Stars/Surface fusion

RHESSI observes high-energy phenomena from a solar flare. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio.

Stellar surface fusion occurs above a star's photosphere to a limited extent as found in studies of near coronal cloud activity.

Surface fusion is produced by reactions during or preceding a stellar flare and at much lower levels elsewhere above the photosphere of a star.

"Nuclear interactions of ions accelerated at the surface of flaring stars can produce fresh isotopes in stellar atmospheres."

Radiation

Main resource: Radiation

"This energy [10³² to 10³³ ergs] appears in the form of electromagnetic radiation over the entire spectrum from γ-rays to radio burst, in fast electrons and nuclei up to relativistic energies, in the creation of a hot coronal cloud, and in large-scale mass motions including the ejections of material from the Sun."

Particle accelerators

Main resources: Laboratories/Particle accelerators and Particle accelerators

"The new reaction ²⁰⁸Pb(⁵⁹Co,n)²⁶⁶Mt was studied using the Berkeley Gas-filled Separator [BGS] at the Lawrence Berkeley National Laboratory [LBNL] 88-Inch Cyclotron."

²⁶⁶Mt has been produced using the ²⁰⁹Bi(⁵⁸Fe,n)²⁶⁶Mt reaction.

"Reactions with various medium-mass projectiles on nearly spherical, shell-stabilized ²⁰⁸Pb or ²⁰⁹Bi targets have been used in the investigations of transactinide (TAN) elements and their decay properties for many years. These so-called “cold fusion” reactions produce weakly excited (10-15 MeV) [1] compound nuclei (CNs) at bombarding energies at or near the Coulomb barrier that de-excite by the emission of one to two neutrons."

"The laboratory-frame, center-of-target energy used was 291.5 MeV, corresponding to a CN excitation energy of 14.9 MeV."

"At the start of the experiment the BGS magnet settings were chosen to guide products with a magnetic rigidity of 2.143 T·m to the center of the [focal plane detector] FPD. After the first event of ²⁶⁶Mt was detected in strip 45 (near one edge of the FPD), the magnetic field strength was decreased to 2.098 T·m in an effort to shift the distribution of products toward the center of the detector."

"²⁵⁸Db [has been produced] via the ²⁰⁹Bi(⁵⁰Ti,n) and ²⁰⁸Pb(⁵¹V,n) reactions [15], and ²⁶²Bh via the ²⁰⁹Bi(⁵⁴Cr,n) and ²⁰⁸Pb(⁵⁵Mn,n) reactions [13, 16]."

"Hofmann et al. at Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany, and Morita et al., at the Institute of Physical and Chemical Research (RIKEN) in Saitama, Japan, have studied the ²⁰⁹Bi(⁶⁴Ni,n)²⁷²Rg reaction [7, 17, 18]. The complementary ²⁰⁸Pb(⁶⁵Cu,n)²⁷²Rg reaction was studied by Folden et al. at the Lawrence Berkeley National Laboratory (LBNL) [19]."

"Based on the observation of the long-lived isotopes of roentgenium, ²⁶¹Rg and ²⁶⁵Rg (Z = 111, t_1/2 ≥ 10⁸ y) in natural Au, an experiment was performed to enrich Rg in 99.999% Au. 16 mg of Au were heated in vacuum for two weeks at a temperature of 1127°C (63°C above the melting point of Au). The content of ¹⁹⁷Au and ²⁶¹Rg in the residue was studied with high resolution inductively coupled plasma-sector field mass spectrometry (ICP-SFMS). The residue of Au was 3 × 10⁻⁶ of its original quantity. The recovery of Rg was a few percent. The abundance of Rg compared to Au in the enriched solution was about 2 × 10⁻⁶, which is a three to four orders of magnitude enrichment."

Theoretical stellar surface fusion

Def. a "nuclear reaction in which nuclei combine to form more massive nuclei with the concomitant release of energy" is called fusion, or nuclear fusion.

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

Here's a theoretical definition:

Def. nuclear reactions occurring at or above a photosphere in which nuclei combine to form more massive nuclei with the concomitant release of energy is called stellar surface fusion.

Accretions

Main resources: Stars/Dwarfs/Whites/Accrretions and Accretions

If a white dwarf has a close companion star that overflows its Roche lobe, the white dwarf will steadily accrete gas from the companion's outer atmosphere. The companion may be a main sequence star, or one that is aging and expanding into a red giant. The captured gases consist primarily of hydrogen and helium, the two principal constituents of ordinary matter in the universe. The gases are compacted on the white dwarf's surface by its intense gravity, compressed and heated to very high temperatures as additional material is drawn in. The white dwarf consists of degenerate matter, and so does not inflate at increased heat, while the accreted hydrogen is compressed upon the surface. The dependence of the hydrogen fusion rate on temperature and pressure means that it is only when it is compressed and heated at the surface of the white dwarf to a temperature of some 20 million kelvin that a nuclear fusion reaction occurs; at these temperatures, hydrogen burns via the CNO cycle.

"While hydrogen fusion can occur in a stable manner on the surface of the white dwarf for a narrow range of accretion rates, for most binary system parameters the hydrogen burning is thermally unstable and rapidly converts a large amount of the hydrogen into other heavier elements in a runaway reaction, liberating an enormous amount of energy, blowing the remaining gases away from the white dwarf's surface and producing an extremely bright outburst of light. The rise to peak brightness can be very rapid or gradual which is related to the speed class of the nova; after the peak, the brightness declines steadily. The time taken for a nova to decay by 2 or 3 magnitudes from maximum optical brightness is used to classify a nova via its speed class. A fast nova will typically take less than 25 days to decay by 2 magnitudes and a slow nova will take over 80 days.

"An accreting white dwarf undergoes [near surface] nuclear burning when the accretion rate exceeds a certain limit." Due to the near surface nuclear burning, "the stellar luminosity is dominated by hydrogen burning, since the energy liberated by hydrogen burning exceeds that due to accretion on a white dwarf by an order of magnitude or more, depending on the mass of the white dwarf."

"[A]bove an accretion rate (with a hydrogen abundance of 0.7 by mass) M_RG ≈ 8.5 10^-7 (M_WD/M_ʘ -0.52)M_ʘ yr^-1 (M_WD=mass of the white dwarf) the accreted matter forms a red-giant like envelope around the white dwarf, with the luminosity being generated from hydrogen shell burning."

Flares

Main resources: Plasmas/Plasma objects/Flares and Flares

A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares. The brightness increase is across the spectrum, from X rays to radio waves. The first known flare stars (V1396 Cygni and AT Microscopii) were discovered in 1924. Most flare stars are dim red dwarfs. The more massive RS Canum Venaticorum variables (RS CVn) are also known to flare. Additionally, nine stars similar to the Sun have also been seen to undergo flare events. Flare stars are intrinsically faint, but have been found to distances of 1,000 light years from Earth.

"All parameters seem to have broad or unimodal distributions, suggesting that flares and CMEs form a continuum with the same underlying physics."

Nova-like activities

Main resources: Stars/Nova-likes, Nova-like stars, and Nova-like activities

"The term cataclysmic variable [...] comprises several related types of objects. For one there are the so-called novae, objects whose brightness has changed by ten to twenty magnitudes once in historical times, or recurrent novae whose amplitudes are on the small side but which have been seen to erupt more often than once; furthermore, dwarf novae whose brightness keeps changing by three to five magnitudes in semi-periodic intervals of time of some ten to one hundred days; and finally nova-like stars, which do not undergo outbursts but only irregular small-scale brightness changes or occasional drops in luminosity, but which in all other aspects are similar to the former group."

In "dwarf novae and nova-like stars the binary system itself is visible, [with] processes which can be traced back directly to the presence of an accretion disk in these systems."

The "primary component of which is a white dwarf."

"The secondary components of cataclysmic variables are cool main sequence stars of spectral type approximately solar of later. Such stars are known to possess fairly active surfaces having large star spots associated with appreciable magnetic activity. Even in single stars the physical structure of such an atmosphere is not well understood, and a consistent theory is still to be developed."

"There exist two sub-classes of nova-like stars, the DQ Herculis stars and the AM Herculis stars, whose white dwarfs possess magnetic fields of appreciable strength which dominate the accretion disk and basically all phenomena related to it."

A nova-like star is a close binary system with a white dwarf as a primary and a "late-type main-sequence secondary" star filling its Roche lobe. "The secondary loses mass through the inner Lagrangian point and in order to conserve angular momentum the transferred material usually forms an accretion disk around the white dwarf component. A hot spot originates at the place where the mass-transfer stream impacts the disk." In these star systems the degree of magnetic fields ranges from non-magnetic to highly magnetic. "For systems in which the primaries have strong magnetic fields, the process of forming the accretion disk is disturbed. The transferred material is forced to follow the field lines and creates accretion columns near one or both of the white dwarfs magnetic poles."

"The shortest orbital periods imply typical dimensions for the systems to be of the order of a solar diameter."

"Spectroscopic investigations (Walker and Herbig (1954)) [...] were of sufficiently good quality for the determination of radial velocities [and included] photoelectric measurements [for UX UMa.] But in 1954 the orbital light curve of the old nova DQ Her was solved, and its similarity to that of UX UMa became obvious; its orbital period is shorter by only 5 minutes, and the shape of the eclipse and the hump near to it make the two so similar to one another that one can easily be mistaken for the other (Walker 1954 and 1956)."

As far as long-term brightness changes are concerned, magnetic and non-magnetic nova-like stars behave in the same way. That the central star in some systems has magnetic properties has nothing to do with the outburst behavior. But, in the AM Herculis stars, the magnetic field of the white dwarf prevents the formation of an accretion disk.

A bright accretion disk forms in non-magnetic nova-like stars. Matter swirling along field lines releases energy in magnetic systems.

The evolution of non-magnetic dwarf novae and nova-like stars can be different from the magnetic systems (polars and intermediate polars). Magnetic and non-magnetic systems display different kinematical properties since some flow velocities come from magnetically channeled plasma.

Non-magnetic systems appear to be much more prevalent than magnetic ones, although the number of magnetic systems is small and near the limit of statistical significance when compared to the non-magnetic systems.

Dwarf novas

Main resources: Stars/Novas/Dwarfs and Dwarf novas

"The first known detection of a dwarf nova [U Geminorum] was recorded by Hind (1856), who describes how on 1855 December 15 he discovered a ninth-magnitude star in a field which he knew well and which he had been monitoring for 5 (!) years."

"Another dwarf nova (we now call it SS Cygni) was detected in 1886; by 1918 the number had increased to eight (Müller and Hartwig, 1918)".

"Currently we know of some 200 dwarf novae and of several hundred nova-like stars and novae."

"Joy pointed out (Joy 1954b) that the spectra of the dwarf novae SS Cyg and RU Peg were rather similar to those of AE Agr and that a physical relationship seemed possible. [In] intervals of about one year AE Aqr underwent outburst-like brightness increases, by one to two magnitudes (Zinner, 1938), that resemble dwarf nova outbursts [...] the explosive U Geminorum requires [...] two stars in a short-period orbit as a necessary, though not sufficient, condition."

EY Cyg is a dwarf nova.

"The most spectacular events in the lives of dwarf novae are the outbursts."

"Instabilities on the surface of the white dwarf lead to nova eruptions."

Dwarf novae are distinct from classical novae in other ways; their luminosity is lower, and they are typically recurrent on a scale from days to decades.

The luminosity of the outburst increases with the recurrence interval as well as the orbital period.

Recurrent novas

Main resources: Stars/Novas/Recurrents and Recurrent novas

A recurrent nova is produced by a white dwarf star and a red giant circling about each other in a close orbit. About every 20 years, enough material from the red giant builds up on the surface of the white dwarf to produce a thermonuclear explosion. The white dwarf orbits close to the red giant, with an accretion disc concentrating the overflowing atmosphere of the red giant onto the white dwarf. If the white dwarf accretes enough mass to reach the Chandrasekhar limit, about 1.4 solar mass, it may explode as a Type Ia supernova.

V1017 Sgr is a recurrent nova.

Supernovas

Main resources: Stars/Supernovas and Supernovas

Def. a star which explodes, increasing its brightness to typically a billion times that of our sun, though attenuated by the great distance from our sun is called a supernova.

From the burst until it fades after some weeks or months a supernova can radiate as much energy as the Sun is expected to emit over its entire life span.

The explosion expels most or all of a star's material at a velocity of up to 30000 km/s (10% of light speed, powering a shock wave into the interstellar medium.

Symbiotic novas

Main resources: Stars/Novas/Symbiots and Symbiotic novas

Symbiotic novae are slow irregular eruptive variable stars with very slow nova-like outbursts with an amplitude of between 9 and 11 magnitudes. The symbiotic nova remains at maximum for one or a few decades, and then declines towards its original luminosity. Variables of this type are double star systems with one red giant, which probably is a mira variable, and one white dwarf, with markedly contrasting spectra and whose proximity and mass characteristics indicate it as a symbiotic star. The red giant fills its Roche lobe so that matter is transferred to the white dwarf and accumulates until a nova-like outburst occurs, caused by ignition of thermonuclear fusion. The temperature at maximum is estimated to rise up to 200,000 K, similar to the energy source of novae, but dissimilar to the dwarf novae. The slow luminosity increase would then be simply due to time needed for growth of the ionization front in the outburst.

It is believed that the white dwarf component of a symbiotic nova remains below the Chandrasekhar limit, so that it remains a white dwarf after its outburst.

One example of a symbiotic nova is V1016 Cygni, whose outburst in 1971–2007 clearly indicated a thermonuclear explosion. Other examples are HM Sagittae and RR Telescopii.

"Though typical symbiotic systems consist of a M giant and a white dwarf companion, systems containing a G or K giant ("yellow symbiotic") are known as well."

Fusion sources

Main resources: Sources/Fusions/Astronomy and Fusion sources

This movie shows the evolution of active region 1520, including coronal loops. Credit: NASA/Goddard Space Flight Center.

This image of coronal loops observed by the Transition Region And Coronal Explorer (TRACE) shows that not all rays travel in straight lines. Credit: NASA.

The image shows the cooling post-flare arcade (rotated by -90 degrees so that north is to the right) 6h after the flare (at 00:11 UT on September 8. Credit: TRACE/NASA.

"This false-color temperature map shows solar active region AR10923, observed close to center of the sun's disk. Blue regions indicate plasma near 10 million degrees K." Credit: Reale, et al. (2009), NASA.

Coronal loops have become very important when trying to understand the current coronal heating problem. Coronal loops are highly radiating sources of plasma and therefore easy to observe by instruments such as TRACE; they are highly observable laboratories to study phenomena such as solar oscillations, wave activity and nanoflares. However, it remains difficult to find a solution to the coronal heating problem as these structures are being observed remotely, where many ambiguities are present (i.e. radiation contributions along the [line-of-sight propagation] LOS). In-situ measurements are required before a definitive answer can be arrived at, but due to the high plasma temperatures in the corona, in-situ measurements are impossible (at least for the time being). The next mission of the Nasa Solar Probe Plus will approach the Sun very closely allowing more direct observations.

"The peak continuum intensity was always at the loop tops."

The population of coronal loops can be directly linked with the solar cycle; it is for this reason coronal loops are often found with sunspots at their footpoints. Coronal loops project through the chromosphere and transition region, extending high into the corona.

Coronal loops have a wide variety of temperatures along their lengths. Loops existing at temperatures below 1 MK are generally known as cool loops, those existing at around 1 MK are known as warm loops, and those beyond 1 MK are known as hot loops. Naturally, these different categories radiate at different wavelengths.

Coronal loops populate both active and quiet regions of the solar surface. Active regions on the solar surface take up small areas but produce the majority of activity and 82% of the total coronal heating energy. The quiet Sun, although less active than active regions, is awash with dynamic processes and transient events (bright points, nanoflares and jets). As a general rule, the quiet Sun exists in regions of closed magnetic structures, and active regions are highly dynamic sources of explosive events. It is important to note that observations suggest the whole corona is massively populated by open and closed magnetic fieldlines. A closed fieldline does not constitute a coronal loop; however, closed flux must be filled with plasma before it can be called a coronal loop.

The image at right shows particle rays leaving the surface of the Sun (darker ends of the loops), traveling in a loop controlled by a local magnetic field similar to how particle accelerators accelerate, steer, and aim a stream of particles at a target (the much brighter regions in the chromosphere). The loops have a temperature of approximately 10⁶ K and are emitting X-rays (synchrotron and cyclotron radiation).

Coronal loops form the basic structure of the lower corona andtransition region of the Sun. These highly structured and elegant loops are a direct consequence of the twisted solar magnetic flux within the solar body. The population of coronal loops can be directly linked with the solar cycle; it is for this reason coronal loops are often found with sunspots at their footpoints. The upwelling magnetic flux pushes through the photosphere, exposing the cooler plasma below.

Loops of magnetic flux (closed flux tubes) well up from the solar body and fill with hot solar plasma. Due to the heightened magnetic activity in these coronal loop regions, coronal loops can often be the precursor to solar flares and coronal mass ejections (CMEs).

"Almost as soon as Active Region 10808 rotated onto the solar disk, it spawned a major X17 flare. TRACE was pointed at the other edge of the Sun at the time, but was repointed 6 hours after the flare started. The image on the left shows the cooling post-flare arcade (rotated by -90 degrees so that north is to the right) 6h after the flare (at 00:11 UT on September 8); the loop tops still glow so brightly that the diffraction pattern repeats them on diagonals away from the brightest spots. Some 18h after the flare, the arcade is still glowing, as seen in the image on the right (at 11:42 UT on September 8). In such big flares, magnetic loops generally light up successively higher in the corona, as can be seen here too: the second image shows loops that are significantly higher than those seen in the first. Note also that the image on the right also contains a much smaller version of the cooling arcade in a small, very bright loop low over the polarity inversion line of the region."

Nearly all of the TRACE images of coronal loops and the transition region indicate that material in these loops and loop-like structures returns to the chromosphere.

"Normally, solar energetic particle (SEP) events associated with disturbances in the eastern hemisphere are characterized by slow onset and lack of high-energy particles. The SEP event associated with the first major flare (X17) [...] is among very few such events over several decades in that although the source region was on the east limb, the particle flux started to rise only a few hours from the flare onset, while the flux of protons with energies in excess of 100 MeV went up by more than a factor of one hundred. We do not understand how these energetic particles can reach the Earth from that side of the Sun, because there should be no magnetic connectivity."

The image fourth at the right shows the first detection of high temperature nanoflares. The false-color temperature map of solar active region AR10923, observed close to center of the sun's disk, contains nanoflare regions (blue, indicating plasma near 10 million degrees K).

"Nanoflares are small, sudden bursts of heat and energy. "They occur within tiny strands that are bundled together to form a magnetic tube called a coronal loop," says Klimchuk. Coronal loops are the fundamental building blocks of the thin, translucent gas known as the sun's corona. ... Observations from the NASA-funded X-Ray Telescope (XRT) and Extreme-ultraviolet Imaging Spectrometer (EIS) instruments aboard Hinode reveal that ultra-hot plasma is widespread in solar active regions. The XRT measured plasma at 10 million degrees K, and the EIS measured plasma at 5 million degrees K. "These temperatures can only be produced by impulsive energy bursts,"says Klimchuk ... "Coronal loops are bundles of unresolved strands that are heated by storms of nanoflares." ... when a nanoflare suddenly releases its energy, the plasma in the low-temperature, low-density strands becomes very hot—around 10 million degrees K—very quickly. The density remains low, however, so the emission, or brightness, remains faint. Heat flows from up in the strand, where it's hot, down to the base of the coronal loop, where it's not as hot. This heats up the dense plasma at the loop’s base. Because it is so dense at the base, the temperature only reaches about 1 million degrees K. This dense plasma expands up into the strand. Thus, a coronal loop is a collection of 5-10 million degree K faint strands and 1 million degree K bright strands. "What we see is 1 million degree K plasma that has received its energy from the heat flowing down from the superhot plasma," says Klimchuk. "For the first time, we have detected this 10 million degree plasma, which can only be produced by the impulsive energy bursts of nanoflares.""

Barium stars

Main resources: Stars/Bariums and Barium stars

Barium stars exhibit carbon and s-process elements at their surfaces suggesting surface fusion possible during mass transfer or without it.

Barium stars are believed to be the result of mass transfer in a binary star system. The mass transfer occurred when the presently-observed giant star was on the main sequence. Its companion, the donor star, was a carbon star on the asymptotic giant branch (AGB), and had produced carbon and s-process elements in its interior. These nuclear fusion products were mixed by convection to its surface. Some of that matter "polluted" the surface layers of the main sequence star as the donor star lost mass at the end of its AGB evolution, and it subsequently evolved to become a white dwarf. We are observing these systems an indeterminate amount of time after the mass transfer event, when the donor star has long been a white dwarf, and the "polluted" recipient star has evolved to become a red giant.

Carbon stars

Main resources: Stars/Carbons and Carbon stars

CH stars

Main resources: Stars/CHs and CH stars

CH stars are particular type of carbon stars which are characterized by the presence of exceedingly strong CH absorption bands in their spectra. They belong to the star population II, meaning they're metal poor and generally pretty middle-aged stars, and are underluminous compared to the classical C–N carbon stars. Many CH stars are known to be binaries, and it's reasonable to believe this is the case for all CH stars. Like Barium stars, they are probably the result of a mass transfer from a former classical carbon star, now a white dwarf, to the current CH-classed star.

The mass transfer hypothesis may be needed to explain elemental occurrences on their surfaces such as carbon and s-process elements otherwise due to surface fusion.

Galaxy clusters

Main resources: Stars/Galaxies/Clusters and Galaxy clusters

This is a Chandra X-ray image of 3C 295. Image is 42 arcsec across. Credit: .

Here the Chandra observations are of the central regions of the Perseus galaxy cluster. Image is 284 arcsec across. Color code: Energy (Red 0.3-1.2 keV, Green 1.2-2 keV, Blue 2-7 keV). Instrument: ACIS.

3C 295 is a strongly X-ray emitting galaxy cluster in the constellation Boötes. 3C 295 (Cl 1409+524) is one of the most distant galaxy clusters observed by X-ray telescopes. The cluster is filled with a vast cloud of 50 MK gas that radiates strongly in X rays. [The Chandra X-ray Observatory detected] that the central galaxy is a strong, complex source of X rays. The cluster is located at J 2000.0 RA 14^h 11^m 20^s Dec −52° 12' 21". Observation date for the Chandra image is August 30, 1999.

Interacting galaxies

Main resources: Stars/Galaxies/Interacting and Interacting galaxies

File:Antennae before its deleted at commons.jpg

This montage of Chandra images shows a pair of interacting galaxies known as The Antennae. The image at the lower right is processed and color-coded to show regions rich in iron (red), magnesium (green) and silicon (blue). Image is 4.8 arcmin across. Color code: Energy (Red: 0.3-0.65 keV, Green: 0.65-1.5 keV, Blue: 1.5-6.0 keV). Credit: .

In the image at right of The Antennae, the top image, a wide field X-ray view, reveals spectacular loops of hot gas spreading out from the southern part of The Antenna into intergalactic space. In the closeup view on the lower left, the point sources have been taken out to emphasize the hot gas clouds in the central regions of The Antennae. The image at the lower right is processed and color-coded to show regions rich in iron (red), magnesium (green) and silicon (blue).

From the Chandra X-ray analysis of the Antennae Galaxies rich deposits of neon, magnesium, and silicon were discovered. These elements are among those that form the building blocks for habitable planets. The clouds imaged contain magnesium and silicon at 16 and 24 times respectively, the abundance in the Sun.

Yellow supergiants

Main resources: Stars/Yellows/Supergiants and Yellow supergiants

Calculations of "the changes in the surface chemical composition of intermediate-mass stars in the first phase of convection dredge-up ... has been used to determine the changes in the surface chemical composition of stars with masses 2.5, 5, 10, 20 M_ʘ due to nuclear reactions of the pp chains, the triple CNO cycle, and the NeNa and MgAl cycles."

For surface fusion or just above surface fusion a convection dredge-up may not be necessary.

"Boyarchuk and Lyubimkov [2] proposed that the excess sodium observed in yellow supergiants is synthesized in reactions of the NeNa cycle in the interior of stars on the main sequence (MS) and then is carried to the surface during the red-giant stage."

CNO stars

Main resources: Stars/CNOs and CNO stars

"Due to the removal of the outer layers by mass loss, matter produced by the CNO tri-cycle is revealed at stellar surfaces in OB stars, supergiants and WN stars."

Quasars

Main resources: Stars/Quasars and Quasars

Def. an extragalactic object, starlike in appearance, that is among the most luminous objects in the universe is called a quasar.

3C 273 is a quasar located in the constellation Virgo. "In the 3C 273 case the emitting area would be about 3000 AU across if the emission were Planckian at 17,000 K, but this could be spread out over many smaller clouds, as suggested by Krolik and McKee."

Supergiant stars

Main resources: Stars/Supergiants and Supergiant stars

The "ratio N_RSG/N_WR of the numbers of these stars strongly varies with galactocentric distance and is different in the LMC and SMC."

There is a "very strong increase with galactocentric distance of the ratio of the numbers of RSG to WR stars, as well as in the difference of this ratio in the LMC and SMC (cf. Maeder et al., 1980)."

White dwarfs

Main resources: Stars/Dwarfs/Whites, White dwarf stars, and White dwarfs

The white dwarf is surrounded by an expanding shell of gas in an object known as a planetary nebula. Planetary nebulae seem to mark the transition of a medium mass star from red giant to white dwarf. X-ray images reveal clouds of multimillion degree gas that have been compressed and heated by the fast stellar wind.

Wolf-Rayet stars

Main resources: Stars/Wolf-Rayets and Wolf-Rayet stars

Notation: let the symbol WC before the word star represent a Wolf-Rayet star exhibiting strong, broad emission lines of helium, carbon, and oxygen.

Notation: let the symbol WN before the word star represent a Wolf-Rayet star exhibiting strong, broad emission lines of helium and nitrogen.

Notation: let the symbol WNE before the word star represent an "early" WN-class Wolf–Rayet star (about WN2 to WN6).

Notation: let the symbol WNL before the word star represent a "late" WN-class Wolf–Rayet star (about WN6 to WN9).

"Wolf-Rayet stars offer us this most valuable possibility of observing the products of nuclear reactions in the H and He-burning phases revealed at stellar surfaces as a result of mass loss (and maybe of some mixing processes). In particular WC stars are the only kind of stars in which the products of the 3α and associated reactions prominently manifest themselves at the stellar surfaces."

"Comparisons are made between the theoretical C/He, N/He, and C/N ratios and those observed by Smith and Willis (1982) and by Nugis (1982) for WNL, WNE, and WC stars. The general agreement strongly supports the advanced evolutionary stage of WR stars as left-over cores resulting from the peeling of massive stars by stellar winds."

Bands

Main resources: Radiation astronomy/Bands, Band astronomy, and Bands

"In the L-band, a diffuse single source is observed. It is located between the two ribbons observed in the higher-energy bands."

Meteors

Main resources: Radiation astronomy/Meteors, Meteor astronomy, and Meteors

Arcs rise above an active region on the surface of the Sun in this series of images taken by the STEREO (Behind) spacecraft. Credit: Images courtesy of the NASA STEREO Science Center.

"A magnetic cloud is a transient event observed in the solar wind. It was defined in 1981 by Burlaga et al. 1981 as a region of enhanced magnetic field strength, smooth rotation of the magnetic field vector and low proton temperature . Magnetic clouds are a possible manifestation of a Coronal Mass Ejection (CME). The association between CMEs and magnetic clouds was made by Burlaga et al. in 1982 when a magnetic cloud was observed by Helios-1 two days after being observed by SMM. However, because observations near Earth are usually done by a single spacecraft, many CMEs are not seen as being associated with magnetic clouds. The typical structure observed for a fast CME by a satellite such as ACE is a fast-mode shock wave followed by a dense (and hot) sheath of plasma (the downstream region of the shock) and a magnetic cloud.

Cosmic rays

Main resources: Radiation astronomy/Cosmic rays, Cosmic-ray astronomy, and Cosmic rays

"The element ²²Ne appears as a tracer closely related to the existence of WC stars, because a large fraction of the existing ²²Ne seems to result from such stars. Thus, peculiar abundances of ²²Ne as observed in galactic cosmic rays (cf. Mewaldt, 1981) and meteorites (cf. Eberhardt et al., 1981) may indicate material originating from WC stars. Moreover, as there is a strong gradient in the distribution of WC stars in the Galaxy, a similar important gradient of ²²Ne should also exist in the Galaxy."

"[C]oronal magnetic bottles, produced by flares, [may] serve as temporary traps for solar cosmic rays ... It is the expansion of these bottles at velocities of 300–500 km/s which allows fast azimuthal propagation of solar cosmic rays independent of energy. A coronagraph on Os 7 observed a coronal cloud which was associated with bifurcation of the underlying coronal structure."

Alphas

Main resources: Radiation astronomy/Subatomics/Alphas and Alphas

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."

Neutrons

Main resources: Radiation astronomy/Neutrons, Neutron astronomy, and Neutrons

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

"Observations made with the gamma-ray spectrometer (GRS) on the Solar Maximum Mission (SMM) satellite and with the Jungfraujoch neutron monitor are used to determine the directional solar neutron emissivity spectrum from ~100 MeV to ~2 GeV during the solar flare on 1982 June 3. The experimental data require a time-extended emission of the neutrons at the Sun with the majority of the neutrons produced after the impulsive phase."

"The first detection of ~400 MeV solar neutrons near the Earth [occurred] following an impulsive solar flare on 1980 June 21 [...] For three events, solar neutron decay protons have been observed near the Earth".

The "existence of neutrons at the Sun, producing the n-p capture γ-ray line at 2.223 MeV, have been reported for several events".

The "average energy of the solar nucleons causing the flare enhancement must be less than for the cosmic-ray primaries above ~3.5 GeV. This means that the atmospheric cascade, producing the excess count rate, was initiated by solar neutrons in the energy range 300 MeV-3.5 GeV."

Protons

Main resources: Radiation astronomy/Protons, Proton astronomy, and Protons

File:Bahcall Figure 2.jpg

This figure shows a detected 94 % correlation between scaled sunspot numbers and neutrino detections. Credit: John N. Bahcall.

"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."

"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."

Positrons

Main resources: Radiation astronomy/Positrons, Positron astronomy, and Positrons

During solar flares "[s]everal radioactive nuclei that emit positrons are also produced; these positrons 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. Higher energy protons and ®-particles produce charged and neutral pions that decay to produce high-energy electrons/positrons and photons, respectively; these were detected in the 1991 June 11 flare by EGRET (Kanbach et al. 1993)."

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 flare."

There "is only a narrow range of temperatures around 6 × 10³ K, and only in a quiet solar atmosphere, where the line shape is dominated by the formation of positronium in flight (the positron replaces the proton in the hydrogen atom). The positronium can be formed in either the singlet or triplet state (Crannell et al. 1976). When it annihilates from the singlet state, it emits two 511 keV γ rays (2γ) in the center-of-mass frame; the lines are broadened by the velocity of the positronium."

"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 × 10⁵ K. In a quiet solar atmosphere, these temperatures are only reached in the transition region at densities ≤ 10¹² H cm⁻³."

The "positrons annihilate at such low densities [...] positrons produced by ³He interactions form higher in the solar atmosphere; however, in order to explain the line width, it would require a much higher ³He/⁴He ratio than the upper limit set for this flare by RHESSI. Alternatively, all the observations are consistent with densities > 10¹² H cm⁻³. But such densities require formation of a substantial mass of atmosphere at transition region temperatures."

Neutrinos

Main resources: Radiation astronomy/Neutrinos, Neutrino astronomy, and Neutrinos

The diagram contains the reactions in the proton-proton chain including neutrino production. Credit: Dorottya Szam.

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.

A neutrino created with a specific lepton flavor (electron, muon or tau) can later be measured to have a different flavor.

A great deal of evidence for neutrino oscillation has been collected from many sources, over a wide range of neutrino energies and with many different detector technologies.

Solar neutrinos have energies below 20 MeV and travel an astronomical unit between the source in the Sun and detector on the Earth. At energies above 5 MeV, solar neutrino oscillation actually takes place in the Sun through a resonance known as the MSW effect, a different process from the vacuum oscillation.

The presence of electrons in matter changes the energy levels of the propagation eigenstates (mass eigenstates) of neutrinos due to charged current coherent forward scattering of the electron neutrinos (i.e., weak interactions). The coherent forward scattering is analogous to the electromagnetic process leading to the refractive index of light in a medium. This means that neutrinos in matter have a different effective mass than neutrinos in vacuum, and since neutrino oscillations depend upon the squared mass difference of the neutrinos, neutrino oscillations may be different in matter than they are in vacuum. With antineutrinos, the conceptual point is the same but the effective charge that the weak interaction couples to (called weak isospin) has an opposite sign.

The effect is important at the very large electron densities of the Sun where electron neutrinos are produced. The high-energy neutrinos seen, for example, in SNO (Sudbury Neutrino Observatory) and in Super-Kamiokande, are produced mainly as the higher mass eigenstate in matter ν_2m, and remain as such as the density of solar material changes. (When neutrinos go through the MSW resonance the neutrinos have the maximal probability to change their nature, but it happens that this probability is negligibly small—this is sometimes called propagation in the adiabatic regime). Thus, the neutrinos of high energy leaving the sun are in a vacuum propagation eigenstate, ν₂, that has a reduced overlap with the electron neutrino ν_e = ν₁ cosθ + ν₂ sinθ seen by charged current reactions in the detectors.

"For high-energy solar neutrinos the MSW effect is important, and leads to the expectation that P_ee = sin²θ, where θ = 34° is the solar mixing angle. This was dramatically confirmed in the Sudbury Neutrino Observatory (SNO), which has resolved the solar neutrino problem. SNO measured the flux of Solar electron neutrinos to be ~34% of the total neutrino flux (the electron neutrino flux measured via the charged current reaction, and the total flux via the neutral current reaction). The SNO results agree well with the expectations.

For the low-energy solar neutrinos, on the other hand, the matter effect is negligible, and the formalism of oscillations in vacuum is valid. The size of the source (i.e. the Solar core) is significantly larger than the oscillation length, therefore, averaging over the oscillation factor, one obtains P_ee = 1 − sin²2θ / 2. For the same value of the solar mixing angle (θ = 34°) this corresponds to a survival probability of P_ee ≈ 60%. This is consistent with the experimental observations of low energy Solar neutrinos by the Homestake experiment (the first experiment to reveal the solar neutrino problem), followed by GALLEX, GNO, and SAGE (collectively, gallium radiochemical experiments), and, more recently, the Borexino experiment. These experiments provided further evidence of the MSW effect.

The transition between the low energy regime (the MSW effect is negligible) and the high energy regime (the oscillation probability is determind by matter effects) lies in the region of about 2 MeV for the Solar neutrinos.

Here on the Earth's surface the ν_e flux is about 10¹¹ ν_e cm^-2 s^-1 in the direction of the Sun.

"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."

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

$p^{+}+{\bar {p}}^{-}\rightarrow \pi ^{+}+\pi ^{-},$

${\pi }^{+}\rightarrow {\mu }^{+}+{\nu }_{\mu }\rightarrow e^{+}+{\nu }_{e}+{\bar {\nu }}_{\mu }+{\nu }_{\mu },$ and

$D_{S}\rightarrow \tau +{\bar {\nu }}_{\tau }\rightarrow \nu _{\tau }+{\bar {\nu }}_{\tau }.$

"All other sources of ν_τ are estimated to have contributed an additional 15%."

$\tau \rightarrow e+\nu _{\tau }+\nu _{e},$

for two neutrinos.

$\tau \rightarrow h+\nu _{\tau }+X,$

where $h$ is a hadron, for two neutrinos.

Gamma rays

Main resources: Radiation astronomy/Gamma rays, Gamma-ray astronomy, and Gamma rays

"The 2.2 MeV line is formed in the reaction which synthesizes deuterium: ¹H(n,γ)²H ... The line has been observed in a number of solar flares by the SMM, Hinotori and Prognoz satellites".

"The 2.2-MeV line fluence throughout the [May 24, 1990] flare was 345 ± 6 photons/cm², which corresponds to the observed synthesis of over 3 tons [some ~3.3 metric tons] of deuterium on the solar surface."

"Surface fusion is no longer bizarre since the 2.2 MeV gamma ray line of the P(n,γ)D reaction was observed during the solar flare of May 24 1990."

"[M]ost of the sun’s fusion must occur near the surface rather than the core."

Ultrasoft X-rays

Main resources: Radiation astronomy/X-rays/Ultrasofts and Ultrasoft X-rays

"[T]he ultrasoft X-ray emission (peak energy 30-50 eV) observed in the three strong (≥ 4 10³⁷-10³⁸ erg s^-1) LMC X-ray sources CAL83, CAL87 and RXJ0527.8-6954 can be explained by steady nuclear burning of hydrogen accreted onto white dwarfs with masses in the range of 0.7 to 1.2 M_ʘ."

Opticals

Main resources: Radiation astronomy/Opticals, Optical astronomy, and Opticals

The "spectral region around the lithium is blended with molecular lines of CN and TiO."

The "lithium lines are much stronger in sunspots due to the lower degree of ionization and therefore the influence of blends is smaller."

"In addition to the doublets of the lithium resonance lines from the isotopes Li⁶ and Li⁷ at 670.8 nm we selected the magnetically sensitive line Fe I 617.3 nm (Landé factor g = 2.5) to determine the magnetic field strength B, six nonmagnetic (g = 0) lines [Fe I 406.5, 444.3, 512.4, 543.5, 557.6, and Ni I 491.2 nm] with different atmospherical heights of formation and the K I 769.9 nm line which originates from the same atomic transition as lithium and is similar in mots line parameters except abundance."

"The field strength was found to B = 2450 Gauß in an optical height of log τ₅₀₀ = -2.5."

The "total lithium abundance is ε_Li = 1.02 and the ratio Li⁶/Li⁷ = 0.02."

Blues

Main resources: Radiation astronomy/Blues, Blue astronomy, and Blues

"High time resolution videotapes of the blue continuum show no fluctuations faster than a few seconds."

"[B]ound electrons in the corona [may be] scattering blue light according to RAYLEIGH'S law ... [The scattering coefficients and the intensities scattered] for blue and yellow light are in the proportion 37/11 = 3,36 [thus] the scattering must be due to bound electrons, belonging to ions or atoms."

The continuous spectrum of the corona may result from the "[s]cattering of the photospheric light by the electrons in the corona ["by emission of the corona itself due to recombinations"] [or by] [t]he emission and the "scattering" in the corona ... described as one single process".

"[T]he corona is supposed to be composed of electrons and positive ions, due to ejection from the photosphere".

Plasma objects

Main resources: Plasmas/Plasma objects and Plasma objects

Although a coronal cloud (as part or all of a stellar or galactic corona) is usually "filled with high-temperature plasma at temperatures of T ≈ 1–2 (MK), ... [h]ot active regions and postflare loops have plasma temperatures of T ≈ 2–40 MK."

Metallicities

Main resources: Chemicals/Metallicities and Metallicities

For a moderately sized star originating very early in the age of the universe and undergoing surface fusion due to the presence of a coronal cloud, its metallicity should increase from this fusion and any going on internally plus metal pickup as it travels through supernovae produced dust.

Hydrogens

Main resources: Chemicals/Hydrogens and Hydrogens

Deuteriums

Main resources: Chemicals/Deuteriums and Deuteriums

"The 2.2 MeV line is formed in the reaction which synthesizes deuterium: ¹H(n,γ)²H ... The line has been observed in a number of solar flares by the SMM, Hinotori and Prognoz satellites".

"The 2.2-MeV line fluence throughout the [May 24, 1990] flare was 345 ± 6 photons/cm², which corresponds to the observed synthesis of over 3 tons [some ~3.3 metric tons] of deuterium on the solar surface."

"Surface fusion is no longer bizarre since the 2.2 MeV gamma ray line of the P(n,γ)D reaction was observed during the solar flare of May 24 1990."

"[M]ost of the sun’s fusion must occur near the surface rather than the core."

Heliums

Main resources: Chemicals/Heliums and Heliums

"Concerning the particles which interact at the Sun, evidence for accelerated ³He enrichment was obtained from the detection (Share & Murphy 1998) of a gamma-ray line at 0.937 MeV produced by the reaction ¹⁶O(³He,p)¹⁸F^∗"

For "essentially all of [some 20] flares ³He/⁴He can be as large as 0.1, while for some of them values as high as 1 are possible. In addition, [...] for the particles that interact and produce gamma rays, ³He enrichments are present for both impulsive and gradual flares."

Lithiums

Main resources: Chemicals/Lithiums and Lithiums

"The solar wind lithium isotopic ratio, (⁶Li/⁷Li)_sw = 0.032±0.004, has recently been determined from measurements in lunar soil (Chaussidon & Robert 1999)."

"Light element production by accelerated particle interactions [in] non-solar settings, and for accelerated particles of predominantly low energy, the dominant reactions are ⁴He(α,p)⁷Li, ⁴He(α,n)⁷Be (with ⁷Be decaying to ⁷Li) and ⁴He(α,x)⁶Li (where x stands for either a proton and a neutron, or a deuteron)."

"In solar flares, [...] the reaction ⁴He(³He,p)⁶Li [has a] very low threshold energy and [...] for solar energetic particles ³He/⁴He can be as large as 1 or even larger (e.g. Reames 1998). Such ³He/⁴He enhancements are one of the main characteristics of the acceleration mechanism responsible for impulsive solar energetic particle events, as distinguished from gradual events, based on the duration of the accompanying soft X-ray emission."

Flare "accelerated particle interactions produce enough ⁶Li which, combined with photospheric ⁷Li, can account for the solar wind ⁶Li/⁷Li measured in lunar soil."

$\mathrm {_{2}^{3}He} +\mathrm {_{2}^{4}He} \rightarrow n^{0}+p^{+}+\mathrm {_{3}^{6}Li} +\gamma (3.561MeV).$

Although the 3.56 MeV line is usually missing from the gamma-ray spectrum during solar flares, there is evidence for significant production of ⁶Li in large solar flares by optical observations of sunspots,[17] and measurements of solar wind Li isotopic ratio in lunar soil.[18]

The "fact that as much as 10³⁰ Li atoms are produced in large solar flares, suggests that flare produced lithium may be detected in a small area of the solar surface near the foot points of the flaring loops shortly after the time of the flare (see Livshits 1997). In this connection, it is interesting to point out that Ritzenhoff et al. (1997) don’t rule out the presence of ⁶Li near a sunspot at a value close to their reported upper limit ⁶Li/⁷Li ≤ 0.03, which in fact coincides with the measured solar wind value."

Meteorites

Main resources: Rocks/Meteorites and Meteorites

"⁷Li in the photosphere is depleted by over a factor of 100 relative to its protosolar value (i.e. the photospheric vs. the meteoritic abundance".

Sun

Main resources: Stars/Sun and Sun (star)

"Soon after the instruments opened their doors, the Sun began performing for SDO with this beautiful prominence eruption." Credit: NASA.

A variety of subatomic particle and γ-ray reactions have been observed during solar flares indicating fusion reactions occurring at or above the photosphere. "There are typically 375 gamma-ray flares per solar cycle ... each releasing on average about 10³¹ erg of kinetic energy in accelerated ions of energy ≥ 1 MeV per nucleon [27]."

"The solar-flare gamma-ray line emission testifies that fresh nuclei are synthesized in abundance in energetic solar events."

"[T]he gamma-ray lines at 478 and 429 keV [are] emitted in the reactions ⁴He(α,p)⁷Li and ⁴He(α,n)⁷Be, respectively".

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

"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."

Chromospheres

Main resources: Stars/Chromospheres and Chromospheres

A variety of subatomic particle and γ-ray reactions have been observed during solar flares indicating nuclear fusion reactions occur above the photosphere, most likely in the chromosphere.

Transition regions

Main resources: Stars/Transition regions and Transition regions

The thin region of temperature increase from the chromosphere to the corona is known as the transition region and can range from tens to hundreds of kilometers thick. An analogy of this would be a light bulb heating the air surrounding it hotter than its glass surface. The second law of thermodynamics would be broken.

Coronal clouds

Main resources: Plasmas/Plasma objects/Coronal clouds and Coronal clouds

"Coronal clouds are irregular objects suspended in the corona with matter streaming out of them into nearby active regions."

Polar coronal holes

Main resources: Radiation astronomy/Holes/Coronal/Polars and Polar coronal hole astronomy

"The striking absence of green emission above both polar regions at activity minimum led Waldmeier (1957) to use the German term 'Koronalöcher', ie, coronal holes." "Here we restrict ourselves to a qualitative study of large scale structures of the green emission line corona."

Solar winds

Main resources: Plasmas/Plasma objects/Solar winds and Solar winds

"The Maunder Minimum, which occurred during 1645-1715, was an interval of low sunspot numbers and greatly diminished solar-induced activity like aurorae and solar winds. This period of very low solar activity, and an inferred, earlier solar activity minimum (1450-1550) known as the Spörer Minimum roughly coincided with cooler climates in Europe and Asia known as the Little Ice Age."

"[M]agnetic-induced phenomena (flares, coronal emissions, coronal mass ejections, winds, etc) continue to have important effects on Earth and the Solar System."

"From the study of solar type stars with different ages, [...] the Sun loses angular momentum with time via magnetized winds (magnetic breaking)."

Mercury

Main resources: Wanderers/Mercury and Mercury

The sodium tail of Mercury is mapped out during the MESSENGER's first flyby on January 14, 2008. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

Mercury's calcium and magnesium tail is mapped out during the MESSENGER's third flyby on September 29, 2009. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

"MOST current literature on solar activity assumes that the planets do not affect it, and that conditions internal to the Sun are primarily responsible for the solar cycle. Bigg¹, however, has shown that the period of Mercury's orbit appears in the sunspot data, and that the influence of Mercury depends on the phases of Venus, Earth, and Jupiter."

Some observations show that Mercury is surrounded by a hot corona of calcium atoms with temperature between 12,000 and 20,000 K.

At right is a diagram of Mercury's sodium tail. "As the MESSENGER spacecraft approached Mercury, the UVVS field of view was scanned across the planet's exospheric "tail," which is produced by the solar wind pushing Mercury's exosphere (the planet's extremely thin atmosphere) outward. This figure, recently published in Science magazine, shows a map of the distribution of sodium atoms as they stream away from the planet (see PIA10396); red and yellow colors represent a higher abundance of sodium than darker shades of blue and purple, as shown in the colored scale bar, which gives the brightness intensity in units of kiloRayleighs. The escaping atoms eventually form a comet-like tail that extends in the direction opposite that of the Sun for many planetary radii. The small squares outlined in black correspond to individual measurements that were used to create the full map. These measurements are the highest-spatial-resolution observations ever made of Mercury's tail. In less than six weeks, on October 6, 2008, similar measurements will be made during MESSENGER's second flyby of Mercury. Comparing the measurements from the two flybys will provide an unprecedented look at how Mercury's dynamic exosphere and tail vary with time."

At left are two diagrams showing the approximate distribution of calcium and magnesium in Mercury's tail. "These figures show observations of calcium and magnesium in Mercury's neutral tail during the third MESSENGER Mercury flyby. The distribution of neutral calcium in the tail appears to be centered near the equatorial plane and the emission rapidly decreases to the north and south as well as in the anti-sunward direction. In contrast, the distribution of magnesium in the tail exhibits several strong peaks in emission and a slower decrease in the north, south, and anti-sunward directions. These distributions are similar to those seen during the second flyby, but the densities were higher during the third flyby, a different "seasonal" variation than for sodium. Studying the changes of the "seasons" for a range of species during MESSENGER's orbital mission phase will be key to quantifying the processes that generate and maintain the exosphere and transport volatile material within the Mercury environment."

Venus

Main resources: Wanderers/Venus and Venus

"A non-thermal, or “hot”, Venus corona of H atoms has been observed by Mariners 5 and 10 and Venera 9."

"After more than two years in orbit still no Venus Express observations were published concerning the hot oxygen corona of Venus which could verify the corresponding controversial observations of Venera 11 and PVO, three decades ago."

"Venus has a hot oxygen corona in addition to its hydrogen corona (Nagy et al., 1981) and charge-exchange between protons and oxygen is accidentally resonant."

Earth

Although ⁷Be is usually assumed to have been produced by the Big Bang nuclear fusion, excesses (100x) of the isotope on the leading edge of the Long Duration Exposure Facility (LDEF) relative to the trailing edge suggest that fusion near the surface of the Sun is the most likely source. The particular reaction ³He(α,γ)⁷Be and the associated reaction chains ⁷Be(e^-,ν_e)⁷Li(p,α)α and ⁷Be(p,γ)⁸B => 2α + e⁺ + ν_e generate 14% and 0.1% of the α-particles, respectively, and 10.7% of the present-epoch luminosity of the Sun. Usually, the ⁷Be produced is assumed to be deep within the core of the Sun; however, such ⁷Be would not escape to reach the leading edge of the LDEF.

Moon

Main resources: Wanderers/Moon and Moon

Based on the ³He-flare flux from the Sun's surface and Surveyor 3 samples (implanted ¹⁵N and ¹⁴C in lunar material) from the surface of the Moon, the level of nuclear fusion occurring in the solar atmosphere is approximately at least two to three orders of magnitude greater than that estimated from solar flares such as those of August 1972.

AG Draconis

Main resources: Stars/AG Draconis and AG Draconis

According to SIMBAD, AG Draconis is spectral type K3IIIep, and an X-ray source as detected by the Einstein X-ray Observatory and ROSAT.

"An abundance analysis of the yellow symbiotic system AG Draconis reveals it to be a metal-poor K-giant ([Fe/H] = -1.3) which is enriched in the heavy s-process elements."

"A comparison of the heavy-element abundance distribution in [AG Draconis] with theoretical nucleosynthesis calculations shows that the s-process is defined by a relatively large neutron exposure (τ=1.3 mb^-1), while an analysis of the rubidium abundance suggests that the s-process occurred at a neutron density of about 2 [x] 10⁸ cm^-3."

The "K giant in AG Dra [has a] T_eff ~ 4100 - 4400 K. ... [With a best fit to spectroscopic data of T_eff = 4300 K.]"

Observed heavy-element abundances may be used "to probe two aspects of the s-process:

1. ... determine the neutron exposure τ characterizing the s-process efficiency, and
2. using the abundance of Rb, whose s-process abundance is sensitive to neutron density [to] obtain constraints on the s-process neutron density N_n."

For any ongoing surface fusion on AG Dra the observed s-process heavy-element abundances probably need no correction. However, without acknowledgment of likely surface fusion, the presence of the s-process elements must be accounted for by some processes associated with inner-core fusion to move s-process elements to the surface, or surface fusion on or above a compact companion.

"The composite nature of the spectrum exhibited by symbiotic stars, consisting of a nebular continuum superimposed on hot and cool stellar continua, is best explained by a binary model ... In such a model [K giants may have larger mass-loss rates], either through a wind or through Roche lobe overflow, [that] falls onto a compact companion (generally a white dwarf), possibly via an accretion disk. The energy released by such an accretion process heats the matter, thereby producing the hot photons [X-rays] observed in symbiotic stars."

"The operation of the s-process is commonly associated with He-burning thermal pulses occurring on the asymptotic giant branch (AGB). As a result of the so-called 'third dredge-up' on the AGB, s-process enriched material is brought to the star's surface ... Barium stars [and] CH stars ... are too hot and of too low a luminosity to have undergone third dredge-up on the AGB. [Because both types] are all single-lined spectroscopic binaries, where the unseen component is almost certainly a white dwarf (WD) ... their chemical peculiarities [are] attributed to mass transfer across the binary system. When the current WD companion of the barium star was a thermally-pulsing AGB star, it transferred s-process- and C-rich material onto its companion, which is now viewed as a barium or CH star."

With a binary model in which the two components have changed roles perhaps more than once, "the observed s-process abundances [must be corrected] for the initial heavy-element distribution [ε_i] in AG Dra's unprocessed material. ... plus an s-process enrichment component ε_s

$\epsilon _{obs}=f\epsilon _{s}+(1-f)\epsilon _{i},$

where f is the fraction of the current envelope mass consisting of processed material accreted from the TP-AGB star."

The "initial heavy-element distribution [ε_i] in AG Dra's unprocessed material" requires an assumption. Rather than using a solitary KIII of comparable metallicity, "ε_i [is assumed] to be identical to the solar-system heavy-element abundances ... scaled down to AG Dra's metallicity assuming [s/Fe] ≈ 0.0."

Comparisons are made to abundance tables for s-process nucleosynthesis using a 'goodness of fit' criterion. A "single neutron exposure [provides] a better fit ... [yielding] τ = 1.3 mb^-1".

Epsilon Eridani

Main resources: Stars/Epsilon Eridani and Epsilon Eridani

Epsilon Eridani has a higher level of magnetic activity than the Sun, and hence demonstrates increased activity in the outer parts of the star's atmosphere: the chromosphere and corona. The average magnetic field strength of this star across the entire surface is (1.65 ± 0.30) × 10⁻² T, which is more than forty times greater than the (5–40) × 10⁻⁵ T magnetic field strength in the Sun's photosphere.

The X-ray luminosity of Epsilon Eridani is about 2 × 10²⁸ ergs/s (2 × 10²¹ W). It is brighter in X-ray emission than the Sun at peak activity. The source for this strong X-ray emission is the star's hot corona. Epsilon Eridani's corona appears larger and hotter than the Sun's, with a temperature of 3.4 × 10⁶ K as measured from observation of the corona's ultraviolet and X-ray emission.

"The stellar wind emitted by Epsilon Eridani expands until it collides with the surrounding interstellar medium of sparse gas and dust, resulting in a bubble of heated hydrogen gas. The absorption spectrum from this gas has been measured with the Hubble Space Telescope, allowing the properties of the stellar wind to be estimated. Epsilon Eridani's hot corona results in a mass loss rate from the star's stellar wind that is 30 times higher than the Sun's. This wind is generating an astrosphere (the equivalent of the heliosphere that surrounds the Sun) that spans about 8,000 AU and contains a bow shock that lies 1,600 AU from the star. At its estimated distance from Earth, this astrosphere spans 42 arcminutes, which is wider than the apparent size of the full Moon.

Gliese 176

Main resources: Stars/Gliese 176 and Gliese 176

The corona of this star has a moderate emission of X-rays at 3 × 10²⁷ erg s^–1. This indicates the star is active, and may exhibit starspots and flares much like the Sun. This is considered normal for a main-sequence star of spectral class M.

Proxima Centauri

Main resources: Stars/Proxima Centauri and Proxima Centauri

"The Sun's nearest stellar neighbor Proxima Centauri is a flare star that undergoes random increases in brightness because of magnetic activity.

RS Ophiuchi

Recurrent nova RS Ophiuchus is imaged during nova activity on February 23, 2006, from Mt. Laguna, California. Credit: Robogun.

RS Ophiuchi (RS Oph) is a recurrent nova system approximately 5,000 light-years away in the constellation Ophiuchus. In its quiet phase it has an apparent magnitude of about 12.5. It erupted in 1898, 1933, 1958, 1967, 1985, and 2006 and reached about magnitude 5 on average.

Wolf 359

Main resources: Stars/Wolf 359 and Wolf 359

The flare star Wolf 359 is another near neighbor (2.39 ± 0.01 parsecs). Wolf 359, also known as Gliese 406 and CN Leo, is a red dwarf of spectral class M6.5 that emits X-rays. It is a UV Ceti flare star, and has a relatively high flare rate.

Nuclear fusions

Main resources: Physics/Fusions, Nuclear fusions, and Fusions

Nuclear fusion is usually assumed to occur within a star as a part of stellar nucleosynthesis. The nuclear reactions that are likely to occur under particular stellar conditions follow one or more synthesis paths described in nucleosynthesis.

The first step involves the fusion of two ¹H nuclei (protons) into deuterium, releasing a positron and a neutrino as one proton changes into a neutron. It is a two-stage process; first, two protons fuse to form a diproton:

¹H + ¹H → ²He

followed by the beta-plus decay of the diproton to deuterium:

²He → ²D + e⁺ + ν_e

with the overall formula:

¹H + ¹H → ²D + e⁺ + ν_e + 0.42 MeV

This first step is extremely slow, because the beta-plus decay of the diproton to deuterium is extremely rare (the vast majority of the time, it decays back into hydrogen-1 through proton emission).

The "abundance of ¹⁷O becomes as important as that of ¹⁶O when the products of advanced CNO-processing are seen."

Hypotheses

Main resource: Hypotheses

1. Most or all nuclear fusion occurring in the solar octant originates in the solar chromosphere or above.

See also

• Stellar active region

Further reading

• E.P.J. van den Heuvel; D. Bhattacharya; K. Nomoto; S.A. Rappaport (August 1992). "Accreting white dwarf models for CAL 83, CAL 87 and other ultrasoft X-ray sources in the LMC". Astronomy and Astrophysics 262 (1): 97-105. 

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{{Charge ontology}}{{Principles of radiation astronomy}}