# Stars/Sciences

Betelgeuse is imaged in ultraviolet light by the Hubble Space Telescope and subsequently enhanced by NASA.[1] Credit: NASA and ESA.

A division of astronomical objects between rocky objects, liquid objects, gas objects (including gas giants and stars), and plasma objects 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 objects 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.

## Astronomy

Main source: Astronomy

A nomy (Latin nomia) is a "system of laws governing or [the] sum of knowledge regarding a (specified) field."[2]

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,[2] succeeds even in its smallest measurement, astronomy is the name of the effort and the result.

While natural objects in the sky, especially at night, may be sensed by sight, sound, smell, taste, or touch (vibration), many have been seen from the light they emit, absorb, reflect, transmit, or fluoresce.

Some of the natural emitters are stars.

## Sciences

Main source: Sciences

Nomology is the "science of physical and logical laws."[2]

"[S]tellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure of the temperature in this chromosphere."[3]

"[T]he luminosity class [is] expressed by the Roman numbers I, II, III, IV and V, expressing the width of certain absorption lines in the star's spectrum."[3]

Class Temperature[4]
K
Conventional color Apparent color[5][6][7] Mass[4]
(solar masses, Mʘ)
Luminosity[4]
(bolometric, Lʘ)
Hydrogen
lines
Fraction of all
main sequence stars[8]
O ≥ 33,000 K blue blue ≥ 16 ≥ 6.6 ≥ 30,000 Weak ~0.00003%
B 10,000–33,000 K blue to blue white blue white 2.1–16 1.8–6.6 25–30,000 Medium 0.13%
A 7,500–10,000 K white white to blue white 1.4–2.1 1.4–1.8 5–25 Strong 0.6%
F 6,000–7,500 K yellowish white white 1.04–1.4 1.15–1.4 1.5–5 Medium 3%
G 5,200–6,000 K yellow yellowish white 0.8–1.04 0.96–1.15 0.6–1.5 Weak 7.6%
K 3,700–5,200 K orange yellow orange 0.45–0.8 0.7–0.96 0.08–0.6 Very weak 12.1%
M ≤ 3,700 K red orange red ≤ 0.45 ≤ 0.7 ≤ 0.08 Very weak 76.45%.[3]

## Luminescences

Def. "[t]he natural medium emanating from the sun and other very hot sources (now recognised as electromagnetic radiation with a wavelength of 400-750 nm), within which vision is possible"[9] is called light.

Def. "to shine light on something"[10] is called illuminate.

Def. "emitting light"[11] is called luminous.

Def. "[a]ny emission of light that cannot be attributed merely to the temperature of the emitting body"[12] is called luminescence.

Def.

1. "the state of being luminous or a luminous object",[13]
2. "the ratio of luminous flux to radiant flux at the same wavelength",[13]
3. "the rate at which a star radiates energy in all directions"[13]

is called luminosity.

## Stars

Main source: Stars
The Hubble Space Telescope image shows four high-velocity, runaway stars plowing through their local interstellar medium. Credit: NASA - Hubble's Advanced Camera for Surveys.

"A star is a massive, luminous sphere of plasma held together by gravity."[14] This is a traditional definition of a star. The term "luminous" relates to light, specifically visible light, as it is perceived by the human eye.

From a dictionary:

Def.

1.a: "any natural luminous body visible in the sky [especially] at night",[2]
1.b: "a self-luminous gaseous celestial body of great mass whose shape is [usually] spheroidal and whose size may be as small as the earth or larger than the earth's orbit".[2]

is called a star.

From astrophysics:

Def. "any object forming on a dynamical timescale, by gravitational instability", is called a star.[15]

Def. a star that exists alone, is secluded or isolated from other stars, a reclusive or hermitary star, is called a solitary star.

Def. a separate, distinct, or individual star from others in a group is called a single star.

A solitary star differs from a single star in that the former exists alone, secluded or isolated from other stars. For example, Psi2 Aquarii (93 Aquarii) is a solitary star. Radial velocity measurements have not yet revealed the presence of planets orbiting it.

18 Scorpii is another solitary star.

Def. a star that shares a barycenter with one or more astronomical substellar objects is called a unary star.

A unary star contrasts with a binary star, trinary (three stars), and a multiple star. It is not necessarily alone like the solitary star.

Def. "a star moving faster than 65 km/s to 100 km/s relative to the average motion of the stars in the Sun's neighbourhood"[16] is called a high-velocity star.

Def. a high-velocity star "moving through space with an abnormally high velocity relative to the surrounding interstellar medium"[16] is called a runaway star.

Def. a star whose elliptical orbit takes it "well outside the plane of [its galaxy] at steep angles"[16] is called a halo star.

## Astrognosy

Main sources: Astrognosy and Stellar astrognosy
This computer-generated diagram of internal rotation in the Sun shows differential rotation in the outer convective region and almost uniform rotation in the central radiative region. Credit: Global Oscillation Network Group (GONG).

Stellar astrognosy, or perhaps stellagnosy as Latin for "star" is stella, deals with the materials of stars and their general exterior and interior constitution.

At right is a diagram of the internal rotation in the Sun, showing differential rotation in the outer convective region and almost uniform rotation in the central radiative region. The transition between these regions is called the tachocline.

"Until the advent of helioseismology, the study of wave oscillations in the Sun, very little was known about the internal rotation of the Sun. The differential profile of the surface was thought to extend into the solar interior as rotating cylinders of constant angular momentum.[17] Through helioseismology this is now known not to be the case and the rotation profile of the Sun has been found. On the surface the Sun rotates slowly at the poles and quickly at the equator. This profile extends on roughly radial lines through the solar convection zone to the interior. At the tachocline the rotation abruptly changes to solid body rotation in the solar radiation zone.[18]"[19]

From the experimentally derived diagram at the upper right using helioseismology, the apparent radiative zone begins at about 0.64 Rʘ and continues inward.

## Tachoclines

Main sources: Stars/Tachoclines and Tachoclines

With reference to the above helioseismology diagram, the tachocline extends outward from the radiative zone to at most 0.70 Rʘ.

## Convection zones

The zone or spherical shell between the tachocline and the photosphere appears to consist of two shells: an inner apparent convective sphere and an outer shell beneath the photosphere of which the photosphere may be a part.

## Photospheres

Def. "[a] visible surface layer of a star, and especially that of a sun"[20] is called a photosphere.

"When we speak of the surface of the Sun, we normally mean the photosphere."[21] "[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."[21] The photosphere emits visual, or visible, radiation.

## Chromospheres

"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."[22]

"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.[23] However, without special equipment, the chromosphere cannot normally be seen due to the overwhelming brightness of the photosphere."[22]

"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.[24]"[22]

"The temperature begins to decrease from the inner boundary of about 6,000 K[25] to a minimum of approximately 3,800 K,[26] before increasing to upwards of 35,000 K[25] at the outer boundary with the transition layer of the corona."[22]

## Transition regions

[Transition Region and Coronal Explorer] (TRACE) produced a 19.5 nm wavelength image of the transition region as a low, bright fog over the surface of the Sun and as a thin bright nimbus around the prominence itself. Credit: TRACE Data Center.

"The solar transition region is a region of the Sun's atmosphere, between the chromosphere and corona.[27] It is visible from space using telescopes that can sense ultraviolet. It is important because it is the site of several unrelated but important transitions in the physics of the solar atmosphere:"[28]

• "Below, most of the helium is not fully ionized, so that it radiates energy very effectively; above, it is fully ionized."[28]
• "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."[28]

## Stellar geography

As geography is the graphy of the geo or Earth, let stellagraphy be the graphy of the surface of any star.

"When we speak of the surface of the Sun [or a star], we normally mean the photosphere."[21] "[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."[21]

The surface of a star may be often described by features observed. These are located using stellagraphic coordinates based on stellagraphic north and south poles. The surface of a star usually rotates, has a rotational north and south pole, and there may have an assignable central meridian.

## Stellametry

Def. the measurement of the physical properties of a star may be called stellametry.

## Temperatures

Def.

1. "[a] measure of cold or heat"[29]
2. the degree or intensity of heat present in a substance or object

is called a temperature.

Def. "[t]he internal energy of a system in thermodynamic equilibrium"[30] is called heat[31], or thermal energy[30].

Def. "[h]aving a low temperature"[32] is called cold.

Def.

1. "[a] quantity that denotes the ability to do work and is measured in a unit dimensioned in mass × distance²/time² (ML²/T²) or the equivalent"[33]
2. "[t]he impetus behind all motion and all activity"[33]
3. "[t]he capacity to do work"[33]

is called energy.

## Pressures

Main sources: Physics/Pressures and Pressures

A density (ρ) of up to 150 g cm-3 is predicted to produce nuclear fusion.[34]

This may relate approximately to a pressure (p) given by

${\displaystyle p=\rho gh,}$

"where h is the height z − z0 of the liquid column between the test volume and the zero reference point of the pressure. Note that this reference point should lie at or below the surface of the liquid."[35]

Notation: let the symbol ${\displaystyle R_{\odot }}$ indicate the solar radius.

The "[s]olar radius is a unit of distance used to express the size of stars in astronomy equal to the current radius of the Sun:

${\displaystyle R_{\odot }=6.955\times 10^{8}\,{\hbox{m}}=0.0046491\,{\hbox{AU}}=2.254\times 10^{-8}\,{\hbox{pc}}}$

The solar radius is approximately 695,500 kilometres (432,450 miles) or about 110 times the radius of the Earth (${\displaystyle R_{\oplus }}$), or 10 times the average radius of Jupiter. It varies slightly from pole to equator due to its rotation, which induces an oblateness of order 10 parts per million."[36]

## Stellar groupings

Messier 92 is a star cluster in the constellation Hercules. Credit: Daniel Bramich (ING) and Nik Szymanek.
This is a Hubble Space Telescope image of the spiral galaxy NGC 1672. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration.

Def. "[t]wo stars that appear to be one when seen with the naked eye"[37] is called a double star.

Def. "[a] star that appears as a double due to an optical illusion; in reality, the stars may be far apart from each other"[38] is called an optical double.

Def. two and only two stars orbiting around their apparent barycenter is called a binary star.

Def. "[a] binary star whose components can be visually resolved"[39] is called a visual binary.

Def. "[a] group of gravitationally bound stars"[40] is called a star cluster.

Def. "a more or less irregular star cluster containing tens to thousands of stars"[41] is called an open cluster.

Def. "a spherical star cluster containing thousands to millions of stars"[42] is called a globular cluster.

Def. "[a] large group of many stars spread over a very many light-years of space [a region of greater than average stellar density]"[43] is called a star cloud.

Def. a group of stars "moving together through space"[16] is called a stellar association.

Def. "an association of stars ... [stretched out along its orbit of] a galaxy"[16] is called a stellar stream.

Def. "a stellar association [drifting] through the galaxy as a somewhat coherent assemblage"[16] is called a moving group.

Def. "[a]ny of the collections of many millions of stars ... existing as independent and coherent systems"[44] is called a galaxy.

Def. "[a]ny galaxy, considerably smaller than the Milky Way, that has only several billions of stars"[45] is called a dwarf galaxy.

Def. "[a] small group of stars that forms a visible pattern but is not an official constellation"[46] is called an asterism.

Def. "[a]ny of the [89] officially recognized regions of the sky, including all stars"[47] is called a constellation.

## Evolution

Main source: Evolution
This figure summarizes sunspot number observations. Credit: Robert A. Rohde.
Changes in 14C concentration in the Earth's atmosphere serve as a long term proxy of solar activity. Note the present day is on the right-hand side. Credit: USGS.

Def. "gradual directional change especially one leading to a more advanced or complex form"[48] is called evolution.

"Studies of stratigraphic data have suggested that the solar cycles have been active for hundreds of millions of years, if not longer; measuring varves in precambrian sedimentary rock has revealed repeating peaks in layer thickness, with a pattern repeating approximately every eleven years. It is possible that the early atmosphere on Earth was more sensitive to changes in solar radiation than today, so that greater glacial melting (and thicker sediment deposits) could have occurred during years with greater sunspot activity.[49][50] This would presume annual layering; however, alternate explanations (diurnal) have also been proposed.[51]"[52]

"Analysis of tree rings has revealed a detailed picture of past solar cycles: Dendrochronologically dated [carbon-14] radiocarbon concentrations have allowed for a reconstruction of sunspot activity dating back 11,400 years, far beyond the four centuries of available, reliable records from direct solar observation.[53]"[52]

"The earliest surviving record of sunspot observation dates from 364 BC, based on comments by Chinese astronomer Gan De in a star catalogue.[54] By 28 BC, Chinese astronomers were regularly recording sunspot observations in official imperial records.[55]"[52]

Variations in the Sun's past indicate that at least one star may have undergone some changes over relatively long time periods.

Viewing distant stars millions of light years away may suggest historical changes, but whether these changes constitute an evolution by direct evidence seems unlikely.

## Theoretical stars

"The evolutionary tracks of stars with different initial masses on the Hertzsprung–Russell diagram. The tracks start once the star has evolved to the main sequence and stop when fusion stops.
A yellow track is shown for the Sun, which will become a red giant after its main-sequence phase ends before expanding further along the asymptotic giant branch, which will be the last phase in which the Sun undergoes fusion."[56]

"Stellar evolution is not studied by observing the life of a single star, as most stellar changes occur too slowly to be detected, even over many centuries. Instead, astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, and by simulating stellar structure using computer models."[56]

## Entities

Main sources: Physics/Entities and Entities

"Where the aim is to understand the collective behavior of vast numbers of interacting entities, computation offers a more direct mode of investigation than has ever been possible in the past."[57]

"The system implements intelligent agents, defined as computational entities that perform their actions with some level of proactivity and/or responsiveness."[58]

"Are the Thin and Thick Disks Distinct Entities? Is the disk system adequately modeled by the superposition of a thin-disk and thick-disk population; that is, are these two components demonstrably distinct from one another?"[59]

## Sources

Main sources: Physics/Sources and Sources

"To minimise the facility's impact on the seeing obtained with the telescope, large heat sources such as power supplies, computers and instrumentation within the enclosure are cooled by chilled water supplied from an equipment pad 40m downhlll from the telescope."[60]

"The CHARA Array observations that produced these results [the determination of the overall diameter and projected shape of Alderamin, direct observation of the stellar disk, and the measured angular size in conjunction with the bolometric flux and distance] are discussed in § 2, detailing source selection and observation."[61]

"Observations of Alderamin were always bracketed within 20 minutes with the calibration source, and every other Alderamin calibration set included an observation of HD 211833 [a primary calibration star]."[61]

"Multiple observations of the calibration source were averaged together in a time-weighted sense, with the error variance being doubled for a 1 hr time separation."[61]

"[G]iven the known rotational velocities of the calibration and check sources in this investigation, it is entirely reasonable to expect that examination of our check star as a [uniform disk] UD as a function of baseline projection angle should result in a χ2/dof [degree of freedom] of 1.0."[61]

"The dominant source of error in our technique is the mass estimate, which we discuss in the next section."[61]

## Objects

"The objects in this region remained undiscovered until 1992 when the first Trans-Neptunian Object (TNO) was discovered (Jewitt & Luu 1993)."[60]

"Astronomy has its own megacatalog: the Sloan Digital Sky Survey [SDSS] will list 100 million objects."[57]

"Gathering all the spectrophotometric information available for a given object implied, firstly, to identify the services of interest, then, submit the same query for each one the services and, once all the information has been collected, tackle problems related to the unit conversions, flux calibrations and/or data formats."[58]

"Some of these stars are intended spectroscopic targets: there are standards of various kinds, stars targeted for stellar science and/or Galactic structure science, and objects that are assigned spectroscopic fibers due to their unusual location in 5-color space."[62]

## Continua

"We know that within 1.5 billion years after the Big Bang, some galaxies had formed; this is evidenced by galaxies observed to z ∼6. Even at this epoch most of the intergalactic medium was ionized as evidenced by the lack of continuum absorption redward of Lyman α (the Gunn-Peterson effect, Gunn & Peterson 1965). It must be concluded that some objects must have existed earlier that produced sufficient UV flux to ionise nearly all the baryonic matter in the Universe."[60] Bold added.

"It [the Solar Oscillation Imager (SOI) onboard Ulysses] will provide high precision solar images 1024x1024 of line-of-sight velocity, line intensity, continuum intensity, longitudinal magnetic field and limb position."[63] Bold added.

"Whereas white–light flares (flares detected in optical continuum emission) are rare on the Sun, they are common on active stars. Even the quiescent coronae in active stars can have X–ray luminosities that are 10-3 of the total stellar luminosity, compared to 10-6 or so for the quiet Sun."[64] Bold added.

## Emissions

"The Sun is characterised by high variability in its X-ray emission. This variability is attributed to flares that are defined as short duration X-ray bursts. ... [X-ray Solar Monitor (XSM) onboard SMART-1] XSM provides X-ray measurements for independent solar and stellar science."[65]

"In particular emission from newly formed stars of type earlier than A, observed at wavelengths below 160nm, will allow study of the co-moving star formation density in the low-Z Universe."[66]

"[A]t the relatively low level of coronal activity exhibited by the Sun, the FUV C IV λλ1548,50 doublet radiates nearly as much luminosity as the entire 0.2-2 keV corona. This is a rather curious fact, with regard to the traditional picture of coronal loops in which the X-ray radiation from the bulk of the ~ 106 K magnetically confined plasma is comparable to the conductive flux down through the loop footpoints ..., which ultimately is degraded into FUV transition zone emission (i.e., near 105 K). In particular, C IV is only one of several important TZ emissions (including the bright O V λ630 and C III λ977 lines), so LC IV is a lower limit to the true radiative loss from the subcoronal layers."[67]

## Absorptions

"Of particular importance has been access to high resolution R~40,000-100,000 echelle spectra providing an ability to study the dynamics of hot plasma and separate multiple stellar and interstellar absorption components."[66]

"In these spectral regions, located near the gain peaks of Yb and Er, standard emission lamps and absorption cells have sparse coverage."[68]

"The use of this isotopologue of methane shifts absorption features away from the more common telluric 12CH4 absorption features."[69]

## Bands

"Only the r- and i-band filters have additional short wavepass coatings to define the bandpass."[60]

"WSO/UV can considerably improve our understanding of accretion processes onto supermassive black holes in active galaxies, as their main energy output is in the far UV/extreme UV domain, and rich emission lines and occasionally absorption lines associated with surrounding gas are visible in the far UV band."[66]

## Backgrounds

"The background was subtracted for RHESSI, whereas XSM background is negligible."[65]

"We do believe that unresolved background stars explain the occasional cases where different colors predict very different temperatures. ... A significant contribution from background stars would in general combine light from stars with different temperatures."[70]

"Do we know enough about the intergalactic medium to trust measurements of background sources seen through foreground structure?"[71]

## Meteors

This ultraviolet-wavelength image mosaic, taken by NASA's Galaxy Evolution Explorer (GALEX), shows a comet-like "tail" stretching 13 light years across space behind the star Mira. Credit: NASA.
A close-up view of a star racing through space faster than a speeding bullet can be seen in this image from NASA's Galaxy Evolution Explorer. Credit: NASA/JPL-Caltech/C. Martin (Caltech)/M. Seibert(OCIW).
The Chandra image shows Mira A (right), a highly evolved red giant star, and Mira B (left), a white dwarf. Scalebar: 0.3 arcsec. Credit: NASA/CXC/SAO/M. Karovska et al.

At left is a radiated object, the binary star Mira, and its associated phenomena.

"Ultra-violet studies of Mira by NASA's Galaxy Evolution Explorer (Galex) space telescope have revealed that it sheds a trail of material from the outer envelope, leaving a tail 13 light-years in length, formed over tens of thousands of years.[72][73] It is thought that a hot bow-wave of compressed plasma/gas is the cause of the tail; the bow-wave is a result of the interaction of the stellar wind from Mira A with gas in interstellar space, through which Mira is moving at an extremely high speed of 130 kilometres/second (291,000 miles per hour).[74][75] The tail consists of material stripped from the head of the bow-wave, which is also visible in ultra-violet observations. Mira's bow-shock will eventually evolve into a planetary nebula, the form of which will be considerably affected by the motion through the interstellar medium (ISM).[76]"[77]

At second right is the only available X-ray image, by the Chandra X-ray Observatory, of Mira A on the right and Mira B (left). "Mira A is losing gas rapidly from its upper atmosphere [apparently] via a stellar wind. [Mira B is asserted to be a white dwarf. In theory] Mira B exerts a gravitational tug that creates a gaseous bridge between the two stars. Gas from the wind and bridge accumulates in an accretion disk around Mira B and collisions between rapidly moving particles in the disk produce X-rays."[78]

Mira A, spectral type M7 IIIe[79], has an effective surface temperature of 2918–3192[80]. Mira A is not a known X-ray source according to SIMBAD, but here is shown to be one.

## Cosmic rays

There is "a correlation between the arrival directions of cosmic rays with energy above 6 x 1019 electron volts and the positions of active galactic nuclei (AGN) lying within ~75 megaparsecs."[81]

"[T]he relative abundances of solar cosmic rays reflect those of the solar photosphere for multicharged nuclei with approximately the same nuclear charge-to-mass ratio."[82]

## Plasma objects

In addition to the Sun there are "many other plasma objects such as stars of different types [and] spherical lightning".[83]

"The [beryllium] Be plasma concentrations and the changes in chemical and physical erosion were measured using a visible light spectroscopy system."[84]

"During all experiments a bias voltage of −50 V was applied to the C target, the target temperatures were in the range from 500 to 1280 K and the Be plasma concentration was in the range from 0.02% to 0.2%."[84]

## Gaseous objects

"[T]he evolution of star accretion onto a supermassive gaseous object in the central region of an active galactic nucleus [may be addressed using] a gaseous model of relaxing dense stellar systems".[85]

## Liquid objects

"The influence of the reflection asymmetry degrees of freedom on the stability of rotating charged or gravitating liquid objects [shows] that the Poincaré instability can, indeed, appear in rotating gravitating objects, but is quite unlikely in rotating nuclei."[86]

"The formation of liquid metallic hydrogen brings with it a new candidate for the interior of the Sun and the stars."[87]

## Rocky objects

"The presence of elements heavier than helium in white dwarf atmospheres is often a signpost for the existence of rocky objects that currently or previously orbited these stars."[88]

"Al can be quite abundant in rocky objects formed at high temperatures".[88]

## Hydrogens

Main sources: Chemicals/Hydrogens and Hydrogens

Early spectroscopy[89] of the Sun using estimates of "the line intensities of several lines by eye [to derive] the abundances of ... elements ... [concluded] that the Sun [is] largely made of hydrogen."[90]

## Heliums

Main sources: Chemicals/Heliums and Heliums

For the star HD 124448, "[s]pectrograms extending from Hα to about λ 3530, obtained at the McDonald Observatory, show no hydrogen lines either in absorption or in emission, although the helium lines are sharp and strong."[91]

"The abundance of hydrogen appears to be very low in the atmosphere of this star. Besides the strong He I spectrum, lines of O II and C II are present."[91]

"Two objects (SB 21 and TON-S 103) turn out to be extreme helium stars, with y ≈ 1.0."[92]

## Lithiums

Main sources: Chemicals/Lithiums and Lithiums

TX Piscium (19 Piscium) is "a Li-rich carbon star".[93]

"Super Lithium Rich giants (SLIR) are an elusive class of objects, serendipitously discovered."[94]

"J37 has a Li abundance well above the meteoritic value ... Al, S, Si, Ca, Fe, and Ni are supersolar and that C is subsolar. ... Na, Sc, Ti are supersolar, while O is subsolar."[95]

"J37 has a Li abundance near A(Li) = 4.3

${\displaystyle A(X)=12+log(N_{X}/N_{H}),}$

1 dex above the solar system meteoritic value."[95]

## Berylliums

"The atmospheres of the two program stars (HR 6158 = 28 Her = HD 149212; HR 8915 = 69 Peg = HD 220933) contain an inordinate amount of beryllium (Be); in fact, the Be abundances in these stars are among the highest known. ... lithium (Li) is detected in neither HR 6158 nor HR 8915."[96]

## Borons

Main sources: Chemicals/Borons and Borons

A "sample of 23 stars contained objects with (1) strong Be and strong B, (2) weak Be and strong B, (3) strong Be and weak B, as well as (4) weak Be and B."[96]

"Boron is estimated to be overabundant by at least 2.0 dex in these three stars [κ Cnc, HR 7361, and 20 Tau] ... Three [additional] stars (HR 2676, γ Crv, and HR 7143) show strong B II lines as in the case of κ Cnc".[97]

## Carbons

Main sources: Chemicals/Carbons and Carbons

"[T[he hot PG1159 stars ... have a surface composition that is a mixture of helium, carbon, oxygen, and little or no hydrogen (Teff between ~75,000 and 200,000 K; typical abundances18), in mass fraction, of He, C, O and Ne are 33%, 50%, 15% and 2%)."[98]

"H1504+65 ... is the hottest specimen of its class at Teff ≈ 200,000 K, and its atmospheric composition very unusual, with a mass fraction of ~ 50% C and ~ 50% O plus small traces of heavier elements, but no detectable helium or hydrogen."[98]

## Nitrogens

Main sources: Chemicals/Nitrogens and Nitrogens

"[T]he sdO-star HD 127493 (Fig. 4) is nitrogen-rich. ... stars which exhibit a large 4542-discrepancy are nitrogen rich."[92]

## Oxygens

Main sources: Chemicals/Oxygens and Oxygens

"H1504+65 ... [has] a mass fraction of ~ 50% C and ~ 50% O plus small traces of heavier elements, but no detectable helium or hydrogen."[98]

## Fluorines

Main sources: Chemicals/Fluorines and Fluorines

"[E]xtreme fluorine abundances were determined in some stars (up to 200 times solar)".[99]

## Neons

Main sources: Chemicals/Neons and Neons

"H 1504+65 is the hottest known white dwarf (Teff = 200 000 K). ... The atmosphere is primarily composed of carbon and oxygen, by equal amounts Werner (1991). In addition, a high abundance of neon [yields an] exotic surface chemistry (C = 49%, O = 49%, Ne = 2%, mass fractions) [modified by] soft X-ray spectrum ... with an abundance of about 2% [magnesium]".[99]

## Atmospheric sciences

The composite shows upper atmospheric lightning and electrical discharge phenomena. Credit: Abestrobi.

"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."[100]

"Aeronomy is the study of the upper layers of the atmosphere, where dissociation and ionization are important."[100]

## Plasmas

Main source: Plasmas

"[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.[101]"[102]

"For plasma to exist, ionization is necessary. The term "plasma density" by itself usually refers to the "electron density", that is, the number of free electrons per unit volume. The degree of ionization of a plasma is the proportion of atoms that have lost or gained electrons, and is controlled mostly by the temperature. Even a partially ionized gas in which as little as 1% of the particles are ionized can have the characteristics of a plasma (i.e., response to magnetic fields and high electrical conductivity). The degree of ionization, α is defined as α = ni/(ni + na) where ni is the number density of ions and na is the number density of neutral atoms. The electron density is related to this by the average charge state <Z> of the ions through ne = <Z> ni where ne is the number density of electrons."[102]

## 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.

## Io

Main source: Io
Gases above Io's surface produced a ghostly glow that could be seen at visible wavelengths (red, green, and violet). Credit: NASA/JPL/University of Arizona.

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."[103]

## Titan

Main source: Titan
This is a natural color image of Titan. Credit: NASA/JPL/Space Science Institute.

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."[104]

"The atmosphere of Titan is largely composed of nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog."[104]

Titan has a mean radius of 2576 ± 2 km.[105]

## Mars

Main source: Mars
On July 4, 2001, this Chandra X-ray Observatory image became the first look at X-rays from Mars. Credit: NASA/CXC/MPE/K.Dennerl et al.

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.[106] 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.[107]

## Sirius B

Main sources: Stars/Sirius/B and Sirius B
An X-ray image of the Sirius star system located 8.6 light years from Earth is shown. Sirius B is the brighter object at lower left of the two. Credit: NASA/SAO/CXC.

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."[108]

Sirius B clearly outshines Sirius A. The surface effective temperature of Sirius B (a white dwarf, DA2) is 25,200 K.[109]

Sirius B has a radius of 5800 ± 200 km.[110]

## Venus

Main source: Venus
This is an image of Venus in true color. The surface is obscured by a thick blanket of clouds. Credit: NASA/Ricardo Nunes, http://www.astrosurf.com/nunes.
An ultraviolet image of the planet Venus is taken on February 26, 1979, by the Pioneer Venus Orbiter. Credit: NASA.

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.[111]

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.[107]

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

Main sources: Stars/Procyon B and Procyon B

Procyon B has a radius of 8,542 ± 223 km and an effective surface temperature of 7,740 ± 50 K.[112]

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),[113] ... 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.[114]"[115]

According to SIMBAD, Procyon B is ROSAT X-ray source 2RXP J073918.2+051334 and 2XMMi J073917.7+051324 by X-ray Multi-Mirror Mission - Newton.

## Van Maanen's star

The image is an optical negative centered on the SIMBAD coordinates J2000.0 for Van Maanen's star. Image is from the Palomar 48-inch Schmidt reflecting telescope. Van Maanen's star is the largest black dot center top right. Credit: NASA/IPAC Extragalactic Database.

"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.[116][117]"[118]

"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,[119] with a relatively high proper motion of 2.98" annually.[6] It is too faint to be seen with the naked eye.[119]"[118]

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.

Van Maanen's star has a radius of 9,000 ± 1,400 km.[120] It's effective surface temperature is 6,220 ± 240 K.[121]

## 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.

## Neptune

Main source: Neptune
This picture from the Voyager 2 sequence shows two of the four cloud features which have been tracked by the Voyager cameras during the past two months. Credit: NASA.
These are infrared images of Neptune. Credit: VLT/ESO/NASA/JPL/Paris Observatory.

"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,[122] 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.[123]"[124]

Neptune has an equatorial radius of 24,764 ± 15 km and a polar radius of 24,341 ± 30 km.[107]

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."[125]

## Uranus

Main source: Uranus
This is an image of the planet Uranus taken by the spacecraft Voyager 2 in 1986. Credit: NASA/JPL/Voyager mission.

"In larger amateur telescopes with an objective diameter of between 15 and 23 cm, the planet appears as a pale cyan disk with distinct limb darkening."[126]

"Methane possesses prominent absorption bands in the visible and near-infrared (IR) making Uranus aquamarine or cyan in color.[127]"[126]

Uranus has an equatorial radius of 25,559 ± 4 km and a polar radius of 24,973 ± 20 km.[107]

## Saturn

Main source: Saturn
The planet Saturn is seen in approximate natural color by the Hubble Space Telescope. Credit: .

"Saturday is the day of Saturn, and the color of Saturn, according to astronomers, is said to be black"[128].

"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.[129][130] ... 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[131] known moons orbit the planet; fifty-three are officially named. This does not include the hundreds of "moonlets" within the rings."[132]

Saturn has an equatorial radius of 60,268 ± 4 km and a polar radius of 54,364 ± 10 km.[107]

## Jupiter

Main source: Jupiter

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.[107]

### X-rays

This image of Jupiter 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. Credit: NASA/CXC/SWRI/G.R.Gladstone et al.
Jupiter shows intense X-ray emission associated with auroras in its polar regions (Chandra observatory X-ray image on the left). The accompanying schematic illustrates how Jupiter's unusually frequent and spectacular auroral activity is produced. Observation period: 17 hrs, February 24-26, 2003. Credit: X-ray: NASA/CXC/MSFC/R.Elsner et al.; Illustration: CXC/M.Weiss.

The "image of Jupiter [at right] shows concentrations of auroral X-rays near the north and south magnetic poles."[133] 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."[134]

### Ultraviolets

Aurora at Jupiter's north pole is seen in ultraviolet light by the Hubble Space Telescope. Credit: .

"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."[135]

### Visuals

Cloud bands are clearly visible on Jupiter. Credit: NASA/JPL/USGS.

"[O]range [is] the color of Jupiter"[136].

"The Great Red Spot (GRS) is a persistent anticyclonic storm, 22° south of Jupiter's equator, which has lasted for at least 187 years and possibly longer than 352 years.[137][138] 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."[139]

"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.[140] 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.[141]"[139]

### Infrareds

An infrared image of GRS (top) shows its warm center, taken by the ground based Very Large Telescope. An image made by the Hubble Space Telescope (bottom) is shown for comparison. Credit: .
This is an infrared image of Jupiter taken by the ESO's Very Large Telescope. Credit: ESO/F. Marchis, M. Wong, E. Marchetti, P. Amico, S. Tordo.

"Spectra from the Voyager I IRIS experiment confirm the existence of enhanced infrared emission near Jupiter's north magnetic pole in March 1979."[142] "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."[142]

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."[143]

## Gliese 229B

Main sources: Stars/Gliese 229B and Gliese 229B
This brown dwarf (Gliese 229B, smaller object) orbits the star Gliese 229, which is located in the constellation Lepus about 19 light years from Earth. Credit: NASA, Hubblesite STScI-1995-48.

Gliese 229B is a brown dwarf and has a radius approximately the same as Jupiter's.[144]

## Proxima Centauri

Proxima Centauri is the orange star next to the cross-hairs for the SIMBAD equatorial coordinates. Credit: Aladin at SIMBAD.

"Proxima Centauri ... is a red dwarf star about 4.22 light-years (4.0×1013 km) distant in the constellation of Centaurus."[145]

Proxima Centauri has a radius of 98,100 ± 4,900 km.[146] Its surface effective temperature is 3,042 ± 117 K.[147]

"Although it has a very low average luminosity, Proxima is a flare star that undergoes random dramatic increases in brightness because of magnetic activity.[148]"[145]

"More than 85% of its radiated power is at infrared wavelengths.[149]"[145]

"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.[150] 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.[151]"[145]

"Proxima Centauri has a relatively weak stellar wind, resulting in no more than 20% of the Sun's mass loss rate from the solar wind."[145]

## TWA 5B

Main sources: Stars/TWA 5B and TWA 5B
This is a Chandra X-ray Observatory image of the brown dwarf TWA 5B. Credit: NASA/CXC/Chuo U./Y. Tsuboi et al.

TWA 5B has an inferred radius between 167,000 and 216,000 km.[152]

"Using NASA's Chandra X-ray Observatory, scientists have detected X-rays from a low-mass brown dwarf in a multiple star system.[153] 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.[153] "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.[153] "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.[153] "This observation, thus, raises the possibility that even massive planets might emit X-rays by themselves during their youth!"[153]"[154]

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."[155] 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"[155], 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.[155]

"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"[155], so these results could have implications for the possible X-ray detection of very massive planets around stars.

## Sun

Main sources: Stars/Sun and Sun (star)
A visual image of the Sun shows sunspots occasionally. The two small spots in the middle have about the same diameter as our planet Earth. Credit: NASA.
This figure shows the extraterrestrial solar spectral irradiance of the Sun. Credit: Sch.

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."[21] "[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."[21]

"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.[156] The effective temperature of the surface of the Sun's photosphere is 5,778 K.[157]"[14] 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".[157] The photosphere is "~400 km" in thickness.[157]

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[158]

## Betelgeuse

Main sources: Stars/Betelgeuse and Betelgeuse
This image is from ESO's Very Large Telescope showing not only the stellar disk, but also an extended atmosphere with a previously unknown plume of surrounding gas.[159] Credit: ESO/P. Kervella.

"The yellow/red "image" or "photo" of Betelgeuse usually seen is actually not a picture of the red giant but rather a mathematically generated image based on the photograph. The photograph was actually of much lower resolution: The entire Betelgeuse image fit entirely within a 10x10 pixel area on the Hubble Space Telescopes Faint Object Camera. The actual images were oversampled by a factor of 5 with bicubic spline interpolation, then deconvolved."[1]

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."[160]

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

1. The rhythmic expansion and contraction of the photosphere [may mean] the diameter is never constant;
2. There is no definable "edge" to the star as limb darkening causes the optical emissions to vary in color and decrease the farther one extends out from the center;
3. Betelgeuse is surrounded by a circumstellar envelope composed of matter being ejected from the star—matter which both absorbs and emits light—making it difficult to define the edge of the photosphere;[161]
4. Measurements can be taken at varying wavelengths within the electromagnetic spectrum, with each wavelength revealing something different. Studies have shown that angular diameters are considerably larger at visible wavelengths, decrease to a minimum in the near-infrared, only to increase again in the mid-infrared.[162][163] 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;[161]
5. Atmospheric twinkling limits the resolution obtainable from ground-based telescopes since turbulence degrades angular resolution.[164]

"Assuming a distance of 197±45pc, an angular distance of 43.33±0.04 mas would equate to a radius of 4.3 AU".[162]

"Images of hotspots on the surface of Betelgeuse [are] taken at visible and infra-red wavelengths using high resolution ground-based interferometers".[163]

## Chemistry

Main sources: Chemicals/Chemistry and Chemistry

Laboratory conditions are often expressed in terms of standard temperature and pressure.

"Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted standards. Other organizations have established a variety of alternative definitions for their standard reference conditions."[165]

"In chemistry, IUPAC established standard temperature and pressure (informally abbreviated as STP) as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm, 1 bar),[166] An unofficial, but commonly used standard is standard ambient temperature and pressure (SATP) as a temperature of 298.15 K (25 °C, 77 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm). The STP and the SATP should not be confused with the standard state commonly used in thermodynamic evaluations of the Gibbs free energy of a reaction."[165]

"Standard conditions for gases: Temperature, 273.15 K [...] and pressure of 105 pascals. The previous standard absolute pressure of 1 atm (equivalent to 1.01325 × 105 Pa) was changed to 100 kPa in 1982. IUPAC recommends that the former pressure should be discontinued."[166]

"NIST uses a temperature of 20 °C (293.15 K, 68 °F) and an absolute pressure of 101.325 kPa (14.696 psi, 1 atm). The International Standard Metric Conditions for natural gas and similar fluids are 288.15 K (59.00 °F, 15.00 °C) and 101.325 kPa.[167]"[165]

## Mountain tops

The Canada-France-Hawaii Telescope is located at the Mauna Kea Observatory in Hawai'i. Credit: Fabian_RRRR.

"The Canada-France-Hawaii Telescope (CFHT) is a 3.6 m optical-infrared telescope located on the summit of Mauna Kea on the island of Hawaii."[168]

## Ancient history

The ancient history period dates from around 8,000 to 3,000 b2k.

"Ancient astronomers were able to differentiate between stars and planets, as stars remain relatively fixed over the centuries while planets will move an appreciable amount during a comparatively short time."[169]

## Mathematics

Main source: Mathematics
The first synchrotron function can be used to represent the spectrum (intensity vs. wavelength, usually normalized to 1.0) of synchrotron radiation produced by a plasma. Credit: Dijon.

Ions accelerating past each other at relativistic speeds generate synchrotron radiation. Depending on the ion concentration, this radiation may contribute significantly to stellar spectra as may cyclotron or bremsstrahlung radiation from similar ion and electron interactions at slower speeds.

## Physics

Main source: Physics
M81 is one of the brightest galaxies that can be seen from the Earth. Credit: Hubble data: NASA, ESA, and A. Zezas (Harvard-Smithsonian Center for Astrophysics); GALEX data: NASA, JPL-Caltech, GALEX Team, J. Huchra et al. (Harvard-Smithsonian Center for Astrophysics); Spitzer data: NASA/JPL/Caltech/S. Willner (Harvard-Smithsonian Center for Astrophysics.

This beautiful galaxy imaged at right is tilted at an oblique angle on to our line of sight, giving a "birds-eye view" of the spiral structure.

This image combines data from the Hubble Space Telescope, the Spitzer Space Telescope, and the Galaxy Evolution Explorer (GALEX) missions. The GALEX ultraviolet data were from the far-UV portion of the spectrum (135 to 175 nanometers). The Spitzer infrared data were taken with the IRAC 4 detector (8 microns). The Hubble data were taken at the blue portion of the spectrum.

## Technology

Main source: Technology
The Hubble Space Telescope is seen from the departing Space Shuttle Atlantis, flying Servicing Mission 4 (STS-125), the fifth and final human spaceflight to visit the observatory. Credit: Ruffnax (Crew of STS-125).

The Hubble Space Telescope (HST) is an excellent example of a radiation astronomy satellite designed for more than one purpose: the various astronomies of optical astronomy.

The HST is an optical astronomy telescope that “incorporated a set of 48 filters isolating spectral lines of particular astrophysical interest."[170]

"The Wide Field Camera 3 (WFC3) is the Hubble Space Telescope's last and most technologically advanced instrument to take images in the visible spectrum. It was installed as a replacement for the Wide Field and Planetary Camera 2 during the first spacewalk of Space Shuttle mission STS-125 on May 14, 2009."[171]

## Hypotheses

Main source: Hypotheses
1. All known stars that give off X-rays at least from a nearby coronal cloud have nuclear fusion taking place above their photospheres.

## References

1. Ronald L. Gilliland, Andrea K. Dupree (May 1996). "First Image of the Surface of a Star with the Hubble Space Telescope" (PDF). Astrophysical Journal Letters 463 (1): L29-32. doi:10.1086/310043. Retrieved 1 August 2010.
2. Philip B. Gove, ed (1963). Webster's Seventh New Collegiate Dictionary. Springfield, Massachusetts: G. & C. Merriam Company. pp. 1221.
3. "Stellar classification, In: Wikipedia". San Francisco, California: Wikimedia Foundation, Inc. August 8, 2012. Retrieved 2012-08-08.
4. Tables VII, VIII, Empirical bolometric corrections for the main-sequence, G. M. H. J. Habets and J. R. W. Heinze, Astronomy and Astrophysics Supplement Series 46 (November 1981), pp. 193–237, bibcode=1981A&AS...46..193H. Luminosities are derived from Mbol figures, using Mbol(ʘ)=4.75.
5. The Guinness book of astronomy facts & feats, Patrick Moore, 1992, 0-900424-76-1
6. "The Colour of Stars". Australia Telescope Outreach and Education. 2004-12-21. Retrieved 2007-09-26. — Explains the reason for the difference in color perception.
7. What color are the stars?, Mitchell Charity. Accessed online March 19, 2008.
8. Glenn LeDrew (February 2001). "The Real Starry Sky". Journal of the Royal Astronomical Society of Canada 95 (1 (whole No. 686, February 2001), pp. 32–33. Note: Table 2 has an error and so this article will use 824 as the assumed correct total of main-sequence stars).
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