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.
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.
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. The effective temperature of the surface of the Sun's photosphere is 5,778 K. 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.
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 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 , , and , 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.
When any effort to acquire a system of laws or knowledge focusing on an astr, aster, or astro, that is, any natural body in the sky especially at night, discovers an entity emitting, absorbing, transmitting, reflecting, or fluorescing ultraviolet, succeeds even in its smallest measurement, ultraviolet astronomy is the name of the effort and the result.
Ultraviolet astronomy consists of three fundamental parts:
- logical laws with respect to incoming ultraviolet rays, or ultraviolet radiation,
- natural ultraviolet sources, and
- the sky and associated realms with respect to ultraviolet rays.
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.
The draft ISO standard on determining solar irradiances (ISO-DIS-21348)5 describes the following ranges:
|Name||Abbreviation||Wavelength range in nanometers||Energy per photon|
|Before UV spectrum||Visible light||above 400 nm||below 3.10 eV|
|Ultraviolet A, long wave, or black light||UVA||400 nm–315 nm||3.10–3.94 eV|
|Near||NUV||400 nm–300 nm||3.10–4.13 eV|
|Ultraviolet B or medium wave||UVB||315 nm–280 nm||3.94–4.43 eV|
|Middle||MUV||300 nm–200 nm||4.13–6.20 eV|
|Ultraviolet C, short wave, or germicidal||UVC||280 nm–100 nm||4.43–12.4 eV|
|Far||FUV||200 nm–122 nm||6.20–10.2 eV|
|Vacuum||VUV||200 nm–100 nm||6.20–12.4 eV|
|Low||LUV||100 nm–88 nm||12.4–14.1 eV|
|Super||SUV||150 nm–10 nm||8.28–124 eV|
|Extreme||EUV||121 nm–10 nm||10.2–124 eV|
|Beyond UV range||X-rays||below 10 nm||above 124 eV|
"Vacuum UV" is so named because it is absorbed strongly by air and is, therefore, used in a vacuum. In the long-wave limit of this region, roughly 150–200 nm, the principal absorber is the oxygen in air.
Ultraviolet lamps are also used in analyzing minerals and gems. Materials may look the same under visible light, but fluoresce to different degrees under ultraviolet light, or may fluoresce differently under short wave ultraviolet versus long wave ultraviolet.
Ultraviolet lamps may cause certain minerals to fluoresce, and is a key tool in prospecting for tungsten mineralisation.
Many samples of fluorite exhibit fluorescence under ultraviolet light, a property that takes its name from fluorite. Many minerals, as well as other substances, fluoresce. Fluorescence involves the elevation of electron energy levels by quanta of ultraviolet light, followed by the progressive falling back of the electrons into their previous energy state, releasing quanta of visible light in the process. In fluorite, the visible light emitted is most commonly blue, but red, purple, yellow, green and white also occur. The fluorescence of fluorite may be due to mineral impurities such as yttrium, ytterbium, or organic matter in the crystal lattice. In particular, the blue fluorescence seen in fluorites from certain parts of Great Britain responsible for the naming of the phenomenon of fluorescence itself, has been attributed to the presence of inclusions of divalent europium in the crystal.
Between 190 and 1700 nm, the ordinary refractive index varies roughly between 1.9 and 1.5, while the extraordinary refractive index varies between 1.6 and 1.4.
Under longwave (365 nm) ultraviolet light, diamond may fluoresce a blue, yellow, green, mauve, or red of varying intensity. The most common fluorescence is blue, and such stones may also phosphoresce yellow—this is thought to be a unique combination among gemstones. There is usually little if any response to shortwave ultraviolet.
The figure at right "is the first laser spectrum from the Chemistry and Camera (ChemCam) instrument on NASA's Curiosity rover, sent back from Mars on August 19, 2012. The plot shows emission lines from different elements present in the target, a rock near the rover's landing site dubbed "Coronation" (see inset)."
"ChemCam's detectors observe light in the ultraviolet (UV), violet, visible and near-infrared ranges using three spectrometers, covering wavelengths from 240 to 850 nanometers. The light is produced when ChemCam's laser pulse strikes a target, generating ionized gases in the form of plasma, which is then analyzed by the spectrometers and their detectors for the presence of specific elements. The detectors can collect up to 16,000 counts produced by the light in any of its 6,144 channels for each laser shot."
"The plot is a composite of spectra taken over 30 laser shots at a single 0.016-inch (0.4-millimeter) diameter spot on the target. An inset on the left shows detail for the minor elements titanium and manganese in the 398-to-404-nanometer range. An inset at the right shows the hydrogen and carbon peaks. The carbon peak was from the carbon dioxide in Mars' air. The hydrogen peak was only present on the first laser shot, indicating that the element was only on the very surface of the rock. Magnesium was also slightly enriched on the surface. The heights of the peaks do not directly indicate the relative abundances of the elements in the rock, as some emission lines are more easily excited than others."
"A preliminarily analysis indicates the spectrum is consistent with basalt, a type of volcanic rock, which is known from previous missions to be abundant on Mars. Coronation is about three inches (7.6 centimeters) across, and located about 5 feet (1.5 meters) from the rover and about nine feet (2.7 meters) from ChemCam on the mast."
Theoretical ultraviolet astronomyEdit
The speed varies from around 1 km/s at the border between the umbra and the penumbra to a maximum of around double this in the middle of the penumbra and falls off to zero at the outer edge of the penumbra.
Measurements of the spectral emission lines emitted in the ultraviolet wavelengths have shown a systematic red-shift. The Evershed effect is common to every spectral line formed at a temperature below 105 K; this fact would imply a constant downflow from the transition region towards the chromosphere. The observed velocity is about 5 km/s. Of course, this is impossible, since if it were true, the corona would disappear in a short time instead of being suspended over the Sun at temperatures of million degrees over distances much larger than a solar radius.
Many theories have been proposed to explain this redshift in line profiles of the transition region, but the problem is still unsolved, since a coherent theory should take into account all the physical observations: UV line profiles are redshifted on average, but they show back and forth velocity oscillations at the same time.
In synthesis, the proposed mechanisms are:
Charles Stuart Bowyer is generally given credit for starting this field.
UV light is found in sunlight (where it constitutes about 10% of the energy in vacuum) and is emitted by electric arcs and specialized lights such as mercury lamps and black lights.
In the diagram at right, the levels of ozone at various altitudes yellow line and blocking of different bands of ultraviolet radiation are shown. Essentially all UVC is blocked by dioxygen (from 100–200 nm) or by ozone (200–280 nm) in the atmosphere. The ozone layer then blocks most UVB. Meanwhile, UVA is hardly affected by ozone and most of it reaches the ground.
This diagram contains "a typical profile of ozone density versus altitude (yellow line) in the midlatitudes of the Northern Hemisphere (units=Dobson Units/kilometer). The stratosphere lies between the tropopause and stratopause (marked in red). Superimposed on the figure are plots of UV radiation as a function of altitude for UVa (320-400 nm, cyan), UVb (280-320 nm, green), and UVc (200-280 nm, magenta). The width of the bar indicates the amount of energy as a function of altitude. The UVc energy decreases dramatically as ozone increases because of the strong absorption in the 200-280 nm wavelength band. The UVb is also strongly absorbed, with a small fraction reaching the surface. The UVa is only weakly absorbed by ozone, with some scattering of radiation near the surface."
Sunlight in space at the top of Earth's atmosphere, at a solar constant output of about 1366 watts/m2, is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light, for a total ultraviolet power of about 140 watts/m2 in vacuum. However, at ground level total sunlight power decreases to about 1000–1100 watts/m2, and by energy fractions, is composed of 44% visible light, 3% ultraviolet (with the Sun at its zenith), and the remainder infrared.
A PG 1159 star, often also called a pre-degenerate, is a star with a hydrogen-deficient atmosphere which is in transition between being the central star of a planetary nebula and being a hot white dwarf. These stars are hot, with surface temperatures between 75,000 K and 200,000 K, and are characterized by atmospheres with little hydrogen and absorption lines for helium, carbon and oxygen. ... The PG 1159 stars are named after their prototype, PG 1159-035. This star, found in the Palomar-Green survey of ultraviolet-excess stellar objects, was the first PG 1159 star discovered.
Lyc photon or Ly continuum photon or Lyman continuum photon are a kind of photon emitted from stars. Hydrogen is ionized by absorption of Lyc photons. Lyc photons are in the ultraviolet portion of the electromagnetic spectrum of the hydrogen atom and immediately next to the limit of the Lyman series of the spectrum with wavelengths that are shorter than 91.1267 nanometres and with energy above 13.6 eV.
The Sun's emission in the lowest UV bands, the UVA, UVB, and UVC bands, are of interest, as these are the UV bands commonly encountered from artificial sources on Earth. The shorter bands of UVC, as well as even more energetic radiation as produced by the Sun, generate the ozone in the ozone layer when single oxygen atoms produced by UV photolysis of dioxygen react with more dioxygen. The ozone layer is especially important in blocking UVB and part of UVC, since the shortest wavelengths of UVC (and those even shorter) are blocked by ordinary air.
"In 1969, Chi Carinae was classified as chemically peculiar Ap star because its absorption lines of silicon appeared unusually strong relative to the lines for helium. However, subsequent examination in the ultraviolet band showed the silicon bands were as expected and it was determined the spectra is normal for a star of its type.
In the figure at right, CUVOB stands for the cosmic ultraviolet and optical background.
"The first spectroscopic measurement of the diffuse cosmic ultraviolet background in the range 1700-2850 Å has resulted in the detection at high Galactic latitude of an intensity of 300 ± 100 photons (cm2 s sr Å)-1 at 1800 Å without any correction necessary for starlight or airglow, a similar intensity over the range 1900-2500 Å after correction for measured airglow, and a similar intensity over the range 2500-2800 Å after correction for zodiacal light."
The temperature for a lightning bolt channel is 28 kK or 28,000 K with a peak emittance wavelength of black-body radiation at approximately 100 nm (far ultraviolet light.
The 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.
"[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).
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 on the right, to on the left (there are infinitely many spectral lines, but they become very dense as they approach to (Lyman limit), so only some of the first lines and the last one appear).
The wavelengths (nm) in the Lyman series are all ultraviolet:
|Wavelength (nm)||121.6||102.6||97.3||95.0||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:
According to Bohr's third assumption, whenever an electron falls from an initial energy level( ) to a final energy level( ), the atom must emit radiation with a wavelength of:
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:
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:
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.
There is another He II line at 320.3 nm.
"[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]."
"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."
"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."
"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."
Boron (B I) line is at 249.67 nm.
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" "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."
"[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."
There is a C III line at 97.7 nm.
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.
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.
Molecular nitrogen and nitrogen compounds have been detected in interstellar space by astronomers using the Far Ultraviolet Spectroscopic Explorer.
Oxygen has three emission lines common in comets at 130.22, 130.49, and 130.60 nm from O I.
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.
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".
"[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)." "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.
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.
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.
The calcium line Ca XII at 332.8 nm occurs in the solar corona.
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.
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.
Iron has two lines occurring in the solar corona in the near ultraviolet: 338.81 and 345.41 nm of Fe XIII.
Nickel has lines occurring in the solar corona at 360.10 nm of Ni XVI and 364.29 nm of Ni XIII.
Copper has two emission lines that occur in plasmas at 324.754 and 327.396 nm from Cu I.
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.
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.
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.
Ultraviolet telescopes such as TRACE and SOHO/EIT can observe individual micro-flares as small brightenings in extreme ultraviolet light, but there seem to be too few of these small events to account for the energy released into the corona.
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.
Ultraviolet irradiance (EUV) varies by approximately 1.5 percent from solar maxima to minima, for 200 to 300 nm UV.
"1 percent of the sun's energy is emitted at ultraviolet wavelengths between 200 and 300 nanometers, the decrease in this radiation from 1 July 1981 to 30 June 1985 accounted for 19 percent of the decrease in the total irradiance".
Energy changes in the UV wavelengths involved in production and loss of ozone have atmospheric effects.
The 30 hPa atmospheric pressure level has changed height in phase with solar activity during the last 4 solar cycles.
UV irradiance increase causes higher ozone production, leading to stratospheric heating and to poleward displacements in the stratospheric and tropospheric wind systems.
A proxy study estimates that UV has increased by 3.0% since the Maunder Minimum.
"Solar satellite observatories such as ESA/NASA's Solar and Heliospheric Observatory (SOHO) have been studying the sun for over 10 years, and have created images of the entire solar surface using spectroscopic techniques. [The second image at right] shows a recent full-sun image created by the Extreme-ultraviolet Imaging Telescope (EIT) taken during sunspot-minimum conditions in 2008. [...] By using the techniques of imaging spectroscopy, solar physicists can isolate gases heated to temperatures of 1,500,000 K and study their motions and evolution over time."
The second image at right is "taken on December 16, 2008 during sunspot-minimum conditions, was created by isolating the light produced at a wavelength of 195 Angstroms (19.5 nanometers) by the ion Fe XII. By selecting the light from only one spectral line, a spectroheliograph works like a high-precision light filter and lets astronomers map, or image, a distant object in the light from a single spectral line. This information can be used to map the temperature and density changes in the gas."
"[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".
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."
When imaged in the ultraviolet (right), Venus appears like a gas dwarf object rather than a rocky object.
"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."
In 1911, Professor Robert W. Wood used ultraviolet photography to take images of the crater area. He discovered the plateau had an anomalous appearance in the ultraviolet, and an area to the north appeared to give indications of a sulfur deposit. This colorful area is sometimes referred to as "Wood's Spot", an alternate name for the Aristarchus Plateau.
Spectra taken of this crater during the Clementine mission were used to perform mineral mapping. The data indicated that the central peak is a type of rock called anorthosite, which is a slow-cooling form of igneous rock composed of plagioclase feldspar. By contrast the outer wall is troctolite, a rock composed of equal parts plagioclase and olivine.
The Aristarchus region was part of a Hubble Space Telescope study in 2005 that was investigating the presence of oxygen-rich glassy soils in the form of the mineral ilmenite. Baseline measurements were made of the Apollo 15 and Apollo 17 landing sites, where the chemistry is known, and these were compared to Aristarchus. The Hubble Advanced Camera for Surveys was used to photograph the crater in visual and ultraviolet light. The crater was determined to have especially rich concentrations of ilmenite, a titanium oxide mineral that could potentially be used in the future by a lunar settlement for extracting oxygen.
"Far-ultraviolet spectra ... Mars in the range 820-1840 Å at ~4 Å resolution were obtained on 1995 March 13 and 12, respectively, by the Hopkins Ultraviolet Telescope (HUT), which was part of the Astro-2 observatory on the space shuttle Endeavour. Longward of 1250 Å, the spectra ... are dominated by emission of the CO fourth positive (A1Π-X1Σ+) band system and strong O I and C I multiplets. ... The Ar I λλ1048, 1066 doublet is detected only in the spectrum of Mars ... CO fluorescence in both the B−X (0,0) and C−X (0,0) Birge-Hopfield bands is identified in ... Mars ... Below 2000 Å, the ultraviolet dayglow of ... Mars is dominated by emissions of carbon monoxide and carbon (Durrance 1981; Fox 1992)."
On the right is an ultraviolet photograph of Mars. It shows clouds and other aspects of the atmosphere.
High-resolution ultraviolet Hubble Space Telescope images taken in 1995 showed a dark spot on its surface which was nicknamed "Piazzi" in honour of the discoverer of Ceres. This was thought to be a crater. Later near-infrared images with a higher resolution taken over a whole rotation with the Keck telescope using adaptive optics showed several bright and dark features moving with the dwarf planet's rotation.
C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids, and an even higher percentage in the outer part of the asteroid belt beyond 2.7 AU, which is dominated by this asteroid type. The proportion of C-types may actually be greater than this, because C-types are much darker than most other asteroid types except D-types and others common only at the extreme outer edge of the asteroid belt. ... Their spectra contain moderately strong ultraviolet absorption at wavelengths below about 0.4 μm to 0.5 μm, while at longer wavelengths they are largely featureless but slightly reddish. The so-called "water" absorption feature around 3 μm, which can be an indication of water content in minerals is also present.
"F-type asteroids have spectra generally similar to those of the B-type asteroids, but lack the "water" absorption feature around 3 μm indicative of hydrated minerals, and differ in the low wavelength part of the ultraviolet spectrum below 0.4 μm.
G-type asteroids are a relatively uncommon type of carbonaceous asteroid. The most notable asteroid in this class is 1 Ceres. Generally similar to the C-type objects, but containing a strong ultraviolet absorption feature below 0.5 μm.
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.
"Experiments on the Voyager 1 and 2 spacecraft and observations made by the International Ultraviolet Explorer (IUE) have provided evidence for the existence of energetic particle precipitation into the upper atmosphere of Jupiter from the magnetosphere."
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."
"The Cassini ultraviolet images [of Io] ... reveal two gigantic, actively erupting plumes of gas and dust. Near the equator, just the top of Pele's plume is visible where it projects into sunlight. None of it would be illuminated if it were less than 240 kilometers (150 miles) high. [The Cassini ultraviolet] images indicate a total height for Pele of 390 kilometers (242 miles). [One] Cassini image ... shows a bright spot over Pele's vent. Although the Pele hot spot has a high temperature, silicate lava cannot be hot enough to explain a bright spot in the ultraviolet, so the origin of this bright spot is a mystery, but it may indicate that Pele was unusually active when the picture was taken."
At right is a Hubble Space Telescope image in the ultraviolet of Io.
"Measurements at two ultraviolet wavelengths indicate that the ejecta consist of sulfur dioxide 'snow,' making the plume appear green in this false-color image. Astronomers increased the colour contrast and added false colours to the image to make the faint plume visible."
"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."
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."
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. Double layers are associated with (filamentary) currents, and their electric fields accelerate ions and electrons.
"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."
"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."
"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."
"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."
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."
The image at second left Saturn's northern UV auroras. These exhibit changes in shape over the course of the observing interval.
"Saturn’s magnetosphere -- the big magnetic bubble that surrounds the planet -- is compressed on the side facing the sun, and it streams out into a long “magnetotail” on the planet’s nightside. Just like with comets, the magnetotails of Earth and Saturn are made of electrified gas from the sun."
"Now it appears that when strong bursts of particles from the sun hit Saturn, the magnetotail collapses and then reconfigures itself -- a disturbance of the magnetic field that’s reflected in the dynamics of auroras."
“We have always suspected this was what also happens on Saturn. This evidence really strengthens the argument.”
"The ultraviolet images were taken by Hubble’s Advanced Camera for Surveys during April and May of last year from the space telescope’s perspective in orbit around Earth. The images are able to provide the first detailed look at dynamics in the “choreography” of auroral glow because Hubble captured them right at that very moment when Saturn’s magnetic field is blasted by particles streaming from the sun."
"Hubble managed to capture a particularly dynamic light show: Some bursts of light shooting around the polar regions traveled at least three times faster than the speed of Saturn’s rotation. (The planet has a 10-hour rotation period.)"
“We can see that the magnetotail is undergoing huge turmoil and reconfiguration, caused by buffering from solar wind. It’s the smoking gun that shows us that the tail is collapsing.”
"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."
"Discovery of the S2 molecule in comets came from UV spectroscopy of the comet IRAS – Araki – Alcock ( 1983d) which passed close to the Earth . ... 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  suggests it is ubiquitous and only its narrow survival zone close to the nucleus inhibits regular detection."
"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.
"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)."
"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".
The Fermi glow are ultraviolet-glowing particles, mostly hydrogen, 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, 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 and the Hubble Space Telescope.
"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."
Subdwarf B starEdit
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. They are from the "extreme horizontal branch stars" of the Hertzsprung–Russell diagram.
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. 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.
"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."
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."
Betelgeuse has an estimated diameter of ~8.21 x 106 km, but "[t]he precise diameter has been hard to define for several reasons:
- The rhythmic expansion and contraction of the photosphere [may mean] the diameter is never constant;
- 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;
- 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;
- 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. 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;
- Atmospheric twinkling limits the resolution obtainable from ground-based telescopes since turbulence degrades angular resolution.
"Assuming a distance of 197±45pc, an angular distance of 43.33±0.04 mas would equate to a radius of 4.3 AU".
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.
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.
The ultraviolet image at right is "of the giant spiral galaxy Messier 101 (M101) was obtained by the Ultraviolet Imaging Telescope during the Astro-2 mission of the Space Shuttle Endeavour."
"M101 is an Sc-type galaxy, meaning a spiral galaxy with a relatively small central bulge and a system of spiral arms that is not tightly wound. At a distance of about 16 million light years, it is considered relatively close to the Earth."
"M101 is known to contain many giant HII regions, meaning huge glowing nebulae shine as a result of ultraviolet radiation from the massive stars within them. The UIT images will be used to determine the far-ultraviolet energy outputs of these stars and nebulae. Also, the astronomers will study the ages of the nebulae, their dust contents, and the "initial mass functions" of their stars, meaning the relative numbers of stars of different masses when they first formed in the nebulae. This is equivalent to finding the relative numbers of newborn babies of different weights. The investigators will also determine the total mass of all the young massive stars in each HII region or nebula."
"UIT is a 15-inch (0.38-m) telescope which was designed and built at the Goddard Space Flight Center, Greenbelt, MD. ... The exposure time was 1310 seconds and the photograph was made at an effective wavelength of 1520 angstroms (152 nanometers), with a bandwidth of 354 angstroms (35.4 nanometers). The photograph was obtained during nighttime portion of Endeavour's orbit on March 11, 1995. The region shown here is about two-thirds the apparent diameter of the full moon. The original UIT image was recorded on black and white film; the image is displayed here with color coding indicating intensity of the ultraviolet light."
The Large Binocular Telescope is located on Mount Graham (10,700-foot (3,300 m)) in the Pinaleno Mountains of southeastern Arizona, and is a part of the Mount Graham International Observatory.
The first image taken shown at right combined ultraviolet and green light, and emphasizes the clumpy regions of newly formed hot stars in the spiral arms.
The Very Large Telescope (VLT) is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The UTs are equipped with a large set of instruments permitting observations to be performed in the near-ultraviolet. It includes large-field imagers, adaptive optics corrected cameras and spectrographs, as well as high-resolution and multi-object spectrographs and covers a broad spectral region, from the deep ultraviolet (300 nm).
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.
Opacity depends on the frequency of the light being considered. For instance, some kinds of glass, while transparent in the visual range, are largely opaque to ultraviolet light.
If a beam of light with frequency travels through a medium with opacity and mass density , both constant, then the intensity will be reduced with distance x according to the formula
- x is the distance the light has traveled through the medium
- is the intensity of light remaining at distance x
- is the initial intensity of light, at .
For a given medium at a given frequency, the opacity has a numerical value that may range between 0 and infinity, with units of length2/mass.
"The absorption of a photon of wavelength λ(nm) in a superconductor is followed by a series of fast processes in which the photon energy is converted into a population of free charge carriers known as quasiparticles in excess of any thermal population. For typical transition metal superconductors this conversion process is of order of a few nanoseconds. At sufficiently low temperatures (typically about an order of magnitude lower than the superconductor's critical temperature Tc) the number density of thermal carriers is very small and the number of excess carriers N0 created as a result of the absorption of a photon of wavelength λ is inversely proportional to the photon wavelength."
"In general N0 can be written [approximately as]:"
"Here the wavelength is in nm and the energy gap is in meV."
The Markarian galaxies are a class of galaxies that have nuclei with excessive amounts of ultraviolet emissions compared with other galaxies.
The nuclei of the galaxies had a blue colour that in a star would be classed from A to F. This blue core did not match the rest of the galaxy. The spectrum in detail tends to show a continuum that Markarian concluded was produced non-thermally.
The First Byurakan Survey commenced in 1965 using the Schmidt telescope at the Byurakan Astrophysical Observatory. ... The purpose of the survey was to find galaxies with an ultraviolet excess.
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).
Mars Science Laboratory (MSL) is a robotic space probe mission to Mars launched by NASA on November 26, 2011, which successfully landed Curiosity, a Mars rover, in Gale Crater on August 6, 2012.
Rover Environmental Monitoring Station (REMS): Meteorological package and an ultraviolet sensor provided by Spain and Finland. It measures humidity, pressure, temperatures, wind speeds, and ultraviolet radiation.
Mars Hand Lens Imager (MAHLI): This system consists of a camera mounted to a robotic arm on the rover, used to acquire microscopic images of rock and soil. It has white and ultraviolet LEDs for illumination.
"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.
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.
Astron was based on the Venera spacecraft design and was operational for six years as the largest ultraviolet space telescope during its lifetime.
Far Ultraviolet Spectroscopic ExplorerEdit
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. The 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.
The Galileo spacecraft has on board an ultraviolet spectrometer (UVS) and an extreme ultraviolet spectrometer (EUV).
The Cassegrain telescope of the UVS had a 250 mm aperture and collected light from the observation target. Both the UVS and EUV instruments used a ruled grating to disperse this light for spectral analysis. This light then passed through an exit slit into photomultiplier tubes that produced pulses or "sprays" of electrons. These electron pulses were counted, and these count numbers constituted the data that were sent to Earth. The UVS was mounted on Galileo's scan platform and could be pointed to an object in inertial space. The EUV was mounted on the spun section. As Galileo rotated, EUV observed a narrow ribbon of space perpendicular to the spin axis. The two instruments combined weighed about 9.7 kilograms and used 5.9 watts of power.
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, and one for ultraviolet astronomy. 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.
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