# 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.
But, the emission of X-rays is most often associated with a coronal cloud, a corona, or at least a high temperature plasma (about 10$^6$ K).

## X-rays

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

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

"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."

## Metallicities

The metallicity (also called Z) 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;
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:
    $[\mathrm {Fe} /\mathrm {H} ]=\log _{10}{\left({\frac {N_{\mathrm {Fe} }}{N_{\mathrm {H} }}}\right)_{star}}-\log _{10}{\left({\frac {N_{\mathrm {Fe} }}{N_{\mathrm {H} }}}\right)_{sun}}$
where $N_{\mathrm {Fe} }$ and $N_{\mathrm {H} }$ 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. 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 (10$^1$). Conversely, those with a value of -1 have one tenth (10 $^{−1}$), while those with -2 have a hundredth (10$^{−2}$), and so on. 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

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:
Def.
    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".
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" is called light.
Def. "to shine light on something" is called illuminate.
Def. "emitting light" is called luminous.
From astrophysics:
Def. "any object forming on a dynamical timescale, by gravitational instability", is called a star.
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" is called a photosphere.
Def. "[t]he faint pink extension of a star's atmospheric envelope between the corona and the photosphere" 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" 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]" 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" is called a solar wind.
Def. "[t]he equivalent of solar wind associated with a star other than our Sun" is called a stellar wind.
Def. "[t]he region of space where interstellar medium is blown away by solar wind" 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 (T$_{eff}$), 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

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.

## Meteors

"[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."
"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".
"Many CMEs have also been observed to be unassociated with any obvious solar surface activity".
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."
"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."
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."
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) and complex ejecta, which can be the result of the interaction of multiple CMEs.

## Cosmic rays

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

## Protons

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

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" is called a flux.
"Mass flux of species α [is in units of] g/cm$^2$ s" or, in dimensions [quantity of species α]·[area]$^{−1}$·[time]$^{−1}$, where an area may be cm$^2$.

## Electron winds

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."

### Surface area

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

R$_{⊙eq}$ ≈ 6.955 x 10$^5$ km. The thickness of the photosphere is about 400 km. R$_{⊙p}$ ≈ 6.951 x 10$^5$ km.
    $V_{\odot p}={\frac {4\pi }{3}}[R_{\odot eq}^{3}-R_{\odot p}^{3}]km^{3},$
    $V_{\odot p}={\frac {4\pi }{3}}[6.955^{3}-6.951^{3}]\times 10^{15}km^{3},$
    $V_{\odot p}={\frac {4\pi }{3}}(0.580)\times 10^{15}km^{3},$
    $V_{\odot p}=7.288\times 10^{15}km^{3}.$

### Photosphere hydrogens

The density of the Sun is about 2 x 10$^{-4}$ kg m$^{-3}$. Or,
    $\rho _{\odot p}=2\times 10^{-4}kg\cdot m^{-3},$
    $\rho _{\odot p}=2\times 10^{-4}kg\cdot [10^{-3}km]^{-3},$
    $\rho _{\odot p}=2\times 10^{5}kg\cdot km^{-3}.$
One mole of H$_2$ (gas) has a mass of 2.016 x 10$^{-3}$ kg. The molar density of the photosphere may be
    $\rho _{\odot p}={\frac {2\times 10^{5}kg}{2.016\times 10^{-3}kg/mole}}km^{-3},$
    $\rho _{\odot p}={\frac {2}{2.016}}{\frac {10^{5}}{10^{-3}}}{\frac {kg}{kg/mole}}km^{-3},$
    $\rho _{\odot p}=0.992\times 10^{8}moles\cdot km^{-3},$
    $\rho _{\odot p}=10^{8}moles\cdot km^{-3}.$
    $V_{\odot p}=7.288\times 10^{15}km^{3}.$
    $H_{2\odot p}=(10^{8}moles\cdot km^{-3})\cdot (7.288\times 10^{15}km^{3}),$
    $H_{2\odot p}=7.288\times 10^{23}moles.$

### Constant volume specific heat capacity

For H$_2$ (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,
    $C_{V,m}=(2.83\times 10^{-3})TJ/(mol\cdot K^{2})+19.3J/(mol\cdot K),$
for 5777 K, yields
    $C_{V,m}=(2.83\times 10^{-3})(5777)J/(mol\cdot K)+19.3J/(mol\cdot K),$
    $C_{V,m}=35.6J/(mol\cdot K).$
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.
    $\Delta Q=[35.6J/(mol\cdot K)]\cdot (5777-100)K,$
    $\Delta Q=(35.6)\cdot (5677){\frac {J}{mole\cdot K}}{K},$
    $\Delta Q=2.02\times 10^{5}J/mole.$
    $1J=6.24\times 10^{18}eV.$
    $\Delta Q=(2.02\times 10^{5}J/mole)\cdot (6.24\times 10^{18}eV/J),$
    $\Delta Q=(2.02)\cdot (6.24)\times 10^{5}\times 10^{18}eV/mole,$
    $\Delta Q=12.6\times 10^{23}eV/mole,$
    $\Delta Q=1.26\times 10^{24}eV/mole.$

### Photosphere heating

    $\Delta Q=1.26\times 10^{24}eV/mole.$
    $H_{2\odot p}=7.288\times 10^{23}moles.$
Voyager 1 is 17,932,000,000 km (119.9 AU) from the Sun at RA 17.163$^h$ 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.
    $\Phi _{e^{-}}=2e^{-}\cdot cm^{-2}\cdot s^{-1}\cdot {\frac {(10^{-2}\cdot m\times 10^{-3}\cdot km/m)^{-2}}{cm^{-2}}},$
    $\Phi _{e^{-}}=2e^{-}\cdot cm^{-2}\cdot s^{-1}\cdot {\frac {10^{10}\cdot km^{-2}}{cm^{-2}}},$
    $\Phi _{e^{-}}=2\times 10^{10}e^{-}\cdot km^{-2}\cdot s^{-1}.$
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
    $\Delta Q=(1.26\times 10^{24}eV/mole)\cdot (7.288\times 10^{23}moles),$
    $\Delta Q=(1.26)\cdot (7.288)\times (10^{24}\times 10^{23})\cdot eV,$
    $\Delta Q=9.18\times 10^{47}eV.$
The power (P) that may be deposited on the photospheric surface of the Sun is
    $P_{e^{-}}=4\pi R_{Voyager1}^{2}\cdot \Phi _{e^{-}}\cdot (10MeV/e^{-}),$
    $P_{e^{-}}=4\pi (1.7932\times 10^{10}km)^{2}\cdot (2\times 10^{10}e^{-}\cdot km^{-2}\cdot s^{-1})\cdot (10MeV/e^{-}),$
    $P_{e^{-}}=(4\pi )\cdot (1.7932)^{2}\cdot 2\times (10^{20}\times 10^{10}\times 10^{7})\cdot (km^{2}\cdot e^{-}\cdot km^{-2}\cdot s^{-1}\cdot eV/e^{-}),$
    $P_{e^{-}}=80.8\times 10^{37}\cdot eV\cdot s^{-1},$
    $P_{e^{-}}=8.08\times 10^{38}eV\cdot s^{-1}.$
The luminosity (in Watts, W) of the Sun is 3.846 x 10$^{26}$ W. In eV/s this is
    $L_{\odot }=3.846\times 10^{26}W\cdot (10^{7}erg/(s\cdot W))\cdot 6.24\times 10^{11}eV/erg,$
    $L_{\odot }=(3.846)\cdot (6.24)\times (10^{26}\times 10^{7}\times 10^{11})\cdot (W\cdot erg/(s\cdot W))\cdot eV/erg),$
    $L_{\odot }=24.0\times 10^{44}\cdot eV\cdot s^{-1},$
    $L_{\odot }=2.40\times 10^{45}\cdot eV\cdot s^{-1}.$
If the energy of the incoming electrons is 700 MeV and the flux is 8.48 x 10$^4$ e$^-$ cm$^{-2}$ s$^{-1}$, then the power from the incoming electrons would be
    $P_{e^{-}}=(8.08\times 10^{38}eV\cdot s^{-1})\cdot (70)\cdot (8.48/2\times 10^{4}),$
    $P_{e^{-}}=(8.08)\cdot (70)\cdot (4.24)\times (10^{4}\times 10^{38})eV\cdot s^{-1},$
    $P_{e^{-}}=2400\times 10^{42}eV\cdot s^{-1},$
    $P_{e^{-}}=2.40\times 10^{45}eV\cdot s^{-1}.$
The power calculated for the electron influx is compared with the first experiment and with the known luminosity of the Sun.

## Coronal heating

The power to heat the solar corona is on the order of 10$^{39}$ 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 10$^4$ 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 10$^4$th increase in the influx estimate. Actual data on interstellar electrons does suggest MeV levels.

## Positrons

The solar flare at Active Region 10039 on July 23, 2002, exhibits many exceptional high-energy phenomena including the 2.223 MeV neutron capture line and the 511 keV electron-positron (antimatter) annihilation line. In the image at right, the RHESSI low-energy channels (12-25 keV) are represented in red and appear predominantly in coronal loops. The high-energy flux appears as blue at the footpoints of the coronal loops. Violet is used to indicate the location and relative intensity of the 2.2 MeV emission.
During solar flares “[s]everal radioactive nuclei that emit positrons are also produced; [which] slow down and annihilate in flight with the emission of two 511 keV photons or form positronium with the emission of either a three gamma continuum (each photon < 511 keV) or two 511 keV photons." 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. The observations are somewhat consistent with electron-positron annihilation in a quiet solar atmosphere via positronium as well as during flares. Line-broadening is due to "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 x 10$^5$ K. ... The RHESSI and all but two of the SMM measurements are consistent with densities ≤ 10$^{12}$ H cm$^{-3}$ [but] <10% of the p and α interactions producing positrons occur at these low densities. ... positrons produced by $^3$He interactions form higher in the solar atmosphere ... all observations are consistent with densities > 10$^{12}$ H cm$^{-3}$. But such densities require formation of a substantial mass of atmosphere at transition region temperatures."

## Neutrinos

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

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

"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."

## Opticals

Sample calibration colors :
| Class | B–V | U–B | V–R | R–I | T$_{eff}$ (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

Sample calibration colors :
| Class | B–V | V–R | T$_{eff}$ (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, 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).

## Greens

The Na I green lines at 568.2 and 568.8 nm arise "in the photospheric layers between log τ$_{5000}$ ≈ -1 and -2."

## Yellows

"[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". 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".

## Plasma objects

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.
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 = n$_i$/(n$_i$ + n$_a$) where n$_i$ is the number density of ions and n$_a$ is the number density of neutral atoms. The electron density is related to this by the average charge state <Z> of the ions through n$_e$ = <Z> n$_i$ where n$_e$ is the number density of electrons.
"Plasma is the fourth state of matter, consisting of electrons, ions and neutral atoms, usually at temperatures above 10$^4$ degrees Kelvin." "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."

## Magnetohydrodynamics

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.
"[M]otions resulting from [a linear magnetohydrodynamic] instability act as a dynamo to sustain the magnetic field." "Supersonic flows are initially generated by the Balbus-Hawley magnetic shear instability." 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

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 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."

## Heliums

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

"[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"].

## Atmospheres

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 (with an effective temperature of 5777 K) and a density of about [2 x 10$^{-4}$kg/m$^3$; other stars may have hotter or cooler photospheres.

## Sun

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 10$^6$ K.

## Photospheres

Def. "[a] visible surface layer of a star, and especially that of a sun" is called a photosphere.
"When we speak of the surface of the Sun, we normally mean the photosphere." "[T]he photosphere may be thought of as the imaginary surface from which the solar light that we see appears to be emitted. The diameter quoted for the Sun usually refers to the diameter of the photosphere." The photosphere emits visual, or visible, radiation.
The solar photosphere is a "weakly ionized [n$_i$/(n$_i$ + n$_a$)] ~ 10$^{-4}$, relatively cold and dense plasma".

## Coronal clouds

The high temperature of the coronal cloud indicates a plasma's temperature in excess of 10$^6$ 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."
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

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

## Surface fusions

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 $^3$He-flare flux from the Sun's surface and Surveyor 3 samples (implanted $^{15}$N and $^{14}$C 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.
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

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

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." 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

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".

## Moon

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

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. 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

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

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

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, while that of Sirius B (a white dwarf, DA2) is 25,200 K. On the surface temperature of the photosphere alone, Sirius B would be a Class B star.

## Stellar sciences

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

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. The effective temperature of the surface of the Sun's photosphere is 5,778 K.
"When we speak of the surface of the Sun, we normally mean the photosphere." "[T]he photosphere may be thought of as the imaginary surface from which the solar light that we see appears to be emitted. The diameter quoted for the Sun usually refers to the diameter of the photosphere."
| Class | Temperature K | Conventional color | Apparent color | Mass (solar masses, M$_ʘ$) | Radius (solar radii, R$_ʘ$) | Luminosity (bolometric, L$_ʘ$) | Hydrogen lines | Fraction of all main sequence stars |
| 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:
* Class W: surface effective temperature up to 70,000 K,
* Class L: 1,300–2,000 K,
* Class T: ~700-1,300 K, and
* Class Y: < 600 K.
Others are based on spectral lines.

## Brown dwarfs

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

## O stars

"Both the Einstein and ROSAT all-sky surveys have shown that nearly all O stars are X-ray emitters", with luminosities given by the relation:
L$_x$ ≈ 10$^{-7}$ L$_{bol}$,
where wind attenuation of X-rays may be significant.
Interpretation of the Einstein X-ray spectra indicate emission from a hot gas at temperatures of typically 3 x 10$^6$ to 9 x 10$^6$ 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

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

## Proxima Centauri

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

Lalande 21185 is still a primary standard for M2 V. Robert Garrison does not list any "anchor" standards among the M dwarf stars, but Lalande 21185 has survived as a M2 V standard through many compendia.
Lalande 21185 (a known flare-star) is X-ray source XBS J110320.1+355803, detected by the X-ray astronomy observatory XMM-Newton.

## Standard red dwarfs

A group at Steward Observatory (Kirkpatrick, Henry, & McCarthy 1991) 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), and D. Kirkpatrick has recently reviewed the classification of M dwarf stars and standard stars in Gray & Corbally's 2009 monograph. 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).
Per SIMBAD:
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

[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. White dwarf effective surface temperatures extend from over 150,000 K to barely under 4,000 K. One of the coolest so far observed, WD 0346+246, has a surface temperature of approximately 3,900 K.
Examples:
* 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.
* 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.
White dwarf spectral types :
| 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.
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. Magnetic fields have been discovered in well over 100 white dwarfs, ranging from 2 x 10$^3$ to 10$^9$ 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"
The first variable white dwarf found was HL Tau 76; in 1965 and 1966, it varied with a period of approximately 12.5 minutes.
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;$^{,pp.891,895}$ DBV, or V777 Her, stars, with helium-dominated atmospheres and the spectral type DB;$^{,p.3525}$ and GW Vir stars (sometimes subdivided into DOV and PNNV stars), with atmospheres dominated by helium, carbon, and oxygen.$^{,§1.1,1.2;}$$^{,§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.$^{,§1.1;}$ 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.
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

The pair 40 Eridani B/C was discovered by William Herschel on 31 January 1783;$^{,p.73}$ it was again observed by Friedrich Georg Wilhelm Struve in 1825 and by Otto Wilhelm von Struve in 1851. 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. The spectral type of 40 Eridani B was officially described in 1914 by Walter Adams.
Per SIMBAD, 40 Eridani B is an X-ray source detected by the Einstein X-ray Observatory.

## Sirius B

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. Walter Adams announced in 1915 that he had found the spectrum of Sirius B to be similar to that of Sirius.
Per SIMBAD, Sirius B is an X-ray source detected by the Einstein X-ray Observatory, HEAO 1, and ROSAT.

## Van Maanen's Star

In 1917, Adriaan Van Maanen discovered Van Maanen's Star, an isolated white dwarf.
Van Maanen's Star is not a known X-ray source per SIMBAD.

## Hypotheses

* 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.