Stellar class O stars have surface temperatures high enough that most of their luminescence is in the ultraviolet.
"NGC 2024, also known as the Flame Nebula, is located at a distance of 900-1200 light years and is part of the Orion Molecular Cloud Complex (Orion B). To the south of the NGC 2024 region lies NGC 2023 (a well-studied photo-dissociation region) and the Horsehead Nebula. The hydrogen emission region that is bright in visible light (and provides a strong contrasting backdrop for the dark Horsehead) is very faint in the infrared; only a faint trace of reflected light traces its outline."
"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".
Betelgeuse is a spectral type M1-M2Ia-Iab initially detected in the UV by the satellite TD1.
"Spectroscopically it is classified as O4 I according to Sota et al. (2007), and its trigonometric parallax from the original Hipparcos catalogue (ESA 1997) is 2.33 ± 0.51 mas."
"From a back-tracing of ζ Pup, Hoogerwerf et al. (2001) found that this star had a possible encounter with the cluster Trumpler 10 some 2 Myr ago provided that its dynamical distance was ddyn = 250 ... 300 pc."
"Our back-tracing confirms the results from Hoogerwerf et al. (2001), giving Trumpler 10 as the host and ddyn = 300 pc, tenc = 2.5 Myr, denc = 0.9 pc, and pkin = 0.94. This result is consistent with the new Hipparcos parallax (3.00 ± 0.1 mas) from the re-reduction of Hipparcos data by van Leeuwen (2007). If we adopt the new Hipparcos distance of 333 pc, we find a solution with tenc = 1.8 Myr and denc = 7.1 pc, which has only a slightly smaller probability (pkin = 0.91)."
"From a non-LTE analysis of the spectrum, Kudritzki et al. (1983) found that the effective temperature of ζ Pup is Teff = 42 000 K instead of 50 000 K according to a spectral type of O4. This, together with the low log g = 3.5, means that ζ Pup is already away from the ZAMS."
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.
"Identified features in the ultraviolet spectrum of Procyon A include Mg I, Mg II, C I, Fe I and Fe II, and Ca H and K."
Procyon A is of spectral type F5 IV-V.
On the right is an ultraviolet spectrum of Procyon A by the International Ultraviolet Explorer (IUE). The spectral lines marked from left to right are N V, O I, C II, Si IV pair, and C IV.
"The ordinate is a flux ratio that represents the monochromatic fraction of the stellar bolometric output at each wavelength (units are Å-1); that is, the absolute monochromatic flux measured at the Earth fλ (ergs cm-2 s~1 Å~1) divided by the stellar bolometric luminosity lbol(ergs cm-2 s~1), also measured at the Earth. As such, the normalized spectra of dwarfs and giants can be compared directly. The background consists of the unresolved photospheric spectrum and some scattered light from longer wavelengths."
The "transition-region (TR) and coronal emission in the G-K dwarfs and G giants is well correlated with the Mg II λ2800 doublet emission strength, which in turn is symptomatic of chromospheric energy losses."
The "chromospheric (e.g., O I λ1305) or transition-region emission features (C II λ1335, C IV λ1550) that are prominent in the F-star spectra (and in the solar ultraviolet spectrum)."
HD 140283 is a spectral type F9VkA5mA1 Peculiar Star that was detected in the UV by the satellite TD1.
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."
On the right "is a Digitized Sky Survey image of the oldest star with a well-determined age in our galaxy. The very old star, catalogued as HD 140283, lies over 190 light-years away. The NASA/ESA Hubble Space Telescope was used to narrow the measurement uncertainty on the star's distance, and this helped to refine the calculation of a more precise age of 14.5 billion years (plus or minus 800 million years). The star is rapidly passing through our local stellar neighborhood. The star's orbit carries it through the plane of our galaxy from the galactic halo that has a population of ancient stars. The Anglo-Australian Observatory (AAO) UK Schmidt telescope photographed the star in blue light."
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:
- siphon flows in coronal loops driven by a pressure difference,
- different cross-sections of the coronal loops footpoints,
- the return of spicules,
- multiple flows,
- nanoflares, and
- thermal instabilities during chromospheric condensation.
The calcium line Ca XII at 332.8 nm occurs in the solar corona.
Nickel has lines occurring in the solar corona at 360.10 nm of Ni XVI and 364.29 nm of Ni XIII.
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."
Alpha Centauri A
Alpha Centauri A is the principal member, or primary, of the binary system. It is a solar-like main-sequence star with a similar yellowish colour, whose stellar classification is spectral type G2 V;
"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."
Alpha Centauri B
Alpha Centauri B is a main-sequence star of spectral type K1 V, making it more an orange colour than the primary star.
Mira B is spectral type DA white dwarf.
Mira B was resolved by the Hubble Space Telescope in 1995, when it was 70 astronomical units from the primary; and results were announced in 1997. The HST ultraviolet images and later X-ray images by the Chandra X-ray Observatory show a spiral of gas rising off Mira in the direction of Mira B. The companion's orbital period around Mira is approximately 400 years.
PG 1159-035 is a spectral type DOQZ1 star detected in the UV by the Extreme Ultraviolet Explorer (catalog number 2EUVE J1201-03.7).
PG 1159-035 was discovered in the Palomar-Green survey of ultraviolet-excess stellar objects and, like the other PG 1159 stars, is apparently in transition between being the central star of a planetary nebula and being a white dwarf.
The luminosity of PG 1159-035 was observed to vary in 1979, and it was given the variable star designation GW Virginis (GW Vir) in 1985. Variable PG 1159 stars may be called GW Vir stars, or the class may be split into DOV and PNNV stars. The variability of PG 1139-035 arises from non-radial gravity wave pulsations within itself. Its light curve has been observed intensively by the Whole Earth Telescope over a 264-hour period in March 1989, and over 100 of its vibrational modes have been found in the resulting vibrational spectrum, with periods ranging from 300 to 1,000 seconds. 
Procyon B is a rare spectral type DQZ white dwarf with an effective temperature of 7740 ± 50 K and a radius of 0.01234 ± 0.00032 R⊙.
On the right is an STIS low-resolution spectrum (line), compared with the medium resolution spectra (points) and the WFPC2 photometry (error bars) of Provencal et al. (1997). The broad bump at 9000 Å in the low-resolution spectrum is a reduction artifact. Ultaviolet rays are less than 4,000 Å.
"The most conspicuous features are the carbon Swan bands near 4700 and 5200 Å and the Mg II resonance lines near 2800 Å. At the short-wavelength end of the ultraviolet spectrum, [are] the asymmetric C I λ1930 line, as well as numerous iron features between 2200 and 2800 Å."
"Also present [...] are the ultraviolet lines C I λλ1930 and 2478. C I λ1930 displays an asymmetric profile, given the reality of the flux shortward of 1900 Å [...], with negligible absorption in the red wing."
"The strength of the ultraviolet C I lines suggests abundances that are not in agreement with the molecular carbon abundances. In the case of Procyon B, our best fit to the ultraviolet C I lines is log[C/He] = -7.0, a factor of ~30 lower than the abundance derived from the molecular Swan bands."
Van Maanen's 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. They are from the "extreme horizontal branch stars" of the Hertzsprung–Russell diagram."
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