# Ultraviolet astronomy

This is a false-color image of the Sun's corona as seen in extreme ultraviolet (at 17.1 nm) by the Extreme ultraviolet Imaging Telescope Credit: NASA.

Ultraviolet astronomy is radiation astronomy applied to the ultraviolet phenomena of the sky, 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) ultraviolet astronomy.

 Educational level: this is a secondary education resource.

Acquiring a suntan may be a student's first encounter with an astronomical ultraviolet source. This probably occurs at the secondary level. How this source generates ultraviolet rays is usually introduced at the university or tertiary educational level.

 Educational level: this is a tertiary (university) resource.

"The color of a star, as determined by the peak frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.[1]"[2] The effective temperature of the surface of the Sun's photosphere is 5,778 K.[3] That's a peak emittance wavelength of 501.5 nm (~0.5 eV) making the photosphere a primarily green radiation source. The temperature of the photosphere is way too cool to generate appreciable amounts of ultraviolet. In fact, the Sun's photosphere probably generates little or no ultraviolet rays.

 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.

Notation: let the symbol dex represent the difference between powers of ten.

"An order or factor of ten[, dex is] used both to refer to the function $dex(x) = 10^x$ and the number of (possibly fractional) orders of magnitude separating two numbers. When dealing with log to the base 10 transform of a number set, the transform of 10, 100, and 1 000 000 is $\log_{10}(10) = 1$, $\log_{10}(100) = 2$, and $\log_{10}(1 000 000) = 6$, so the difference between 10 and 100 in base 10 is 1 dex and the difference between 1 and 1 000 000 is 6 dex."[4]

# 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.[5]

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

Def. "the spectral region bounded on the long wavelength side at about λ3000 by the onset of atmospheric ozone absorption and on the short wavelength side at λ912 by the photoionization of interstellar hydrogen" is called the ultraviolet.[6]

# Ultraviolet

"The draft ISO standard on determining solar irradiances (ISO-DIS-21348)5 describes the following ranges:"[7]

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

""Vacuum UV" is so named because it is absorbed strongly by air and is, therefore, used in a vacuum. In the long-wave limit of this region, roughly 150–200 nm, the principal absorber is the oxygen in air."[7]

# Glass

"Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths, whereas silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm.[8][9][10]"[7]

# Optical astronomy

Optical astronomy includes those portions of ultraviolet, visual, and infrared astronomy that benefit from the use of quartz crystal or silica glass telescope components.

"[T]he color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. To measure the index, one observes the magnitude of an object successively through two different filters, such as U and B, or B and V, where U is sensitive to ultraviolet rays, B is sensitive to blue light, and V is sensitive to visible (green-yellow) light (see also: UBV system). The set of passbands or filters is called a photometric system. The difference in magnitudes found with these filters is called the U-B or B–V color index, respectively. The smaller the color index, the more blue (or hotter) the object is. Conversely, the larger the color index, the more red (or cooler) the object is. This is a consequence of the logarithmic magnitude scale, in which brighter objects have smaller (more negative) magnitudes than dimmer ones."[11]

# Technology

"XUV is strongly absorbed by most known materials, but it is possible to synthesize multilayer optics that reflect up to about 50% of XUV radiation at normal incidence. This technology, which was pioneered by the NIXT and MSSTA sounding rockets in the 1990s, has been used to make telescopes for solar imaging (current examples are SOHO/EIT and TRACE)"[7]

# Telescope

"Although optical telescopes can image the near ultraviolet, the ozone layer in the stratosphere absorbs ultraviolet radiation shorter than 300 nm so most ultra-violet astronomy is conducted with satellites. Ultraviolet telescopes [10 nm - 400 nm] resemble optical telescopes, but conventional aluminium-coated mirrors cannot be used and alternative coatings such as magnesium fluoride or lithium fluoride are used instead. The OSO 1 satellite carried out observations in the ultra-violet as early as 1962. The International Ultraviolet Explorer (1978) systematically surveyed the sky for eighteen years, using a 45 cm (18 in) aperture telescope with two spectroscopes. Extreme-ultraviolet astronomy (10–100 nm) is a discipline in its own right and involves many of the techniques of X-ray astronomy; the Extreme Ultraviolet Explorer (1992) was a satellite operating at these wavelengths."[12]

# Hydrogen

"[T]he Lyman series is the series of transitions and resulting ultraviolet emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1 (where n is the principal quantum number referring to the energy level of the electron)."[13]

"The version of the Rydberg formula that generated the Lyman series was[14]:

${1 \over \lambda} = R_H \left( 1 - {1 \over n^2} \right) \qquad \left( R_H = 1.0968 \times 10^7 \mbox{m}^{-1} = {13.6eV \over hc} \right)$

Where n is a natural number greater than or equal to 2 (i.e. n = 2,3,4,...).

Therefore, the lines seen in the image above are the wavelengths corresponding to $n=2\,$ on the right, to $n= \infty$ on the left (there are infinitely many spectral lines, but they become very dense as they approach to $n= \infty$ (Lyman limit), so only some of the first lines and the last one appear).

The wavelengths (nm) in the Lyman series are all ultraviolet:"[13]

 $n$ Wavelength (nm) 2 3 4 5 6 7 8 9 10 11 $\infty$ 121.6 102.6 97.3 95 93.8 93.1 92.6 92.3 92.1 91.9 91.18 (Lyman limit)

"In 1913, when Niels Bohr produced his Bohr model theory, the reason why hydrogen spectral lines fit Rydberg's formula was explained. Bohr found that the electron bound to the hydrogen atom must have quantized energy levels described by the following formula:

$E_n = - {{m e^4} \over {2 \left( 4 \pi \varepsilon_0 \hbar \right)^2}} {1 \over n^2} = - {13.6 \over n^2} [\mbox{eV}].$

According to Bohr's third assumption, whenever an electron falls from an initial energy level($E_i$) to a final energy level($E_f$), the atom must emit radiation with a wavelength of:

$\lambda = {{h c} \over {E_i - E_f}}.$

There is also a more comfortable notation when dealing with energy in units of electronvolts and wavelengths in units of angstroms:

$\lambda = {12430 \over {E_i - E_f}}.$

Replacing the energy in the above formula with the expression for the energy in the hydrogen atom where the initial energy corresponds to energy level n and the final energy corresponds to energy level m:

${1 \over \lambda} = {{E i-E f} \over 12430} = \left( {12430 \over 13.6} \right)^{-1} \left({1 \over m^2} - {1 \over n^2} \right) = R \left({1 \over m^2} - {1 \over n^2} \right)$

where R_H is the same Rydberg constant for hydrogen of Rydberg's long known formula.

For the connection between Bohr, Rydberg, and Lyman, one must replace m by 1 to obtain:

${1 \over \lambda} = R_H \left( 1 - {1 \over n^2} \right)$

which is Rydberg's formula for the Lyman series. Therefore, each wavelength of the emission lines corresponds to an electron dropping from a certain energy level (greater than 1) to the first energy level."[13]

# Helium

There is a He I (Is21S-Is2p2P) transition at 58.4 nm and another Is21S-Is3p1P) at 53.7 nm.[15] The He II lines are at 25.6, 30.4, and 164.0 nm.[15]

There is another He II line at 320.3 nm.[16]

# Beryllium

"[T]he Be II 3130 Å region ... contains the Be II resonance doublet with components at 3130.42 [313.042 nm] and 3131.07 Å [313.107 nm]."[17]

"High-resolution, high signal-to-noise ratio spectra, obtained at the Cerro Tololo Inter-American Observatory 4 m telescope, of the Be II 3131 Å [313.1 nm] region [are from] the metal-rich solar analog α Centauri A and its companion α Centauri B."[17]

"The photospheric abundances of the light elements Li, Be, and B provide important clues about stellar structure and evolution, as they are destroyed by (p,α)-reactions at temperatures exceeding a few million degrees."[17]

"For Cen A, ... [Be/H] = +0.20 ± 0.15, where the error reflects random uncertainties at the 1σ confidence level; systematic errors of 0.1 dex are also possible."[17]

# Boron

Boron (B I) line is at 249.67 nm.[18]

Boron is detected in the Population II star HD 140283 by observing the "wavelength region around the resonance lines of B I at 2497 Å ... with the Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope on September 5, 1992, ... and continued on February 15, and 21, 1993"[18] "The resulting B/Be ratio is in the range 9-34 with 17 being the most probable value. This is in very good agreement with predictions for cosmic ray spallation."[18]

"[T]he solar system meteoritic NB/NBe ratio 28 ± 4 (Anders & Grevesse 1989) is within our limits of uncertainty, implying that the same process could in principle be responsible for the production of B and Be throughout the history of the Galaxy."[18]

# Carbon

There is a C III line at 97.7 nm.[15]

Carbon has several emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas: 229.687 nm from C III, 227.089, 227.727, and 227.792 nm from C V, 207.025, 208.216, 313.864, and 343.366 nm from C VI.[19]

# Nitrogen

Several emission lines occur in plasmas at 347.872, 348.30, and 348.493 nm from N IV and 252.255, 344.211, and 388.678 nm from N VII.[19]

"Molecular nitrogen and nitrogen compounds have been detected in interstellar space by astronomers using the Far Ultraviolet Spectroscopic Explorer.[20]"[21]

# Oxygen

Oxygen has three emission lines common in comets at 130.22, 130.49, and 130.60 nm from O I.[22]

Oxygen has several emission lines that occur in plasmas at 278.101, 278.699, and 278.985 nm from O V, and 253.04, 297.569, 348.767 nm from O VIII.[19]

Atomic oxygen "airglow emissions [have been] measured by using vertical-viewing photometers [on-board a sounding rocket for the] Herzberg I bands near 300 nm"[23].

"[T]he presence of the [oxygen] green line can still be questioned, unless the 2972 Å trans-auroral line [1S - 3P] is detected (Herbig, 1976)."[24] "The transitions involved ... in the spectrum of the oxygen atoms in a cometary atmosphere" include 295.8 and 297.2 nm, 98.9 nm (a triplet), and 1304 nm (a triplet), 102.7 nm (a triplet) and 1128.7 nm.[24]

# Fluorine

Fluorine has several emission lines that occur in plasmas at 311.361, 311.57, 312.154, 312.478, 313.422, 314.278, 317.418, 317.476, and 321.397 nm from F III, 270.23, 270.717, 271.288, 272.106, 273.20, 273.691, and 275.62 nm from F V, 231.539, 232.335, and 232.728 nm from F VI, and 342.938 nm from F IX.[19]

# Argon

Argon has several emission lines that occur in an electron cyclotron resonance (ECR) heated plasmas: 333.613, 334.472, 335.211, 335.849, and 336.128 nm from Ar III.[19]

# Calcium

The calcium line Ca XII at 332.8 nm occurs in the solar corona.[25]

# Chromium

Chromium has several emission lines that occur in plasmas at 357.869, 359.349, and 360.533 nm from Cr I, and 222.67 and 223.59 nm from Cr III.[19]

# Iron

Iron has several emission lines that occur in plasmas at 371.994 nm from Fe I, 234.35 nm from Fe II, and 273.9, 274.9, and 276.9 nm from Fe XV.[19]

Iron has two lines occurring in the solar corona in the near ultraviolet: 338.81 and 345.41 nm of Fe XIII.[25]

# Nickel

Nickel has lines occurring in the solar corona at 360.10 nm of Ni XVI and 364.29 nm of Ni XIII.[25]

# Copper

Copper has two emission lines that occur in plasmas at 324.754 and 327.396 nm from Cu I.[19]

# Sun

This is a FUV image of the Sun. Credit: NASA/SDO/AIA.

"[Ultraviolet] light is found in sunlight ... The sun emits ultraviolet radiation in the UVA, UVB, and UVC bands. The Earth's ozone layer blocks 97–99% of this UV radiation from penetrating through the atmosphere.[26]"[7]

"The solar transition region is a region of the Sun's atmosphere, between the chromosphere and corona[27]. It is visible from space using telescopes that can sense ultraviolet."[28]

"The transition region is visible in far-ultraviolet (FUV) images from the TRACE spacecraft, as a faint nimbus above the dark (in FUV) surface of the Sun and the corona. The nimbus also surrounds FUV-dark features such as solar prominences, which consist of condensed material that is suspended at coronal altitudes by the magnetic field."[28]

# Solar coronal cloud

STEREO—First images is a slow animation of a mosaic of the extreme ultraviolet images taken on December 4, 2006. These false color images show the Sun's atmospheres at a range of different temperatures. Clockwise from top left: 1 million degrees C (171 Å—blue), 1.5 million °C (195 Å—green), 60,000–80,000 °C (304 Å—red), and 2.5 million °C (286 Å—yellow). Credit: NASA.

"In the corona thermal conduction occurs from the external hotter atmosphere towards the inner cooler layers. Responsible for the diffusion process of the heat are the electrons, which are much lighter than ions and move faster"[29].

"Ultraviolet telescopes such as TRACE and SOHO/EIT can observe individual micro-flares as small brightenings in extreme ultraviolet light,[30] but there seem to be too few of these small events to account for the energy released into the corona."[29]

"The first direct observation of waves propagating into and through the solar corona was made in 1997 with the SOHO space-borne solar observatory, the first platform capable of observing the Sun in the extreme ultraviolet (EUV) for long periods of time with stable photometry. Those were magneto-acoustic waves with a frequency of about 1 millihertz (mHz, corresponding to a 1,000 second wave period), that carry only about 10% of the energy required to heat the corona. Many observations exist of localized wave phenomena, such as Alfvén waves launched by solar flares, but those events are transient and cannot explain the uniform coronal heat."[29]

# Mercury

"[U]ltraviolet observations by Mariner 10 provided evidence for the presence of H and He in the atmosphere (~1011 and 1012 atoms cm-2 respectively3"[31].

Aboard Mariner 10, "[t]he extreme ultraviolet spectrometer consisted of two instruments: an occultation spectrometer that was body-fixed to the spacecraft and an airglow spectrometer that was mounted on the scan platform. When the sun was obscured by the limbs of the planet, the occultation spectrometer measured the extinction properties of the atmosphere. The occultation spectrometer had a plane grating which operated at grazing incidence. The fluxes were measured at 47.0, 74.0, 81.0, and 89.0 nm using channel electron multipliers. Pinholes defined the effective field of view of the instrument which was 0.15 degree full width at half maximum (FWHM). Isolated spectral bands at approximately 75 nm (FWHM) were also measured. The objective grating airglow spectrometer was flown to measure airglow radiation from Venus and Mercury in the spectral range from 20.0--170.0 nm. With a spectral resolution of 2.0 nm, the instrument measured radiation at the following wavelengths: 30.4, 43.0, 58.4, 74.0, 86.9, 104.8, 121.6, 130.4, 148.0, and 165.7 nm. In addition, to provide a check on the total incident extreme UV flux to the spectrometer, two zero-order channels were flown. The effective field of view of the instrument was 0.13 by 3.6 degree. Data also include the interplanetary region."[3]

# Venus

An ultraviolet image of the planet Venus is taken on February 26, 1979, by the Pioneer Venus Orbiter. Credit: NASA.

When imaged in the ultraviolet (right), Venus appears like a gas dwarf object rather than a rocky object.

# Earth

This image shows how the Earth glows in the ultraviolet. Credit: NASA.

"This unusual false-color image [at right] shows how the Earth glows in ultraviolet (UV) light. The Far UV Camera/Spectrograph deployed and left on the Moon by the crew of Apollo 16 captured this image. The part of the Earth facing the Sun reflects much UV light and bands of UV emission are also apparent on the side facing away from the Sun. These bands are the result of aurora caused by charged particles given off by the Sun. They spiral towards the Earth along Earth's magnetic field lines."[32]

# Jupiter

Aurora at Jupiter's north pole is seen in ultraviolet light by the Hubble Space Telescope. Credit: John T. Clarke (U. Michigan), ESA, NASA.
This ultraviolet image of Jupiter is taken by the Wide Field Camera of the Hubble Space Telescope. Credit: NASA/Hubble Space Telescope Comet Team.

"In astronomy, very hot objects preferentially emit UV radiation (see Wien's law). Because the ozone layer blocks many UV frequencies from reaching telescopes on the surface of the Earth, most UV observations are made from space."[7]

The image at lower right "shows Jupiter's atmosphere at a wavelength of 2550 Angstroms after many impacts by fragments of comet Shoemaker-Levy 9. The most recent impactor is fragment R which is below the center of Jupiter (third dark spot from the right). This photo was taken 3:55 EDT on July 21, about 2.5 hours after R's impact. A large dark patch from the impact of fragment H is visible rising on the morning (left) side. Proceding to the right, other dark spots were caused by impacts of fragments Q1, R, D and G (now one large spot), and L, with L covering the largest area of any seen thus far. Small dark spots from B, N, and Q2 are visible with careful inspection of the image. The spots are very dark in the ultraviolet because a large quantity of dust is being deposited high in Jupiter's stratosphere, and the dust absorbs sunlight."[33]

# Saturn

This image of Saturn is taken in ultraviolet light. Credit: NASA and E. Karkoschka (University of Arizona).
This is a movie of Saturn in the ultraviolet from the Hubble Space Telescope. Credit: NASA, ESA, and Jonathan Nichols (University of Leicester).
Saturnian aurora whose Lyman alpha false reddish colour in this image is characteristic of ionized hydrogen plasma. Credit: J. Trauger (JPL), NASA.
This is an image of Saturn's A Ring, taken by the Cassini Orbiter using an Ultraviolet Imaging Spectrograph. Credit: NASA/JPL/University of Colorado.

"One of a series, this image [at right] of Saturn was taken when the planet's rings were at their maximum tilt of 27 degrees toward Earth. Saturn experiences seasonal tilts away from and toward the sun, much the same way Earth does. This happens over the course of its 29.5-year orbit. Every 30 years, Earth observers can catch their best glimpse of Saturn's south pole and the southern side of the planet's rings. ... NASA's Hubble Space Telescope [captured detailed images of Saturn's Southern Hemisphere and the southern face of its rings."[34]

The movie at right records Saturn "when its rings were edge-on, resulting in a unique movie featuring the nearly symmetrical light show at both of the giant planet's poles. It takes Saturn almost thirty years to orbit the Sun, with the opportunity to image both of its poles occurring only twice during that time. The light shows, called aurorae, are produced when electrically charged particles race along the planet's magnetic field and into the upper atmosphere where they excite atmospheric gases, causing them to glow. Saturn's aurorae resemble the same phenomena that take place at the Earth's poles."[35]

"Powered by the Saturnian equivalent of (filamentary) Birkeland currents, streams of charged particles from the interplanetary medium interact with the planet's magnetic field and funnel down to the poles.[36] Double layers are associated with (filamentary) currents,[37][38] and their electric fields accelerate ions and electrons.[39]"[40]

"Towering more than 1,000 miles above the cloud tops, these Saturnian auroral displays are analogous to Earth's. ... In this false color image, the dramatic red aurora identify emission from atomic hydrogen, while the more concentrated white areas are due to hydrogen molecules."[41]

"The best view of Saturn's rings in the ultraviolet indicates there is more ice toward the outer part of the rings, than in the inner part, hinting at the origins of the rings and their evolution."[42]

"Images taken during the Cassini spacecraft's orbital insertion on June 30 show compositional variation in the A, B and C rings. From the inside out, the "Cassini Division" in faint red at left is followed by the A ring in its entirety. The Cassini Division at left contains thinner, dirtier rings than the turquoise A ring, indicating a more icy composition. The red band roughly three-fourths of the way outward in the A ring is known as the Encke gap."[42]

"The ring system begins from the inside out with the D, C, B and A rings followed by the F, G and E rings. The red in the image indicates sparser ringlets likely made of "dirty," and possibly smaller, particles than in the icier turquoise ringlets."[42]

The image at right "was taken with the Ultraviolet Imaging Spectrograph instrument, which is capable of resolving the rings to show features up to 97 kilometers (60 miles) across, roughly 100 times the resolution of ultraviolet data obtained by the Voyager 2 spacecraft."[42]

# Comets

"Ultraviolet spectra of comets show strong emission in the hydrogen Lyman-α line, the O I 130.2 nm resonance lines, and the OH Α 2Π-Χ 2Σ+ bands."[22]

"Discovery of the S2 molecule in comets came from UV spectroscopy of the comet IRAS – Araki – Alcock ( 1983d) which passed close to the Earth [59]. ... Emission from S2 was shown to be confined to a small region ( < 100 km) around the nucleus. Outside this region, S2 is destroyed. ... its presence in comet Hyakutake [60] suggests it is ubiquitous and only its narrow survival zone close to the nucleus inhibits regular detection."[22]

"The transitions involved (allowed and forbidden) in the spectrum of the oxygen atoms in a cometary atmosphere" are 557.7 nm, 630.0 and 636.4 nm, 295.8 and 297.2 nm, 98.9 nm (a triplet), 799.0 nm, 844.7 nm, and 1304 nm (a triplet), 102.7 nm (a triplet) and 1128.7 nm.[24]

"Measurements of the spatial distribution of the hydroxyl radical in cometary atmospheres [may be] made by observations of ultraviolet emission at 309 nm ... The distribution depends upon the velocities of the parent water molecules from which OH is produced by photodissociation and on the lifetime of OH ... The ultraviolet data ... yield a lifetime of OH at 1 AU from the Sun for Comet Bennett (1970 LI) of 2(+1,-1)105 sec (Keller and Lillie, 1974), for Comet Kobayashi-Berger-Milon (1975 IX) a lifetime of 2.3(+1.5,-1.3)105 sec (Festou, 1981b), for Comet Kohoutek (1973 XII) a lifetime of 2(+2,-0.7)105 sec (Blamont and Festou, 1974; Festou, 1981b), and for Comet Bradfield (1979X) a lifetime between 5 x 104 and 1.6 x 105 sec (Weaver et al., 1981 a)."[43]

"Measurements of hydrogen Lyman alpha emission from comets indicate the presence of two populations of hydrogen atoms, one with a velocity of about 20 km sec-1, the second with a velocity of about 8 km sec-1 ... the high-velocity component [may arise] from photodissociation of H2O and the low-velocity component from photodissociation of OH"[43].

# Fermi glow

"The Fermi glow are ultraviolet-glowing[44] particles, mostly hydrogen,[45] originating from the Solar System's Bow shock, created when light from stars and the Sun enter the region between the heliopause and the interstellar medium and undergo Fermi acceleration[45], bouncing around the transition area several times, gaining energy via collisions with atoms of the interstellar medium. The first evidence of the Fermi glow, and hence the bow shock, was obtained with the help from Voyager 1[44] and the Hubble Space Telescope[44]."[46]

# Interstellar clouds

"Carbon monoxide is the second most abundant molecule, after H2, in interstellar clouds. In diffuse clouds, the amount of CO is mainly derived from measurements of absorption at UV wavelengths."[22]

# Subdwarf B star

"The subdwarf B star is a kind of subdwarf star with spectral type B. They differ from the typical subdwarf star by being much hotter and brighter.[47] They are from the "extreme horizontal branch stars" of the Hertzsprung–Russell diagram."[48]

"Subdwarf B stars, being more luminous than white dwarfs, are a significant component in the hot star population of old stellar systems, such as globular clusters, spiral galaxy bulges and elliptical galaxies.[49] They are prominent on ultraviolet images. The hot subdwarfs are proposed to be the cause of the UV-upturn in the light output of elliptical galaxies.[47]"[48]

# Betelgeuse

Betelgeuse is imaged in ultraviolet light by the Hubble Space Telescope and subsequently enhanced by NASA.[50] Credit: NASA and ESA.
This image is from ESO's Very Large Telescope showing not only the stellar disk, but also an extended atmosphere with a previously unknown plume of surrounding gas.[51] Credit: ESO/P. Kervella.

"The yellow/red "image" or "photo" of Betelgeuse usually seen is actually not a picture of the red giant but rather a mathematically generated image based on the photograph. The photograph was actually of much lower resolution: The entire Betelgeuse image fit entirely within a 10x10 pixel area on the Hubble Space Telescopes Faint Object Camera. The actual images were oversampled by a factor of 5 with bicubic spline interpolation, then deconvolved."[50]

The image at lower right is "of the supergiant star Betelgeuse obtained with the NACO adaptive optics instrument on ESO’s Very Large Telescope. The use of NACO combined with a so-called “lucky imaging” technique, allows the astronomers to obtain the sharpest ever image of Betelgeuse, even with Earth’s turbulent, image-distorting atmosphere in the way. The resolution is as fine as 37 milliarcseconds, which is roughly the size of a tennis ball on the International Space Station (ISS), as seen from the ground. The image is based on data obtained in the near-infrared, through different filters. The field of view is about half an arcsecond wide, North is up, East is left."[52]

Betelgeuse has an estimated diameter of ~8.21 x 106 km, but "[t]he precise diameter has been hard to define for several reasons:

1. The rhythmic expansion and contraction of the photosphere [may mean] the diameter is never constant;
2. There is no definable "edge" to the star as limb darkening causes the optical emissions to vary in color and decrease the farther one extends out from the center;
3. Betelgeuse is surrounded by a circumstellar envelope composed of matter being ejected from the star—matter which both absorbs and emits light—making it difficult to define the edge of the photosphere;[53]
4. Measurements can be taken at varying wavelengths within the electromagnetic spectrum, with each wavelength revealing something different. Studies have shown that angular diameters are considerably larger at visible wavelengths, decrease to a minimum in the near-infrared, only to increase again in the mid-infrared.[54][55] The difference in reported diameters can be as much as 30–35%, yet because each wavelength measures something different, comparing one finding with another is problematic;[53]
5. Atmospheric twinkling limits the resolution obtainable from ground-based telescopes since turbulence degrades angular resolution.[56]

"Assuming a distance of 197±45pc, an angular distance of 43.33±0.04 mas would equate to a radius of 4.3 AU".[54]

"Images of hotspots on the surface of Betelgeuse [are] taken at visible and infra-red wavelengths using high resolution ground-based interferometers"[55].

# Mira

This ultraviolet-wavelength image mosaic, taken by NASA's GALEX, shows a comet-like "tail" stretching 13 light years across space. Credit: NASA.

"Mira A is a red giant variable star in the constellation Cetus. This ultraviolet-wavelength image mosaic, taken by NASA's GALEX, shows a comet-like "tail" stretching 13 light year across space. The "tail" consists of hydrogen gas blown off of the star, with the material at the furthest end of the "tail" having been emitted about 30,000 years ago. The tail-like configuration of the emitted material appears to result from Mira's uncommonly high speed (about 130 km/s (81 mi/s)) relative to the Milky Way galaxy's ambient gas. Mira itself is seen as a small white dot inside a blue bulb."[57]

# Lyman-alpha blob

Composite of two different images taken with the FORS instrument on the Very Large Telescope of the Lyman-alpha blob LAB-1. Credit ESO/M. Hayes.

"[A] Lyman-alpha blob (LAB) is a huge concentration of a gas emitting the Lyman-alpha emission line. LABs are some of the largest known individual objects in the Universe. Some of these gaseous structures are more than 400,000 light years across. So far they have only been found in the high-redshift universe because of the ultraviolet nature of the Lyman-alpha emission line. Since the Earth's atmosphere is very effective at filtering out UV photons, the Lyman-alpha photons must be redshifted in order to be transmitted through the atmosphere."[58]

# Astron

"Astron was ... based on the Venera spacecraft design and was operational for six years as the largest ultraviolet space telescope during its lifetime."[59]

# Far Ultraviolet Spectroscopic Explorer

The Far Ultraviolet Spectroscopic Explorer is shown in a pre-launch clean room. Credit: NASA.

"The Far Ultraviolet Spectroscopic Explorer (FUSE) ... detected light in the far ultraviolet portion of the electromagnetic spectrum, between 90.5-119.5 nanometres, which is mostly unobservable by other telescopes. Its primary mission was to characterize universal deuterium in an effort to learn about the stellar processing times of deuterium left over from the Big Bang. ... [T]he ... telescope comprises four individual mirrors. Each of the four mirrors is a 39-by-35 cm (15.4-by-13.8 in) off-axis parabola. Two mirror segments are coated with silicon carbide for reflectivity at the shortest ultraviolet wavelengths, and two mirror segments are coated with lithium fluoride over aluminum that reflects better at longer wavelengths. ... Each mirror has a corresponding astigmatism-corrected, holographically-ruled diffraction grating, each one on a curved substrate so as to produce four 1.65 m (5.4 ft) Rowland circle spectrographs. The dispersed ultraviolet light is detected by two microchannel plate intensified double delay-line detectors, whose surfaces are curved to match the curvature of the focal plane.[60]"[61]

# Kosmos 215

"Kosmos 215 ... was used to study radiation and conduct optical observations of the Earth's atmosphere. It was equipped with eight telescopes for optical observation,[62] and one for ultraviolet astronomy.[63] It was primarily used to study the Sun, although several other X-ray emissions were detected. ... Kosmos 215 performed ultraviolet photometry of 36 A and B stars from parallel telescopes and two UV photometers with maximum responses at 274.0 and 227.5 nanometres.[64]"[65]

# History

This is a distant view of the V-2 launch complex as NRL readies for launching V-2 number 6. William Baum, United States Navy, Naval Research Laboratory, National Air and Space Museum.
This color image shows the missile being checked by a technician for the last time before launch. Credit: Naval Research Laboratory and the National Air and Space Museum.
The color image shows the crater formed when V-2 number 6 returned to Earth. Credit: Naval Research Laboratory.

The first launch of a V-2 by the Naval Research Laboratory was an effort to place an ultraviolet spectrograph over 160 km above the desert at White Sands Proving Ground in New Mexico. The spectrograph would record how much high-energy solar radiation reaches the Earth's upper atmosphere.

The first image at right shows the launch complex. On either side of the rocket are extension ladders to allow technicians access to the nose of the rocket for a final instrument check, color image. On June 27th, liquid oxygen is pumped into one tank and kerosene into a separate fuel tank. This was followed by filling the smaller hydrogen peroxide and permanganate tanks for the turbopumps.

On June 28, 1946, the missle is launched on a flight that lasts 354 s, reaches an altitude of some 100 km, and crashes back to Earth.

The third image show the impact crater created by the V-2 when it returned from its suborbital flight. An approximate size of the crater is indicated by the crane brought in to help find the photographic film taken during the measurements. Excavation of the crater continued throughout the summer, but the cassette carrying the film was never found.

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