Orange astronomy is astronomy applied to the various extraterrestrial orange sources of radiation, especially at night. It is also conducted above the Earth's atmosphere and at locations away from the Earth as a part of explorational (or exploratory) orange astronomy.

An orange sunset in the Mahim Bay is shown around the Haji Ali Dargah in India. Credit: Humayunn Peerzaada.

Seeing an orange Sun due to atmospheric effects and feeling the warmth of its rays is probably a student's first encounter with an apparent astronomical orange radiation source.

There are orange objects and emission lines in the orange portion of the visible spectrum.

## Astronomy

This image shows a detailed view of the spiral arms on one side of the galaxy Messier 99. Credit: ESA/Hubble & NASA.

"This image [at right], taken by the NASA/ESA Hubble Space Telescope, shows a detailed view of the spiral arms on one side of the galaxy Messier 99. Messier 99 is a so-called grand design spiral, with long, large and clearly defined spiral arms — giving it a structure somewhat similar to the Milky Way."[1]

"Lying around 50 million light-years away, Messier 99 is one of over a thousand galaxies that make up the Virgo Cluster, the closest cluster of galaxies to us. Messier 99 itself is relatively bright and large, meaning it was one of the first galaxies to be discovered, way back in the 18th century. This earned it a place in Charles Messier’s famous catalogue of astronomical objects."[1]

"In recent years, a number of unexplained phenomena in Messier 99 have been studied by astronomers. Among these is the nature of one of the brighter stars visible in this image. Catalogued as PTF 10fqs, and visible as a yellow-orange star in the top-left corner of this image, it was first spotted by the Palomar Transient Facility, which scans the skies for sudden changes in brightness (or transient phenomena, to use astronomers’ jargon). These can be caused by different kinds of event, including variable stars and supernova explosions."[1]

"What is unusual about PTF 10fqs is that it has so far defied classification: it is brighter than a nova (a bright eruption on a star’s surface), but fainter than a supernova (the explosion that marks the end of life for a large star). Scientists have offered a number of possible explanations, including the intriguing suggestion that it could have been caused by a giant planet plunging into its parent star."[1]

"This Hubble image was made in June 2010, during the period when the outburst was fading, so PTF 10fqs’s location could be pinpointed with great precision. These measurements will allow other telescopes to home in on the star in future, even when the afterglow of the outburst has faded to nothing."[1]

In traditional colour theory, orange is a range of colours between red and yellow. Credit: Wilinckx.

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.

## Planetary sciences

This photo shows the view to the east from Paranal Observatory, seconds after the Sun has disappeared behind the horizon. Credit: C. Liefke/ESO.

The image appears to have captured the Belt of Venus. Credit: Arenagamma.

"This photo shows the view to the east from Paranal Observatory, seconds after the Sun has disappeared behind the horizon. The orange glow of the sunset can be seen against the 1.8-metre VLT Auxiliary Telescopes, and the almost full Moon is hanging in the sky. But the image is more interesting still, thanks to an atmospheric phenomenon known as the Belt of Venus."[2]

"The grey-bluish shadow above the horizon is the shadow of the Earth, and right above it is a pinkish glow. This phenomenon is produced by the reddened light of the setting Sun being backscattered by the Earth's atmosphere. As well as right after sunset, this atmospheric effect can also be seen shortly before sunrise. A very similar effect can also be observed during a total solar eclipse."[2]

"The telescopes shown in the image are three of the four 1.8-metre Auxiliary Telescopes, housed in ultra-compact mobile enclosures. These telescopes are dedicated to interferometric observations, when two or more telescopes work together, forming a virtual mirror and thus allowing astronomers to see much finer details than can be seen with the individual telescopes working independently."[2]

## Colors

The box shows nine variations of the color orange. Credit: Mizunoryu, Badseed, Jacobolus.

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

## Orange minerals

The mineral orpiment from an arsenic mine in southern Russia is a source of yellow and orange pigments and is highly toxic. Credit: United States Geological Survey and the Mineral Information Institute.

Realgar, an arsenic sulfide mineral 1.5-2.5 Mohs hardness, is highly toxic and is used to make red-orange pigment. Credit: Reno Chris.

A sample of crocoite crystals from Dundas extended mine in Tasmania and is used to make the first synthetic orange pigment, chrome orange. Credit: JJ Harrison.

Calcite is a common calcium carbonate mineral that occurs as orange. Credit: Parent Géry.

The mineral orpiment [at top right] is a source of yellow and orange pigments. Realgar is an arsenic sulfide mineral of 1.5-2.5 Mohs hardness is used to make red-orange pigment. Crocoite crystals from Dundas extended mine in Tasmania is used to make the first synthetic orange pigment, chrome orange.

Calcite [at second left] is a common calcium carbonate mineral that occurs in orange.

## Theoretical orange astronomy

The quasar is the orange object at the center of the large, irregular-shaped galaxy. Credit: NASA/JPL-Caltech.

"A growing black hole, called a quasar, can be seen at the center of a faraway galaxy in this artist's concept. Astronomers using NASA's Spitzer and Chandra space telescopes discovered swarms of similar quasars hiding in dusty galaxies in the distant universe."[3]

"The quasar is the orange object at the center of the large, irregular-shaped galaxy. It consists of a dusty, doughnut-shaped cloud of gas and dust that feeds a central supermassive black hole. As the black hole feeds, the gas and dust heat up and spray out X-rays, as illustrated by the white rays. Beyond the quasar, stars can be seen forming in clumps throughout the galaxy. Other similar galaxies hosting quasars are visible in the background."[3]

"The newfound quasars belong to a long-lost population that had been theorized to be buried inside dusty, distant galaxies, but were never actually seen. While some quasars are easy to detect because they are oriented in such a way that their X-rays point toward Earth, others are oriented with their surrounding doughnut-clouds blocking the X-rays from our point of view. In addition, dust and gas in the galaxy itself can block the X-rays."[3]

"Astronomers had observed the most energetic of this dusty, or obscured, bunch before, but the "masses," or more typical members of the population, remained missing. Using data from Spitzer and Chandra, the scientists uncovered many of these lost quasars in the bellies of massive galaxies between 9 and 11 billion light-years away. Because the galaxies were also busy making stars, the scientists now believe most massive galaxies spent their adolescence building up their stars and black holes simultaneously."[3]

## Sources

This is an animation of the Abney effect. Credit: LadyofHats.

The Abney effect describes the perceived hue shift that occurs when white light is added to a monochromatic light source.[4]

The addition of white light will cause a desaturation of the monochromatic source, as perceived by the human eye. However, a less intuitive effect of the white light addition that is perceived by the human eye is the change in the apparent hue. This hue shift is physiological rather than physical in nature.

A white light source is created by the combination of red light, blue light, and green light.

The term hue discrimination is used to describe the change in wavelength that must be obtained in order for the eye to detect a shift in hue. An expression λ + Δλ defines the required wavelength adjustment that must take place.[5] A less than two nanometer change in wavelength causes most spectral colors to appear to take on a different hue. However, for blue light and red light, a much larger wavelength shift must occur in order for a person to be able to identify a difference in hue.

A white point (often referred to as reference white or target white in technical documents) is a set of tristimulus values or chromaticity coordinates that serve to define the color "white" in image capture, encoding, or reproduction.[6] Depending on the application, different definitions of white are needed to give acceptable results. For example, photographs taken indoors may be lit by incandescent lights, which are relatively orange compared to daylight. Defining "white" as daylight will give unacceptable results when attempting to color-correct a photograph taken with incandescent lighting.

## Objects

The NASA/ESA Hubble Space Telescope has used its powerful optics to separate the globular cluster NGC 6401 into its constituent stars. Credit: ESA/Hubble & NASA.

"The NASA/ESA Hubble Space Telescope has used its powerful optics to separate the globular cluster NGC 6401 into its constituent stars. What was once only visible as a ghostly mist in the eyepieces of astronomical instruments has been transformed into a stunning stellar landscape."[7]

"NGC 6401 can be found within the constellation of Ophiuchus (The Serpent Bearer). The globular cluster itself is relatively faint, so a telescope and some observational experience are required to see it. Globular clusters are very rich, and generally spherical, collections of stars, hence the name. They orbit the cores of galaxies, with the force of gravity also keeping the stars bound as a group. There are around 160 globular clusters associated with our Milky Way, of which NGC 6401 is one. These objects are very old, containing some of the most ancient stars known. However, there are many mysteries surrounding them, with the origin of globular clusters and their role within galaxy evolution not being completely understood."[7]

"The famous astronomer William Herschel discovered this cluster in 1784 with his 47 cm telescope, but mistakenly believed it to be a bright nebula. Later his son, John Herschel, was to make the same error — evidently the technology of the day was insufficient to allow the individual stars to be resolved visually."[7]

"NGC 6401 has confused more modern astronomers as well. In 1977 it was thought that a low-mass star in the cluster had been discovered venting its outer layers (known as a planetary nebula). However, a further study in 1990 concluded that the object is in fact a symbiotic star: a binary composed of a red giant and a small hot star such as a white dwarf, with surrounding nebulosity. It could be that the study in 1977 was simply a few thousand years ahead of its time, as symbiotic stars are thought to become a type of planetary nebula."[7]

"This picture was created from images taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Images through a yellow-orange filter (F606W, coloured blue) were combined with images taken in the near-infrared (F814W, coloured red). The total exposure times were 680 s and 580 s, respectively and the field of view is 3.3 x 1.5 arcminutes."[7]

## Emissions

This is a wide-field image in the region of NGC 3603 taken on the ground by the Digitized Sky Survey 2. Credit: NASA, ESA, and the Digitized Sky Survey 2.

"A wide-field image in the region of NGC 3603 [is] taken on the ground by the Digitized Sky Survey 2. The glowing clouds of hydrogen gas (seen here in orange) compose a vast emission nebula. The field of view is approximately 2.6 x 2.8 degrees."[8]

Doppler broadening is the broadening of spectral lines due to the Doppler effect caused by a distribution of velocities of atoms or molecules. Different velocities of the emitting particles result in different (Doppler) shifts, the cumulative effect of which is the line broadening.[9]

Thermal Doppler broadening is one of the explanations for the broadening of spectral lines, and as such gives an indication for the temperature of observed material. It should be noted, though, that other causes of velocity distributions may exist, e.g., due to turbulent motion. For a fully developed turbulence, the resulting line profile is generally very difficult to distinguish from the thermal one.[10] Another cause could be a large range of macroscopic velocities resulting, e.g., from the receding and approaching portions of a rapidly spinning accretion disk. Finally, there are many other factors which can also broaden the lines. For example, a sufficiently high particle number density may lead to significant Stark broadening.

"Linear Stark broadening occurs via the linear Stark effect which results from the interaction of an emitter with an electric field, which causes a shift in energy which is linear in the field strength. (${\displaystyle \Delta E\sim 1/r^{2}}$ )" from the Wikipedia entry spectral line.

Quadratic Stark broadening occurs via the quadratic Stark effect which results from the interaction of an emitter with an electric field, which causes a shift in energy which is quadratic in the field strength. (${\displaystyle \Delta E\sim 1/r^{4}}$ ).

Notation: "van der Waals profile" appears as lowercase in almost all sources

Van der Waals broadening occurs when the emitting particle is being perturbed by van der Waals forces. For the quasistatic case, a van der Waals profile[11][12] is often useful in describing the profile. The energy shift as a function of distance is given in the wings by e.g. the Lennard-Jones potential (${\displaystyle \Delta E\sim 1/r^{6}}$ ).

### Temperatures

"Temperatures [are] estimated from intensity ratios of atomic lines (used mainly for early C stars), color in the orange region of the spectrum, strength of the Na D-lines, and C2 band intensity gradients."[13]

## Bands

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

Band spectra are the combinations of many different spectral lines, resulting from rotational, vibrational and electronic transition.

The "ERE manifests itself through a broad, featureless emission band of 60 < FWHM < 100 nm, with a peak appearing in the general wavelength range 610 < λp < 820 nm."[14]

In the Red Rectangle Nebula, diffraction-limited speckle images of it in visible and near infrared light reveal a highly symmetric, compact bipolar nebula with X-shaped spikes which imply toroidal dispersion of the circumstellar material. The central binary system is completely obscured, providing no direct light.[15]

"The star HD 44179 is surrounded by an extraordinary structure known as the Red Rectangle. It acquired its moniker because of its shape and its apparent colour when seen in early images from Earth. This strikingly detailed new Hubble image reveals how, when seen from space, the nebula, rather than being rectangular, is shaped like an X with additional complex structures of spaced lines of glowing gas, a little like the rungs of a ladder. The star at the centre is similar to the Sun, but at the end of its lifetime, pumping out gas and other material to make the nebula, and giving it the distinctive shape. It also appears that the star is a close binary that is surrounded by a dense torus of dust — both of which may help to explain the very curious shape. Precisely how the central engine of this remarkable and unique object spun the gossamer threads of nebulosity remains mysterious. It is likely that precessing jets of material played a role."[16]

The Red Rectangle is an unusual example of what is known as a proto-planetary nebula. These are old stars, on their way to becoming planetary nebulae. Once the expulsion of mass is complete a very hot white dwarf star will remain and its brilliant ultraviolet radiation will cause the surrounding gas to glow. The Red Rectangle is found about 2 300 light-years away in the constellation Monoceros (the Unicorn).

The High Resolution Channel of the NASA/ESA Hubble Space Telescope’s Advanced Camera for Surveys captured this view of HD 44179 and the surrounding Red Rectangle nebula — the sharpest view so far. Red light from glowing Hydrogen was captured through the F658N filter and coloured red. Orange-red light over a wider range of wavelengths through a F625W filter was coloured blue.

Using various flames such as from a Bunsen burner, "[s]trontium yields two red bands and one orange band."[17]

The orange system is a number of emission lines very close together forming a band in the orange portion of the visible spectrum. These lines are usually associated with particular molecular species, including ScO, YO, and TiO.[18] The emission features for ScO (the 0-0 band) are near 603.6 and 607.9 nm.[18] There is also a 1-1 band at 607.2 and 611.5 nm.[18]

There may be a TiO band present at 615.8 nm, which "suggests that the star may be a late K or early M star."[19]

The orange band from molecular CaCl is "observed in the spectra of many carbon stars."[20] "[T]he concentration of CaCl is strongly temperature and pressure dependent, but almost independent of the C/O ratio at a fixed pressure."[21] "The probable absence of CaCl bands in spectra of carbon stars with C/O ≫ 1 can be explained by CN opacity effects near 6000 Å, ... whereas the absence of CaCl bands in spectra of the coolest M and S stars can probably be attributed largely to molecular band masking."[21] "[T]he CaCl bands are a useful, but not infallible, temperature criteria."[22]

"The YO bands at 6132 [the 0-0 emission band] and 5972 Å appear only in the cooler stars, but in MS stars and weak S stars they are so sensitive to heavy-element abundance that they are not very useful as temperature indicators."[22]

## Meteors

This image is a real color image of the planetary nebula NGC 6751 in Aquila. Credit: NASA/Hubble Space Telescope.

"Faint green and orange colored meteors prevailed in the early part of the shower, and those seen on the 15th were orange, red, or deep yellow."[23] "Five Perseids of magnitude greater than the first appeared to explode at points about two-thirds the length of their respective shots. The large oval explosion traces, yellow or orange in color, could be seen for twenty or thirty seconds."[23]

"The Aquarids generally left long, yellow or orange streaks."[23]

"Astronomers using NASA's Hubble Space Telescope have obtained images of the strikingly unusual planetary nebula, NGC 6751. Glowing in the constellation Aquila like a giant eye, the nebula is a cloud of gas ejected several thousand years ago from the hot star visible in its center. The Hubble observations were obtained in 1998 with the Wide Field and Planetary Camera 2 (WFPC2) by a team of astronomers led by Arsen Hajian of the U.S. Naval Observatory in Washington, DC. The Hubble Heritage team, working at the Space Telescope Science Institute in Baltimore, has prepared this color rendition by combining the Hajian team's WFPC2 images taken through three different color filters that isolate nebular gases of different temperatures. The nebula shows several remarkable and poorly understood features. Blue regions mark the hottest glowing gas, which forms a roughly circular ring around the central stellar remnant. Orange and red show the locations of cooler gas. The cool gas tends to lie in long streamers pointing away from the central star, and in a surrounding, tattered-looking ring at the outer edge of the nebula. The origin of these cooler clouds within the nebula is still uncertain, but the streamers are clear evidence that their shapes are affected by radiation and stellar winds from the hot star at the center."[24]

## Ultraviolets

The photograph, taken by NASA's Hubble Space Telescope, captures a small region within M17, a hotbed of star formation. Credit: NASA, ESA and J. Hester (ASU).

"Like the fury of a raging sea, this anniversary image from the NASA/ESA Hubble Space Telescope shows a bubbly ocean of glowing hydrogen, oxygen, and sulphur gas in the extremely massive and luminous molecular nebula Messier 17. This Hubble photograph captures a small region within Messier 17 (M17), a hotbed of star formation. M17, also known as the Omega or Swan Nebula, is located about 5500 light-years away in the Sagittarius constellation. The release of this image commemorates the thirteenth anniversary of Hubble's launch on 24 April 1990. The wave-like patterns of gas have been sculpted and illuminated by a torrent of ultraviolet radiation from young, massive stars (which lie outside the picture to the upper left). The glow of these patterns highlights the 3D structure of the gases. The ultraviolet radiation is carving and heating the surfaces of cold hydrogen gas clouds. The warmed surfaces glow orange and red in this image. The intense heat and pressure cause some material to stream away from the surface, creating the glowing veil of even hotter green-coloured gas that masks background structures. The pressure on the tips of the waves may trigger new star formation within them. The image, roughly 3 light-years across, was taken on 29-30 May 1999, with Hubble's Wide Field Planetary Camera 2. The colours in the image represent various gases. Red represents sulphur; green, hydrogen; and blue, oxygen."[25]

## Opticals

The strange and irregular bundle of jets and clouds in this curious image from the NASA/ESA Hubble Space Telescope is the result of a burst of activity late in the life of a star. Credit: ESA/Hubble & NASA.

"The strange and irregular bundle of jets and clouds in this curious image from the NASA/ESA Hubble Space Telescope is the result of a burst of activity late in the life of a star. As its core runs out of nuclear fuel, the star’s unstable outer layers are puffing out a toxic concoction of gases including carbon monoxide and hydrogen cyanide."[26]

"The Westbrook Nebula — also known as PK166-06, CRL 618 and AFGL 618 — is a protoplanetary nebula, an opaque, dark and relatively short-lived cloud of gas that is ejected by a star as it runs out of nuclear fuel. As the star hidden deep in the centre of the nebula evolves further it will turn into a hot white dwarf and the gas around it will become a glowing planetary nebula, before eventually dispersing. Because this is a relatively brief stage in the evolution process of stars, only a few hundred protoplanetary nebulae are known in the Milky Way."[26]

"Protoplanetary nebulae are cool, and so emit little visible light. This makes them very faint, posing challenges to scientists who wish to study them. What this picture shows, therefore, is a composite image representing the different tricks that the astronomers used to unravel what is going on within this strange nebula. The picture includes exposures in visible light which shows light reflected from the cloud of gas, combined with other exposures in the near-infrared part of the spectrum, showing us the dim glow, invisible to human eyes, that is coming from different elements deep in the cloud itself."[26]

"One of the nebula’s names, AFGL 618, comes from its discovery by a precursor to the Hubble Space Telescope: the letters stand for Air Force Geophysics Laboratory. This US research organisation launched a series of suborbital rockets with infrared telescopes on board in the 1970s, cataloguing hundreds of objects that were impossible or difficult to observe from the ground. In some respects, these were a proof of concept for later orbital infrared astronomical facilities including Hubble and ESA’s Herschel Space Observatory."[26]

"This image was prepared from many separate exposures taken using Hubble’s newest camera, the Wide Field Camera 3. Exposures through a green filter (F547M) were coloured blue, those through a yellow/orange filter (F606W) were coloured green and exposures through a filter that isolates the glow from ionised nitrogen (F658N) have been coloured red. Images through filters that capture the glows from singly and doubly ionised sulphur (F673N and F953N) are also shown in red. The total exposure times were about nine minutes through each filter and the field of view is approximately 20 arcseconds across."[26]

## Visuals

The image shows the lower 45 kilometers of the Tsauchab River. Credit: ISS Expedition 22 crew.

"Taken on Christmas Eve of 2009, this image shows the lower 45 kilometers of the Tsauchab River, a famous landmark for Namibians, tourists, and for orbiting astronauts. The Tsauchab River bed is seen jutting into the sea of red dunes near Namibia’s hyper-arid coast. The riverbed ends in a series of light-colored, silty mud holes on the dry lake floor, known locally as Sossus Vlei (“small lake”)."[27]

"Because of the present arid climate, few people have ever seen the Tsauchab River with flowing water or a lake in Sossus Vlei. In times past, however, the Tsauchab appears to have reached the Atlantic coast, another 55 kilometers farther west. Like several other rivers of the coastal Namib Desert, the Tsauchab brings sediment down from the hinterland to the coastal lowland. This sediment is then blown from the river beds, and over probably tens of millions of years, has accumulated as the red dunes of the impressive Namib Sand Sea."[27]

"Astronaut photograph ISS022-E-15154 was acquired on December 24, 2009, with a Nikon D2Xs digital camera fitted with an 180 mm lens, and is provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center. The image was taken by the Expedition 22 crew."[27]

"This astronaut photo shows sand heaped up in numerous star dunes, each of them with long arms extending in several directions. Unlike crescent-shaped barchan dunes, which form in areas where winds generally blow from one direction, star dunes are apparently generated where winds are variable. In this part of the Namib Sand Sea, winds are mainly from the south, but easterly winds, channeled along the Tsauchab valley, provide another component. And warm dry winter winds—similar to the Santa Ana winds of California—blow from the northeast."[27]

"These northeasterly winds are likely responsible for the regular dune arms that point into the valley from both sides. These large dunes facing the river valley are promoted as the highest dunes in the world. Although continuous dune slopes allow hikers to ascend to altitudes more than 300 meters above the river bottom, not all of that elevation gain has to be walked; the main base of the dunes lies on a terrace 180 meters above the river."[27]

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

## Reds

The Red Square Nebula (MWC 922) is a bipolar nebula appearing as an orange square in its center with red bowl-shaped gas and dust toward the top right and bottom left of the image. Credit: Peter Tuthill & James Lloyd.

"What could cause a nebula to appear square? No one is quite sure. The hot star system known as MWC 922, however, appears to be embedded in a nebula with just such a shape. The above image combines infrared exposures from the Hale Telescope on Mt. Palomar in California, and the Keck-2 Telescope on Mauna Kea in Hawaii. A leading progenitor hypothesis for the square nebula is that the central star or stars somehow expelled cones of gas during a late developmental stage. For MWC 922, these cones happen to incorporate nearly right angles and be visible from the sides. Supporting evidence for the cone hypothesis includes radial spokes in the image that might run along the cone walls. Researchers speculate that the cones viewed from another angle would appear similar to the gigantic rings of supernova 1987A, possibly indicating that a star in MWC 922 might one day itself explode in a similar supernova."[28]

## Plasma objects

This is an aurora borealis photographed as occurring above Finland. Credit: Pekka Parviainen.`{{fairuse}}`

This is an extensively orange aurora that occurred over Maine. Credit: Unknown, or unstated.`{{fairuse}}`

This is an orange aurora over New York. Credit: Unknown, or unstated.

The aurora imaged on the right occurred over Finland in early October 2002. Note the pastel orange colors in the veil or curtain-like aurora.

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 which also occurred over Finland is the extensively orange veil aurora over Maine on the left.

## Gaseous objects

Most stars in this image of the Sagittarius Star Cloud are orange. Credit: Hubble Heritage Team (AURA/ STScI/ NASA).

"Stars come in all different colors. The color of a star indicates its surface temperature, an important property used to assign each star a spectral type. Most stars in the above Sagittarius Star Cloud are orange or red and relatively faint, as our Sun would appear. The blue and greenish stars are hotter, many being relatively young and massive. The bright red stars are cool Red Giants, bloated stars once similar to our Sun that have entered a more advanced stage of evolution. Stars of this Sagittarius Cloud lie towards the center of our Galaxy - tantalizing cosmic jewels viewed through a rift in the dark, pervasive, interstellar dust. This famous stellar grouping houses some of the oldest stars known."[29]

## Rocky objects

The two billowing structures in this NASA/ESA Hubble Space Telescope image of IRAS 13208-6020 are formed from material that is shed by a central star. Credit: ESA/Hubble & NASA.

"The two billowing structures in this NASA/ESA Hubble Space Telescope image of IRAS 13208-6020 are formed from material that is shed by a central star. This is a relatively short-lived phenomenon that gives astronomers an opportunity to watch the early stages of planetary nebula formation, hence the name protoplanetary, or preplanetary nebula. Planetary nebulae are unrelated to planets and the name arose because of the visual similarity between some planetary nebulae and the small discs of the outer planets in the Solar System when viewed through early telescopes."[30]

"This object has a very clear bipolar form, with two very similar outflows of material in opposite directions and a dusty ring around the star."[30]

"Protoplanetary nebulae do not shine, but are illuminated by light from the central star that is reflected back to us. But as the star continues to evolve, it becomes hot enough to emit strong ultraviolet radiation that can ionise the surrounding gas, making it glow as a spectacular planetary nebula. But before the nebula begins to shine, fierce winds of material ejected from the star will continue to shape the surrounding gas into intricate patterns that can only be truly appreciated later once the nebula begins to glow."[30]

"This picture was created from images taken using the High Resolution Channel of Hubble’s Advanced Camera for Surveys. Images taken through an orange filter (F606W, coloured blue) and a near-infrared filter (F814W, coloured red) have been combined to create this picture. The exposure times were 1130 s and 150 s respectively and the field of view is just 22 x 17 arcseconds."[30]

## Lithiums

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

Lithium I has an orange line at 610.3 nm. In some 824 red giant stars, the Li I 670.78 nm line was detected in several stars, "but only the five objects ... presented a strong line. Indeed, the Li subordinate line at 6103.6 Å was detected in these stars only."[31]

The Spite plateau (or Spite lithium plateau) is a baseline in the abundance of lithium found in old stars orbiting the galactic halo.

The curve on a graph of the abundance of lithium versus effective surface temperature formed a plateau among old halo stars for effective temperatures below about:

log Teff ~ 3.75

or roughly 5,600 K. This suggested that the plateau represented the primordial abundance level of lithium in the Milky Way with an estimate that the abundance of lithium at the beginning of the galaxy was:

NLi = (11.2 ± 3.8) × 10−11 NH

where NH is the abundance of hydrogen.[32]

"Lithium depletion through atomic diffusion has been suggested as a solution to the discrepancy between the Spite plateau abundance and the predicted value of the primordial lithium abundance"[33].

## Borons

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

Boron has a line in the orange.

## Carbons

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

There is a C2 band at 619.1 nm.[19] Sometimes there is a hint of 13C12C at 618.8 nm.[19]

## Nitrogens

The spectrum shows the lines in the visible due to emission from elemental nitrogen. Credit:Kurgus.

Nitrogen has a weak line in the orange.

## Oxygens

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

Oxygen has emission lines at 615.6-8 nm from O I.[34]

## Fluorines

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

Fluorine does have a number of lines in the orange as the spectrum above shows.

## Neons

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

Neon has two emission lines at 614.3 and 616.3 nm from Ne I.[34]

## Materials

Various layers of material are expelled by the central star. Credit: ESA/Hubble and NASA.

"It may look like something from "The Lord of the Rings," but this fiery swirl is actually a planetary nebula known as ESO 456-67. Set against a backdrop of bright stars, the rust-colored object lies in the constellation of Sagittarius (The Archer), in the southern sky."[35]

"In this image of ESO 456-67, it is possible to see the various layers of material expelled by the central star. Each appears in a different hue - red, orange, yellow, and green-tinted bands of gas are visible, with clear patches of space at the heart of the nebula. It is not fully understood how planetary nebulae form such a wide variety of shapes and structures; some appear to be spherical, some elliptical, others shoot material in waves from their polar regions, some look like hourglasses or figures of eight, and others resemble large, messy stellar explosions - to name but a few."[35]

## Sun

The image shows a sunrise in Kodachadri. Credit: Chinmayahd.

The image shows an orange sun in Boracay, Philippines. Credit: Sarahr.

But truly, the now famous sun set at the peak where you will witness the panorama of rolling green hills and also a glimpse of Arabian sea and the bright orange sun going down the hills might force one to contemplate on the nature and its complex beauty. And to witness the same sun rise above the foggy and misty hills and over a blanket of silver clouds the next early morning would be the perfect way to start a day.

## Earth

This image shows the Namib Desert. Credit: USGS EROS Data Center Satellite Systems Branch/NASA.

The image shows the Namib-Naukluft National Park is full color. Credit: NASA/GSFC/MITI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team.

This is an image of the Namib Desert by the MODIS satellite. Credit: Jeff Schmaltz, MODIS Land Rapid Response Team, NASA GSFC.

Sossusvlei, one of the Namib's major tourist attractions, is a salt and clay pan surrounded by large dunes. The flats pictured here were caused by the Tsauchab stream after summer rains. Credit: Ikiwaner.

The "Long Wall" is a prominent coastal area of the Namib that runs along the Sperrgebiet in southwestern Namibia. Credit: Brian McMorrow.

A satellite image of the Namib Sand Sea by NASA World Wind; the Sand Sea, a major feature of the Namib, covers an area of 34,000 km (21,000 mi)[36] and is bounded on the west by the Atlantic and on the east by the Great Escarpment. Credit: NASA.

The dune sea of the Namib Desert. The crests of the dunes are aligned in a marked northwest-southeast orientation. The dunes act as obstacles, causing the winds to be deflected significantly to the right, in effect reorienting the southerly wind to become a southwesterly wind. Credit: ISS Crew Earth Observations experiment and the Image Science & Analysis Group, Johnson Space Center.

Namib Desert is seen from the Spot satellite. Credit: Spot Image.

Camel Thorn Tree (Acacia erioloba) in Sossusvlei region. Credit: Luca Galuzzi.

At right is an image from LandSat 7 of the Namib Desert. "Namib-Naukluft National Park is an ecological preserve in Namibia's vast Namib Desert. Coastal winds create the tallest sand dunes in the world here, with some dunes reaching 980 feet (300 meters) in height."[37]

"This image was acquired by Landsat 7's Enhanced Thematic Mapper plus (ETM+) sensor on August 12, 2000. This is a false-color composite image made using near infrared, green, and blue wavelengths. The image has also been sharpened using the sensor's panchromatic band."[37]

"Namib-Naukluft National Park [second right] is an ecological preserve in the Namib Desert in southwest Africa, thought to be Earth’s oldest desert. The park is the largest game park in Africa, and a surprising collection of creatures manages to survive in the hyper-arid region, including snakes, geckos, unusual insects, hyenas, and jackals. More moisture comes in as a fog off the Atlantic Ocean than falls as rain, with the average 63 millimeters of rainfall per year concentrated in the months of February and April."[38]

"The winds that bring in the fog are also responsible for creating the park’s towering sand dunes, whose burnt orange color is a sign of their age. The orange color develops over time as iron in the sand is oxidized (like rusty metal); the older the dune, the brighter the color. These dunes are the tallest in the world, in places rising above the desert floor more than 300 meters (almost 1000 feet). The dunes taper off near the coast, and lagoons, wetlands, and mudflats located along the shore attract hundreds of thousands of birds."[38]

The Namib is a coastal desert in southern Africa.

Southern Namib (between Lüderitz and the Kuiseb River) comprises a vast dune sea with some of the tallest and most spectacular dunes of the world, ranging in color from pink to vivid orange. In the Sossusvlei area, several dunes exceed 300 meters (984 ft) in height. The complexity and regularity of dune patterns in its dune sea have attracted the attention of geologists for decades, but it remains poorly understood.

The Namib-Naukluft National Park, that extends over a large part of the Namib Desert, is the largest game reserve in Africa and one of the largest of the world. While most of the park is hardly accessible, several well-known visitor attractions are found in the desert. The prominent attraction is the famous Sossusvlei area, where high orange sand dunes surround vivid white salt pans, creating a fascinating landscape.

## Moon

This is a real color photograph of the moon rising over the Lick Observatory in California on March 7, 2012. Credit: Rick Baldridge.

"Crystallized spheres of orange glass from Shorty Crater at the Apollo 17 site are ... the characteristic ingredient of the dark mantling deposit of the Taurus-Littrow region."[39]

"The reflectance properties of the orange glass are highly distinctive. There are two broad absorption bands, one near 1.15 µm and the other near 1.9 µm that arise from Fe2+ on octahedral and tetrahedral sites, respectively ... The weak absorption near 0.5 µm probably arises from Ti3+, and the absorption edge extending into the visible region is due largely to oxygen-titanium charge transfer".[39]

Occasionally, atmospheric effects are just right to give the Moon an orange or orange-yellow color as shown in the image at right.

## Vesta

"The [NASA's Dawn spacecraft] Framing Camera (FC) discovered enigmatic orange material on Vesta. FC images revealed diffuse orange ejecta around two impact craters, 34-km diameter Oppia, and 30-km diameter Octavia, as well as numerous sharp-edge orange units in the equatorial region."[40] The spacecraft "entered orbit around asteroid (4) Vesta in July 2011 for a year-long mapping orbit."[40]

## Jupiter

Cloud bands are clearly visible on Jupiter. Credit: NASA/JPL/USGS.

"[O]range [is] the color of Jupiter".[41]

The orange and brown coloration in the clouds of Jupiter are caused by upwelling compounds that change color when they are exposed to ultraviolet light from the Sun. The exact makeup remains uncertain, but the substances are believed to be phosphorus, sulfur or possibly hydrocarbons.[42][43] These colorful compounds, known as chromophores, mix with the warmer, lower deck of clouds. The zones are formed when rising convection cells form crystallizing ammonia that masks out these lower clouds from view.[44]

## Io

An active volcanic eruption on Jupiter's moon Io was captured in this image taken on February 22, 2000 by NASA's Galileo spacecraft. Credit: NASA/JPL/University of Arizona.

"An active volcanic eruption on Jupiter's moon Io was captured in this image taken on February 22, 2000 by NASA's Galileo spacecraft. Tvashtar Catena, a chain of giant volcanic calderas centered at 60 degrees north, 120 degrees west, was the location of an energetic eruption caught in action in November 1999. A dark, "L"-shaped lava flow to the left of the center in this more recent image marks the location of the November eruption. White and orange areas on the left side of the picture show newly erupted hot lava, seen in this false color image because of infrared emission. The two small bright spots are sites where molten rock is exposed to the surface at the toes of lava flows. The larger orange and yellow ribbon is a cooling lava flow that is more than more than 60 kilometers (37 miles) long. Dark, diffuse deposits surrounding the active lava flows were not there during the November 1999 flyby of Io."[45]

"This color mosaic was created by combining images taken in the near-infrared, clear, and violet filters from Galileo's camera. The range of wavelengths is slightly more than that of the human eye. The mosaic has been processed to enhance subtle color variations. The bright orange, yellow, and white areas at the left of the mosaic use images in two more infrared filters to show temperature variations, orange being the coolest and white the hottest material. This picture is about 250 kilometers (about 155 miles) across. North is toward the top and illumination from the Sun is from the west (left)."[45]

This color Voyager 2 image mosaic shows the water-ice-covered surface of Enceladus, one of Saturn's icy moons. Credit: NASA/JPL/USGS.

"This color Voyager 2 image mosaic shows the water-ice-covered surface of Enceladus, one of Saturn's icy moons. Enceladus' diameter of just 500 km would fit across the state of Arizona, yet despite its small size Enceladus exhibits one of the most interesting surfaces of all the icy satellites. Enceladus reflects about 90% of the incident sunlight (about like fresh-fallen snow), placing it among the most reflective objects in the Solar System. Several geologic terrains have superposed crater densities that span a factor of at least 500, thereby indicating huge differences in the ages of these terrains. It is possible that the high reflectivity of Enceladus' surface results from continuous deposition of icy particles from Saturn's E-ring, which in fact may originate from icy volcanoes on Enceladus' surface. Some terrains are dominated by sinuous mountain ridges from 1 to 2 km high (3300 to 6600 feet), whereas other terrains are scarred by linear cracks, some of which show evidence for possible sideways fault motion such as that of California's infamous San Andreas fault. Some terrains appear to have formed by separation of icy plates along cracks, and other terrains are exceedingly smooth at the resolution of this image. The implication carried by Enceladus' surface is that this tiny ice ball has been geologically active and perhaps partially liquid in its interior for much of its history. The heat engine that powers geologic activity here is thought to be elastic deformation caused by tides induced by Enceladus' orbital motion around Saturn and the motion of another moon, Dione."[46]

## Tethys

Voyager 2 imaged Tethys on August 25, 1981. Credit: NASA/JPL.

This is Tethys as imaged by Voyager 2. Credit: NASA/JPL.

"Voyager 2 obtained this [upper right] image of Tethys on Aug. 25, when the spacecraft was 594,000 kilometers (368,000 miles) from this satellite of Saturn. This photograph was compiled from images taken through the violet, clear and green filters of Voyager's narrow-angle camera."[47]

"Tethys shows two distinct types of terrain--bright, densely cratered regions; and relatively dark, lightly cratered planes that extend in a broad belt across the satellite. The densely cratered terrain is believed to be part of the ancient crust of the satellite; the lightly cratered planes are thought to have been formed later by internal processes. Also clearly seen is a trough that runs parallel to the terminator (the day-night boundary, seen at right). This trough is an extension of the huge canyon system Voyager 1 saw last fall. This system extends nearly two-thirds the distance around Tethys."[47]

At lower right "Tethys is shown here in a Voyager 2 image taken 26 August 1981 from 282,600 km away. Tethys is 1,060 km in diameter. Ithaca Chasma is a large canyon running diagonally in the left of this image; Ithaca Chasma is up to 100 km wide, several km deep, and stretches at least three-fourths of the distance around Tethys. "[48]

## Titan

This is a natural color image of Titan. Credit: NASA/JPL/Space Science Institute.

The robotic Cassini spacecraft orbiting Saturn captured the heavily cratered Tethys slipping behind Saturn's atmosphere-shrouded Titan late last year. Credit: Cassini Imaging Team, ISS, JPL, ESA, NASA.

Much as with Venus prior to the Space Age, the dense, opaque atmosphere prevented understanding of Titan's surface until new information accumulated with the arrival of the Cassini–Huygens mission in 2004, including the discovery of liquid hydrocarbon lakes in the polar regions.

The atmosphere of Titan is largely composed of nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog.

"With its thick, distended atmosphere, Titan's orange globe shines softly".[49]

In the second image at right, "Titan shows not only its thick and opaque orange lower atmosphere, but also an unusual upper layer of blue-tinted haze."[50]

## Uranus

This is a Hubble Space Telescope image at 619.0 nm of Uranus. Credit: Heidi Hammel (Massachusetts Institute of Technology), NASA.

The picture of Uranus is a composite of images taken through blue, green and orange filters. Credit: NASA/JPL.

"Spring has finally come to the northern hemisphere of Uranus. The newest images, both the visible-wavelength ones described here and those taken a few days earlier with the Near Infrared and Multi-Object Spectrometer (NICMOS) by Erich Karkoschka (University of Arizona), show a planet with banded structure and detectable clouds."[51]

"The "red" image (on the right) is taken at 6,190 Angstroms, and is sensitive to absorption by methane molecules in the planet's atmosphere. The banded structure of Uranus is evident, and the small cloud near the northern limb is now visible."[51]

The picture of Uranus in true color was "compiled from images returned Jan. 17, 1986, by the narrow-angle camera of Voyager 2. The spacecraft was 9.1 million kilometers (5.7 million miles) from the planet, several days from closest approach. The picture [...] has been processed to show Uranus as human eyes would see it from the vantage point of the spacecraft. The picture is a composite of images taken through blue, green and orange filters. The darker shadings at the upper right of the disk correspond to the day-night boundary on the planet. Beyond this boundary lies the hidden northern hemisphere of Uranus, which currently remains in total darkness as the planet rotates. The blue-green color results from the absorption of red light by methane gas in Uranus' deep, cold and remarkably clear atmosphere."[52]

## Stellar classifications

Most stars are currently classified using the letters O, B, A, F, G, K, and M ..., where O stars are the hottest and the letter sequence indicates successively cooler stars up to the coolest M class. K stars are "orange", even though the actual star colors perceived by an observer may deviate from these colors depending on visual conditions and individual stars observed.

The Secchi Class III consists of orange to red stars with complex band spectra.

The Harvard classification system is a one-dimensional classification scheme. Stars vary in surface temperature from about 2,000 to 40,000 kelvins. Physically, the classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest, as is done in the following table:

Class Temperature[53]
K
Conventional color Apparent color[54][55][56] Mass[53]
(solar masses, Mʘ)
Luminosity[53]
(bolometric, Lʘ)
Hydrogen
lines
Fraction of all
main sequence stars[57]
O ≥ 33,000 K blue blue ≥ 16 ≥ 6.6 ≥ 30,000 Weak ~0.00003%
B 10,000–33,000 K blue to blue white blue white 2.1–16 1.8–6.6 25–30,000 Medium 0.13%
A 7,500–10,000 K white white to blue white 1.4–2.1 1.4–1.8 5–25 Strong 0.6%
F 6,000–7,500 K yellowish white white 1.04–1.4 1.15–1.4 1.5–5 Medium 3%
G 5,200–6,000 K yellow yellowish white 0.8–1.04 0.96–1.15 0.6–1.5 Weak 7.6%
K 3,700–5,200 K orange yellow orange 0.45–0.8 0.7–0.96 0.08–0.6 Very weak 12.1%
M ≤ 3,700 K red orange red ≤ 0.45 ≤ 0.7 ≤ 0.08 Very weak 76.45%

The orange color of a number of stars is noted in observations from 1885: "Certain orange stars have been included, either because they are finely coloured, or because of a neighbouring blue star--a combination of frequent occurrence, the reason of which is at present unexplained."[58]

## Orange supergiants

Betelgeuse is shown at the upper left. Credit: Rogelio Bernal Andreo.

In the photograph at right, the orange super giant star Betelgeuse is shown in relationship with the dense nebulas of the Orion Molecular Cloud Complex and Orion's Belt.

Eta Pavonis on the lower right is an example of a super giant orange star. It is specifically spectral type K2II according to SIMBAD.

## Orange giants

HD 152334, or Zeta2 Scorpii is a giant orange star in Scorpius. Credit: PlanetStar.

Arcturus (α Boötis) is an orange (K1.5III) giant per SIMBAD.

According to SIMBAD HD 152334, or Zeta2 Scorpii is a spectral type K4III star. In the image at right, it is the orange star on the left. It is a single star. Its actual color has less red in it as does that of zeta1 Scorpii, the blue giant on the right. The image on SIMBAD by AladinLite is from DSS2. It has the correct colors.

## Orange stars

This is a visual image in real color of SY Persei (an orange star). Credit: Aladin at SIMBAD.

The bright orange star in the upper left is Suhail in Vela. Credit: ESO/B. Tafreshi/TWAN.

The variability of BD +50 961 (SY Persei, an orange star) is confirmed.[59]

"ESO Photo Ambassador Babak Tafreshi snapped this remarkable image [at left] of the antennas of the Atacama Large Millimeter/submillimeter Array (ALMA), set against the splendour of the Milky Way. The richness of the sky in this picture attests to the unsurpassed conditions for astronomy on the 5000-metre-high Chajnantor plateau in Chile’s Atacama region."[60]

"This view shows the constellations of Carina (The Keel) and Vela (The Sails). The dark, wispy dust clouds of the Milky Way streak from middle top left to middle bottom right. The bright orange star in the upper left is Suhail in Vela, while the similarly orange star in the upper middle is Avior, in Carina. Of the three bright blue stars that form an “L” near these stars, the left two belong to Vela, and the right one to Carina. And exactly in the centre of the image below these stars gleams the pink glow of the Carina Nebula"[60]

## Brown dwarfs

This brown dwarf (smaller object) orbits the star Gliese 229, which is located in the constellation Lepus about 19 light years from Earth. The brown dwarf, called Gliese 229B, is about 20 to 50 times the mass of Jupiter. Credit: NASA and Hubblesite.

This image shows Gliese 105C at the upper right. Credit: NASA, HST, WFPC 2, D. Golimowski (JHU).

Brown dwarfs are sub-stellar objects that have fully convective surfaces and interiors, with no chemical differentiation by depth. Brown dwarfs occupy the mass range between that of large gas giant planets and the lowest-mass stars; this upper limit is between 75[1] and 80 Jupiter masses (${\displaystyle M_{J}}$ ).

Astronomers have reported that spectral class T brown dwarves (the ones with the coolest temperatures) are colored magenta because of absorption by sodium and potassium atoms of light in the green portion of the spectrum.[61][62][63]

"The star on the left is so much brighter than the "coolest star" that it creates the white streak and dramatic pattern visible in the image."[64]

### 2MASS J10475385+2124234

"2MASS J10475385+2124234 [is] a brown dwarf more than 33 light-years away in the constellation Leo. The dwarf ... has a surface temperature of just ... 900 Kelvin ... Jupiter's lights are linked to its rapid rotation ... Since brown dwarfs are comparable in size to Jupiter, the brown dwarf flare mechanisms might arise similarly. ... [When] first examined using the ... fixed radio dish at Arecibo Observatory in Puerto Rico In several observations, ... flares of radio activity [occurred] ... [Using the] Karl G. Jansky Very Large Array (VLA) of telescopes ... The radio waves emanating ... are about 4.5 times fainter than the previous record ... observing ... LPP 944-20."[65]

## Sub-brown dwarfs

A sub-brown dwarf is an astronomical object of planetary mass that is not orbiting a star and is not considered to be a brown dwarf because its mass is below the limiting mass of about 13 Jupiter masses).[66] Sub-brown dwarfs are formed in the manner of stars, through the collapse of a gas cloud (perhaps with the help of photo-erosion), and not through accretion or core collapse from a circumstellar disc Johnstonalthough not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification.[67] The smallest mass of gas cloud that could collapse to form a sub-brown dwarf is about 1 MJ.[68] This is because to collapse by gravitational contraction requires radiating away energy as heat and this is limited by the opacity of the gas.[69]

## Novas

The orange filaments are the tattered remains of the star and consist mostly of hydrogen. Credit: NASA, ESA, J. Hester and A. Loll (Arizona State University).

On September 3, 1885, the Nova Andromedae exhibited "Colour orange. Nebula around Nova very ruddy."[70] On September 5, 1885, the same nova appeared, "Very light, but not bright orange. Nebula ruddy."[70] By the 6th, it was "Dull orange."[70]

In the image at right, the orange filaments are the tattered remains of the star and consist mostly of hydrogen.

## Star-forming regions

Towards the upper right of this image from the Carina Nebula are orange dust clumps. Credit: ESA/Hubble & NASA.

"Looking like an elegant abstract art piece painted by talented hands, this picture is actually a NASA/ESA Hubble Space Telescope image of a small section of the Carina Nebula. Part of this huge nebula was documented in the well-known Mystic Mountain picture (heic1007a) and this picture takes an even closer look at another piece of this bizarre astronomical landscape (heic0707a)."[71]

"The Carina Nebula itself is a star-forming region about 7500 light-years from Earth in the southern constellation of Carina (The Keel: part of Jason’s ship the Argo). Infant stars blaze with a ferocity so severe that the radiation emitted carves away at the surrounding gas, sculpting it into strange structures. The dust clumps towards the upper right of the image, looking like ink dropped into milk, were formed in this way. It has been suggested that they are cocoons for newly forming stars."[71]

"The Carina Nebula is mostly made from hydrogen, but there are other elements present, such as oxygen and sulphur. This provides evidence that the nebula is at least partly formed from the remnants of earlier generations of stars where most elements heavier than helium were synthesised."[71]

"The brightest stars in the image are not actually part of the Carina Nebula. They are much closer to us, essentially being the foreground to the Carina Nebula’s background."[71]

"This picture was created from images taken with Hubble’s Wide Field Planetary Camera 2. Images through a blue filter (F450W) were coloured blue and images through a yellow/orange filter (F606W) were coloured red. The field of view is 2.4 by 1.3 arcminutes."[71]

## Globular clusters

This picture was put together from images taken with the Wide Field Channel of Hubble's Advanced Camera for Surveys. Credit: ESA/Hubble & NASA.

This bright spray of stars in the small but evocative constellation of Delphinus (the Dolphin) is the globular cluster NGC 6934. Credit: ESA/Hubble & NASA.

"The dazzling stars in Messier 15 [at right] look fresh and new in this image from the NASA/Hubble Space Telescope, but they are actually all roughly 13 billion years old, making them some of the most ancient objects in the Universe. Unlike another recent Hubble Picture of the Week, which featured the unusually sparse cluster Palomar 1, Messier 15 is rich and bright despite its age."[72]

"Messier 15 is a globular cluster — a spherical conglomeration of old stars that formed together from the same cloud of gas, found in the outer reaches of the Milky Way in a region known as the halo and orbiting the Galactic Centre. This globular lies about 35 000 light-years from the Earth, in the constellation of Pegasus (The Flying Horse)."[72]

"Messier 15 is one of the densest globulars known, with the vast majority of the cluster’s mass concentrated in the core. Astronomers think that particularly dense globulars, like this one, underwent a process called core collapse, in which gravitational interactions between stars led to many members of the cluster migrating towards the centre."[72]

"Messier 15 is also the first globular cluster known to harbour a planetary nebula, and it is still one of only four globulars known to do so. The planetary nebula, called Pease 1, can be seen in this image as a small blue blob to the lower left of the globular’s core."[72]

"This picture was put together from images taken with the Wide Field Channel of Hubble's Advanced Camera for Surveys. Images through yellow/orange (F606W, coloured blue) and near-infrared (F814W, coloured red) filters were combined. The total exposure times were 535 s and 615 s respectively and the field of view is 3.4 arcminutes across."[72]

"This bright spray of stars [at second right] in the small but evocative constellation of Delphinus (the Dolphin) is the globular cluster NGC 6934. Globular clusters are large balls of (typically) a few hundred thousand ancient stars that exist on the edges of galaxies."[73]

"Lying 50 000 light-years from Earth, in the outer reaches of our Milky Way galaxy, NGC 6934 is home to some of the most distant stars still to be part of our galactic system — in a sense, it is a far-flung suburb to the Milky Way’s city centre."[73]

"NGC 6934 was first seen by William Herschel in the late eighteenth century. He classified it as a “bright nebula” and was not able to resolve it into stars. The cluster is not bright enough to see with the naked eye, and even in ideal conditions it is very difficult to view with binoculars. However, it is a popular target for amateur astronomers as it can easily be observed using relatively inexpensive telescopes. Broadcaster Patrick Moore, presenter of BBC TV’s The Sky at Night for more than 50 years, included this cluster in his “Caldwell catalogue” of celestial objects that amateur astronomers should look out for."[73]

"NGC 6934’s faintness is down to its distance — not how bright it really is. With its many thousands of stars, the cluster is no minnow. The fact that the huge core of our galaxy dwarfs it, along with the other 150 or so globular clusters that orbit the Milky Way’s galactic centre, is a reminder of the breathtaking scale of the cosmos."[73]

"This picture was taken with the Wide Field Channel of the NASA/ESA Hubble Space Telescope’s Advanced Camera for Surveys. It was created from images taken through filters F814W (near infrared) and F606W (orange), coloured red and blue respectively. The exposure times were 29 minutes per filter, and the field of view is 3.3 arcminutes across."[73]

## Locations on Earth

Dune 45 is so called because it lies 45 km past Sesriem on the road to Sossusvlei. Credit: Harald Süpfle.

Sossusvlei is seen from the dunes. Credit: Moongateclimber.

Sossusvlei is imaged during an occasional flood in autumn 2006. Credit: GIRAUD Patrick.

This is a true-color Moderate Resolution Imaging Spectroradiometer (MODIS) image acquired by the Terra satellite on August 19, 2002. Credit: Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC.

The Sossusvlei area belongs to a wider region of southern Namib with homogeneous features (about 32.000 km²) extending between rivers Koichab and Kuiseb. This area is characterized by high sand dunes of vivid pink-to-orange color, a consequence of a high percentage of iron in the sand and consequent oxidation processes. The oldest dunes are those of a more intense reddish color. These dunes are among the highest in the world; many of them are above 200 metres, the highest being the one nicknamed Big Daddy, about 380 metres high.

Sossusvlei is about 66 km past the Sesriem gate. The last 6 km can only be traversed with 4WD vehicles as the concrete road ends and sand begins (the place where the concrete road ends is known as "2x4 parking" as any non-4WD vehicle must stop there). Sossusvlei is a clay pan, of roughly elliptical shape, covered in a crust of salt-rich sand.[74] While the pan has been shaped over time by the Tsauchab river, the actual flooding of the pan is a relatively rare event, and sometimes several years pass between one flood and the next one. The river is dry most of the year, and even when it is not, it carries relatively little water to the vlei. The vlei is surrounded by high orange-reddish dunes, partially covered by a vegetation comprising grass, bushes, and some tree (mostly of species Acacia erioloba).

Deadvlei is another clay pan, about 2 km from Sossusvlei. A notable feature of Deadvlei is that it used to be an oasis with several acacia trees; afterwards, the river that watered the oasis changed its course. The pan is thus punctuated by blackened, dead acacia trees, in vivid contrast to the shiny white of the salty floor of the pan and the intense orange of the dunes. This creates a particularly fascinating and surrealistic landscape, that appears in uncountable pictures and that has been used as a setting for films and videos.

"The striking orange-red colored southern Australian coast contrasts against the deep sapphire-blue waters of the Southern Ocean in this true-color Moderate Resolution Imaging Spectroradiometer (MODIS) image acquired by the Terra satellite on August 19, 2002. In the northern portion of the image, a handful of fires (marked in red) were detected burning in the Great Victoria Desert by the MODIS instrument. South of the desert is the lighter-orange Nullarbor Plain, which stretches for over 1000 kilometers (about 600 miles) from end to end."[75]

## Hypotheses

1. Orange stars actually appear in visual or optical astronomy as orange in color subject to blue or red shift.

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