# Stars/Sun/Astronomy

< Stars‎ | Sun(Redirected from Solar astronomy)
This is a red image of the Sun taken through a solar telescope. Credit: Totallyhaywire2.

Sol passes overhead every day in most locations on the surface of the Earth.

The Sun moves across the sky during the day time only. An entity or two may be responsible for this.

Solar astronomy is the radiation astronomy of the star, Sol, often called the Sun.

## Sun

Def. "1.9891 x 1030 kg" is called the mass of the Sun.[1]

## 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.

## Theoretical solar astronomy

Def. the astronomy of the "star at the center of the Solar System, represented in astronomy and astrology by ⨀"[3] is called the Sun astronomy, or solar astronomy.

## Meteors

This movie of the Sun shows a coronal mass ejection. Credit: .

"Sun-grazing comets almost never re-emerge, but their sublimative destruction near the sun has only recently been observed directly, while chromospheric impacts have not yet been seen, nor impact theory developed."[4] "[N]uclei are ... destroyed by ablation or explosion ... in the chromosphere, producing flare-like events with cometary abundance spectra."[4]

"The death of a comet at r ~ R has been seen directly only very recently (Schrijver et al 2011) using the SDO AIA XUV instrument. This recorded sublimative destruction of Comet C/2011 N3 as it crossed the solar disk very near periheloin q = 1.139Rʘ."[4]

"The phenomenon of flare induced sunquakes - waves in the photosphere - discovered by Kosovichev and Zharkova (1998) and now widely studied (e.g. Kosovichev 2006) should also result from the momentum impulse delivered by a cometary impact."[4]

"Coronal clouds, type IIIg, form in space above a spot area and rain streamers upon it."[5]

The solar wind originates through the polar coronal holes.

## Cosmic rays

The surface of the Sun has not been detected with cosmic rays.

"[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."[6]

## Neutrals

As stars are defined as luminous balls of plasma, the Sun may not qualify as its photosphere has a plasma concentration of approximately 10-4. The rest is composed of neutral atoms or molecules at about 5800 K.

## Subatomics

The "evidence for the overwhelming majority of the Li-atoms in photospheres has its origin not only in nuclear synthesis near the stellar centers, but also by active processes in stellar atmospheres. [...] the lithium [resonance] line [is] near 478 keV."[7]

"Approximately 90% of lithium atoms originate from α - α reactions for the typical spectra of an accelerated particle on the Sun [...] During impulsive flares, interaction between the accelerated particles and the ambient medium occurs mainly at low altitudes, i.e., close to the footprints of loops."[7]

"Due to the very low energy of the colliding protons in the Sun, only states with no angular momentum (s-waves) contribute significantly. One can consider it as a head-on collision, so that angular momentum plays no role. Consequently, the total angular momentum is the sum of the spins, and the spins alone control the reaction. Because of Pauli's exclusion principle, the incoming protons must have opposite spins. On the other hand, in the only bound state of deuterium, the spins of the neutron and proton are aligned. Hence a spin flip must take place [...] The strength of the nuclear force which holds the neutron and the proton together depends on the spin of the particles. The force between an aligned proton and neutron is sufficient to give a bound state, but the interaction between two protons does not yield a bound state under any circumstances. Deuterium has only one bound state."[8]

The "force acting between the protons and the neutrons [is] the strong force".[8]

"A potential of 36 MeV is needed to get just one energy state."[8]

The width of a bound proton and neutron is "2.02 x 10-13 cm".[8]

## Neutrons

The image is a schematic view of the Mount Norikura solar neutron telescope. Credit: Y. Muraki, K. Murakami, M. Miyazaki, K. Mitsui. S. Shibata, S. Sakakibara, T. Sakai, T. Takahashi, T. Yamada, and K. Yamaguchi.
RHESSI observes high-energy phenomena from a solar flare. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio.
"This graph shows the neutrons detected by a neutron detector at the University of Oulu in Finland from May 16 through May 18, 2012. The peak on May 17 represents an increase in the number of neutrons detected, a phenomenon dubbed a ground level enhancement or GLE. This was the first GLE since December of 2006. Credit: University of Oulu/NASA's Integrated Space Weather Analysis System"[9].

Fairly large fluxes of neutrons have been observed during solar flares such as that of November 12, 1960, with a flux of 30-70 neutrons per cm-2 s-1.[10]

"The neutrons are produced by the energetic protons interacting with a number of different nuclei."[11]

A "new detector to observe solar neutrons [has been in operation] since 1990 October 17 [...] at the Mount Norikura Cosmic Ray Laboratory (CRL) of [the] Institute for cosmic Ray Research, the University of Tokyo."[12]

"On 1991 June 1, an active sunspot appeared at N25 E90 on the Sun (NOAA region 6659). The commencement of an enormous bright flare was observed at 03:37 UT on 1991 June 4 [...] The flare was classified as 3 B and the location was at N31 E70 of the solar surface."[12]

"The solar neutron telescope [image at right] consists of 10 blocks of scintillator [...] and several lead plates which are used to place kinetic energies Tn of incoming particles into three bands (50-360 MeV, 280-500 MeV, and ≥ 390 MeV)."[12] The telescope is inclined to the direction of the Sun by 15°.[12] The plane area of the detector is 1.0 m2 and protected by lead plates (Pb) to eliminate gamma-ray and muon background from the side of the detector.[12] The anti-coincident counter (A) is used to reject the muons and gamma rays, coming from the side of the detector and the top scintillators.[12] (P) and (G) are used to identify the proton events and gamma rays.[12] The central scintillator blocks are optically separated into 10 units.[12]

"The horizontal scintillator just above the 10 vertical scintillators distinguishes neutral particles (neutrons) from the charged particles (mainly muons, protons and electrons)."[12]

"Mount Norikura Cosmic-Ray Laboratory has an elevation of 2770 m above sea level. The geographical latitude is 36.10° N and the longitude is 137.55° E. The zenith angle of the Sun at 03:37 UT on June 4 is 18.9° and the solar neutron telescope was set at a zenith angle of 15° on this day."[12]

The solar flare at Active Region 10039 on July 23, 2002, exhibits many exceptional high-energy phenomena including the 2.223 MeV neutron capture line and the 511 keV electron-positron (antimatter) annihilation line. In the image at right, the RHESSI low-energy channels (12-25 keV) are represented in red and appear predominantly in coronal loops. The high-energy flux appears as blue at the footpoints of the coronal loops. Violet is used to indicate the location and relative intensity of the 2.2 MeV emission.

During solar flares “[s]everal radioactive nuclei that emit positrons are also produced; [which] slow down and annihilate in flight with the emission of two 511 keV photons or form positronium with the emission of either a three gamma continuum (each photon < 511 keV) or two 511 keV photons."[13] The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) made the first high-resolution observation of the solar positron-electron annihilation line during the July 23, 2003 solar flare.[13] The observations are somewhat consistent with electron-positron annihilation in a quiet solar atmosphere via positronium as well as during flares.[13] Line-broadening is due to "the velocity of the positronium."[13] "The width of the annihilation line is also consistent ... with thermal broadening (Gaussian width of 8.1 ± 1.1 keV) in a plasma at 4-7 x 105 K. ... The RHESSI and all but two of the SMM measurements are consistent with densities ≤ 1012 H cm-3 [but] <10% of the p and α interactions producing positrons occur at these low densities. ... positrons produced by 3He interactions form higher in the solar atmosphere ... all observations are consistent with densities > 1012 H cm-3. But such densities require formation of a substantial mass of atmosphere at transition region temperatures."[13]

"On May 17, 2012 an M-class flare exploded from the sun. The eruption also shot out a burst of solar particles traveling at nearly the speed of light that reached Earth about 20 minutes after the light from the flare. An M-class flare is considered a "moderate" flare, at least ten times less powerful than the largest X-class flares, but the particles sent out on May 17 were so fast and energetic that when they collided with atoms in Earth's atmosphere, they caused a shower of particles to cascade down toward Earth's surface. The shower created what's called a ground level enhancement (GLE)."[9]

"[O]n Saturday, May 5, ... a large sunspot rotated into view on the left side of the sun. ... [J]ust before [Active Region 1476] disappeared over the right side of the sun, it ... erupted with an M-class flare."[9]

## Protons

This graph displays the flux of high energy protons measured by GOES 11 over four days from November 2, 2004, to November 4, 2003. Credit: NOAA.

The Sun and the solar wind, at least that portion that originates through the polar coronal holes apparently from the photosphere, may be major sources of protons within the solar system.

At right is a temporal distribution of solar proton flux in units of particles cm-2 s-1 sr-1 as measured by GOES 11 over the four days from November 2, 2003, to November 4, 2003, in three windows of energy: ≥ 100 MeV (green), ≥ 50 MeV (blue), and ≥ 10 MeV (red). The percentage originating from the surface of the Sun either directly or through the contribution to the solar wind is not indicated.

## Mesons

An "analysis of the energy-loss distributions in the GRS HEM during the impulsive phase of this event indicates that γ-rays from the decay of π0 mesons were detected [...] The production of pions, which is accompanied (on average) by neutrons, has an energy threshold of ~290 MeV for p-p and ~180 MeV for p-α interactions, giving, therefore, a lower limit to the maximum energy of the particles accelerated at the Sun."[14]

## Beta particles

"Above the photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun entirely. The change in opacity is due to the decreasing amount of H ions, which absorb visible light easily.[15] Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H ions.[16][17] ... The photosphere has a particle density of ~1023 m−3 (this is about 0.37% of the particle number per volume of Earth's atmosphere at sea level; however, photosphere particles are electrons and protons, so the average particle in air is 58 times as heavy)."[18]

"Positrons entering a gaseous medium at [0.6 to 4.5 MeV] are quickly slowed by ionizing collisions with neutral atoms and by long-range Coulomb interactions with any ionized component."[19]

## Electrons

"Energetic photons, ions and electrons from the solar wind, together with galactic and extragalactic cosmic rays, constantly bombard surfaces of planets, planetary satellites, dust particles, comets and asteroids."[20]

Jovian electrons propagate "along the spiral magnetic field of the interplanetary medium [from Jupiter and its magnetosphere to the Sun]".[21]

## Positrons

The solar flare at Active Region 10039 on July 23, 2002, exhibits many exceptional high-energy phenomena including the 2.223 MeV neutron capture line and the 511 keV electron-positron (antimatter) annihilation line. In the image at right, the RHESSI low-energy channels (12-25 keV) are represented in red and appear predominantly in coronal loops. The high-energy flux appears as blue at the footpoints of the coronal loops. Violet is used to indicate the location and relative intensity of the 2.2 MeV emission.

During solar flares “[s]everal radioactive nuclei that emit positrons are also produced; [which] slow down and annihilate in flight with the emission of two 511 keV photons or form positronium with the emission of either a three gamma continuum (each photon < 511 keV) or two 511 keV photons."[13] The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) made the first high-resolution observation of the solar positron-electron annihilation line during the July 23, 2003 solar flare.[13] The observations are somewhat consistent with electron-positron annihilation in a quiet solar atmosphere via positronium as well as during flares.[13] Line-broadening is due to "the velocity of the positronium."[13] "The width of the annihilation line is also consistent ... with thermal broadening (Gaussian width of 8.1 ± 1.1 keV) in a plasma at 4-7 x 105 K. ... The RHESSI and all but two of the SMM measurements are consistent with densities ≤ 1012 H cm-3 [but] <10% of the p and α interactions producing positrons occur at these low densities. ... positrons produced by 3He interactions form higher in the solar atmosphere ... all observations are consistent with densities > 1012 H cm-3. But such densities require formation of a substantial mass of atmosphere at transition region temperatures."[13]

## Neutrinos

This "neutrino image" of the Sun is produced by using the Super-Kamiokande to detect the neutrinos from nuclear fusion coming from the Sun. Credit: R. Svoboda and K. Gordan (LSU).
This figure shows a detected 94 % correlation between scaled sunspot numbers and neutrino detections. Credit: John N. Bahcall.

Neutrinos are hard to detect. The Super-Kamiokande, or "Super-K" is a large-scale experiment constructed in an unused mine in Japan to detect and study neutrinos. The image at right required 500 days worth of data to produce the "neutrino image" of the Sun. The image is centered on the Sun's calculated position. It covers a 90° x 90° octant of the sky (in right ascension and declination). The higher the brightness of the color, the larger is the neutrino flux.

The surface of the Sun is not a known source of neutrinos. Those detected may be from nucleosynthesis within the coronal cloud in the near vicinity of the Sun or perhaps from nucleosynthesis occurring interior to the Sun.

"[N]eutrino flux increases noted in Homestake results [coincide] with major solar flares [14]."[22]

This result together with those in the next two paragraphs establishes that neutrinos are being produced by processes above the photosphere and probably within 2-4 solar radii as most solar flares give off energy close to and into the chromosphere.

"The correlation between a great solar flare and Homestake neutrino enhancement was tested in 1991. Six major flares occurred from May 25 to June 15 including the great June 4 flare associated with a coronal mass ejection and production of the strongest interplanetary shock wave ever recorded (later detected from spacecraft at 34, 35, 48, and 53 AU) [15]. It also caused the largest and most persistent (several months) signal ever detected by terrestrial cosmic ray neutron monitors in 30 years of operation [16]. The Homestake exposure (June 1–7) measured a mean 37Ar production rate of 3.2 ± 1.5 atoms/day (≈19 37Ar atoms produced in 6 days) [13]; about 5 times the rate of ≈ 0.65 day −1 for the preceding and following runs, > 6 times the long term mean of ≈ 0.5 day−1 and > 2 1/2 times the highest rates recorded in ∼ 25 operating years."[22]

The highest flux of solar neutrinos come directly from the proton-proton interaction, and have a low energy, up to 400 keV. There are also several other significant production mechanisms, with energies up to 18 MeV.[23]

"The parts of the Sun above the photosphere are referred to collectively as the solar atmosphere.[15]"[18]

"Neutrinos can be produced by energetic protons accelerated in solar magnetic fields. Such protons produce pions, and therefore muons, hence also neutrinos as a decay product, in the solar atmosphere."[24]

"Energetic protons in the solar corona could explain Figure 2 [at right] only if (1) they tap a substantial fraction of the entire energy generated in the corona, (2) the energy generated in the corona is at least 3 times what has been deduced from the observations, (3) the vast majority of energetic protons do not escape the Sun, (4) the proton energy spectrum is unusually hard (p0 = 300 MeV c-1, and (5) the sign of the variation is opposite to what one would predict. As the likelihood of all of these conditions being fulfilled seems extremely small, we do not believe that neutrinos produced by energetic protons in the solar atmosphere contribute significantly to the neutrino capture in the 37Cl experiment."[24]

"The total number of neutrinos of all types agrees with the number predicted by the computer model of the Sun. Electron neutrinos constitute about a third of the total number of neutrinos. [...] The missing neutrinos were actually present, but in the form of the more difficult to detect muon and tau neutrinos."[25]

The reactions that produce the higher energy neutrinos: νµ and ντ are.

For antiproton-proton annihilation at rest, a meson result is, for example,

${\displaystyle p^{+}+{\bar {p}}^{-}\rightarrow \pi ^{+}+\pi ^{-},}$[26]
${\displaystyle {\pi }^{+}\rightarrow {\mu }^{+}+{\nu }_{\mu }\rightarrow e^{+}+{\nu }_{e}+{\bar {\nu }}_{\mu }+{\nu }_{\mu },}$[27] and
${\displaystyle D_{S}\rightarrow \tau +{\bar {\nu }}_{\tau }\rightarrow \nu _{\tau }+{\bar {\nu }}_{\tau }.}$[28]

"All other sources of ντ are estimated to have contributed an additional 15%."[28]

${\displaystyle \tau \rightarrow e+\nu _{\tau }+\nu _{e},}$[28]

for two neutrinos.[28]

${\displaystyle \tau \rightarrow h+\nu _{\tau }+X,}$[28]

where ${\displaystyle h}$ is a hadron, for two neutrinos.[28]

## Gamma rays

The Sun is seen in gamma rays by COMPTEL during a June 15, 1991, solar flare. Credit: COMPTEL team, University of New Hampshire.

The surface of the Sun has yet to be detected as a gamma ray source, reflector, or in fluorescence.

RHESSI was the first satellite to image solar gamma rays from a solar flare.[29]

## X-rays

First Public Image is from GOES 14 taken with the Solar X-ray Imager (SXI). Credit: NWS Internet Services Team of the NOAA/Space Weather Prediction Center.
The coronal clouds of the Sun are captured with X-rays. Credit: NASA Goddard Laboratory for Atmospheres.

The image at right is the first public image taken by the solar X-ray imager (SXI) aboard the GOES 14 satellite. These geostationary operational environmental satellites (GOES) monitor the Sun’s X-rays for the early detection of solar flares, coronal mass ejections (CMEs), and other phenomena that impact the geospace environment. This early warning is important because travelling solar disturbances affect not only the safety of humans in high-altitude missions, such as human spaceflight, but also military and commercial satellite communications. In addition, CMEs can damage long-distance electric power grids, causing extensive power blackouts.

The GOES satellites circle the Earth in geosynchronous orbits.

"X-ray photons can be effectively backscattered by photosphere atoms and electrons (Tomblin 1972; Bai & Ramaty 1978). ... [A]t energies not dominated by absorption the backscattered albedo flux must be seen virtually in every solar flare spectrum, the degree of the albedo contribution depending on the directivity of the primary X-ray flux (Kontar et al. 2006). The solar flare photons backscattered by the solar photosphere can contribute significantly (the reflected flux is 50-90 % of the primary in the 30 - 50 keV range for isotropic sources) to the total observed photon spectrum. for the simple case of a power-law-like primary solar flare spectrum (without albedo), the photons reflected by the photosphere produce a broad 'hump' component. Photospheric albedo makes the observed spectrum flatter below ~ 35 keV and slightly steeper above, in comparison with the primary spectrum."[30]

## Ultraviolets

This is a false-color image of the Sun's corona as seen in extreme ultraviolet (at 17.1 nm) by the Extreme ultraviolet Imaging Telescope aboard Stereo B. Credit: NASA.
This is a 3D image of Sun provided by STEREO satellites. Credit: NASA.

"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.[31] The effective temperature of the surface of the Sun's photosphere is 5,778 K.[32] That's a peak emittance wavelength of 501.5 nm (~0.5 eV) making the photosphere a primarily green radiation source. The temperature of the photosphere is way too cool to generate appreciable amounts of ultraviolet. In fact, the Sun's photosphere probably generates little or no ultraviolet rays.

To produce 3D images of the Sun [at right], the STEREO spacecraft take images in the ultraviolet from two different locations, STEREO A leading the Earth and STEREO B trailing the Earth. One view is displayed in magenta and the other in cyan.

## Opticals

Sample calibration colors[33]
Class B–V U–B V–R R–I Teff (K)
O5V –0.33 –1.19 –0.15 –0.32 42,000
B0V –0.30 –1.08 –0.13 –0.29 30,000
A0V –0.02 –0.02 0.02 –0.02 9,790
F0V 0.30 0.03 0.30 0.17 7,300
G0V 0.58 0.06 0.50 0.31 5,940
K0V 0.81 0.45 0.64 0.42 5,150
M0V 1.40 1.22 1.28 0.91 3,840

"[T]he color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. To measure the index, one observes the magnitude of an object successively through two different filters, such as U and B, or B and V, where U is sensitive to ultraviolet rays, B is sensitive to blue light, and V is sensitive to visible (green-yellow) light (see also: UBV system). The set of passbands or filters is called a photometric system. The difference in magnitudes found with these filters is called the U-B or B–V color index, respectively. The smaller the color index, the more blue (or hotter) the object is. Conversely, the larger the color index, the more red (or cooler) the object is. This is a consequence of the logarithmic magnitude scale, in which brighter objects have smaller (more negative) magnitudes than dimmer ones. ... The passbands most optical astronomers use are the UBVRI filters, where the U, B, and V filters are as mentioned above, the R filter passes red light, and the I filter passes infrared light. ... These filters were specified as particular combinations of glass filters and photomultiplier tubes."[34]

## Visuals

This is a visual image of the Sun with some sunspots visible. The two small spots in the middle have about the same diameter as our planet Earth. Credit: NASA.
This is the appearance of the Sun in visual radiation centered in the yellow-green. Credit: Jim E. Brau, Pearson Prentice Hall, Inc..
A visual image is shown in black and white. Credit: .
Sample calibration colors[33]
Class B–V V–R Teff (K)
O5V –0.33 –0.15 42,000
B0V –0.30 –0.13 30,000
A0V –0.02 0.02 9,790
F0V 0.30 0.30 7,300
G0V 0.58 0.50 5,940
K0V 0.81 0.64 5,150
M0V 1.40 1.28 3,840

"[T]he [visual] color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. To measure the [visual] index, one observes the magnitude of an object successively through two different filters, such as ... B and V, where ... B is sensitive to blue light, and V is sensitive to visible (green-yellow) light ... The difference in magnitudes found with these filters is called the ... B–V color index ... The smaller the color index, the more blue (or hotter) the object is. Conversely, the larger the color index, the more red (or cooler) the object is. ... For comparison, the yellowish Sun has a B–V index of 0.656 ± 0.005,[35] while the bluish Rigel has a B–V of –0.03 (its B magnitude is 0.09 and its V magnitude is 0.12, B–V = –0.03).[36]"[34]

## Violets

The Sun in purple or violet is shown. Credit: .

The Sun's photosphere emits over the violet (380-450 nm) band.

As an astronomical object sets or rises in relation to the horizon, the light it emits travels through Earth's atmosphere, which works as a prism separating the light into different colors. The color of the upper rim of an astronomical object could go from green to blue to violet depending on the decrease in concentration of pollutants, as they spread throughout an increasing volume of atmosphere.[37]

## Blues

This an image of the Sun demonstrating blue emission. Credit: A. Friedman.

The surface of the Sun emits blue (450-475 nm) electromagnetic radiation.

"Very occasionally, the amount of blue light is sufficient to be visible as a "blue flash".[38]"[39]

## Cyans

The surface of the Sun emits cyan (476-495 nm) electromagnetic radiation.

## Greens

The upper rim is green while the lower one is red, as the Sun sets behind the Golden Gate Bridge. Credit: Brocken Inaglory.
The green rim and flashes of a setting Sun are imaged. Credit: Brocken Inaglory.

The Na I green lines at 568.2 and 568.8 nm arise "in the photospheric layers between log τ5000 ≈ -1 and -2."[40]

"As an astronomical object sets or rises in relation to the horizon, the light it emits travels through Earth's atmosphere, which works as a prism separating the light into different colors. The color of the upper rim of an astronomical object could go from green to blue to violet depending on the decrease in concentration of pollutants, as they spread throughout an increasing volume of atmosphere.[37] The lower rim of an astronomical object is always red.
A green rim is very thin, and is difficult or impossible to see with the naked eye. In usual conditions, a green rim of an astronomical object gets fainter, when an astronomical object is very low above the horizon because of atmospheric reddening,[41] but sometimes the conditions are right to see a green rim just above the horizon."[39]

## Yellows

The Sun often appears yellow. Credit: NASA.

Seeing the yellow Sun and feeling the warmth of its rays is probably a student's first encounter with an astronomical yellow radiation source.

The surface of the Sun emits yellow (570-590 nm) radiation.

"Stars of spectral classes F and G, such as our sun Sol, have color temperatures that make them look "yellowish".[42]

Depending primarily upon gas temperature, the presence of gas may be used to determine the composition of the gas body observed, at least the outer layer. Early spectroscopy[21] 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."[43]

## Oranges

The image shows a sunrise in Kodachadri. Credit: Chinmayahd.
Atmospheric effects may produce an orange Sun. Credit: Sarahr.
This is an orange colorized intensity gram of the Sun. Credit: NASA, Solar Dynamics Observatory.

The surface of the Sun emits in the orange (590 to 620 nm) wavelengths.

The Na I lines at 615.4 and 616.0 nm arise "in the photospheric layers between log τ5000 ≈ -1 and -2."[40]

But truly, the now famous sun set at the peak where you will witness the panorama of rolling green hills and also a glimpse of Arabian sea and the bright orange sun going down the hills might force one to contemplate on the nature and its complex beauty. And to witness the same sun rise above the foggy and misty hills and over a blanket of silver clouds the next early morning would be the perfect way to start a day.

## Reds

The Sun is observed through a telescope with an H-alpha filter. Credit: Marshall Space Flight Center, NASA.
The sun with Venus in transit, from Mount Wilson Observatory, June 5, 2012. Credit: Alan Friedman, Mount Wilson Observatory.

The surface of the Sun emits in the red (621 to 750 nm) wavelengths.

"[S]ome observed properties of the Sun still defy explanation, such as the degree of Li depletion" [the "solar Li abundance is roughly a factor of 200 below the meteoritic abundance"].[44]

The second image down on the right is an image of the Sun during a transit of Venus at a wavelength close 620 nm.

## Infrareds

This graph depicts the expected intensity of infrared emission versus wavelength at various radial distances from the Sun. Credit: NASA (Dr. Paulett Liewer).
The surface of the Sun emits infrared. Credit: .
This is an image of the Sun in infrared. Credit: Jim E. Brau, Pearson Prentice Hall, Inc..

The surface of the Sun emits in the infrared.

At right is a graph of the expected solar and interstellar infrared emission versus wavelength at various radial distances from the Sun.

Both images of the Sun are in the infrared.

The Na I lines at 818.3, 819.4, and 1140 nm in the near infrared arise "in the photospheric layers between log τ5000 ≈ -1 and -2."[40]

## Submillimeters

An "intense solar flare spectral radiation component, peaking somewhere in the shorter submillimeter to far-infrared range, [is] identified during the 2003 November 4 large flare. The new solar submillimeter telescope, designed to extend the frequency range to above 100 GHz, detected this new component with increasing fluxes between 212 and 405 GHz."[45]

## Microwaves

This is the Sun at 5 GHz. Credit: S.G. Djorgovski et al.

The surface of the Sun emits radio waves, including microwaves.

The quiet Sun at 4.6 GHz is imaged by the VLA with a resolution of 12 arcsec, or about 8400 km on the surface of the Sun, in the image on the right.

This is an image of the Sun in radio waves. Credit: Jim E. Brau, Pearson Prentice Hall, Inc.

On the right is an image of the Sun in radio waves.

## Heliognosy

This graph shows the temperature and density of the Sun's atmosphere from Skylab observations. Credit: John A. Eddy, NASA.

While it seldom seems that listening to an entity or object in the sky is beneficial, application of the science of acoustics to studying the Sun in optical astronomy provides insight into the Sun's interior.

The image at right describes graphically the temperature and density of the Sun's atmosphere from the photosphere upwards. "The Sun's photosphere has a temperature between 4500 and 6000 K[46] (with an effective temperature of 5777 K) and a density of about [2 x 10-4kg/m3;[47] other stars may have hotter or cooler photospheres. The Sun's photosphere is composed of convection cells called granules—cells of gas each approximately 1000 km in diameter[48] with hot rising gas in the center and cooler gas falling in the narrow spaces between them. Each granule has a lifespan of only about eight minutes, resulting in a continually shifting "boiling" pattern. Grouping the typical granules are super granules up to 30,000 kilometers in diameter with lifespans of up to 24 hours. These details are too fine to see on other stars."[49]

## Heliogony

This is an artist’s impression of a baby star still surrounded by a protoplanetary disc in which planets are forming. Credit: ESO/L. Calçada.

"Cosmogony is a term which admits some ambiguity or, at least, flexibility. One dictionary definition states 'a theory of the creation of the Universe' while another, more parochially, says 'a theory of the world's origin and growth'. A large collection of definitions fall between these two extremes but the most common feature they possess is the phrase 'origin of the solar system'."[50]

"The German philosopher Kant was the first to conceive the idea that the Sun originated as a condensation from a nebula, and the same idea in a more elaborate and refined form was later put forward by Laplace (1796)."[50]

An "initially spherical and contracting nebula would spin faster as it collapsed. At a certain critical stage it would have a lenticular shape with equatorial material in orbit around a central mass. Any further contraction would leave a disk of freely rotating material in the equatorial plane. In fact Laplace postulated that the evolution of the disk would be discontinuous and that the material would be shed in a series of discrete rings. Condensation in the rings would gradually merge with each other so that each ring would be the originator of a single planet."[50]

The apparent "outstanding failure of this early nebula theory was in explaining the distribution of angular momentum in the solar system. The planets, with 0.13 per cent of the mass of the system, account for about 99.5 per cent of its angular momentum and no spontaneous way of so partitioning mass and angular momentum seems possible."[50]

## Heliography

This is a rotating projection of the entire surface of the Sun on February 10, 2011, as seen by the twin STEREO satellites. Credit: NASA STEREO mission.

As geography describes the features of the surface of the Earth, heliography describes the surface features of Helios or the Sun, Sol.

## Heliology

This is an image of the Sun taken from the surface of the Earth through a camera lens. Credit: Lykaestria.

Def. the "scientific study of the Sun"[51] is called heliology.

Def. the star that the Earth revolves around and from which it receives light and warmth.[52] is called the sun.

## Heliometry

Solar irradiance spectrum is diagrammed above atmosphere and at the Earth's surface. Credit: Robert A. Rohde.
One composite is graphed of the last 30 years of solar variability. Credit: Robert A. Rohde.

"Direct irradiance measurements have only been available during the last three cycles and are based on a composite of many different observing satellites.[53] [54] However, the correlation between irradiance measurements and other proxies of solar activity make it reasonable to estimate past solar activity. Most important among these proxies is the record of sunspot observations that has been recorded since ~1610. Since sunspots and associated faculae are directly responsible for small changes in the brightness of the sun, they are closely correlated to changes in solar output. Direct measurements of radio emissions from the Sun at 10.7 cm also provide a proxy of solar activity that can be measured from the ground since the Earth's atmosphere is transparent at this wavelength. Lastly, solar flares are a type of solar activity that can impact human life on Earth by affecting electrical systems, especially satellites. Flares usually occur in the presence of sunspots, and hence the two are correlated, but flares themselves make only tiny perturbations of the solar luminosity."[55]

## Helionomy

Use of Stonyhurst disk is shown to determine the heliographic coordinates of sunspots. Credit: Cortie, A. L. 1908.

The surface of the Sun is often described by features observed. These are located using heliographic coordinates based on heliographic north and south poles. The surface of the Sun rotates, has a rotational north and south pole, and there is a central meridian.

The Stonyhurst Disk is superimposed on an image of the Sun to determine the heliographic coordinates. These are constructed by analogy with the geographical and are characterized by two values, latitude (Φ) and longitude (λ).[56] Latitude is measured from the plane of the solar equator. The first longitude (λ1) is measured from the plane of the "central meridian" as it passes through the rotation axis of the Sun and the line connecting the center of the Sun to the observer.[56] The Carrington longitude (λ2) is measured from the central meridian as it passes through the ascending node of the solar equator at Greenwich noon on January 1, 1854 (JD 2398220.0) and rotating with the sidereal period of 25.38 Earth days.[56]

The two longitudes are associated approximately by the ratio

${\displaystyle \lambda _{2}=\left\{{\frac {\lambda _{1}}{360^{\circ }}}-{\frac {{\rm {JD}}-2398220}{27.2753}}\right\}\cdot 360^{\circ }}$.[56]

The Sun has an equatorial radius of 695,500 km[57]

## Heliophysics

On July 21, 2012, the magnetometer instrument indicated that Voyager 1 had entered a region where the wind is from the southern hemisphere. Credit: NASA/JPL-Caltech.

"Heliophysics is a fast-developing scientific discipline that integrates studies of the Sun's variability, the surrounding heliosphere, and the environment and climate of the planets."[58]

## Helioseismology

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).

"[D]ifferent parts of the Sun ... rotate at different rates".[59]

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.[60] 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.[61]

## Classical planets

"In antiquity the classical planets were the non-fixed objects visible in the sky, known to various ancient cultures. The classical planets were therefore the Sun and Moon and the five non-earth planets of our solar system closest to the sun (and closest to the Earth); all easily visible without a telescope. They are Mercury, Venus, Mars, Jupiter, and Saturn."[62]

## Solar System

Def. the "Sun and all the heavenly bodies that orbit around it, including the eight planets, their moons, the asteroids and comets"[63] is called the Solar System.

Def. any "collection of heavenly bodies including a star or binary star, and any lighter stars, brown dwarfs, planets, and other objects in orbit"[64] is called a solar system.

Usage notes

• "As Sol is the name of our star, this phrase is usually used to refer specifically to our own sun and planets (the Sol system), in which case it is used with the and generally capitalised (as the Solar system or the Solar System). Other systems are then known as star systems or planetary systems, or specified by the name of the individual star (the Alpha Centauri system)."[64]

Planets "of the Solar System [are] Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune".[64]

Dwarf "planets of the Solar System [are] Ceres, Orcus, Pluto, Salacia, Varuna, Haumea, Quaoar, Makemake, 2007 OR10, Eris, Sedna".[64]

## Comets

The Sun emits visual radiation that may reflect off a comet's tail. The coronal cloud in close proximity to the Sun also emits X-rays that produce visual fluorescence from gases in a comet's coma and tail.

## Heliospheres

This artist's concept shows Voyager going interstellar. Credit: NASA/JPL.

"The heliosphere is a bubble in space "blown" into the interstellar medium (the hydrogen and helium gas that permeates the galaxy) by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself."[65]

## Heliopauses

"[T]he point where the interstellar medium and solar wind pressures balance is called the heliopause".[65]

"[T]he point where the interstellar medium, traveling in the opposite direction, slows down as it collides with the heliosphere is the bow shock".[65]

"The outer edge of the solar system is the boundary between the flow of the solar wind and the interstellar medium. This boundary is known as the heliopause and is believed to be a fairly sharp transition of the order of 110 to 160 astronomical units from the sun. The interplanetary medium thus fills the roughly spherical volume contained within the heliopause."[66]

## Kuiper belts

Known objects in the Kuiper belt, are derived from data from the Minor Planet Center. Credit: .

In the image at right, objects in the main part of the Kuiper belt are coloured green, while scattered objects are coloured orange. The four outer planets are blue. Neptune's few known trojans are yellow, while Jupiter's are pink. The scattered objects between Jupiter's orbit and the Kuiper belt are known as centaurs. The scale is in astronomical units. The pronounced gap at the bottom is due to difficulties in detection against the background of the plane of the Milky Way.

"The Kuiper belt ... is a region of the [solar system] ... extending from the orbit of Neptune (at 30 AU to approximately 60 AU from the Sun.[67] ... [I]t consists mainly of small bodies"[68]

## Oort clouds

Here, the presumed distance of the Oort cloud is compared to the rest of the Solar System using the orbit of Sedna. Credit: NASA / JPL-Caltech / R. Hurt.
Sedna, a possible inner Oort cloud object, is a discovery in 2003. Credit: .

The Oort cloud or the Öpik–Oort cloud[69] is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Stars/Sun.[70] This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the region of the Sun's gravitational dominance.[71]

## Scattered discs

The dwarf planet Eris is the largest known scattered-disc object (center), with its moon Dysnomia (left of object). Credit: .

The scattered disc (or scattered disk) is a distant region of the solar system that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects (SDOs) have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units (4.5 x 109 km; 2.8 x 109 mi.). ... While the nearest distance to the Sun approached by scattered objects is about 30–35 AU, their orbits can extend well beyond 100 AU. This makes scattered objects "among the most distant and cold objects in the Solar System".[72]

## Solar nebulas

File:Solarnebula.jpg
Shock waves through icy parts of the solar nebula may be the mechanism that enriched ancient meteorites. Credit: William K. Hartmann, Planetary Science Institute, Tucson, Arizona.

Def. a "disc-shaped cloud of gas and dust left over from the formation of the Sun"[73] is called a solar nebula.

In the artist's impression on the right: "Shock waves through icy parts of the solar nebula may be the mechanism that enriched ancient meteorites (called chondrites) with water -- water that some believe provided an otherwise dry Earth with oceans."[74]

## Solar binary

The Sun-Jupiter binary may serve to establish an upper limit for interstellar cometary capture when three bodies are extremely unequal in mass, such as the Sun, Jupiter, and a third body (potential comet) at a large distance from the binary.[75] The basic problem with a capture scenario even from passage through “a cloud of some 10 million years, or from a medium enveloping the solar system, is the low relative velocity [~0.5 km s-1] required between the solar system and the cometary medium.”[76] The capture of interstellar comets by Saturn, Uranus, and Neptune together cause about as many captures as Jupiter alone.[76]

## Archeoastronomy

The rising Sun illuminates the inner chamber of Newgrange, Ireland, only at the winter solstice. Credit: Richard Gallagher.

At the Newgrange observation post shown in the image on the right, the rising Sun illuminates the inner chamber of Newgrange, Ireland, only at the winter solstice.

"Archaeoastronomy (also spelled archeoastronomy) is the study of how people in the past "have understood the phenomena in the sky how they used phenomena in the sky and what role the sky played in their cultures."[77]

## Sun-synchronous orbital rocketry

Diagram shows the orientation of a Sun-synchronous orbit (green) in four points of the year. A non-sun-synchronous orbit (magenta) is also shown for reference. Credit: Brandir.
The photograph shows a full-size model of ERS-2. Credit:Poppy.
The ERS-2 is carried into a sun-synchronous polar orbit by an Ariane 4 similar to the one imaged. Credit: NASA.

A Sun-synchronous orbit (sometimes called a heliosynchronous orbit[78]) is a geocentric orbit which combines altitude and inclination in such a way that an object on that orbit ascends or descends over any given Earth latitude at the same local mean solar time. The surface illumination angle will be nearly the same every time. This consistent lighting is a useful characteristic for satellites that image the Earth's surface in visible or infrared wavelengths (e.g. weather and spy satellites) and for other remote sensing satellites (e.g. those carrying ocean and atmospheric remote sensing instruments that require sunlight). For example, a satellite in sun-synchronous orbit might ascend across the equator twelve times a day each time at approximately 15:00 mean local time. This is achieved by having the osculating orbital plane precess (rotate) approximately one degree each day with respect to the celestial sphere, eastward, to keep pace with the Earth's movement around the Sun.[79]

The uniformity of Sun angle is achieved by tuning the inclination to the altitude of the orbit ... such that the extra mass near the equator causes the orbital plane of the spacecraft to precess with the desired rate: the plane of the orbit is not fixed in space relative to the distant stars, but rotates slowly about the Earth's axis. Typical sun-synchronous orbits are about 600–800 km in altitude, with periods in the 96–100 minute range, and inclinations of around 98° (i.e. slightly retrograde compared to the direction of Earth's rotation: 0° represents an equatorial orbit and 90° represents a polar orbit).[79]

European remote sensing satellite (ERS) was the European Space Agency's first Earth-observing satellite. It was launched on July 17, 1991 into a Sun-synchronous polar orbit at a height of 782–785 km.

ERS-1 carried an array of earth-observation instruments that gathered information about the Earth (land, water, ice and atmosphere) using a variety of measurement principles. These included:

• ATSR-1 (Along-Track Scanning Radiometer) is a 4 channel infrared radiometer and microwave sounder for measuring temperatures at the sea-surface and the top of clouds.
• SAR (synthetic aperture radar) operating in C band can detect changes in surface heights with sub-millimeter precision.
• Wind Scatterometer used to calculate information on wind speed and direction.
• MWR is a Microwave Radiometer used in measuring atmospheric water, as well as providing a correction for the atmospheric water for the altimeter.

To accurately determine its orbit, the satellite included a Laser Retroreflector. The Retroreflector was used for calibrating the Radar Altimeter to within 10 cm.

Its successor, ERS-2, was launched on April 21, 1995, on an Ariane 4, from ESA's Guiana Space Centre near Kourou, French Guiana. Largely identical to ERS-1, it added additional instruments and included improvements to existing instruments including:

• GOME (Global Ozone Monitoring Experiment) is a nadir scanning ultraviolet and visible spectrometer.
• ATSR-2 included 3 visible spectrum bands specialized for Chlorophyll and Vegetation

## Heliocentric rocketry

The image shows the Spitzer Space Telescope prior to launch. Credit: NASA/JPL/Caltech.
NASA's Space Infrared Telescope Facility (SIRTF, now Spitzer) lifts off from Launch Pad 17-B, Cape Canaveral Air Force Station, aboard a Delta rocket, on August 25, 2003 at 1:35:39 a.m. EDT. Credit: NASA.
Ulysses is photographed after deployment from STS-41. Credit: NASA.
Ulysses' second orbit (1999–2004) included a swing-by Jupiter. Credit: NASA.
A technician stands next to one of the twin Helios spacecraft during testing. Credit: NASA/Max Planck.
Shown is Helios 1 sitting atop the Titan IIIE / Centaur launch vehicle. Credit: NASA.
Trajectory of the Helio space probes is diagrammed. Credit: NASA.

The Spitzer Space Telescope (SST), formerly the Space Infrared Telescope Facility (SIRTF) is an infrared space observatory launched ... from Cape Canaveral Air Force Station, on a Delta II 7920H ELV rocket, Monday, 25 August 2003 at 13:35:39 UTC-5 (EDT).[80]

Cryogenic satellites that require liquid helium (LHe, T ≈ 4 K) temperatures in near-Earth orbit are typically exposed to a large heat load from the Earth, and consequently entail large usage of LHe coolant, which then tends to dominate the total payload mass and limits mission life. Placing the satellite in solar orbit far from Earth allowed innovative passive cooling such as the sun shield, against the single remaining major heat source to drastically reduce the total mass of helium needed, resulting in an overall smaller lighter payload, with major cost savings. This orbit also simplifies telescope pointing, but does require the Deep Space Network for communications.

Ulysses is a "robotic space probe ... designed to study the Sun as a joint venture of NASA and the European Space Agency (ESA)."[81] To obtain an Out-Of-The-Ecliptic (OOE) heliocentric orbit Ulysses swung by Jupiter. Between 1994 and 1995 it explored both the southern (June - October 1994) and northern (June - September 1995) solar polar regions. "Between 2000 and 2001 it explored the southern solar polar regions, which gave many unexpected results. In particular the southern magnetic pole was found to be much more dynamic than the north pole and without any fixed clear location."[81] It operates over the Sun's poles for the third and last time in 2007 and 2008. "After it became clear that the power output from the spacecraft's RTG would be insufficient to operate science instruments and keep the attitude control fuel, hydrazine, from freezing, instrument power sharing was initiated. Up until then, the most important instruments had been kept online constantly, whilst others were deactivated. When the probe neared the Sun, its power-hungry heaters were turned off and all instruments were turned on.[82]"[81]

"Helios 1 and Helios 2 ... are a pair of probes launched into heliocentric orbit for the purpose of studying solar processes. ... The probes are notable for having set a maximum speed record among spacecraft at 252,792 km/h[83] (157,078 mi/h or 43.63 mi/s or 70.22 km/s or 0.000234c). Helios 2 flew three million kilometers closer to the Sun than Helios 1, achieving perihelion on 17 April 1976 at a record distance of 0.29 AU (or 43.432 million kilometers),[84] slightly inside the orbit of Mercury. Helios 2 was sent into orbit 13 months after the launch of Helios 1. ... The probes are no longer functional but still remain in their elliptical orbit around the Sun." On board, each probe carried an instrument for cosmic radiation investigation (the CRI) for measuring protons, electrons, and X-rays "to determine the distribution of cosmic rays."[85]

## Earth-trailing astronomy

Spitzer's Earth-trailing solar orbit (ETSO) for a 62-month mission lifetime. Credit: Premkumar R. Menon, JPL/NASA.

"An Earth Trailing Solar Orbit (ETSO)" causes Spitzer "to drift from Earth at a rate of about 0.1 AU per year."[86]

The figure at right shows the Earth-trailing solar orbit (ETSO) for Spitzer with the Earth at the origin and the Sun at left in the rotating coordinate frame "for an 8/25/03 launch projected onto the Ecliptic plane during the 62-month mission lifetime".[87]

This is a photograph of one of the two STEREO spacecraft. Credit: NASA.
The images show the two orbital paths of the STEREO A and B spacecraft. Credit: NASA.

"To obtain their unique stereo view of the sun, the two observatories must be placed in different orbits, where they are offset from each other and Earth. Spacecraft "A" will be in an orbit moving ahead of Earth, and "B" will lag behind, as the planet orbits the sun."[88]

"STEREO [one of the two satellites imaged on the left] is the first NASA mission to use separate lunar swingbys to place two observatories into vastly different orbits around the sun. The observatories will fly in an orbit from a point close to Earth to one that extends just beyond the moon."[88]

"Approximately two months after launch, mission operations personnel at the Johns Hopkins University Applied Physics Laboratory, Laurel, Md., will use a close flyby of the moon to modify the orbits. The moon's gravity will be used to direct one observatory to its position trailing Earth. Approximately one month later, the second observatory will be redirected after another lunar swingby to its position ahead of Earth. These maneuvers will enable the spacecraft to take permanent orbits around the sun."[88]

## Earth-Sun Lagrange astronomy

The WIND spacecraft spent "several months at the L1 Langrangian point--the point where the gravitational and centrifugal pull of the Sun and Earth cancel each other".[89]

"SOHO [Solar and Heliospheric Observatory] moves around the Sun in step with the Earth, by slowly orbiting around the First Lagrangian Point (L1), where the combined gravity of the Earth and Sun keep SOHO in an orbit locked to the Earth-Sun line. The L1 point is approximately 1.5 million kilometres away from Earth (about four times the distance of the Moon), in the direction of the Sun. There, SOHO enjoys an uninterrupted view of our daylight star."[90]

## Satellites

This diagram shows the mounting of PAMELA on the Resurs-DK1 satellite. Credit: -=HyPeRzOnD=- as modified by Aldebaran66.

The Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) "is an operational cosmic ray research module attached to"[91] the Resurs-DK1 commercial Earth observation satellite. PAMELA "is the first satellite-based experiment dedicated to the detection of cosmic rays, with a particular focus on their antimatter component, in the form of positrons and antiprotons. Other objectives include long-term monitoring of the solar modulation of cosmic rays, measurements of energetic particles from the Sun, high-energy particles in Earth's magnetosphere and Jovian electrons."[91]

"The instrument is built around a permanent magnet spectrometer with a silicon microstrip tracker that provides rigidity and dE/dx information. At its bottom is a silicon-tungsten imaging calorimeter, a neutron detector and a shower tail scintillator to perform lepton/hadron discrimination. A Time of Flight (ToF), made of three layers of plastic scintillators, is used to measure the beta and charge of the particle. An anticounter system made of scintillators surrounding the apparatus is used to reject false triggers and albedo particles during off-line analysis.[92]"[91]

## Clocks

This is a sundial from Ai Khanoum, Afghanistan. Credit: Musee Guimet, World Imaging.
This image shows another sundial from Ai Khanoum, Afghanistan. Credit: Musee Guimet, World Imaging.

The image at right shows a sun dial from Ai Khanoum, Afghanistan, dated to the 3rd century BCE, ~2300 b2k. The image at left is also from Ai Khanoum, Afghanistan, showing its workings.

"[T]he earliest known sundial [is] from an Egyptian burial dated in the fifteenth century B.C. Sometimes called a shadow clock, or an L-board because of its shape [with] relatively crude performance."[93] A "[f]ragment of a late Egyptian sundial [from] about 3000 B.C." exists.[93]

## Hypotheses

1. Solar astronomy includes the various radiation astronomies focused on the Sun.
2. Solar astronomy includes the effects on the Sun of revolving around the Milky Way.

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