejected as recently as 50 years ago."[34]

Visuals[edit | edit source]

This yellow aurora near the horizon has many vertical rays, sometimes called "light pillars", though these are probably not from ice crystals. Credit: Unknown, or unstated.
These two rays in the foreground are distinctive from the background aurora. Credit: Unknown, or unstated.
The image shows rays from an unexpected aurora. Credit: Anderson.
sRGB rendering of the spectrum of visible light
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

The visible spectrum is the portion of the electromagnetic spectrum that is visible to (can be detected by) the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm.[36] In terms of frequency, this corresponds to a band in the vicinity of 400–790 THz. A light-adapted eye generally has its maximum sensitivity at around 555 nm (540 THz), in the green region of the optical spectrum (see: luminosity function).

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.

The image on the right shows a yellow aurora near the horizon that has many vertical rays, sometimes called "light pillars", though these are probably not from ice crystals.

The second image down on the right shows two distinctive rays in the foreground that terminate in yellow over Queenstown, New Zealand, in July 2012.

"This aurora [on the left] was a bit of a surprise. For starters, on this Friday morning in August 2002, no intense auroral activity was expected at all. Possibly more surprising, however, the aurora appeared to show an usual structure of green rays from some locations. In the [left] image, captured from North Dakota, USA, a picket fence of green rays stretches toward the horizon. Mirroring the green rays is a red band, somewhat rare in its own right. Lights from the cities of Bismarck and Mandan are visible near the horizon. Large sunspot groups indicate that activity from an active Sun is relatively likely, possibly causing other streams of energetic particles to cascade onto the Earth and so causing more auroras."[37]

"The ray structure often seen in arcs and bands marks out the orientation of the magnetic field, nearly vertical at high latitude. The vertical extent of arcs and bands is also along this direction. Though the rays appear to converge upward, they are, in reality, essentially parallel shafts of light."[38]

"If rayed aurora is directly overhead, the point to which the rays appear to converge is the magnetic zenith. A line from that point to the observer marks out the local direction of the earth's magnetic field."[38]

"Standing in the aurora like pickets in a fence, the rays sometimes move sideways across the arcs and bands at high speeds. Sometimes one even sees them appear to move past each other both to the left and the right."[39]

"Rays line up along the direction of the earth's magnetic field, which points nearly vertically and somewhat to the northeast over Alaska and western Canada. To recognize the cross-sectional shapes of the rays, one needs to see them directly overhead in the sky. When they are in that position, they don't look like rays anymore; one reason why it took so long to discover their true shapes."[39]

"Not until very sensitive, high-speed television cameras were aimed at the bottoms of rays overhead was the mystery resolved. [The] rays were tightly wound up spirals only a kilometer or two across. Their form is difficult to recognize with the naked eye because the curled up shapes develop so quickly--sometimes in a second or so--and they often move very rapidly."[39]

"With a television camera capable of taking 30 pictures each second, it was possible to record the development of the spiral-shaped rays and measure their motion. Sometimes they move across the sky at speeds one hundred times that of a jet aircraft. To the observer on the ground, they do not appear to move quite that fast because the rays are so far away."[39]

Colors[edit | edit source]

The image shows separate curtains of an aurora borealis. Credit: surangaw / Fotolia.

Usually auroras seen locally are arcs that are part of an auroral oval around or near the magnetic poles. In the image on the right are separate curtains apparently from one aurora borealis.

Ices[edit | edit source]

The image shows light pillars over Finland. Credit: Thomas Kast.

"Pictured [on the right] are not aurora but nearby light pillars, a local phenomenon that can appear as a distant one. In most places on Earth, a lucky viewer can see a Sun-pillar, a column of light appearing to extend up from the Sun caused by flat fluttering ice-crystals reflecting sunlight from the upper atmosphere. Usually these ice crystals evaporate before reaching the ground. During freezing temperatures, however, flat fluttering ice crystals may form near the ground in a form of light snow, sometimes known as a crystal fog. These ice crystals may then reflect ground lights in columns not unlike a Sun-pillar."[40]

These "light pillars [are] extending up from bright parking lot lights in Oulu, Finland."[40]

Def. a "visual phenomenon created by the reflection of light from ice crystals with near-horizontal parallel planar surfaces"[41] is called a light pillar.

Fluorescences[edit | edit source]

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons; this two-photon absorption can lead to emission of radiation having a shorter wavelength than the absorbed radiation. The emitted radiation may also be of the same wavelength as the absorbed radiation, termed "resonance fluorescence".[42]

The most striking examples of fluorescence occur when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye, and the emitted light is in the visible region.

The common fluorescent lamp relies on fluorescence. Inside the glass tube is a partial vacuum and a small amount of mercury. An electric discharge in the tube causes the mercury atoms to emit ultraviolet light. The tube is lined with a coating of a fluorescent material, called the phosphor, which absorbs the ultraviolet and re-emits visible light. Fluorescent lighting is more energy-efficient than incandescent lighting elements. However, the uneven spectrum of traditional fluorescent lamps may cause certain colors to appear different than when illuminated by incandescent light or daylight. The mercury vapor emission spectrum is dominated by a short-wave UV line at 254 nm (which provides most of the energy to the phosphors), accompanied by visible light emission at 436 nm (blue), 546 nm (green) and 579 nm (yellow-orange). These three lines can be observed superimposed on the white continuum using a hand spectroscope, for light emitted by the usual white fluorescent tubes. These same visible lines, accompanied by the emission lines of trivalent europium and trivalent terbium, and further accompanied by the emission continuum of divalent europium in the blue region, comprise the more discontinuous light emission of the modern trichromatic phosphor systems used in many compact fluorescent lamp and traditional lamps where better color rendition is a goal.[43]

Luminescences[edit | edit source]

Luminol glows in an alkalic solution when you add Hemoglobin and H2O2. Credit: everyone's idle from berlin, germany.

Def. any "emission of light that cannot be attributed merely to the temperature of the emitting body"[44] is called luminescence.

Whites[edit | edit source]

Lhotse is seen from the climb up to Chhukung Ri. Credit: Jamie O'Shaughnessy.
This is a white aurora. Credit: hensen.
This is a predominantly white aurora over Canada. Credit: Paul Zizka.

White is the color of fresh milk and snow.[45][46] 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.[47]

"de la couleur de la neige, du lait. Lumiere resultant de la combinaison de toutes les couleurs du spectre solaire."[48] (of the color of snow, of milk. Light resulting from the combination of all the colors of the solar spectrum.)

The images on the right and left show white auroras.

Multicolored auroras[edit | edit source]

This is a multicolor aurora. Credit: tommy-eliassen and leonafaye.
This shows a multicolored aurora over Finland. Credit: S. D. Simonson.

"Auroras are known to be generated by beams of electrons which are accelerated along Earth's magnetic field lines. The fast-moving electrons collide with atoms in the ionosphere at altitudes of between 100 to 600 km. This interaction with oxygen atoms results in a green or, more rarely, red glow in the night sky, while nitrogen atoms yield blue and purple colours."[49]

On the right and left are two images of multicolored auroras. The second down on the right occurred over Finland in October 2012.

Blacks[edit | edit source]

Anthracite coal is black. Credit: USGS and the Mineral Information Institute.
Basalt is a black rock, albite is a white mineral silicate, and epidote is green. Credit: Siim Sepp.
This image appears to contain a black aurora between the red and green. Credit: Unknown, or not stated.
This aurora over Tromso, Norway, appears to contains a black aurora. Credit: Unknown, or unstated.
This is apparently the first of two images of auroras over Fairbanks, Alaska. Credit: Unknown, or unstated.
This is apparently the second of two images of auroras over Fairbanks, Alaska. Credit: Unknown, or unstated.

Black is the color of coal, ebony, and of outer space. It is the darkest color, the result of the absence of or complete absorption of light. It is the opposite of white and often represents darkness in contrast with light.[50][51][52]

"Opposite to white: colourless from the absence or complete absorption of light. Also, so near this as to have no distinguishable colour, very dark."[50]

Black is "[t]he darkest color".[51]

"Se dit de la couleur la plus foncée, due à l'absence ou à l'absorption totale des rayons lumineux."[52]("said of the very darkest color, due to the absence or complete absorption of all rays of light.")

"Most people have heard of auroras - more commonly known as the Northern and Southern Lights - but, except on rare occasions, such as the recent widespread apparition on 17 March, they are not usually visible outside the polar regions. Less familiar are phenomena known as black auroras, dark patches which often subdivide the glowing curtains of red and green light."[49]

"Whereas bright auroras are created by electrons plunging downward into the ionosphere, neighbouring black auroras are caused by electrons escaping from the ionosphere - like a kind of anti-aurora. However, until now, scientists have been struggling to explain the relationship between the two auroral types."[49]

"We found strong evidence of a two-way interaction between the ionosphere and the magnetosphere."[53]

"Auroral arcs are created by electric currents. The beam of electrons shooting down towards Earth along magnetic field lines is actually an electric current aligned with Earth's magnetic field. It is called an upward, field-aligned current because the negatively charged electrons are moving downward."[53]

"On the other hand, when a downward magnetospheric current meets the ionosphere, electrons are driven upwards and 'sucked' from the ionosphere, creating a black aurora. However, when the electron density in the ionosphere drops markedly the black aurora becomes less intense."[53]

"This evacuation of the ionosphere is essential in shaping the black auroras. The process is much more important on Earth's nightside than on the dayside because sunlight creates new electrons which fill the 'hole'."[53]

The "two-way electrodynamic coupling between the magnetosphere and ionosphere [...] is made possible by a horizontal drift of ions in the ionosphere, known as the Pedersen current, which closes the current system."[53]

"According to convention, negatively charged electrons flow downward, from the magnetosphere to the ionosphere, in an upward field-aligned current. Electrons flow upward, from the ionosphere to the magnetosphere, in a downward field-aligned current."[49]

The third and fourth images down on the right are apparently two successive images of the same aurora showing changes with time and black auroras.

Grays[edit | edit source]

These are the various shades of gray. Credit: Mizunoryu, Badseed, Jacobolus.
This image shows some red pebbles among gray pebbles of the same rock type. Credit: Titus Tscharntke.
Gray clay shows the cracking from water loss. Credit: Barnes Dr Thomas G, U.S. Fish and Wildlife Service.

Grey or gray is an intermediate color between black and white, a neutral or achromatic color, meaning literally a color "without color." [54] It is the color of a cloud-covered sky, of ash and of lead.[55]

The first image at right shows some red pebbles among gray pebbles, which are all the same rock type.

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

The second image at left of gray clay shows the cracking from water loss.

Violets[edit | edit source]

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.
The aurora borealis imaged shows blue, violet, and purple colors. Credit: Ragnar Sigurdsson.
This multicolored aurora has a strong violet band above the pink band. Credit: Black Swamp Storm Intercept Team.

Violet is a bright bluish purple color that takes its name from the violet flower.[51] ... Violet is at the lower end of spectrum of light, with a wavelength between approximately 380-450 nanometers.[56]

Def. a bluish-purple colour is called violet.

Def. a colour/color that is a dark blend of red and blue; dark magenta is called purple.

Purple is a range of hues of color occurring between red and blue.[57] The Oxford English Dictionary describes it as a deep, rich shade between crimson and violet.[58]

The aurora borealis imaged on the left shows blue, violet, and purple colors with the Milky Way in the background.

The second aurora on the right contains an intense violet band above the pink band.

Blues[edit | edit source]

Blue is between violet and green in the spectrum of visible light. Credit: Gringer.
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.
This is a blue aurora borealis that occurred over Iceland. Credit: Daniel Nelson.
The image contains an extensive blue aurora over Canada. Credit: Unknown, or unstated.
This is a blue aurora with some purple at the lower left. Credit: Micha.

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. Blue is the colour of the clear sky and the deep sea.[59]

"De la couleur du ciel sans nuages, de l'azur"[60]

The image on the right shows blue aurora borealis that occurred over Iceland.

The second image down on the right shows an extensive blue aurora above the green over Canada.

The image on the left shows an extensive blue aurora.

Cyans[edit | edit source]

Planck's equation (colored curves) accurately describes black body radiation. Credit: .
The color box shows some of the variations of cyan. Credit: .
This is a white and aqua aurora over Finland. Credit: Unknown, or unstated.
This aurora borealis is a greenish-blue or cyan. Credit: beautiful-portals.tumblr.com.
This aurora contains a band of aqua-blue. Credit: Unknown, or unstated.

Cyan light has a wavelength of between 490 and 520 nanometers, between the wavelengths of blue and green.[61]

Planck's equation describes the amount of spectral radiance at a certain wavelength radiated by a black body in thermal equilibrium.

In terms of wavelength (λ), Planck's equation is written as

where B is the spectral radiance, T is the absolute temperature of the black body, kB is the Boltzmann constant, h is the Planck constant, and c is the speed of light.

This form of the equation contains several constants that are usually not subject to variation with wavelength. These are h, c, and kB. They may be represented by simple coefficients: c1 = 2h c2 and c2 = h c/kB.

By setting the first partial derivative of Planck's equation in wavelength form equal to zero, iterative calculations may be used to find pairs of (λ,T) that to some significant digits represent the peak wavelength for a given temperature and vice versa.


Use c2 = 1.438833 cm K.

For a star to have a peak in the cyan, iterative calculations using the last equation yield the pairs: approximately (476 nm, 6300 K) and (495 nm, 6100 K).

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 cyan source.

Electric blue is a color close to cyan that is a representation of the color of lightning, an electric spark, and argon signs.

The electric blue glow of electricity results from the spectral emission of the excited ionized atoms (or excited molecules) of air (mostly oxygen and nitrogen) falling back to unexcited states, which happens to produce an abundance of electric blue light. This is the reason electrical sparks in air, including lightning, appear electric blue. It is a coincidence that the color of Cherenkov radiation and light emitted by ionized air are a very similar blue despite their very different methods of production.

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".[62]

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

This is a white aurora at the lower center and an aqua aurora in the upper part of the image on the right.

The aurora borealis on the left is probably the usual green aurora but appears greenish-blue or cyan.

Greens[edit | edit source]

These color squares show a variety of greens. Credit: FedericoMP.
Green, blue and red are additive colors. All the colors you see on your computer screen are made by mixing them in different intensities. Credit: Bb3cxv.
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.[63] Credit: Rob Lavinsky.
Cassini imaged the surface of Saturn's moon Helene as the spacecraft flew by the moon on Jan. 31, 2011. Credit: NASA / Jet Propulsion Lab / Space Science Institute.
The image shows Aurora Borealis at its finest in Alaska. Credit: jewishbrick.
This green aurora appears to have a black aurora at its center. Credit: Sebastian Saarloos.
This green aurora has the appearance of coming at the viewer. Credit: Unknown, or unstated.
This is a loop aurora over Summit, Alaska. Credit: Jason Ahrns.

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.[64]

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 is called green.

"...in nature chiefly conspicuous as the colour of growing herbage and leaves..."[65]

Green is the color of emeralds, jade, and growing grass.[65] 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.[66]

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.[67]

The perception of greenness (in opposition to redness forming one of the opponent mechanisms in human color vision) is evoked by light which triggers the medium-wavelength M cone cells in the eye more than the long-wavelength L cones. Light which triggers this greenness response more than the yellowness or blueness of the other color opponent mechanism is called green. A green light source typically has a spectral power distribution dominated by energy with a wavelength of roughly 487–570 nm. More specifically, "blue green" 487–493 nm, "bluish green" 493–498 nm, "green" 498–530 nm, "yellowish green" 530–559 nm, "yellow green" 559–570 nm.[68]

Green earth is a natural pigment. It s composed of clay colored by iron oxide, magnesium, aluminum silicate, or potassium. Large deposits were found in the South of France near Nice, and in Italy around Verona, on Cyprus, and in Bohemia. The clay was crushed, washed to remove impurities, then powdered. It was sometimes called Green of Verona.[69]

"Cassini imaged [on the left] the surface of Saturn's moon Helene as the spacecraft flew by the moon on Jan. 31, 2011."[70]

"This small moon leads Dione by 60 degrees in the moons' shared orbit. Helene is a "Trojan" moon of Dione, named for the Trojan asteroids that orbit 60 degrees ahead of and behind Jupiter as it circles the Sun."[70]

"This view looks toward the leading hemisphere of Helene (33 kilometers, 21 miles across). North on Helene is up and rotated 2 degrees to the left."[70]

"The image was taken with the Cassini spacecraft narrow-angle camera using a combination of spectral filters sensitive to wavelengths of polarized green light centered at 617 and 568 nanometers. The view was obtained at a distance of approximately 31,000 kilometers (19,000 miles) from Helene and at a Sun-Helene-spacecraft, or phase, angle of 65 degrees. Scale in the original image was 187 meters (612 feet) per pixel. The image was contrast enhanced and magnified by a factor of 1.5 to enhance the visibility of surface features."[70]

This aurora borealis on the right that occurred over Alaska is almost all green.

The second green aurora down on the right is over Urenroe, Russia. It shows the radiation pattern of being directly overhead.

"Last night Earth experienced a geomagnetic storm and aurora were visible in the Northern U.S. states. [This image on the left] of [an] aurora [was] captured on March 17, 2015, around 5:30 a.m. EDT in Donnelly Creek, Alaska by Sebastian Saarloos. These aurora might have been caused by the fast solar wind streaming from two solar coronal holes."[71]

An earlier green aurora is shown second down on the left from apparently January 2015.

Yellows[edit | edit source]

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.
This photo of yellow and green auroras shows convincingly that yellow is a distinctive result of the auroral process. Credit: Belinda Witzenhausen.
This image captures an unusual aurora from Urengoi, Russia. Credit: Unknown.
This aurora image from Alaska shows distinctive yellow associated with the horizon. Credit: Unknown.
This is another aurora from Alaska containing yellow. Credit: Unknown.
This orange and yellow aurora occurred above Beaghmore Stone Circles. Credit: Martin McKenna.

"To see day objects with most distinctness, I require a less concave lens by one degree than for seeing the stars best by night, the cause of which seems to be, that the bottom of the eye being illuminated by the day objects, and thereby rendered a light ground, obscures the fainter colours blue indigo and violet in the circle of dissipation, and therefore the best image of the object will be found in the focus of the bright yellow rays, and not in that of the mean refrangible ones, or the dark green, agreeable to Newton's remark, and consequently nearer the retina of a short-sighted person; but the parts of the retina surrounding the circle of dissipation of a star being in the dark, the fainter colours, blue, indigo, and violet, will have some share in forming the image, and consequently the focus will be shorter."[72] Bold added.

"The error due to color loses its disturbing effect because the photographic plate is not sensitive for the red and yellow rays, while the photographically active rays of shorter wave-length are well united by the objective."[73]

"The star brightness increase in 1964 was considerably different in yellow and blue rays. ... Extensive tables and graphs represent the mean photographic and photovisual curve of V1329 Cyg observed in Moscow and Odessa, brightness curves in blue and yellow rays, brightness increases, and brightness minima before and after an outburst."[74]

"The GE Reveal bulb is marketed as the bulb that is made to “specially filter out yellow rays that hide life's true colors.” This is accomplished by the use of neodymium in the glass."[75]

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


a bright yellow colour, resembling the metal gold

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.

Any doubt that a yellow aurora can occur should be put to rest with the image on the right.

The image on the left shows individual rays of radiation apparently impacting an upper atmospheric layer to produce a bead-like pattern.

The second image down on the left shows yellow of an aurora near the horizon with apparently the midnight Sun off to the left.

The third image on the left contains yellow aurora that is closer to true yellow.

The second image down on the right shows a yellow aurora following the skyline with an orange aurora above.

"On February 25th 2014 a violent X4.9-class solar flare erupted from a large sunspot group which had just rotated into view around the SE limb of the solar disk. The CME it unfurled was a massive full halo feature in the form of an expanding cloud of highly charged particles and plasma en route to the inner planets at a staggering velocity of over 2000km/sec. At this speed the CME would sweep across 93 million miles of space and impact planet Earth in only two days. However there was bad news as the source of this flare - and subsequent CME event - was located so close to the limb of the sun that the CME was very unlikely to impact Earth because it was located too far from the meridian and hence was not termed geoeffective which meant there was no chance of any Earth directed component at all. A few hours later a more detailed look by spaceweather scientists followed which offered some cautious optimism for in some of their forecasting models there was a slight chance that the CME could hit Earth a glancing blow with a possibility of minor geomagnetic storms on Feb 27th however the consensus was that the CME would probably miss entirely or if there was a hit then it wasn't expected to be significant."[76]

"The Bz is the secret to a good aurora show, this is [where] its at, the Bz (pronounced Bee Sub Zee) is a value indicating the tilt of the Interplanetary Magnetic Field or IMF. If the Bz is N then you can forget about a good show, even if the KP is good it won't make a difference, however if the Bz tilts S then the Earth and Sun's magnetic fields become aligned and in effect what you are doing is opening a gate way [...] allowing the highly charged solar particles to interact with the Earth's magnetosphere undisturbed - this open channel will manifest as a strong geomagnetic storm. The fact that it was - 20 got me extremely excited, this value meant the aurora was going to be strong and would be seen from far more southern latitudes than usual."[76]

Oranges[edit | edit source]

In traditional colour theory, orange is a range of colours between red and yellow. Credit: Wilinckx.
The box shows nine variations of the color orange. Credit: .
This is an aurora borealis photographed as occurring above Finland. Credit: Pekka Parviainen.
This is an orange aurora over New York. Credit: Unknown, or unstated.
This is an extensively orange aurora that occurred over Maine. Credit: Unknown, or unstated.

The orange portion of the visible spectrum is from 590 to 620 nm in wavelength.

In optics, orange is the colour seen by the eye when looking at light with a wavelength between approximately 585–620 nm. It has a hue of 30° in HSV colour space.

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

The aurora imaged on the right occurred over Finland in early October 2002. Note the pastel orange colors.

The second image down on the right shows a reddish-orange aurora observed over New York in October 2011.

To compare and contrast with the orange-containing aurora on the right is the extensively orange aurora on the left which also occurred over Finland.

Reds[edit | edit source]

This is a red aurora borealis. Credit: Isarl.
A view of an all-red aurora is captured in Independence, Mo., on October 24, 2011. Credit: Tobias Billings.

In wavelengths, red astronomy covers 620 - 750 nm.

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

Infrared (IR) light is electromagnetic radiation with longer wavelengths than those of visible light, extending from the nominal red edge of the visible spectrum at 700 nanometres (nm) to 1 mm. This range of wavelengths corresponds to a frequency range of approximately 430 THz down to 300 GHz,[77] and includes most of the thermal radiation emitted by objects near room temperature. Infrared light is emitted or absorbed by molecules when they change their rotational-vibrational movements.

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

On the right is an example of a red aurora borealis.

"A coronal mass ejection (CME) shot off the sun late in the evening of October 21 [2011] and hit Earth on October 24 at about 2 PM ET. The CME caused strong magnetic field fluctuations near Earth's surface – technically, this level of magnetic fluctuation rated a 7 out of 9 on what is called the "KP index" – that resulted in aurora that could be seen in the US as far south as Alabama. This image [on the left] was captured in Independence, Mo. Such completely red aurora are not as common as green aurora, however they can happen during strong solar activity and they occur a little more often at low latitudes such as where this was taken."[78]

"The strength, speed, and mass of this CME also pushed the boundary of Earth's magnetic fields – a boundary known as the magnetopause – from its normal position at about 40,000 miles away from Earth in to about 26,000 miles. This is the area where spacecraft in geosynchronous orbit reside, so these spacecraft were briefly orbiting outside of Earth's normal environment, traveling through material and magnetic fields far different from usual."[78]

Infrareds[edit | edit source]

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:[79]

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:[80]

  • Near infrared: from 0.7 to 1.0  µm (from the approximate end of the response of the human eye to that of silicon).
  • 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:[81]

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.

Submillimeters[edit | edit source]

Submillimeter waves lie at the far end of the infrared band, just before the start of the microwave band. Credit: Tatoute.
This image composite shows a warped and magnified view of a galaxy discovered by the Herschel Space Observatory. Credit: ESA/NASA/JPL-Caltech/Keck/SMA.

[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),"[82] 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.

Terahertz radiation is emitted as part of the black body radiation from anything with temperatures greater than about 10 kelvin. While this thermal emission is very weak, observations at these frequencies are important for characterizing the cold 10-20K dust in the interstellar medium in the Milky Way galaxy, and in distant starburst galaxies. Telescopes operating in this band include the James Clerk Maxwell Telescope, the Caltech Submillimeter Observatory and the Submillimeter Array at the Mauna Kea Observatory in Hawaii, the BLAST balloon borne telescope, the Herschel Space Observatory, and the Heinrich Hertz Submillimeter Telescope at the Mount Graham International Observatory in Arizona. The Atacama Large Millimeter Array, under construction, will operate in the submillimeter range. The opacity of the Earth's atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, or to space.

"This image composite [on the right] shows a warped and magnified view of a galaxy discovered by the Herschel Space Observatory, one of five such galaxies uncovered by the infrared telescope. The galaxy -- referred to as "SDP 81" -- is the yellow dot in the left image taken by Herschel. It can also be seen as the pink smudges in the right image, a composite of ground-based observations showing more detail."[83]

"Herschel was able to find the galaxy, which is buried in dust, because it happens to be positioned behind another galaxy (blue blob at right), which is acting like a cosmic lens to make it appear brighter. The gravity of the foreground galaxy is distorting and magnifying the distant galaxy's light, causing it to appear in multiple places, as seen as the pink smudges. The distant galaxy is so far away that its light took about 11 billion years to reach us."[83]

"Herschel couldn't detect the foreground galaxy, but astronomers were able to spot it in visible light using the W.M. Keck Observatory. Several follow-up observations by ground telescopes helped to get a better view of the distant galaxy. For example, the pink smudges at the right show wavelengths that are even longer than what Herschel sees in the submillimeter portion of the electromagnetic spectrum. Those observations were made by the Smithsonian Astrophysical Observatory's Submillimeter Array in Hawaii."[83]

Microwaves[edit | edit source]

The image shows sea ice coverage in 1980 (bottom) and 2012 (top). Credit: Josefino C. Comiso, NASA.

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.[84] This broad definition includes both [ultra high frequency] UHF and [extremely high frequency] EHF (millimeter waves), and various sources use different boundaries.[85] 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
L band 1 to 2 GHz 15 cm to 30 cm military telemetry, GPS, mobile phones (GSM), amateur radio
S band 2 to 4 GHz 7.5 cm to 15 cm weather radar, surface ship radar, and some communications satellites (microwave ovens, microwave devices/communications, radio astronomy, mobile phones, wireless LAN, Bluetooth, ZigBee, GPS, amateur radio)
C band 4 to 8 GHz 3.75 cm to 7.5 cm long-distance radio telecommunications
X band 8 to 12 GHz 25 mm to 37.5 mm satellite communications, radar, terrestrial broadband, space communications, amateur radio
Ku band or P band 12 to 18 GHz 16.7 mm to 25 mm satellite communications
K band 18 to 26.5 GHz 11.3 mm to 16.7 mm radar, satellite communications, astronomical observations, automotive radar
Ka band 26.5 to 40 GHz 5.0 mm to 11.3 mm satellite communications
Q band 33 to 50 GHz 6.0 mm to 9.0 mm satellite communications, terrestrial microwave communications, radio astronomy, automotive radar
U band 40 to 60 GHz 5.0 mm to 7.5 mm
V band 50 to 75 GHz 4.0 mm to 6.0 mm millimeter wave radar research and other kinds of scientific research
W band 75 to 110 GHz 2.7 mm to 4.0 mm satellite communications, millimeter-wave radar research, military radar targeting and tracking applications, and some non-military applications, automotive radar
F band 90 to 140 GHz 2.1 mm to 3.3 mm SHF transmissions: Radio astronomy, microwave devices/communications, wireless LAN, most modern radars, communications satellites, satellite television broadcasting, DBS, amateur radio
D band 110 to 170 GHz 1.8 mm to 2.7 mm EHF transmissions: Radio astronomy, high-frequency microwave radio relay, microwave remote sensing, amateur radio, directed-energy weapon, millimeter wave scanner

The "oldest and thickest Arctic sea ice is disappearing at a faster rate than the younger and thinner ice at the edges of the ice cap. The rapid disappearance of older ice makes the Arctic Ocean's sea ice cap more vulnerable to further decline."[86]

"Arctic multi-year ice “extent”—which includes all areas where at least 15 percent of the ocean surface is covered by multi-year ice—has been vanishing at a rate of –15.1 percent per decade [...] Over the same period, the “area” covered by multi-year ice—which discards open water among ice floes and focuses exclusively on regions that are completely covered—has been shrinking by –17.2 percent per decade."[86]

"The images above show sea ice coverage in 1980 and 2012, as observed by passive microwave sensors on NASA’s Nimbus-7 satellite and by the Special Sensor Microwave Imager/Sounder (SSMIS) from the Defense Meteorological Satellite Program (DMSP). Multi-year ice is shown in bright white, while average sea ice cover is shown in light blue to milky white. The data shows the ice cover for the period of November 1 through January 31 in their respective years."[86]

"The thickest “multi-year” ice survives through two or more summers, while young, seasonal ice forms over a winter and typically melts just as quickly as it formed. [...] “perennial” ice is all ice cover that has survived at least one summer. All multi-year ice is perennial ice, but not all perennial ice is multi-year ice. [...] perennial ice extent has been shrinking at a rate of –12.2 percent per decade, while its area is declining at a rate of –13.5 percent per decade. These numbers indicate that multiyear ice is declining faster than the perennial ice that surrounds it."[86]

"As perennial ice has retreated over the past three decades, it has opened up new areas of the Arctic Ocean that could then be covered by seasonal ice. A larger volume of seasonal ice meant that a larger portion of it could make it through the summer to form second-year ice. This is likely the reason why the perennial ice cover, which includes second year ice, is not declining as rapidly as multiyear ice cover."[86]

“The Arctic sea ice cover is getting thinner because it’s rapidly losing its thick component. At the same time, the surface temperature in the Arctic is going up, which results in a shorter ice-forming season. It would take a persistent cold spell for multi-year sea ice to grow thick enough again to be able to survive the summer melt season and reverse the trend.”[86]

Radars[edit | edit source]

Two passages across the crater Aitken (16.8ºS, 173.4ºE) produced radar sounder images of the area. Credit: NASA.

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
HF 3–30 MHz 10–100 m Coastal radar systems, over-the-horizon radar (OTH) radars; 'high frequency'
VHF 30–300 MHz 1–10 m Very long range, ground penetrating; 'very high frequency'
P < 300 MHz > 1 m 'P' for 'previous', applied retrospectively to early radar systems; essentially HF + VHF
UHF 300–1000 MHz 0.3–1 m Very long range (e.g. ballistic missile early warning), ground penetrating, foliage penetrating; 'ultra high frequency'
L 1–2 GHz 15–30 cm Long range air traffic control and surveillance; 'L' for 'long'
S 2–4 GHz 7.5–15 cm Moderate range surveillance, Terminal air traffic control, long-range weather, marine radar; 'S' for 'short'
C 4–8 GHz 3.75–7.5 cm Satellite transponders; a compromise (hence 'C') between X and S bands; weather; long range tracking
X 8–12 GHz 2.5–3.75 cm Missile guidance, marine radar, weather, medium-resolution mapping and ground surveillance; in the USA the narrow range 10.525 GHz ±25 MHz is used for airport radar; short range tracking. Named X band because the frequency was a secret during WW2.
Ku 12–18 GHz 1.67–2.5 cm High-resolution, also used for satellite transponders, frequency under K band (hence 'u')
K 18–24 GHz 1.11–1.67 cm From German kurz, meaning 'short'; limited use due to absorption by water vapour, so Ku and Ka were used instead for surveillance. K-band is used for detecting clouds by meteorologists, and by police for detecting speeding motorists. K-band radar guns operate at 24.150 ± 0.100 GHz.
Ka 24–40 GHz 0.75–1.11 cm Mapping, short range, airport surveillance; frequency just above K band (hence 'a') Photo radar, used to trigger cameras which take pictures of license plates of cars running red lights, operates at 34.300 ± 0.100 GHz.
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.
V 40–75 GHz 4.0–7.5 mm Very strongly absorbed by atmospheric oxygen, which resonates at 60 GHz.
W 75–110 GHz 2.7–4.0 mm Used as a visual sensor for experimental autonomous vehicles, high-resolution meteorological observation, and imaging.

Two passages across the crater Aitken (16.8ºS, 173.4ºE) produced radar sounder images on the right of the area. The image shown here is two sections from the complete data set as processed after retrieval from the scientific instrument module (SIM) bay during the return flight from the Moon.

Radios[edit | edit source]

Details in radiation belts close to Jupiter are mapped from measurements that NASA's Cassini spacecraft made. Credit: NASA Jet Propulsion Laboratory (NASA-JPL).

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 wavelengths from 1 millimeter to 100 kilometers.

"Details in radiation belts close to Jupiter are mapped from measurements that NASA's Cassini spacecraft made of radio emission from high-energy electrons moving at nearly the speed of light within the belts."[87]

"The three views show the belts at different points in Jupiter's 10-hour rotation. A picture of Jupiter is superimposed to show the size of the belts relative to the planet. Cassini's radar instrument, operating in a listen-only mode, measured the strength of microwave radio emissions at a frequency of 13.8 gigahertz (13.8 billion cycles per second or 2.2 centimeter wavelength). The results indicate the region near Jupiter is one of the harshest radiation environments in the solar system."[87]

"From Earth-based radio telescopes, the telltale radio emissions would be swamped out by heat-generated radio emissions from Jupiter's atmosphere, but Cassini was close enough to Jupiter in January 2001 to differentiate between the emissions from the radiation belts and those from the atmosphere."[87]

"The belts appear to wobble as the planet turns because they are controlled by Jupiter's magnetic field, which is tilted in relation to the planet's poles."[87]

See also[edit | edit source]

References[edit | edit source]

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External links[edit | edit source]

{{Charge ontology}}{{Physics resources}}{{Principles of radiation astronomy}}