A division of astronomical objects between rocky bodies and gas bodies (including gas giants and stars) may be natural and informative. This division allows moons like Io to be viewed as rocky objects like Earth as part of planetary science rather than as a satellite around a star like Jupiter.
A further benefit is the view of gaseous objects as potential stars, failed stars, or stars radiant over peak radiation bands. These objects may be best studied as a part of stellar science.
Each of the gas bodies described are by approximate radius, increasing from apparent gas dwarfs, through gas giants, to large stars with examples.
Viewing a gaseous object with multiple radiation astronomy detectors may uncover what the object looks like beneath the gas. In some instances the gaseous object turns out to have a detectable rocky interior.
Accompanying higher temperatures is usually plasma with its ionized atoms. Around a gaseous object this plasma may be a coronal cloud.
Objects with parallax measurements available are especially helpful as such measurements allow the determination of the object's radius.
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.
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
- findings, and
The findings demonstrate a statistically systematic change from the status quo or the control group.
Stellar astrognosy 
Stellar astrognosy deals with the materials of stars and their general exterior and interior constitution.
"The chromosphere (literally, "sphere of color") is the second of the three main layers in the Sun's atmosphere and is roughly 2,000 kilometers deep. It sits just above the photosphere and just below the solar transition region."
"The density of the chromosphere is very small, it being only 10−4 times that of the photosphere, the layer just below it, and 10−8 times that of the atmosphere of Earth. This makes the chromosphere normally invisible and it can only be seen during a total eclipse, where its reddish color is revealed. The color hues are anywhere between pink and red. However, without special equipment, the chromosphere cannot normally be seen due to the overwhelming brightness of the photosphere."
"The density of the chromosphere decreases with distance from the center of the sun. This decreases logarithmically from 1017 particles per cubic centimeter, or approximately 2×10−4
kg/m3 to under 1.6×10−11
kg/m3 at the outer boundary."
"The temperature begins to decrease from the inner boundary of about 6,000 K to a minimum of approximately 3,800 K, before increasing to upwards of 35,000 K at the outer boundary with the transition layer of the corona."
Transition region 
"The solar transition region is a region of the Sun's atmosphere, between the chromosphere]] and corona. It is visible from space using telescopes that can sense ultraviolet. It is important because it is the site of several unrelated but important transitions in the physics of the solar atmosphere:"
- "Below, most of the helium is not fully ionized, so that it radiates energy very effectively; above, it is fully ionized."
- "Below, gas pressure and fluid dynamics dominate the motion and shape of structures; above, magnetic forces dominate the motion and shape of structures, giving rise to different simplifications of magnetohydrodynamics."
Atmospheric sciences 
"Atmospheric sciences is an umbrella term for the study of the atmosphere, its processes, the effects other systems have on the atmosphere, and the effects of the atmosphere on these other systems."
"Aeronomy is the study of the upper layers of the atmosphere, where dissociation and ionization are important."
"[P]lasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms (reduce or increase the number of electrons in them), thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions."
Gas dwarfs 
There are astronomical bodies that initially appear as gas dwarfs. When examined via radiation astronomy, especially away from visual or optical astronomy, they may turn out to be small terrestrial or rocky objects.
How small a gas dwarf may be is unknown but explorable, at least among nearby objects.
At right is an "eerie view of Jupiter's moon Io in eclipse ... acquired by NASA's Galileo spacecraft while the moon was in Jupiter's shadow. Gases above the satellite's surface produced a ghostly glow that could be seen at visible wavelengths (red, green, and violet). The vivid colors, caused by collisions between Io's atmospheric gases and energetic charged particles trapped in Jupiter's magnetic field, had not previously been observed. The green and red emissions are probably produced by mechanisms similar to those in Earth's polar regions that produce the aurora, or northern and southern lights. Bright blue glows mark the sites of dense plumes of volcanic vapor, and may be places where Io is electrically connected to Jupiter."
Titan like Venus is another gas dwarf when viewed in visible light. "Much as with Venus prior to the Space Age, the dense, opaque atmosphere prevented understanding of Titan's surface until new information accumulated with the arrival of the Cassini–Huygens mission in 2004, including the discovery of liquid hydrocarbon lakes in the ... polar regions."
Titan has a mean radius of 2576 ± 2 km.
At right is an X-ray image of Mars. X-radiation from the Sun excites oxygen atoms in the Martian upper atmosphere, about 120 km above its surface, to emit X-ray fluorescence. A faint X-ray halo that extends out to 7,000 km above the surface of Mars has also been found. The Chandra X-ray Observatory image on the right is the first look at X-rays from Mars.
In X-ray astronomy, Mars is a gas dwarf.
Mars has an equatorial radius of 3,396.2 ± 0.1 km and a polar radius of 3,376.2 ± 0.1 km.
Sirius B 
The image at right "shows two sources and a spike-like pattern due to the support structure for the transmission grating. The bright source is Sirius B, a white dwarf star that has a surface temperature of about 25,000 degrees Celsius which produces very low energy X-rays. The dim source at the position of Sirius A ... – may be due to ultraviolet radiation from Sirius A leaking through the filter on the detector."
Sirius B has a radius of 5800 ± 200 km.
Violet photographs of the planet Venus taken in 1927 “recorded two nebulous bright streaks, or bands, running ... approximately at right angles to the terminator” that may be from the upper atmosphere.
When imaged in visible light (upper left) Venus appears like a gas dwarf rather than a rocky body. The same image result occurs when it is viewed in the ultraviolet (right).
Venus has a mean radius of 6,051.8 ± 1.0 km.
When Venus is viewed by radiation astronomy in addition to ultraviolet astronomy and visual astronomy, it is discovered to have a rocky interior suggesting that it is better understood and studied from the perspective of planetary science as a rocky object.
Procyon B 
Procyon B has a radius of 8,542 ± 223 km and an effective surface temperature of 7,740 ± 50 K.
Procyon B is "a faint white dwarf ... of spectral type DA ... it lies at a distance of just 11.46 light-years (3.51 parsecs), ... It is more difficult to observe from Earth than Sirius B, due to a greater apparent magnitude difference and smaller angular separation from its primary. The average separation of the two components is 15.0 AUs, a little less than the distance between Uranus and the Sun, though the eccentric orbit carries them as close as 8.9 AUs and as far as 21.0 AU."
Van Maanen's star 
"Van Maanen's star (van Maanen 2) is a white dwarf star. Out of the white dwarfs known, it is the third closest to the Sun, after Sirius B and Procyon B, in that order, and the closest known solitary white dwarf."
"Van Maanen's star is located 14.1 light-years from the Sun in the constellation Pisces, about 2° to the south of the star Delta Piscium, with a relatively high proper motion of 2.98" annually. It is too faint to be seen with the naked eye."
The optical negative at right was taken earlier than the current coordinates for Van Maanen's star, which are at the center of the negative.
Gas giants 
Gas giants such as those astronomical objects in orbit around the Sun suggest themselves as failed stars or subdwarfs. At the least, they are usually larger than rocky objects. Whether there are rocky objects consistently larger than gas giants remains to be discovered.
A gas giant is an astronomical object within or relatively near in radius to those of the solar system.
"A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune's vivid azure differs from Uranus's milder cyan. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour."
Neptune has an equatorial radius of 24,764 ± 15 km and a polar radius of 24,341 ± 30 km.
At right are three images of Neptune using infrared astronomy. "Thermal images of planet Neptune taken with VISIR on ESO's Very Large Telescope, obtained on 1 and 2 September 2006. These thermal images show a 'hot' south pole on Neptune. These warmer temperatures provide an avenue for methane to escape out of the deep atmosphere. Scientists say Neptune's south pole is 'hotter' than anywhere else on the planet by about 10°C. The average temperature on Neptune is about minus 200 degrees Celsius. The upper left image samples temperatures near the top of Neptune's troposphere (near 100 mbar pressure). The hottest temperatures are located at the lower part of the image at Neptune's south pole (see the graphic at the upper right). The lower two images, taken 6.3 hours apart, sample temperatures at higher altitudes in Neptune's stratosphere. They do show generally warmer temperatures near, but not at, the south pole. In addition they show a warm area which can be seen in the lower left image and rotated completely around the planet in the lower right image."
Uranus has an equatorial radius of 25,559 ± 4 km and a polar radius of 24,973 ± 20 km.
"Saturday is the day of Saturn, and the color of Saturn, according to astronomers, is said to be black".
"Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. [It is n]amed after the Roman god Saturn ... Saturn is a gas giant with an average radius about nine times that of Earth. ... Saturn has a ring system that consists of nine continuous main rings and three discontinuous arcs, composed mostly of ice particles with a smaller amount of rocky debris and dust. Sixty-two known moons orbit the planet; fifty-three are officially named. This does not include the hundreds of "moonlets" within the rings."
Saturn has an equatorial radius of 60,268 ± 4 km and a polar radius of 54,364 ± 10 km.
Jupiter has an equatorial radius of 71,492 ±4 km, a polar radius of 66,854 ±10 km, and a mean radius of 69,911 ± 6 km.
X-ray astronomy 
The "image of Jupiter [at right] shows concentrations of auroral X-rays near the north and south magnetic poles." The Chandra X-ray Observatory accumulated X-ray counts from Jupiter for its entire 10-hour rotation on December 18, 2000. Note that X-rays from the entire globe of Jupiter are detected.
In the second at right is a diagram describing interaction with the local magnetic field. Jupiter's strong, rapidly rotating magnetic field (light blue lines in the figure) generates strong electric fields in the space around the planet. Charged particles (white dots), "trapped in Jupiter's magnetic field, are continually being accelerated (gold particles) down into the atmosphere above the polar regions, so auroras are almost always active on Jupiter. Electric voltages of about 10 million volts, and currents of 10 million amps - a hundred times greater than the most powerful lightning bolts - are required to explain the auroras at Jupiter's poles, which are a thousand times more powerful than those on Earth. On Earth, auroras are triggered by solar storms of energetic particles, which disturb Earth's magnetic field. As shown by the swept-back appearance in the illustration, gusts of particles from the Sun also distort Jupiter's magnetic field, and on occasion produce auroras."
Ultraviolet astronomy 
"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."
Visual astronomy 
"[O]range [is] the color of Jupiter".
"The Great Red Spot (GRS) is a persistent anticyclonic storm, 22° south of Jupiter's equator, which has lasted for at least 183 years and possibly longer than 348 years. The storm is large enough to be visible through Earth-based telescopes. ... Its dimensions are 24–40,000 km west–to–east and 12–14,000 km south–to–north. The spot is large enough to contain two or three planets the size of Earth. At the start of 2004, the Great Red Spot had approximately half the longitudinal extent it had a century ago, when it was 40,000 km in diameter. ... The Great Red Spot's latitude has been stable for the duration of good observational records, typically varying by about a degree."
"It is not known exactly what causes the Great Red Spot's reddish color. Theories supported by laboratory experiments suppose that the color may be caused by complex organic molecules, red phosphorus, or yet another sulfur compound. The Great Red Spot (GRS) varies greatly in hue, from almost brick-red to pale salmon, or even white. The reddest central region is slightly warmer than the surroundings, which is the first evidence that the Spot's color is affected by environmental factors. The spot occasionally disappears from the visible spectrum, becoming evident only through the Red Spot Hollow, which is its niche in the South Equatorial Belt. The visibility of GRS is apparently coupled to the appearance of the SEB; when the belt is bright white, the spot tends to be dark, and when it is dark, the spot is usually light. The periods when the spot is dark or light occur at irregular intervals; as of 1997, during the preceding 50 years, the spot was darkest in the periods 1961–66, 1968–75, 1989–90, and 1992–93."
Infrared astronomy 
"Spectra from the Voyager I IRIS experiment confirm the existence of enhanced infrared emission near Jupiter's north magnetic pole in March 1979." "Some species previously detected on Jupiter, including CH3D, C2H2, and C2H6, have been observed again near the pole. Newly discovered species, not previously observed on Jupiter, include C2H4, C3H4, and C6H6. All of these species except CH3D appear to have enhanced abundances at the north polar region with respect to midlatitudes."
The image at lower right is "of Jupiter taken in infrared light on the night of [August 17, 2008,] with the Multi-Conjugate Adaptive Optics Demonstrator (MAD) prototype instrument mounted on ESO's Very Large Telescope. This false color photo is the combination of a series of images taken over a time span of about 20 minutes, through three different filters (2, 2.14, and 2.16 microns). The image sharpening obtained is about 90 milli-arcseconds across the whole planetary disc, a real record on similar images taken from the ground. This corresponds to seeing details about 186 miles wide on the surface of the giant planet. The great red spot is not visible in this image as it was on the other side of the planet during the observations. The observations were done at infrared wavelengths where absorption due to hydrogen and methane is strong. This explains why the colors are different from how we usually see Jupiter in visible-light. This absorption means that light can be reflected back only from high-altitude hazes, and not from deeper clouds. These hazes lie in the very stable upper part of Jupiter's troposphere, where pressures are between 0.15 and 0.3 bar. Mixing is weak within this stable region, so tiny haze particles can survive for days to years, depending on their size and fall speed. Additionally, near the planet's poles, a higher stratospheric haze (light blue regions) is generated by interactions with particles trapped in Jupiter's intense magnetic field."
Gliese 229B 
Gliese 229B is a brown dwarf and has a radius approximately the same as Jupiter's.
Proxima Centauri 
"The chromosphere of this star is active, and its spectrum displays a strong emission line of singly ionized magnesium at a wavelength of 280 nm. About 88% of the surface of Proxima Centauri may be active, a percentage that is much higher than that of the Sun even at the peak of the solar cycle. Even during quiescent periods with few or no flares, this activity increases the corona temperature of Proxima Centauri to 3.5 million K, compared to the 2 million K of the Sun's corona."
TWA 5B 
TWA 5B has an inferred radius between 167,000 and 216,000 km.
"Using NASA's Chandra X-ray Observatory, scientists have detected X-rays from a low-mass brown dwarf in a multiple star system. This is the first time that a brown dwarf this close to its parent star(s) (Sun-like stars TWA 5A) has been resolved in X-rays. "Our Chandra data show that the X-rays originate from the brown dwarf's coronal plasma which is some 3 million degrees Celsius", said Yohko Tsuboi of Chuo University in Tokyo. "This brown dwarf is as bright as the Sun today in X-ray light, while it is fifty times less massive than the Sun", said Tsuboi. "This observation, thus, raises the possibility that even massive planets might emit X-rays by themselves during their youth!""
At right is an image from NASA's Chandra X-ray Observatory which shows X-rays produced by TWA 5B, a brown dwarf orbiting a young binary star system "known as TWA 5A. The [star] system is 180 light years from the Sun and a member of a group of about a dozen young stars in the constellation Hydra. The brown dwarf orbits the binary star system at a distance about 2.75 times that of Pluto's orbit around the Sun." The sizes of the sources in the image are due to an instrumental effect that causes the spreading of pointlike sources.
"Brown dwarfs are often referred to as "failed stars" [because] they [may be] under the mass limit (about 80 Jupiter masses", or 8 percent of the mass of the Sun) needed to spark the nuclear fusion of hydrogen to helium which supplies the energy for stars such as the Sun. Lacking any central energy source, brown dwarfs are intrinsically faint and draw their energy from a very gradual shrinkage or collapse.
Young brown dwarfs, like young stars, have turbulent interiors. When combined with rapid rotation, this turbulent motion can lead to a tangled magnetic field that can heat their upper atmospheres, or coronas, to a few million degrees Celsius. "Our Chandra data show that the X-rays originate from the brown dwarf's coronal plasma which is some 3 million degrees Celsius," said Yohko Tsuboi of Chuo University in Tokyo.
"TWA 5B is estimated to be only between 15 and 40 times the mass of Jupiter, making it one of the least massive brown dwarfs known. Its mass is rather near the boundary, about 12 Jupiter masses, between planets and brown dwarfs", so these results could have implications for the possible X-ray detection of very massive planets around stars.
At right is a visual image of the Sun, which is the star around which the Earth orbits. This image shows the ball that is the photosphere of the Sun, the surface of the Sun.
"When we speak of the surface of the Sun, we normally mean the photosphere." "[T]he photosphere may be thought of as the imaginary surface from which the solar light that we see appears to be emitted. The diameter quoted for the Sun usually refers to the diameter of the photosphere."
From the Wikipedia article star: "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. The "[t]emperature at [the] bottom of [the Sun's] photosphere [is] 6600 K", while the "[t]emperature at [the] top of [the] photosphere [is] 4400 K". The photosphere is "~400 km" in thickness.
The peak emittance wavelength of 501.5 nm (~0.5 eV) makes the photosphere a primarily green radiation source. The figure at the right shows the extraterrestrial solar spectral irradiance as compared with a blackbody spectrum. There is a sharper than black-body cutoff at the shorter wavelength end.
The Sun has an equatorial radius of 695,500 km
"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 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".
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