Stars/X-ray classification

Any X-ray classification of stars seems unlikely as the visible portion of the electromagnetic spectrum is expected to accompany the emission of X-radiation (X-rays) and phenomena may be associated.

A Chandra X-ray Observatory image of RX J1242-11 obtained on March 9, 2001, shows a single point source in blue. Credit: NASA/Chandra X-Ray Observatory.

But, the emission of X-rays is most often associated with a coronal cloud, a corona, or at least a high temperature plasma (about 106 K).

X-rays edit

X-ray astronomy is a physical subfield of astronomy, more specifically radiation astronomy, that uses a variety of X-ray detectors fashioned into X-ray telescopes to observe natural sources that emit, reflect, transmit, or fluoresce X-rays. X-rays can only penetrate so far into a planetary atmosphere such as that surrounding the crustal and oceanic surface of the Earth. This limitation requires that these detectors and telescopes be lofted above nearly all of the atmosphere to function. Another alternative is to place them on astronomical bodies such as the Moon or in orbit.

X-radiation edit

X-rays span 3 decades in wavelength, frequency and energy. From 10 to 0.1 nanometers (nm) (about 0.12 to 12 keV) they are classified as soft x-rays, and from 0.1 nm to 0.01 nm (about 12 to 120 keV) as hard X-rays.

Although the more energetic X-rays, photons with an energy greater than 30 keV (4,800 aJ) can penetrate the air at least for distances of a few meters (they would never have been detected and medical X-ray machines would not work if this was not the case) the Earth's atmosphere is thick enough that virtually none are able to penetrate from outer space all the way to the Earth's surface. X-rays in the 0.5 to 5 keV (80 to 800 aJ) range, where most celestial sources give off the bulk of their energy, can be stopped by a few sheets of paper; ninety percent of the photons in a beam of 3 keV (480 aJ) X-rays are absorbed by traveling through just 10 cm of air.

Planetary sciences edit

"Apart from the Sun, the known X-ray emitters now include planets (Venus, Earth, Mars, Jupiter, and Saturn), planetary satellites (Moon, Io, Europa, and Ganymede), all active comets, the Io plasma torus, the rings of Saturn, the coronae (exospheres) of Earth and Mars, and the heliosphere."[1]

Metallicities edit

The metallicity (also called Z[2]) of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. The metallicity of an astronomical object may provide an indication of its age. When the universe first formed, according to the Big Bang theory, it consisted almost entirely of hydrogen which, through primordial nucleosynthesis, created a sizeable proportion of helium and only trace amounts of lithium and beryllium and no heavier elements. Therefore, older stars have lower metallicities than younger stars such as our Sun.

Stellar populations are categorized as I, II, and III, with each group having decreasing metal content and increasing age. The populations were named in the order they were discovered, which is the reverse of the order they were created. Thus, the first stars in the universe (low metal content) were population III, and recent stars (high metallicity) are population I. ... The next generation of stars was born out of those materials left by the death of the first. The oldest observed stars, known as Population II, have very low metallicities;[3]

The metallicity of the Sun is approximately 1.8 percent by mass. For other stars, the metallicity is often expressed as "[Fe/H]", which represents the logarithm of the ratio of a star's iron abundance compared to that of the Sun (iron is not the most abundant heavy element, but it is among the easiest to measure with spectral data in the visible spectrum). The formula for the logarithm is expressed thus:


where   and   are the number of iron and hydrogen atoms per unit of volume respectively. The unit often used for metallicity is the "dex" which is a (now-deprecated) contraction of decimal exponent.[4] By this formulation, stars with a higher metallicity than the Sun have a positive logarithmic value, while those with a lower metallicity than the Sun have a negative value. The logarithm is based on powers of ten; stars with a value of +1 have ten times the metallicity of the Sun (101). Conversely, those with a value of -1 have one tenth (10 −1), while those with -2 have a hundredth (10−2), and so on.[5] Young Population I stars have significantly higher iron-to-hydrogen ratios than older Population II stars. Primordial Population III stars are estimated to have a metallicity of less than −6.0, that is, less than a millionth of the abundance of iron which is found in the Sun.

Theoretical stellar classification edit

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

From a dictionary:


1.a: "any natural luminous body visible in the sky [especially] at night",
1.b: "a self-luminous gaseous celestial body of great mass whose shape is [usually] spheroidal and whose size may be as small as the earth or larger than the earth's orbit".[6]

is called a star.

Def. "[t]he natural medium emanating from the sun and other very hot sources (now recognised as electromagnetic radiation with a wavelength of 400-750 nm), within which vision is possible"[7] is called light.

Def. "to shine light on something"[8] is called illuminate.

Def. "emitting light"[9] is called luminous.

From astrophysics:

Def. "any object forming on a dynamical timescale, by gravitational instability", is called a star.[10]

As an objective of original research, an X-ray classification of stars needs to produce evidence that it is possible to classify stars by X-ray astronomy or coronal cloud characteristics. These characteristics need not correspond in any way with the conventional visual classification.

Def. "[a] visible surface layer of a star, and especially that of a sun"[11] is called a photosphere.

Def. "[t]he faint pink extension of a star's atmospheric envelope between the corona and the photosphere"[12] is called the chromosphere.

Def. "[t]he luminous plasma atmosphere of the Sun or other star, extending millions of kilometres into space, most easily seen during a total solar eclipse"[13] is called a corona.

Def. "a low energy discharge caused by ionization of a gas by an electric field [quite common at conductor bends of 12kV or higher]"[13] is called a corona, or an electrical corona.

Def. "[t]he outflow of charged particles from the solar corona into space ... [b]ecause of the high temperature of the particles of the corona, ... are moving at speeds higher than the solar escape velocity"[14] is called a solar wind.

Def. "[t]he equivalent of solar wind associated with a star other than our Sun"[15] is called a stellar wind.

Def. "[t]he region of space where interstellar medium is blown away by solar wind"[16] is called the heliosphere.

It may be the case that coronal clouds around a star are independent of the stellar type based upon photospheric characteristics. But, if heat is being radiated or transferred to the photosphere to heat it to the temperature observed (Teff), then the photospheric temperature may reflect the coronal cloud conditions. Ions characteristic of the chromosphere should not be used to classify stars.

Testing the coronal cloud independence of stellar classification may be statistically possible once a proper separation of chromospheric and above phenomena is performed. The same coronal cloud at different locations may produce different star types depending solely upon what is the star itself (photosphere and beneath).

Continua edit

X-ray continuum emission "can arise both from a jet and from the hot corona of the accretion disc via a scattering process: in both cases it shows a power-law spectrum. In some radio-quiet active galactic nuclei (AGN) there is an excess of soft X-ray emission in addition to the power-law component.

X-ray line emission is a result of illumination of cold heavy elements by the X-ray continuum that causes fluorescence of X-ray emission lines.

Using X-rays to determine a crystal structure results in diffraction intensities that are represented in reciprocal space as peaks. These have a finite width due to a variety of defects away from a perfectly periodic lattice. There may be significant diffuse scattering, a continuum of scattered X-rays that fall between the Bragg peaks.

The X-ray continuum can arise from bremsstrahlung, black-body radiation, synchrotron radiation, or what is called inverse Compton scattering of lower-energy photons by relativistic electrons, knock-on collisions of fast protons with atomic electrons, and atomic recombination, with or without additional electron transitions.[17]

Meteors edit

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] medium-strength flare erupted from the sun on July 19, 2012. The blast also generated the enormous, shimmering plasma loops, which are an example of a phenomenon known as "coronal rain," agency officials said."[18]

"Hot plasma in the corona cooled and condensed along strong magnetic fields in the region" slowly falling back to the solar surface as plasma "rain".[18]

"Many CMEs have also been observed to be unassociated with any obvious solar surface activity"[19].

In the images at right, a CME, or "arcs rise above an active region on the surface of the Sun in this series of images taken by the STEREO (Behind) spacecraft on January 27, 2010. The arcs are plasma, superheated matter made up of moving charged particles (electrons and ions). Just as iron filings arc from one end of a magnet to another, the plasma is sliding in an arc along magnetic field lines. In a movie of STEREO observations made between January 26 and January 29, the dynamic streams were initially just over the Sun’s edge and readily spotted as the Sun rotated them more into view."[20]

"About mid-way through the movie clip, a small coronal mass ejection (a stream of charged particles from the Sun) shoots out and into space at about a million miles per hour, carrying some magnetic field with it. The [first] image shows the beginning of the coronal mass ejection, while the [second] image shows the solar matter leaving the Sun’s corona. Most coronal mass ejections are more bulbous and wide: this one is quite narrow and contained. Nonetheless, NASA solar scientists agree that its speed and characteristics suggest that it was indeed a non-typical coronal mass ejection."[20]

The February 10, 1956, event "was observed at Sacramento Peak. A bright ball appears above the [Sun's] surface, grows in size and Hα brightness, and explodes upward and outward."[21]

Magnetic clouds represent about one third of ejecta observed by satellites at Earth. Other types of ejecta are multiple-magnetic cloud events (a single structure with multiple subclouds distinguishable)[22][23] and complex ejecta, which can be the result of the interaction of multiple CMEs.

Cosmic rays edit

"[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."[24]

Protons edit

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.

Electrons edit

This is an X-ray image of the coronal clouds near the Sun. Credit: NASA Goddard Space Flight Center.

By using estimates of the interstellar electron influx that may have been measured by Voyager 1, a back-of-the-envelope calculation shows that the electron influx may be sufficient to heat the solar corona to MK and the photosphere to 5777 K. These estimates put the coronal heating problem in perspective of the overall external heating of the outer Sun including the photosphere.

The news report of Voyager 1 reaching some 120 AU from the Sun contain interesting discoveries. The most significant may be a flux of electrons diffusing into our solar system from elsewhere in the galaxy. The solar wind has ceased earlier and has even been turned backward. These results suggest a net influx of electrons perhaps toward the Sun that may be involved in heating the solar corona. The possibility is explored using some "back-of-the-envelope" calculations in the learning resource electron beam heating/Laboratory.

The lecture electron beam heating describes at least one example of the use of this technology to heat objects.

Def. the "rate of transfer of energy (or another physical quantity) through a given surface, specifically electric flux, magnetic flux"[25] is called a flux.

"Mass flux of species α [is in units of] g/cm2 s"[26] or, in dimensions [quantity of species α]·[area]−1·[time]−1, where an area may be cm2.

Electron winds edit

As of December 5, 2011, "Voyager 1 is about ... 18 billion kilometers ... from the [S]un [but] the direction of the magnetic field lines has not changed, indicating Voyager is still within the heliosphere ... the outward speed of the solar wind had diminished to zero in April 2010 ... inward pressure from interstellar space is compacting [the magnetic field] ... Voyager has detected a 100-fold increase in the intensity of high-energy electrons from elsewhere in the galaxy diffusing into our solar system from outside ... [while] the [solar] wind even blows back at us."[27]

Surface area edit

From the image at the above right it appears that the coronal clouds about the Sun cover a partial shell from say 80° N to about 80° S that follows the curvature of the photosphere. There does seem to be a physical separation of the shell of coronal clouds from the concentric shell of the photosphere. The coronal holes perhaps being void of incoming electron flux.

Photosphere volumes edit

R⊙eq ≈ 6.955 x 105 km. The thickness of the photosphere is about 400 km. R⊙p ≈ 6.951 x 105 km.


Photosphere hydrogens edit

The density of the Sun is about 2 x 10-4 kg m-3. Or,


One mole of H2 (gas) has a mass of 2.016 x 10-3 kg. The molar density of the photosphere may be


Constant volume specific heat capacity edit

For H2 (gas) the molar constant-volume heat capacity at 298 K is 20.18 J/(mol · K). At 2000 K it is about 25 J/(mol · K). Using a linear extrapolation,


for 5777 K, yields


Before calculating the amount of energy or power necessary to heat the coronal clouds around the Sun, let's see if the influx of electrons from outside the heliosphere may be able to heat the surface of the photosphere (p) to 5777 K from 100 K.


Photosphere heating edit


Voyager 1 is 17,932,000,000 km (119.9 AU) from the Sun at RA 17.163h Dec +12.44°, ecliptic latitude of 34.9°.

For this laboratory example, let the electron flux be 2 e- cm-2 s-1 diffusing into our solar system from elsewhere in the galaxy. Each of these electrons has an energy of 10 MeV.


If the electron flux measured by Voyager 1 is close to 2 e- cm-2 s-1 where each electron averages 10 MeV and these electrons are heading for the Sun, then each electron may strike the photosphere from anywhere in a sphere around the Sun.

To heat the photosphere to 5777 K takes


The power (P) that may be deposited on the photospheric surface of the Sun is


The luminosity (in Watts, W) of the Sun is 3.846 x 1026 W. In eV/s this is


If the energy of the incoming electrons is 700 MeV and the flux is 8.48 x 104 e- cm-2 s-1, then the power from the incoming electrons would be


The power calculated for the electron influx is compared with the first experiment and with the known luminosity of the Sun.

Coronal heating edit

The power to heat the solar corona is on the order of 1039 eV s-1. While this is within range of the estimated electron influx beam heating, X-rays are usually generated between 0.1 and 120 keV. 10 to 700 MeV electrons would likely produce gamma rays rather than X-rays.

The initial guess put the power at about six orders of magnitude too low to match the Sun's current luminosity. However, increasing the electron energy by 70 times and the influx by about 4 x 104 brings the estimate into agreement. It is likely that either increase may be too much.

Other concerns focus on whether any of these electrons can reach the Sun. The Sun has a net surface negative charge. The solar wind contains electrons, protons and heavier nuclei that may alter the energy or influx significantly.

While likely factors exist that may decrease the estimated electron energy or influx, it appears that such an influx may not only heat the solar corona but also the photosphere. Actual numbers from Voyager 1 may shed more light on this conclusion.

Unless the influx is significantly higher at lower energies it seems that 10 to 700 MeV is way too high. The corona of the Sun would be producing gamma-rays not X-rays. No justification is given for the 104th increase in the influx estimate. Actual data on interstellar electrons does suggest MeV levels.

Positrons edit

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

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."[28] 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.[28] The observations are somewhat consistent with electron-positron annihilation in a quiet solar atmosphere via positronium as well as during flares.[28] Line-broadening is due to "the velocity of the positronium."[28] "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."[28]

Neutrinos edit

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).

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.

Gamma rays edit

The Earth's atmosphere is a relatively bright source of gamma rays produced in interactions of ordinary cosmic ray protons with air atoms. In gamma-ray and X-ray astronomy, Earth is a dwarf gaseous object.

X-rays edit

"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."[29]

Opticals edit

Sample calibration colors[30]
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

The 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.

Visuals edit

Sample calibration colors[30]
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

The 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,[31] 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).[32]

Greens edit

The Na I green lines at 568.2 and 568.8 nm arise "in the photospheric layers between log τ5000 ≈ -1 and -2."[33]

Yellows edit

"[H]igh-resolution spectral measurements of Mercury show emission in sodium D lines (Potter and Morgan 1985a). This suggests a substantial sodium population in Mercury's atmosphere ... possibly due to photo-sputtering of the planetary surface"[34]. At least in emission yellow astronomy, Mercury is a dwarf gaseous object.

"Stars of spectral classes F and G, such as our sun Sol, have color temperatures that make them look "yellowish".[35]

Plasma objects edit

Plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms (reduce or increase the number of electrons in them), thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions.[36]

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

"Plasma is the fourth state of matter, consisting of electrons, ions and neutral atoms, usually at temperatures above 104 degrees Kelvin."[37] "The sun and stars are plasmas; the earth's ionosphere, Van Allen belts, magnetosphere, etc., are all plasmas. Indeed, plasma makes up much of the known matter in the universe."[37]

Magnetohydrodynamics edit

Def. "the study of the interaction of electrically conducting fluids with magnetic fields", after magnetohydrodynamics, is called magnetohydrodynamics (MHD).

In a coronal cloud are magnetohydrodynamic plasma flux tubes along magnetic field lines.[38]

"[M]otions resulting from [a linear magnetohydrodynamic] instability act as a dynamo to sustain the magnetic field."[39] "Supersonic flows are initially generated by the Balbus-Hawley magnetic shear instability."[39] From radiative dynamo: "A plasma with local magnetohydrodynamic instabilities creates mechanical turbulence, motion, or shear (a dynamo) which in turn generates or sustains the local magnetic field."

Hydrogens edit

Depending primarily upon gas temperature, the presence of gas may be used to determine the composition of the gas object observed, at least the outer layer. Early spectroscopy[40] 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."[41]

Heliums edit

Helium was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with detecting the element along with Norman Lockyer during the solar eclipse of 1868, and Lockyer was the first to propose that the line was due to a new element, which he named.

Lithiums edit

"[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"].[42]

Atmospheres edit

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

The graph at right shows the temperature and density of the Sun's atmosphere from Skylab observations. Above and in contact with the outer surface of the photosphere is considered to be the solar atmosphere.

"The Sun's photosphere has a temperature between 4500 and 6000 K[43] (with an effective temperature of 5777 K) and a density of about [2 x 10-4kg/m3;[44] other stars may have hotter or cooler photospheres.

Sun edit

The GOES 14 spacecraft carries a Solar X-ray Imager that took this image of the Sun during the most recent quiet period. The Sun appears dark because of the wavelength band of observation. Credit: NOAA/Space Weather Prediction Center and the NWS Internet Services Team.

The GOES 14 spacecraft carries a Solar X-ray Imager that took this image at right of the Sun during the most recent quiet period. The Sun appears dark because the wavelength band of observation is of higher energy than any X-rays being produced.

Except for X-ray emission that suggests a circular disc with some isolated X-ray sources at specific locations, the Sun is almost invisible. X-rays are primarily emitted from plasmas near 106 K.

Photospheres edit

This is a visual image of the Sun with some sunspots visible on the photosphere. The two small spots in the middle have about the same diameter as our planet Earth. Credit: NASA.

Def. "[a] visible surface layer of a star, and especially that of a sun"[11] is called a photosphere.

"When we speak of the surface of the Sun, we normally mean the photosphere."[45] "[T]he photosphere may be thought of as the imaginary surface from which the solar light that we see appears to be emitted. The diameter quoted for the Sun usually refers to the diameter of the photosphere."[45] The photosphere emits visual, or visible, radiation.

The solar photosphere is a "weakly ionized [ni/(ni + na)] ~ 10-4, relatively cold and dense plasma".[46]

Coronal clouds edit

The high temperature of the coronal cloud indicates a plasma's temperature in excess of 106 Kelvin (MK).

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."[38]

An X-ray classification of stars may actually need to focus on the coronal clouds surrounding the stars rather than the stars themselves. A star always has a photosphere that does not emit X-rays, although not all photospheres are as spherical as the Sun.

Solar nanoflares edit

Typical flaring coronal loops are observed by TRACE in the EUV rays. Credit: .

A nanoflare is a very small solar flare which happens in the corona, the external atmosphere of the Sun. Observations show that the solar magnetic field, which is frozen into the motion of the plasma opens into semicirculal structures in the corona. These coronal loops, which can be seen in the EUV and X-ray images (see the figure on the left), confine very hot plasma, emitting as it were at a temperature of a few million degrees. Many flux tubes are stable for several days on the solar corona in the X-ray images, emitting at steady rate. However flickerings, brightenings, small explosions, bright points, flares and mass eruptions are observed very frequently, especially in active regions. These macroscopic signs of solar activity are considered by astrophysicists as the phenomenology related to events of relaxation of stressed magnetic fields, during which part of the coronal heating is released by current dissipation or Joule effect. These nanoflares might be very tiny flares, so close one to each other, both in time and in space, to heat the corona and to cause all the phenomena due to solar activity.

The distribution of the number of flares observed in the hard X-rays is a function of the energy, following a power law with negative spectral index 1.8.[47][48][49] [50] If this distribution would have the same spectral index also at lower energies, flares, micro-flares and nanoflares might provide a considerable part of coronal heating. Actually a negative spectral index of the order of 2 is required in order to maintain the solar corona.

"[T]he importance of the magnetic field is recognized by all the scientists: there is a strict correspondence between the active regions, where the irradiated flux is higher (especially in the X-rays), and the regions of intense magnetic field.[51]

More energy is released in turbulent regimes when nanoflares happen at much smaller scale-lengths, where non-linear effects are not negligible.[52]

Surface fusions edit

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 fusion usually occurs within a star as a part of stellar nucleosynthesis. But, 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.

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.[53]

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.

Sunspot cycles edit

This is a montage of solar activity during a sunspot cycle. Credit: David Chenette, Joseph B. Gurman, Loren W. Acton.

The image at right is a montage of ten years' worth of Yohkoh SXT images, demonstrating the variation in solar activity during a sunspot cycle, from after August 30, 1991, to September 6, 2001.

Venus edit

This Chandra image is the first X-ray image ever made of Venus. Credit: NASA/MPE/K.Dennerl et al..

The lower right image is the first X-ray image ever made of Venus. It "shows a half crescent due to the relative orientation of the Sun, Earth and Venus. The X-rays from Venus are produced by fluorescent radiation from oxygen and other atoms in the atmosphere between 120 and 140 kilometers above the surface of the planet."[54] The fluorescent source of the X-rays places Venus in the gas dwarf category even though a rocky object lies some 120 km beneath this layer.

Earth edit

Bright X-ray arcs are generated during auroral activity. Observation dates: 10 pointings between December 16, 2003 and April 13, 2004. Instrument: HRC. Credit: NASA/MSFC/CXC/A.Bhardwaj & R.Elsner, et al.; Earth model: NASA/GSFC/L.Perkins & G.Shirah.

Bright X-ray arcs of low energy (0.1 - 10 keV) are generated during auroral activity above the Earth. The images at right are superimposed on a simulated image of the Earth. The color code represents brightness, maximum in red. Distance from the North pole to the black circle is 3,340 km (2,080 mi).

"Auroras are produced by solar storms that eject clouds of energetic charged particles. These particles are deflected when they encounter the Earth’s magnetic field, but in the process large electric voltages are created. Electrons trapped in the Earth’s magnetic field are accelerated by these voltages and spiral along the magnetic field into the polar regions. There they collide with atoms high in the atmosphere and emit X-rays".[55]

Moon edit

The Chandra observations at right of the bright portion of the Moon detect X-rays from oxygen, magnesium, aluminum and silicon atoms. Credit: Optical: Robert Gendler; X-ray: NASA/CXC/SAO/J.Drake et al.

The Chandra X-ray Observatory, right image, detects X-rays from the Moon. These X-rays are produced by fluorescence when solar X-rays bombard the Moon's surface. Close inspection of the Chandra X-ray image shows a region of X-rays in the dark region trending toward the lower left corner of the X-ray image. These X-rays only appear to come from the Moon. Instead, they originate from radiation of the Earth's geocorona (an extended outer atmosphere) through which orbiting spacecraft such as the Chandra satellite move.

Mars edit

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

The upper image at right is an X-ray image of Mars. X-radiation from the Sun excites oxygen atoms in the Martian upper atmosphere, about 120 km above its surface, to emit X-ray fluorescence. A faint X-ray halo that extends out to 7,000 km above the surface of Mars has also been found.[56] The Chandra X-ray Observatory image on the right is the first look at X-rays from Mars. If X-ray astronomy was the first astronomy to view Mars, the conclusion that the X-rays are fluorescence rather than emission is important. The first class of X-ray sources may be fluorescence sources rather than emission sources.

Jupiter edit

This image of Jupiter shows concentrations of auroral X-rays near the north and south magnetic poles. The Chandra X-ray Observatory accumulated X-ray counts from Jupiter for its entire 10-hour rotation on December 18, 2000. Credit: NASA/CXC/SWRI/G.R.Gladstone et al.

The "image of Jupiter [at right] shows concentrations of auroral X-rays near the north and south magnetic poles."[57] The Chandra X-ray Observatory accumulated X-ray counts from Jupiter for its entire 10-hour rotation on December 18, 2000. Note that X-rays from the entire globe of Jupiter are detected.

The X-rays from the poles of Jupiter are not fluorescent X-rays but emission X-rays. Those from the main portion of Jupiter may be fluorescence with some reflectance.

Saturn edit

X-rays from Saturn have a spectrum similar to that of the Sun indicating that Saturn's X-radiation is due to reflection of solar X-rays by Saturn's atmosphere.

Sirius edit

This Hubble Space Telescope image shows Sirius A, the brightest star in our nighttime sky, along with its faint, tiny stellar companion, Sirius B. Credit: H. Bond (STScI) and M. Barstow (University of Leicester), NASA, ESA.
An X-ray image of the Sirius star system located 8.6 light years from Earth is shown. Credit: NASA/SAO/CXC.

An example of the differences between visual stellar classification and a possible X-ray classification is the disparity between the image of Sirius A [at above centre in the overexposed Hubble image] with the dim Sirius B [tiny dot at lower left]. [The cross-shaped diffraction spikes and concentric rings around Sirius A, and the small ring around Sirius B, are artifacts produced within the telescope's imaging system.] And, the lower image of the same two stars in X-rays.

This image shows two sources and a spike-like pattern due to the support structure for the transmission grating. The bright source is Sirius B, a white dwarf star that has a surface temperature of about 25,000 degrees Celsius which produces very low energy X-rays. The dim source at the position of Sirius A – a normal star more than twice as massive as the Sun – may be due to ultraviolet radiation from Sirius A leaking through the filter on the detector. In contrast, Sirius A is the brightest star in the northern sky when viewed with an optical telescope, while Sirius B is 10,000 times dimmer.

In the bottom image, Sirius B clearly outshines Sirius A. However, in the visual range the reverse is the case as shown in the top image. The surface effective temperature of Sirius A (spectral type A1V) is only 9,940±210 K,[58] while that of Sirius B (a white dwarf, DA2) is 25,200 K.[59] On the surface temperature of the photosphere alone, Sirius B would be a Class B star.

Stellar sciences edit

In stellar science, a division of astronomical objects between rocky bodies and gas bodies (including gas giants and stars) may be natural and informative. This division allows moons like Io to be viewed as rocky objects like Earth, a part of planetary science or rocky object science rather than as a satellite around a gas giant like Jupiter. A further benefit is the view of gaseous objects as potential stars, failed stars, or stars radiant over peak radiation bands, especially those that emit X-rays. Gaseous objects need not be considered alone but in binaries and multiple gaseous object systems including clusters, galaxies, and galaxy clusters.

Stellar classifications edit

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

The luminosity class [is] expressed by the Roman numbers I, II, III, IV and V, expressing the width of certain absorption lines in the star's spectrum.

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.[60] The effective temperature of the surface of the Sun's photosphere is 5,778 K.[61]

"When we speak of the surface of the Sun, we normally mean the photosphere."[45] "[T]he photosphere may be thought of as the imaginary surface from which the solar light that we see appears to be emitted. The diameter quoted for the Sun usually refers to the diameter of the photosphere."[45]

Class Temperature[62]
Conventional color Apparent color[63][64][65] Mass[62]
(solar masses, Mʘ)
(solar radii, Rʘ)
(bolometric, Lʘ)
Fraction of all
main sequence stars[66]
O ≥ 33,000 K blue blue ≥ 16 ≥ 6.6 ≥ 30,000 Weak ~0.00003%
B 10,000–33,000 K blue to blue white blue white 2.1–16 1.8–6.6 25–30,000 Medium 0.13%
A 7,500–10,000 K white white to blue white 1.4–2.1 1.4–1.8 5–25 Strong 0.6%
F 6,000–7,500 K yellowish white white 1.04–1.4 1.15–1.4 1.5–5 Medium 3%
G 5,200–6,000 K yellow yellowish white 0.8–1.04 0.96–1.15 0.6–1.5 Weak 7.6%
K 3,700–5,200 K orange yellow orange 0.45–0.8 0.7–0.96 0.08–0.6 Very weak 12.1%
M ≤ 3,700 K red orange red ≤ 0.45 ≤ 0.7 ≤ 0.08 Very weak 76.45%.

All of these stellar classes and the ones below are too low in temperature to emit X-rays from their photospheres.

There are extended spectral types:

  1. Class W: surface effective temperature up to 70,000 K,
  2. Class L: 1,300–2,000 K,
  3. Class T: ~700-1,300 K, and
  4. Class Y: < 600 K.

Others are based on spectral lines.

Brown dwarfs edit

Chandra image of LP 944-20 before flare and during flare. Credit: .

Some brown dwarfs emit X-rays. Here are some X-ray milestones from the same article:

  • 1998: First X-ray-emitting brown dwarf found. Cha Halpha 1, an M8 object in the Chamaeleon I dark cloud, is determined to be an X-ray source, similar to convective late-type stars.
  • December 15, 1999: First X-ray flare detected from a brown dwarf. A team at the University of California monitoring LP 944-20 (60 Jupiter masses, 16 ly away) via the Chandra X-ray Observatory, catches a 2-hour flare.

X-ray flares detected from brown dwarfs since late 1999 suggest changing magnetic fields similar to those in very low-mass stars. When combined with the rapid rotation that most brown dwarfs exhibit, conditions [may exist] for the development of a strong, tangled magnetic field near the surface. The flare observed by Chandra from LP 944-20 could have its origin in the turbulent magnetized hot material that may conduct heat to the atmosphere, allowing electric currents to flow and produce an X-ray flare, like a stroke of lightning. The absence of X-rays from LP 944-20 during the non flaring period is also a significant result. It sets the lowest observational limit on steady X-ray power produced by a brown dwarf star, and shows that coronas cease to exist as the surface temperature of a brown dwarf cools below about 2500°C and becomes electrically neutral.

Using NASA's Chandra X-ray Observatory, scientists have detected X-rays from a low-mass brown dwarf in a multiple star system.[67] This is the first time that a brown dwarf this close to its parent star(s) (Sun-like stars TWA 5A) has been resolved in X-rays.[61] "Our Chandra data show that the X-rays originate from the brown dwarf's coronal plasma which is some 3 million degrees Celsius", said Yohko Tsuboi of Chuo University in Tokyo.[61] "This brown dwarf is as bright as the Sun today in X-ray light, while it is fifty times less massive than the Sun", said Tsuboi.[61] "This observation, thus, raises the possibility that even massive planets might emit X-rays by themselves during their youth!"[61]

O stars edit

"Both the Einstein and ROSAT all-sky surveys have shown that nearly all O stars are X-ray emitters"[68], with luminosities given by the relation:

Lx ≈ 10-7 Lbol,

where wind attenuation of X-rays may be significant.[68]

Interpretation of the Einstein X-ray spectra indicate emission from a hot gas at temperatures of typically 3 x 106 to 9 x 106 K.[27] Most of the X-ray emission from OB supergiants comes from regions well above a base coronal zone, since there is too little absorption of the coronal emission by the material in the intervening wind.[24][26]

Red dwarfs edit

Red dwarfs are by far the most common type of star in the Galaxy, at least in the neighborhood of the Sun.

Proxima Centauri edit

Proxima Centauri, the nearest star to the Sun, is a red dwarf (Type M5, apparent magnitude 11.05), as are twenty of the next thirty nearest. One mystery which has not been solved as of 2009 is the absence of red dwarf stars with no metals. (In astronomy, a metal is any element heavier than hydrogen or helium.)

Both the Einstein X-ray Observatory and ROSAT have detected X-rays from Proxima Centauri. It is X-ray sources (1E 1425.9-6228, 2E 1426.0-6227, 2E 3278, and 1ES 1426-62.4) per Einstein and (RE J1429-624, RE J142950-624056, RX J1429.7-6240, 1RXS J142947.9-624058, and [FS2003] 0708) per ROSAT. Proxima Centauri is a known flare star.

Lalande 21185 edit

Lalande 21185 is still a primary standard for M2 V. Robert Garrison[69] does not list any "anchor" standards among the M dwarf stars, but Lalande 21185 has survived as a M2 V standard through many compendia[70][71][72].

Lalande 21185 (a known flare-star) is X-ray source XBS J110320.1+355803, detected by the X-ray astronomy observatory XMM-Newton.[73]

Standard red dwarfs edit

A group at Steward Observatory (Kirkpatrick, Henry, & McCarthy 1991)[74] filled in the spectral sequence from K5 V to M9 V. It is these M type dwarf standard stars which have largely survived intact as the main standards to the modern day. There have been negligible changes in the M dwarf spectral sequence since 1991. Additional M dwarf standards were compiled by Henry et al. (2002)[75], and D. Kirkpatrick has recently reviewed the classification of M dwarf stars and standard stars in Gray & Corbally's 2009 monograph[76]. The M-dwarf primary spectral standards are: GJ 270 (M0 V), GJ 229A (M1 V), Lalande 21185 (M2 V), GJ 752A (M3 V), GJ 402 (M4 V), GJ 51 (M5 V), Wolf 359 (M6 V), Van Biesbroeck 8 (M7 V), VB 10 (M8 V), LHS 2924 (M9 V).


GJ 229A is an X-ray source detected by Einstein and ROSAT,

GJ 752A is an X-ray source detected by Einstein,

GJ 51 is an X-ray source detected by ROSAT,

Wolf 359 is an X-ray source detected by ASCA, Einstein, and ROSAT,

Van Biesbroeck 8 (HD 152751) is an X-ray source detected by Einstein and ROSAT,

not yet detected as X-ray sources are GJ 270, GJ 402, VB 10, and LHS 2924.

White dwarfs edit

[S]pectroscopy typically shows that white dwarf emitted light comes from an atmosphere which is observed to be either hydrogen-dominated or helium-dominated. The dominant element is usually at least 1,000 times more abundant than all other elements.

The system currently in use classifies a spectrum by a symbol which consists of an initial D, a letter describing the primary feature of the spectrum followed by an optional sequence of letters describing secondary features of the spectrum, and a temperature index number, computed by dividing 50,400 K by the effective temperature. The visible radiation emitted by white dwarfs varies over a wide color range, from the blue-white color of an O-type main sequence star to the red of a M-type red dwarf.[77] White dwarf effective surface temperatures extend from over 150,000 K[78] to barely under 4,000 K.[79][80] One of the coolest so far observed, WD 0346+246, has a surface temperature of approximately 3,900 K.[79]


  1. A white dwarf with only He I lines in its spectrum and an effective temperature of 15,000 K could be given the classification of DB3, or, if warranted by the precision of the temperature measurement, DB3.5.
  2. A white dwarf with a polarized magnetic field, an effective temperature of 17,000 K, and a spectrum dominated by He I lines which also had hydrogen features could be given the classification of DBAP3.

The symbols ? and : may also be used if the correct classification is uncertain.[78][77]

White dwarf spectral types[78]
Primary and secondary features
A H lines present; no He I or metal lines
B He I lines; no H or metal lines
C Continuous spectrum; no lines
O He II lines, accompanied by He I or H lines
Z Metal lines; no H or He I lines
Q Carbon lines present
X Unclear or unclassifiable spectrum
Secondary features only
P Magnetic white dwarf with detectable polarization
H Magnetic white dwarf without detectable polarization
E Emission lines present
V Variable

Hot white dwarfs, with surface temperatures in excess of 30,000 K, have been observed to be sources of soft (i.e., lower-energy) X-rays. This enables the composition and structure of their atmospheres to be studied by soft X-ray and extreme ultraviolet observations.[81]

The first magnetic white dwarf to be observed was GJ 742, which was detected to have a magnetic field in 1970 by its emission of circularly polarized light.[82] Magnetic fields have been discovered in well over 100 white dwarfs, ranging from 2 x 103 to 109 gauss (0.2 T to 100 kT). The magnetic fields in a white dwarf star may allow for the existence of a new type of chemical bond, perpendicular paramagnetic bonding, in addition to ionic and covalent bonds, resulting in what has been initially described as "magnetized matter"[83]

The first variable white dwarf found was HL Tau 76; in 1965 and 1966, it varied with a period of approximately 12.5 minutes.[84]

Known types of pulsating white dwarf include the DAV, or ZZ Ceti, stars, including HL Tau 76, with hydrogen-dominated atmospheres and the spectral type DA;[85], pp. 891, 895 DBV, or V777 Her, stars, with helium-dominated atmospheres and the spectral type DB;[80], p. 3525 and GW Vir stars (sometimes subdivided into DOV and PNNV stars), with atmospheres dominated by helium, carbon, and oxygen.[86],§1.1, 1.2;[87],§1. GW Vir stars are not, strictly speaking, white dwarfs, but are stars which are in a position on the Hertzsprung-Russell diagram between the asymptotic giant branch and the white dwarf region. They may be called pre-white dwarfs.[86], § 1.1;[88] These variables all exhibit small (1%–30%) variations in light output, arising from a superposition of vibrational modes with periods of hundreds to thousands of seconds. Observation of these variations gives asteroseismological evidence about the interiors of white dwarfs.[89]

The first white dwarf discovered was in the triple star system of 40 Eridani, which contains the relatively bright main sequence star 40 Eridani A, orbited at a distance by the closer binary system of the white dwarf 40 Eridani B and the main sequence red dwarf 40 Eridani C.

40 Eridani B edit

The pair 40 Eridani B/C was discovered by William Herschel on 31 January 1783;[90], p. 73 it was again observed by Friedrich Georg Wilhelm Struve in 1825 and by Otto Wilhelm von Struve in 1851.[91][92] In 1910, Henry Norris Russell, Edward Charles Pickering and Williamina Fleming discovered that, despite being a dim star, 40 Eridani B was of spectral type A, or white.[93] The spectral type of 40 Eridani B was officially described in 1914 by Walter Adams.[94]

Per SIMBAD, 40 Eridani B is an X-ray source detected by the Einstein X-ray Observatory.

Sirius B edit

It was not until 31 January 1862 that Alvan Graham Clark observed a previously unseen star close to Sirius, later identified as the predicted companion.[95] Walter Adams announced in 1915 that he had found the spectrum of Sirius B to be similar to that of Sirius.[96]

Per SIMBAD, Sirius B is an X-ray source detected by the Einstein X-ray Observatory, HEAO 1, and ROSAT.

Van Maanen's Star edit

In 1917, Adriaan Van Maanen discovered Van Maanen's Star, an isolated white dwarf.[97]

Van Maanen's Star is not a known X-ray source per SIMBAD.

Hypotheses edit

  1. The temperature of the photosphere results from a balance between the charge on the surface of the photosphere and the energy of the incoming electron flux.

See also edit

References edit

  1. Anil Bhardwaj; Ronald F. Elsner; G. Randall Gladstone; Thomas E. Cravens; Carey M. Lisse; Konrad Dennerl; Graziella Branduardi-Raymont; Bradford J. Wargelin et al. (June 2007). "X-rays from solar system objects". Planetary and Space Science 55 (9): 1135-89. doi:10.1016/j.pss.2006.11.009. Retrieved 2013-05-23. 
  3. Lauren J. Bryant. What Makes Stars Tick. Retrieved September 7, 2005. 
  4. A Dictionary of Units of Measurement
  5. John C. Martin. What we learn from a star's metal content. Retrieved September 7, 2005. 
  6. Philip B. Gove, ed (1963). Webster's Seventh New Collegiate Dictionary. Springfield, Massachusetts: G. & C. Merriam Company. pp. 1221. 
  7. light. San Francisco, California: Wikimedia Foundation, Inc. July 13, 2013. Retrieved 2013-07-13. 
  8. illuminate. San Francisco, California: Wikimedia Foundation, Inc. 20 June 2013. Retrieved 2013-07-13. 
  9. luminous. San Francisco, California: Wikimedia Foundation, Inc. May 19, 2013. Retrieved 2013-07-13. 
  10. Anthony Whitworth; Dimitri Stamatellos; Steffi Walch; Murat Kaplan; Simon Goodwin; David Hubber; Richard Parker (2009). R. de Grijs & J. R. D. Lépine. ed. The formation of brown dwarfs, In: Star clusters: basic galactic building blocks, Proceedings IAU Symposium No. 266. International Astronomical Union. pp. 264-71. doi:10.1017/S174392130999113X. Retrieved 2011-10-30. 
  11. 11.0 11.1 photosphere. San Francisco, California: Wikimedia Foundation, Inc. 30 August 2012. Retrieved 2012-11-23. 
  12. chromosphere. San Francisco, California: Wikimedia Foundation, Inc. November 10, 2013. Retrieved 2013-12-29. 
  13. 13.0 13.1 corona. San Francisco, California: Wikimedia Foundation, Inc. May 9, 2012. Retrieved 2013-12-29. 
  14. solar wind. San Francisco, California: Wikimedia Foundation, Inc. October 7, 2013. Retrieved 2013-12-29. 
  15. stellar wind. San Francisco, California: Wikimedia Foundation, Inc. December 10, 2013. Retrieved 2013-12-29. 
  16. heliosphere. San Francisco, California: Wikimedia Foundation, Inc. November 7, 2013. Retrieved 2013-12-29. 
  17. P Morrison. "Extrasolar X-ray Sources". Annual Review of Astronomy and Astrophysics 1967 5 (1): 325–50. doi:10.1146/annurev.aa.05.090167.001545. 
  18. 18.0 18.1 Mike Wall (21 February 2013). Super-Hot Plasma 'Rain' Falls on Sun in Amazing Video. Yahoo! News. Retrieved 2013-02-23. 
  19. David F. Webb, Timothy A. Howard (2012). "Coronal Mass Ejections: Observations". Living Reviews in Solar Physics 9: 3. Retrieved 2012-11-11. 
  20. 20.0 20.1 Paul Przyborski (13 February 2010). Coronal Mass Ejection in late January 2010. NASA Earth Observatory. Retrieved 2012-11-26. 
  21. Harold Zirin (October 1964). "The Limb Flare of November 20, 1960: a Coronal Phenomenon". Astrophysical Journal 140 (10): 1216-35. doi:10.1086/148019. 
  22. Wang, Y. M., et al., Multiple magnetic clouds in interplanetary space, Solar Physics, 211, 333-344, 2002.
  23. Wang, Y. M., et al., Multiple magnetic clouds: Several examples during March - April, 2001, J. Geophys. Res., 108(A10), 1370, 2003.
  24. D. L. Bertsch; C. E. Fichtel; D. V. Reames (July 1969). "Relative Abundance of Iron-Group Nuclei in Solar Cosmic Rays". The Astrophysical Journal 157 (07): L53-6. doi:10.1086/180383. Retrieved 2012-11-27. 
  25. flux. San Francisco, California: Wikimedia Foundation, Inc. 24 January 2014. Retrieved 2014-01-25. 
  26. R. Byron Bird; Warren E. Stewart; Edwin N. Lightfoot (2007). Transport Phenomena. John Wiley & Sons. pp. 905. ISBN 0470115394. 
  27. Steve Cole; Jia-Rui C. Cook; Alan Buis (December 2011). NASA's Voyager Hits New Region at Solar System Edge. Washington, DC: NASA. Retrieved 2012-02-09. 
  28. 28.0 28.1 28.2 28.3 28.4 Gerald H. Share; Ronald J. Murphy (January 2004). Andrea K. Dupree. ed. Solar Gamma-Ray Line Spectroscopy – Physics of a Flaring Star, In: Stars as Suns: Activity, Evolution and Planets. San Francisco, CA: Astronomical Society of the Pacific. pp. 133-44. ISBN 158381163X. Bibcode: 2004IAUS..219..133S. Retrieved 2012-03-15. 
  29. Jana Kašparová; Eduard P. Kontar; John C. Brown (1 May 2007). "Hard X-ray Spectra and Positions of Solar Flares observed by RHESSI: photospheric albedo, directivity and electron spectra". Astronomy & Astrophysics 466 (2): 705-12. doi:10.1051/0004-6361:20066689. Retrieved 2012-11-27. 
  30. 30.0 30.1 Martin V. Zombeck (1990). Calibration of MK spectral types, In: Handbook of Space Astronomy and Astrophysics (2nd ed.). Cambridge University Press. p. 105. ISBN 0-521-34787-4. 
  31. David F. Gray 1992 (November). "The Inferred Color Index of the Sun". Publications of the Astronomical Society of the Pacific 104 (681): 1035-8. 
  32. {{ cite book title=Rigel |url= }}
  33. D. Baumüller; K. Butler; T. Gehren (October 1998). "Sodium in the Sun and in metal-poor stars". Astronomy and Astrophysics 338: 637-50. 
  34. C. T. Russell; D. N. Baker; J. A. Slavin (1 January 1988). Faith Vilas. ed. The Magnetosphere of Mercury, In: Mercury. Tucson, Arizona, United States of America: University of Arizona Press. pp. 514-61. ISBN 0816510857. Bibcode: Retrieved 2012-08-23. 
  35. Ron Miller (2005). Stars and Galaxies. Twenty-First Century Books. p. 22. ISBN 9780761334668. 
  36. Q-Z Luo; N. D'Angelo; R. L. Merlino (1998). Shock formation in a negative ion plasma. 5. Department of Physics and Astronomy. Retrieved 2011-11-20. 
  37. 37.0 37.1 CK Birdsall, A. Bruce Langdon (1 October 2004). Plasma Physics via Computer Simulation. New York: CRC Press. pp. 479. ISBN 9780750310253. Retrieved 2011-12-17. 
  38. 38.0 38.1 Markus J. Aschwanden (2007). Erdelyi R. ed. "Fundamental Physical Processes in Coronae: Waves, Turbulence, Reconnection, and Particle Acceleration In: Waves & Oscillations in the Solar Atmosphere: Heating and Magneto-Seismology". Proceedings IAU Symposium 3 (S247): 257–68. doi:10.1017/S1743921308014956. 
  39. 39.0 39.1 Axel Brandenburg; Åke Nordlund; Robert F. Stein; Ulf Torkelsson (June 1995). "Dynamo-generated Turbulence and Large-Scale Magnetic Fields in a Keplerian Shear Flow". The Astrophysical Journal 446 (6): 741-54. doi:10.1086/175831. 
  40. H. N. Russell (1929). The Astrophysical Journal 70: 11-82. 
  41. Sarbani Basu; H. M. Antia (March 2008). "Helioseismology and Solar Abundances". Physics Reports 457 (5-6): 217-83. doi:10.1016/j.physrep.2007.12.002. 
  42. Jeremy R. King; Constantine P. Deliyannis; Merchant Boesgaard (1 April 1997). "The 9Be Abundances of α Centauri A and B and the Sun: Implications for Stellar Evolution and Mixing". The Astrophysical Journal 478 (2): 778. Retrieved 2012-07-11. 
  43. The Sun - Introduction
  44. SP-402 A New Sun: The Solar Results From Skylab. 
  45. 45.0 45.1 45.2 45.3 Mike Guidry (16 April 1999). The Photosphere of the Sun. University of Tennessee. Retrieved 2006-10-12. 
  46. M. L. Khodachenko; V. V. Zaitsev (1 March 2002). "Formation of Intensive Magnetic Flux Tubes in a Converging Flow of Partially Ionized Solar Photospheric Plasma". Astrophysics and Space Science 279 (4): 389-410. doi:10.1023/A:1015162131331. Retrieved 2013-07-17. 
  47. D.W. Datlowe; M.J.Elean; H.S. Hudson (1974). "OSO-7 observations of solar x-rays in the energy range 10?100 keV". Solar Physics 39: 155. doi:10.1007/BF00154978. 
  48. Lin R.P.; Schwartz R.A.; Kane S.R.; Pelling R.M.; Hurley K.C.. "Solar hard X-ray microflares". The Astrophysical Journal 283: 421. doi:10.1086/162321. 
  49. Brian R. Dennis (1985). "Solar hard X-ray bursts". Solar Physics 100: 465. doi:10.1007/BF00158441. 
  50. Porter J.G.; Fontenla J.M.; Simnett G.M. "Simultaneous ultraviolet and X-ray observations of solar microflares". The Astrophysical Journal 438: 472. doi:10.1086/175091. 
  51. Poletto G; Vaiana GS; Zombeck MV; Krieger AS; Timothy AF (September 1975). "A comparison of coronal X-ray structures of active regions with magnetic fields computed from photospheric observations". Solar Physics 44 (9): 83–99. doi:10.1007/BF00156848. 
  52. Rappazzo, A. F.; Velli, M.; Einaudi, G.; Dahlburg, R. B.. "Nonlinear Dynamics of the Parker Scenario for Coronal Heating". The Astrophysical Journal 2008 677 (2): 1348–1366. doi:10.1086/528786. 
  53. Fireman EL; Damico J; Defelice J (March 1975). Solar-wind tritium limit and nuclear processes in the solar atmosphere, In: Lunar Science Conference Proceedings 6th Houston TX. New York: Pergamon Press. pp. 1811-21. Bibcode: 1975LPSC....6.1811F. 
  54. File:Venus xray 420.jpg. San Francisco, California: Wikimedia Foundation, Inc. 28 October 2010. Retrieved 2012-08-08. 
  55. A. Bhardwaj; R. Elsner (20 February 2009). Earth Aurora: Chandra Looks Back At Earth. Cambridge, Massachusetts, USA: Harvard-Smithsonian Center for Astrophysics. Retrieved 2013-05-10. 
  56. K. Dennerl (November 2002). "Discovery of X-rays from Mars with Chandra". Astronomy & Astrophysics 394 (11): 1119-28. doi:10.1051/0004-6361:20021116. 
  57. NASA/CXC/SWRI/G.R.Gladstone (27 February 2002). Jupiter Hot Spot Makes Trouble For Theory. Cambridge, Massachusetts: Harvard-Smithsonian Center for Astrophysics. Retrieved 2012-07-11. 
  58. Adelman, Saul J. (July 8–13, 2004). The Physical Properties of normal A stars. Poprad, Slovakia: Cambridge University Press. pp. 1–11. Bibcode: 2004IAUS..224....1A. 
  59. Liebert, J.; Young, P. A.; Arnett, D.; Holberg, J. B.; Williams, K. A. (2005). "The Age and Progenitor Mass of Sirius B". The Astrophysical Journal 630 (1): L69–L72. doi:10.1086/462419. 
  60. The Colour of Stars. Australian Telescope Outreach and Education. Retrieved 2006-08-13. 
  61. 61.0 61.1 61.2 61.3 61.4 David R. Williams (September 2004). Sun Fact Sheet. Greenbelt, MD: NASA Goddard Space Flight Center. Retrieved 2011-12-20. 
  62. 62.0 62.1 62.2 62.3 Tables VII, VIII, Empirical bolometric corrections for the main-sequence, G. M. H. J. Habets and J. R. W. Heinze, Astronomy and Astrophysics Supplement Series 46 (November 1981), pp. 193–237, bibcode=1981A&AS...46..193H. Luminosities are derived from Mbol figures, using Mbol(ʘ)=4.75.
  63. The Guinness book of astronomy facts & feats, Patrick Moore, 1992, 0-900424-76-1
  64. The Colour of Stars. Australia Telescope Outreach and Education. 21 December 2004. Retrieved 2007-09-26.  — Explains the reason for the difference in color perception.
  65. What color are the stars?, Mitchell Charity. Accessed online March 19, 2008.
  66. Glenn LeDrew (February 2001). "The Real Starry Sky". Journal of the Royal Astronomical Society of Canada 95 (1 (whole No. 686, February 2001), pp. 32–33. Note: Table 2 has an error and so this article will use 824 as the assumed correct total of main-sequence stars). 
  67. X-rays from a Brown Dwarf's Corona. 14 April 2003. 
  68. 68.0 68.1 D. H. Cohen; J. P. Cassinelli; J. J. MacFarlane (1 October 1997). "ROSAT PSPC Observations of 27 Near-Main-Sequence B Stars". The Astrophysical Journal 487 (2): 867-84. doi:10.1086/304636. Retrieved 2012-03-26. 
  69. MK ANCHOR POINTS, Robert F. Garrison
  70. Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas H.L. Johnson & W.W. Morgan, 1953, Astrophysical Journal, 117, 313
  71. The Perkins Catalog of Revised MK Types for the Cooler Stars, P.C. Keenan & R.C McNeil, "Astrophysical Journal Supplement Series" 71 (October 1989), pp. 245–266.
  72. A standard stellar spectral sequence in the red/near-infrared - Classes K5 to M9 Kirkpatrick, J. D.; Henry, Todd J.; McCarthy, Donald W., Jr., 1991, Astrophysical Journal Supplement Series, vol. 77, Nov. 1991, p. 417
  73. Roberto Della Ceca; T. Maccacaro; A. Caccianiga; P. Severgnini; V. Braito; X. Barcons; F.J. Carrera; M.G. Watson et al. (December 2004). "Exploring the X-ray Sky with the XMM-Newton Bright Serendipitous Survey". Astronomy and Astrophysics 428 (12): 383-99. doi:10.1051/0004-6361:20040252. 
  74. A standard stellar spectral sequence in the red/near-infrared - Classes K5 to M9 Kirkpatrick, J. D.; Henry, Todd J.; McCarthy, Donald W., Jr., 1991, Astrophysical Journal Supplement Series, vol. 77, Nov. 1991, p. 417-440
  75. The Solar Neighborhood. VI. New Southern Nearby Stars Identified by Optical Spectroscopy Henry, Todd J.; Walkowicz, Lucianne M.; Barto, Todd C.; Golimowski, David A., 2002, The Astronomical Journal, Volume 123, Issue 4, pp. 2002-2009
  76. Stellar Spectral Classification Richard O. Gray and Christopher J. Corbally, 2009, Princeton University Press. ISBN 978-0-691-12511-4
  77. 77.0 77.1 Sion, E. M.; Greenstein, J. L.; Landstreet, J. D.; Liebert, J.; Shipman, H. L.; Wegner, G. A. (1983). "A proposed new white dwarf spectral classification system". The Astrophysical Journal 269: 253. doi:10.1086/161036. 
  78. 78.0 78.1 78.2 McCook, George P.; Sion, Edward M. (1999). "A Catalog of Spectroscopically Identified White Dwarfs". The Astrophysical Journal Supplement Series 121: 1. doi:10.1086/313186. 
  79. 79.0 79.1 Hambly, N. C.; Smartt, S. J.; Hodgkin, S. T. (1997). "WD 0346+246: A Very Low Luminosity, Cool Degenerate in Taurus". The Astrophysical Journal 489 (2): L157. doi:10.1086/316797. 
  80. 80.0 80.1 Fontaine, G.; Wesemael, F. (2001). "White dwarfs". In Murdin, P.. Encyclopedia of Astronomy and Astrophysics. IOP Publishing/Nature Publishing Group. ISBN 0-333-75088-8. 
  81. Heise, J. (1985). "X-ray emission from isolated hot white dwarfs". Space Science Reviews 40: 79. doi:10.1007/BF00212870. 
  82. Kemp, J. C.; Swedlund, J. B.; Landstreet, J. D.; Angel, J. R. P. (1970). "Discovery of Circularly Polarized Light from a White Dwarf". The Astrophysical Journal 161: L77. doi:10.1086/180574. 
  83. Stars draw atoms closer together : Nature News & Comment
  84. Landolt, A. U. (1968). "A New Short-Period Blue Variable". The Astrophysical Journal 153: 151. doi:10.1086/149645. 
  85. Koester, D.; Chanmugam, G. (1990). "Physics of white dwarf stars". Reports on Progress in Physics 53 (7): 837. doi:10.1088/0034-4885/53/7/001. 
  86. 86.0 86.1 Quirion, P.‐O.; Fontaine, G.; Brassard, P. (2007). "Mapping the Instability Domains of GW Vir Stars in the Effective Temperature–Surface Gravity Diagram". The Astrophysical Journal Supplement Series 171: 219. doi:10.1086/513870. 
  87. Nagel, T.; Werner, K. (2004). "Detection of non-radial g-mode pulsations in the newly discovered PG 1159 star HE 1429-1209". Astronomy and Astrophysics 426 (2): L45. doi:10.1051/0004-6361:200400079. 
  88. O'Brien, M. S. (2000). "The Extent and Cause of the Pre–White Dwarf Instability Strip". The Astrophysical Journal 532 (2): 1078. doi:10.1086/308613. 
  89. Winget, D. E. (1998). "Asteroseismology of white dwarf stars". Journal of Physics: Condensed Matter 10 (49): 11247. doi:10.1088/0953-8984/10/49/014. 
  90. William Herschel (1785). "Catalogue of Double Stars. By William Herschel, Esq. F. R. S". Philosophical Transactions of the Royal Society of London 75: 40–126. doi:10.1098/rstl.1785.0006. 
  91. W. H. Van Den Bos (1926). "The orbit and the masses of 40 Eridani BC". Bulletin of the Astronomical Institutes of the Netherlands 3: 128. 
  92. W. D. Heintz (1974). "Astrometric study of four visual binaries". The Astronomical Journal 79: 819. doi:10.1086/111614. 
  93. J. B. Holberg (2005). "How Degenerate Stars Came to be Known as White Dwarfs". American Astronomical Society Meeting 207 207: 1503. 
  94. Walter Sydney Adams (1914). "An A-Type Star of Very Low Luminosity". Publications of the Astronomical Society of the Pacific 26: 198. doi:10.1086/122337. 
  95. Camille Flammarion (1877). "The Companion of Sirius". Astronomical register 15: 186. 
  96. W. S. Adams (1915). "The Spectrum of the Companion of Sirius". Publications of the Astronomical Society of the Pacific 27: 236. doi:10.1086/122440. 
  97. A. Van Maanen (1917). "Two Faint Stars with Large Proper Motion". Publications of the Astronomical Society of the Pacific 29: 258. doi:10.1086/122654. 

Further reading edit

  • Manuel Güdel (September 2004). "X-ray astronomy of stellar coronae". The Astronomy and Astrophysics Review 12 (2-3): 71–237. doi:10.1007/s00159-004-0023-2. 

External links edit

{{Charge ontology}}