# Red astronomy

The Sun is observed through a telescope with an H-alpha filter. Credit: Marshall Space Flight Center, NASA.
 Completion status: Almost complete, but you can help make it more thorough.

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

 Educational level: this is a secondary education resource.

“In 1926 ... [t]here were no national observatories (except the Naval Observatory), very little chance for guest observing elsewhere, no radio astronomy, no X-ray astronomy, no satellite astronomy, and very little infrared or even red astronomy!”[1] Bold added. In wavelengths, red astronomy covers 620 - 750 nm.

 Educational level: this is a tertiary (university) resource.
 Educational level: this is a research resource.
 Resource type: this resource is an article.
 Resource type: this resource contains a lecture or lecture notes.
 Subject classification: this is an astronomy resource.

# Notation

Notation: let the symbol Def. indicate that a definition is following.

Notation: let the symbols between [ and ] be replacement for that portion of a quoted text.

# Universals

To help with definitions, their meanings and intents, there is the learning resource theory of definition.

Def. evidence that demonstrates that a concept is possible is called proof of concept.

The proof-of-concept structure consists of

1. background,
2. procedures,
3. findings, and
4. interpretation.[2]

The findings demonstrate a statistically systematic change from the status quo or the control group.

# Absorption/emission lines

"Balmer lines can appear as absorption or emission lines in a spectrum, depending on the nature of the object observed. In stars, the Balmer lines are usually seen in absorption, and they are "strongest" in stars with a surface temperature of about 10,000 kelvin (spectral type A). In the spectra of most spiral and irregular galaxies, AGNs, H II regions and planetary nebulae, the Balmer lines are emission lines."[3]

# Hydrogen spectral series

The spectral series of hydrogen is displayed on a logarithmic scale. Credit: .
Electron transitions and their resulting wavelengths for hydrogen. Energy levels are not to scale. Credit: .

"The emission spectrum of atomic hydrogen is divided into a number of spectral series, with wavelengths given by the Rydberg formula. These observed spectral lines are due to electrons moving between energy levels in the atom. The spectral series are important in astronomy for detecting the presence of hydrogen and calculating red shifts. ... [T]he spectral lines of hydrogen correspond to particular jumps of the electron between energy levels. The simplest model of the hydrogen atom is given by the Bohr model. When an electron jumps from a higher energy to a lower, a photon of a specific wavelength is emitted."[4]

"The spectral lines are grouped into series according to n'. Lines are named sequentially starting from the longest wavelength/lowest frequency of the series, using Greek letters within each series. For example, the 2 → 1 line is called "Lyman-alpha" (Ly-α), while the 7 → 3 line is called "Paschen-delta" (Pa-δ). Some hydrogen spectral lines fall outside these series, such as the 21 cm line; these correspond to much rarer atomic events such as hyperfine transitions.[5] The fine structure also results in single spectral lines appearing as two or more closely grouped thinner lines, due to relativistic corrections.[6]"[4].

"The energy differences between levels in the Bohr model, and hence the wavelengths of emitted/absorbed photons, is given by the Rydberg formula[7]:

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

where n is the initial energy level, n′ is the final energy level, and R is the Rydberg constant. Meaningful values are returned only when n is greater than n′ and the limit of one over infinity is taken to be zero."[4]

# Hydrogen

The spectrum shows the lines in the visible due to emission from elemental hydrogen. Credit:Teravolt.
Milky Way is viewed by H-Alpha Sky Survey. Credit: David Brown and Douglas Finkbeiner.

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

"A hydrogen-alpha filter is an optical filter designed to transmit a narrow bandwidth of light generally centered on the H-alpha wavelength. They are characterized by a bandpass width that measures the width of the wavelength band that is transmitted.[8] These filters are manufactured by multiple (~50) layers of vacuum-deposited layers. These layers are selected to produce interference effects that filter out any wavelengths except at the requisite band.[9] Alternatively, an etalon may be used as the narrow band filter (in conjunction with a "blocking filter" or energy rejection filter) to pass only a narrow (<0.1 nm) range of wavelengths of light centred around the H-alpha emission line. The physics of the etalon and the dichroic interference filters are essentially the same (relying on constructive/destructive interference of light reflecting between surfaces), but the implementation is different (an interference filter relies on the interference of internal reflections). Due to the high velocities sometimes associated with features visible in H-alpha light (such as fast moving prominences and ejections), solar H-alpha etalons can often be tuned (by tilting or changing the temperature) to cope with the associated Doppler effect."[10]

# Helium

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

Helium has at least one weak line in the red.

# Lithium

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

"[T]he standard solar models have enjoyed tremendous success recently in terms of agreement between the predicted outer structure and the results from helioseismology[, but] some observed properties of the Sun still defy explanation, such as the degree of Li depletion" [the "solar Li abundance is roughly a factor of 200 below the meteoritic abundance"].[11]

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."[12]

Some of the incontrovertible brown dwarf substellar objects are "identified by the presence of the 670.8 nm lithium [I] line. The most notable of these objects was Gliese 229B, which was found to have a temperature and luminosity well below the stellar range. Remarkably, its near-infrared spectrum clearly exhibited a methane absorption band at 2 micrometres, a feature that had previously only been observed in gas giant atmospheres and the atmosphere of Saturn's moon, Titan. Methane absorption is not expected at the temperatures of main-sequence stars. This discovery helped to establish yet another spectral class even cooler than L dwarfs known as "T dwarfs" for which Gl 229B is the prototype. ... Lithium is generally present in brown dwarfs and not in low-mass stars. [T]he presence of the lithium line in a candidate brown dwarf's spectrum is a strong indicator that it is indeed substellar. The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the lithium test ... Some brown dwarfs emit X-rays; and all "warm" dwarfs continue to glow tellingly in the red and infrared spectra until they cool to planetlike temperatures (under 1000 K)."[13]

# Beryllium

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

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

# Boron

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

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

# Carbon

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

Carbon has one strong line in the red.

# Nitrogen

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

Nitrogen has an emission line at 658.4 nm.

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

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

# Oxygen

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

Oxygen (O I) has two red lines at 630.0 and 636.4 nm. In the red there are the atomic oxygen transitions of the "forbidden oxygen red doublet at 6300.304 and 6363.776 Å (1D - 3P)"[14]. Atmospheric O2 has a red line at 686.72 nm.

"The oxygen abundance [may be determined] using the oxygen forbidden line at 630nm"[15]. "[R]atios [of] O/Fe ... are in agreement with the ratios found in the metal-poor red giants, suggesting that no real difference exists between dwarfs and giants."[15]

"The forbidden oxygen line (λ 630.03nm) is weak in dwarf stars"[15]

In the spectrum at right several red astronomy emission lines are detected and recorded at normalized intensities (to the oxygen III line) from the Ring Nebula. In the red are the two forbidden lines of oxygen ([O I], 630.0 and 636.4 nm), two forbidden lines of nitrogen ([N II], 654.8 nm and [N II], 658.4 nm), the hydrogen line (Hα, 656.3 nm) and a forbidden line of sulfur ([S II], 671.7 nm).

# Neon

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

Neon has many lines across the red.

# Iron

Iron has two emission lines occurring in the solar corona at 637.451 nm from Fe X and 705.962 nm from Fe XV.[16]

# Nickel

Nickel has an emission line occurring in the solar corona at 670.183 nm from Ni XV.[16]

# Comets

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

"My first thought was Hubble Space Telescope does it again! We caught the fish! This is amazing, very exciting, very neat."[17]

"Actually, I would have been more amazed if Hubble saw no pieces ... They just had to be there. The amount of heat available from sunlight just isn't enough to boil away something the size of a mountain in so short a time".[18]

"On July 27th, ground-based observers had lost sight of the bright core of the comet and were suggesting that the nucleus had totally disintegrated into a pile of dust. ... On Weaver's screen was at least a half dozen "mini-comets" with tails, resembling the shower of glowing fireballs from an aerial firework. They are clustered in the lance-head tip of an elongated stream of dust seen from a ground-based telescope."[19]

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

# Venus

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

"During the MESSENGER mission's second flyby of Venus, the Wide Angle Camera (WAC) of the Mercury Dual Imaging System (MDIS) acquired images through all of its 11 narrow-band color filters of the approaching planet. The surface of Venus is shrouded in clouds, and the WAC images returned from the encounter show this cloud covered view, as seen in a previously released image. However, by processing the WAC images and "stretching" the gray scale used to display the images, subtle differences in the clouds of Venus are revealed, as seen in the image here. This WAC image was taken through a narrow-band filter centered at 630 nanometers, and in this stretched image, global circulation patterns can be seen in the atmosphere of Venus."[21]

# Earth

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

"The Belt of Venus or Venus's Girdle is the Victorian-era name for an atmospheric phenomenon seen at sunrise and sunset. Shortly after sunset or shortly before sunrise, the observer is, or is very nearly, surrounded by a pinkish glow (or anti-twilight arch) that extends roughly 10°–20° above the horizon. Often, the glow is separated from the horizon by a dark layer, the Earth's shadow or "dark segment". The Arch's light rose (pink) color is due to backscattering of reddened light from the rising or setting Sun."[22]

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

In the image at right is the Belt of Venus, a pink band that is visible above the dark blue of the Earth's shadow, in the same part of the sky. No defined line divides the Earth's shadow and the Belt of Venus; one colored band blends into the other in the sky."[23]

In the image at second left, the Belt of Venus is shown at sunset, looking east from the Marin Headlands just north of San Francisco.[23] There is a thin greyish cloud layer partially obscuring the horizon in this image.[23]

The second image at right shows the full Moon rising as seen through the Belt of Venus.[23]

"In the right viewing conditions, a pink (or orange or purple) band is visible in the twilight sky just above the dark blue band of the Earth's shadow. This pink band is called the "anti-twilight arch" or "Belt of Venus". The name "Belt of Venus" is not connected with the planet Venus; the Belt of Venus is part of Earth's upper atmosphere which is illuminated by the setting or rising sun. It is visible either after the sun ceases to be visible (at sunset) or before the sun becomes visible (at sunrise).[24][25]"[23]

"When the sun is near the horizon at sunset or sunrise, the light from the sun is red; this is because the light is reaching the observer through an especially thick layer of the atmosphere, which works as a filter, scattering all but the red light."[23]

"From the viewpoint of the observer, the red sunlight directly illuminates small particles in the lower atmosphere on the other side of the sky from the sun. The red light is backscattered to the observer, and that is why the Belt of Venus appears pink."[23]

"The lower the sunset sun descends, the less clearly distinguished the boundary between the Earth's shadow and Belt of Venus becomes. This is because now the setting sun illuminates a thinner part of the upper atmosphere. The red light is not scattered there because there are fewer particles, and the eye only sees the "normal" (usual) blue sky, which is due to Rayleigh scattering from air molecules. Eventually, both the Earth's shadow and the Belt of Venus dissolve into the darkness of the night sky.[25]"[23]

# Moon

A total lunar eclipse on February 9, 2009, shows the reddish light falling on the moon's surface. Credit: .

"During a lunar eclipse, a very small amount of light from the sun does however still reach the Moon, even when the lunar eclipse is total; this is light which has been refracted or bent as it passes through the Earth's atmosphere. This sunlight has been scattered by the dust in the Earth's atmosphere, and thus that light is red, in the same way that sunset and sunrise light is red. This weak red illumination is what causes the eclipsed Moon to be dimly reddish or copper-colored in appearance.[26]"[23]

# Mars

The tenuous atmosphere of Mars is visible on the horizon in this low-orbit photo. Credit: .

"Mars is the fourth planet from the Sun in the Solar System. Named after the Roman god of war, Mars, it is often described as the "Red Planet" as the iron oxide prevalent on its surface gives it a reddish appearance.[27] ... The red-orange appearance of the Martian surface is caused by iron(III) oxide, more commonly known as hematite, or rust.[28] ... Much of the surface is deeply covered by finely grained iron(III) oxide dust.[29][30]" after the Wikipedia article about the planet Mars.

Mars is imaged from Hubble Space Telescope on October 28, 2005, with dust storm visible. Credit: NASA, ESA, The Hubble Heritage Team (STScI/AURA), J. Bell (Cornell University) and M. Wolff (Space Science Institute).

# Great Red Spot

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

"The Great Red Spot (GRS) is a persistent anticyclonic storm, 22° south of Jupiter's equator, which has lasted for at least 183 years and possibly longer than 348 years.[31][32] The storm is large enough to be visible through Earth-based telescopes. ... Its dimensions are 24–40,000 km west–to–east and 12–14,000 km south–to–north. The spot is large enough to contain two or three planets the size of Earth. At the start of 2004, the Great Red Spot had approximately half the longitudinal extent it had a century ago, when it was 40,000 km in diameter. ... The Great Red Spot's latitude has been stable for the duration of good observational records, typically varying by about a degree." after the Wikipedia article on the atmosphere of Jupiter.

"It is not known exactly what causes the Great Red Spot's reddish color. Theories supported by laboratory experiments suppose that the color may be caused by complex organic molecules, red phosphorus, or yet another sulfur compound. The Great Red Spot (GRS) varies greatly in hue, from almost brick-red to pale salmon, or even white. The reddest central region is slightly warmer than the surroundings, which is the first evidence that the Spot's color is affected by environmental factors.[33] The spot occasionally disappears from the visible spectrum, becoming evident only through the Red Spot Hollow, which is its niche in the South Equatorial Belt. The visibility of GRS is apparently coupled to the appearance of the SEB; when the belt is bright white, the spot tends to be dark, and when it is dark, the spot is usually light. The periods when the spot is dark or light occur at irregular intervals; as of 1997, during the preceding 50 years, the spot was darkest in the periods 1961–66, 1968–75, 1989–90, and 1992–93.[34]" per the Wikipedia article on the atmosphere of Jupiter.

# Comets

"The λλ6300, 6363 Auroral red doublet of [OI] has been measured on digital sky-subtracted spectra of nine cometary nuclei ... The cometary oxygen lines are confined to their nuclear source, so that small apertures include much of the oxygen emission, particularly for small comets with Δ ≳ 1.0 AU."[35]

# Red controversy

"Around 150 AD, the Hellenistic astronomer Claudius Ptolemy described Sirius as reddish, along with five other stars, Betelgeuse, Antares, Aldebaran, Arcturus and Pollux, all of which are clearly of orange or red hue.[36] The discrepancy was first noted by amateur astronomer Thomas Barker, ... who prepared a paper and spoke at a meeting of the Royal Society in London in 1760.[37] The existence of other stars changing in brightness gave credence to the idea that some may change in colour too; Sir John Herschel noted this in 1839, possibly influenced by witnessing Eta Carinae two years earlier.[36] Thomas Jefferson Jackson See resurrected discussion on red Sirius with the publication of several papers in 1892, and a final summary in 1926.[36] He cited not only Ptolemy but also the poet Aratus, the orator Cicero, and general Germanicus as colouring the star red, though acknowledging that none of the latter three authors were astronomers, the last two merely translating Aratus' poem Phaenomena.[36] Seneca, too, had described Sirius as being of a deeper red colour than Mars.[38] However, not all ancient observers saw Sirius as red. The 1st century AD poet Marcus Manilius described it as "sea-blue", as did the 4th century Avienus.[36] It is the standard star for the color white in ancient China, and multiple records from the 2nd century BC up to the 7th century AD all describe Sirius as white in hue.[39][40]

In 1985, German astronomers Wolfhard Schlosser and Werner Bergmann published an account of an 8th century Lombardic manuscript, which contains De cursu stellarum ratio by St. Gregory of Tours. The Latin text taught readers how to determine the times of nighttime prayers from positions of the stars, and Sirius is described within as rubeola — "reddish". The authors proposed this was further evidence Sirius B had been a red giant at the time.[41]" from the Wikipedia article about Sirius.

# Luminous red nova

V838 Monocerotis in this real visual image from the Hubble Space Telescope is a prototypic luminous red nova. Credit: .

"A luminous red nova (abbr. LRN, pl. luminous red novae, pl.abbr. LRNe) is a stellar explosion thought to be caused by the merger of two stars. They are characterised by a distinct red colour, and a light curve that lingers with resurgent brightness in the infrared. Luminous red novae are not to be confused with standard novae, explosions that occur on the surface of white dwarf stars. ... The visible light lasts for weeks or months, and is distinctively red in colour, becoming dimmer and redder over time. As the visible light dims, the infrared light grows and also lasts for an extended period of time, usually dimming and brightening a number of times. ... Some astronomers believe it to be premature to declare a new class of stellar explosions based on such a limited number of observations. For instance, Pastorello et al. 2007[42] explained that the event may be due to a type II-p supernova and Todd et al. 2008[43] pointed out that supernovae undergoing a high level of extinction will naturally be both red and of low luminosity." per the Wikipedia article about the luminous red nova.

# Red dwarf

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

“A red dwarf is a small and relatively cool star on the main sequence, either late K or M spectral type. ... Red dwarfs are by far the most common type of star in the Galaxy, at least in the neighborhood of the Sun. Proxima Centauri, the nearest star to the Sun, is a red dwarf ... [D]ue to their low luminosity, individual red dwarfs cannot easily be observed. From Earth, none are visible to the naked eye.[44]

Typical characteristics[45]
Stellar
Class
Mass
(Mʘ)
(Rʘ)
Luminosity
(Lʘ)
Teff
(K)
M0V 60% 62% 7.2% 3,800
M1V 49% 49% 3.5% 3,600
M2V 44% 44% 2.3% 3,400
M3V 36% 39% 1.5% 3,250
M4V 20% 26% 0.55% 3,100
M5V 14% 20% 0.22% 2,800
M6V 10% 15% 0.09% 2,600
M7V 9% 12% 0.05% 2,500
M8V 8% 11% 0.03% 2,400
M9V 7.5% 8% 0.015% 2,300

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

"[O]ut [of] a sample of 3,897 red dwarfs ... [the Kepler Space Telescope ]has identified 95 exoplanet candidates circling them. Three of these candidates are roughly Earth-size and orbit within their stars' "Goldilocks zone," where liquid water (and possibly life as we know it) can exist."[46]

# Red giant

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

“A red giant is a luminous giant star The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower. The appearance of the red giant is from yellow orange to red, including the spectral types K and M, but also class S stars and most carbon stars. The most common red giants are the so-called red giant branch stars (RGB stars) ... Another case of red giants are the asymptotic giant branch stars (AGB) ... To the AGB stars belong the carbon stars of type C-N and late C-R. ... The stellar limb of a red giant is not sharply-defined, as depicted in many illustrations. Instead, due to the very low mass density of the envelope, such stars lack a well-defined photosphere. The body of the star gradually transitions into a 'corona' with increasing radii.[47][48]

# Red supergiant

"Red supergiants (RSGs) are supergiant stars (luminosity class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive. Betelgeuse and Antares are the best known examples of a red supergiant. ... These stars have very cool surface temperatures (3500–4500 K), and enormous radii. The five largest known red supergiants in the Galaxy are VY Canis Majoris, VV Cephei A, V354 Cephei, RW Cephei and KW Sagittarii, which all have radii about 1500 times that of the [S]un (about 7 astronomical units, or 7 times as far as the Earth is from the [S]un). The radius of most red giants is between 200 and 800 times that of the [S]un. ... Absolute luminosities may reach -10 magnitude compared to +5 for our [S]un."[49]

# Red clump

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

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

# Tip of the red giant branch

"Tip of the red giant branch (TRGB) is a primary distance indicator used in astronomy. It uses the luminosity of the brightest red giant branch stars in a galaxy to gauge the distance to that galaxy. It has been used in conjunction with observations from the Hubble Space Telescope to determine the relative motions of the Local Cluster of galaxies within the Local Supercluster. ... [There] is a sharp discontinuity in the evolutionary track of the star on the HR diagram.[50] This discontinuity is called the tip of the red giant branch. When distant stars at the TRGB are measured in the I-band, their magnitude is somewhat insensitive to their composition of elements with more mass than helium (metallicity) and their mass. This makes the technique especially useful as a distance indicator. The TRGB indicator uses stars in the old stellar populations (Population II).[51]" after the Wikipedia article about the Tip of the red giant branch.

# Interstellar reddening

“In interstellar astronomy, visible spectra can appear redder due to scattering processes in a phenomenon referred to as interstellar reddening[52] — similarly Rayleigh scattering causes the atmospheric reddening of the Sun seen in the sunrise or sunset and causes the rest of the sky to have a blue color. This phenomenon is distinct from redshifting because the spectroscopic lines are not shifted to other wavelengths in reddened objects and there is an additional dimming and distortion associated with the phenomenon due to photons being scattered in and out of the line-of-sight.” from the Wikipedia article on the redshift.

# Star-forming region

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

"The gas in the clouds of NGC 6559, mainly hydrogen, is the raw material for star formation ... When a region inside this nebula gathers enough matter, it starts to collapse under its own gravity. The center of the cloud grows ever denser and hotter, until thermonuclear fusion begins and a star is born. The hydrogen atoms combine to form helium atoms, releasing energy that makes the star shine. ... In regions where it is very dense, the dust completely blocks the light behind it, as is the case for the dark isolated patches and sinuous lanes to the bottom left-hand side and right-hand side of the image".[53]

"The Danish 1.54-metre telescope located at ESO’s La Silla Observatory in Chile has captured a striking image of NGC 6559, an object that showcases the anarchy that reigns when stars form inside an interstellar cloud. This region of sky includes glowing red clouds of mostly hydrogen gas, blue regions where starlight is being reflected from tiny particles of dust and also dark regions where the dust is thick and opaque."[54]

"The two colors of the cloud represent a pair of nebulas. Once the young stars are born, they "energize" the hydrogen surrounding them, ESO officials said. The gas then creates the red wispy cloud — known to astronomers as an "emission nebula" — in the center of the image."[55]

"These young stars are usually of spectral type O and B, with temperatures between 10 000 and 60 000 K, which radiate huge amounts of high energy ultraviolet light that ionises the hydrogen atoms."[56]

"The blue section of the photo — representing a "reflection nebula" — shows light from the newly formed stars in the cosmic nursery being reflected in all directions by the particles of dust made of iron, carbon, silicon and other elements in the interstellar cloud."[55]

"NGC 6559 is planetary nebula located at a distance of about 5000 light-years from Earth, in the constellation of Sagittarius."[57]

"NGC 6559 is a cloud of gas and dust located at a distance of about 5000 light-years from Earth, in the constellation of Sagittarius (The Archer). The glowing region is a relatively small object, just a few light-years across, in contrast to the one hundred light-years and more spanned by its famous neighbour, the Lagoon Nebula (Messier 8, eso0936). Although it is usually overlooked in favour of its distinguished companion, NGC 6559 has the leading role in this new picture."[56]

"The Milky Way fills the background of the image with countless yellowish older stars. Some of them appear fainter and redder because of the dust in NGC 6559."[56]

"This eye-catching image of star formation was captured by the Danish Faint Object Spectrograph and Camera (DFOSC)".[56]

# BL Lacertae objects

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

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

# Red shift

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

"Ideally all intrinsic colours should be found from unreddened stars. This is possible for dwarf and giant stars later than about A0 (Johnson, 1964) ... However, it cannot be used for stars of other spectral classes since they are all relatively infrequent in space, and generally reddened."[58]

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

# Cosmological redshift

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

# Gravitational redshift

Gravitational redshifts are a relativistic effect observed in electromagnetic radiation moving out of gravitational fields.” per the Wikipedia article on the redshift.

# Blueshift

“[A] decrease in wavelength is called blueshift and is generally seen when a light-emitting object moves toward an observer or when electromagnetic radiation moves into a gravitational field.” after the Wikipedia article on the redshift.

# Infrared astronomy

"Infrared astronomy is the branch of astronomy and astrophysics that studies astronomical objects visible in infrared (IR) radiation. 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 and optical astronomy are often practiced using the same telescopes, as the same mirrors or lenses are usually effective over a wavelength range that includes both visible and infrared light. ... Infrared light is absorbed at many wavelengths by water vapor in the Earth's atmosphere, so most infrared telescopes are at high elevations in dry places, above as much of the atmosphere as possible. There are also infrared observatories in space, including the Spitzer Space Telescope and the Herschel Space Observatory. ... Infrared radiation with wavelengths just longer than visible light, known as near-infrared, behaves in a very similar way to visible light, and can be detected using similar solid state devices. For this reason, the near infrared region of the spectrum is commonly incorporated as part of the "optical" spectrum, along with the near ultraviolet. Many optical telescopes, such as those at Keck Observatory, operate effectively in the near infrared as well as at visible wavelengths." after the Wikipedia article about infrared astronomy.

# Orange astronomy

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: .

"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[59] and 80 Jupiter masses ($M_J$)." per the Wikipedia article about the brown dwarf.

"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.[60][61][62]" from the Wikipedia article about magenta.

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