Temporal, spatial, and spectral distributions of radiation are the focus of the science of spectroscopy as applied to astronomy.
"NASA's Spitzer Space Telescope has detected the building blocks of life in the distant universe, albeit in a violent milieu. Training its powerful infrared eye on a faint object located at a distance of 3.2 billion light-years (inset [in the image on the right]), Spitzer has observed the presence of water and organic molecules in the galaxy IRAS F00183-7111. With an active galactic nucleus, this is one of the most luminous galaxies in the universe, rivaling the energy output of a quasar. Because it is heavily obscured by dust, most of its luminosity is radiated at infrared wavelengths."
"The infrared spectrograph instrument onboard Spitzer breaks light into its constituent colors, much as a prism does for visible light. The image shows a low-resolution spectrum of the galaxy obtained by the spectrograph at wavelengths between 4 and 20 microns. Spectra are graphical representations of a celestial object's unique blend of light. Characteristic patterns, or fingerprints, within the spectra allow astronomers to identify the object's chemical composition and to determine such physical properties as temperature and density."
"The broad depression in the center of the spectrum denotes the presence of silicates (chemically similar to beach sand) in the galaxy. An emission peak (red) within the bottom of the trough is the chemical signature for molecular hydrogen. The hydrocarbons (orange) are organic molecules comprised of carbon and hydrogen, two of the most common elements on Earth. Since it has taken more than three billion years for the light from the galaxy to reach Earth, it is intriguing to note the presence of organics in a distant galaxy at a time when life is thought to have started forming on our home planet."
"Additional features in the spectrum reveal the presence of water ice (blue), carbon dioxide ice (green) and carbon monoxide (purple) in both gas and solid forms. The magenta peak corresponds to singly ionized neon gas, a spectral line often used by astronomers as a diagnostic of star formation rates in distant galaxies."
- 1 Spectroscopy
- 2 Theoretical spectroscopy
- 3 Meteors
- 4 Gamma rays
- 5 X-rays
- 6 Ultraviolets
- 7 Polarized light
- 8 Visuals
- 9 Infrareds
- 10 Submillimeters
- 11 Microwaves
- 12 Radios
- 13 Hydrogens
- 14 Ions
- 15 Chromospheres
- 16 Betelgeuse
- 17 CNO stars
- 18 See also
- 19 References
- 20 External links
Spectroscopy ... is the study of the interaction between matter and radiated energy. The concept [comprises] any interaction with radiative energy as a function of its wavelength or frequency. Spectroscopic data is often represented by a spectrum, a plot of the response of interest as a function of wavelength or frequency. Spectrometry and spectrography are terms used to refer to the measurement of radiation intensity as a function of wavelength and are often used to describe experimental spectroscopic methods. Spectral measurement devices are referred to as spectrometers, spectrophotometers, spectrographs or spectral analyzers.
Def. the "scientific study of spectra" is called spectroscopy.
The "Si I λ2516 resonance emission line (Lobel & Dupree 2001) [...] has previously been observed by scanning over the inner chromosphere [of Betelgeuse] at 0, 25, 50, and 75 mas, using a slit size of 100 × 30 mas [the left panel in the graphs on the right]."
"The double-peaked line profiles are observed across the inner chromosphere. The central (self-) absorption core results from scattering opacity in the chromosphere. The asymmetry of the emission component intensities probes the chromospheric flow dynamics in our line of sight. The spectra of GO 9369 are observed across the outer chromosphere using a slitsize of 200 × 63 mas [image on the right]. The profiles beyond 200 mas appear red-shifted with a rather weak short-wavelength emission component. It signals substantial wind outflow opacity in the upper chromoshere, which fastly accelerates beyond a radius of ∼8 R∗."
The set of graphs on the right "compares the profiles of the Si I λ2516 and λ2507 resonance lines (vertical dotted lines are drawn at stellar rest velocity). Both lines share a common upper energy level and their intensities are influenced by pumping through a fluoresced Fe II line. The self-absorption cores of the Si I lines are therefore observed far out, into the upper chromosphere. The shape of these unsaturated emission lines is strongly opacity sensitive to the local chromospheric velocity field. As for the Mg II lines, the outward decreasing intensity of the short-wavelength emission component signals fast acceleration of chromospheric outflow in the upper chromosphere. We also observe this decrease for the resonance line of Mg I λ2852 (not shown). Our previous radiative transfer modeling work based on Si I revealed that α Ori’s inner chromosphere oscillates nonradially, with simultaneous up- and downflows in Sept. 1998."
The Energetic Gamma Ray Experiment Telescope, (EGRET) measured high energy (20 MeV to 30 GeV) gamma ray source positions to a fraction of a degree and photon energy to within 15 percent. EGRET was developed by NASA Goddard Space Flight Center, the Max Planck Institute for Extraterrestrial Physics, and Stanford University. Its detector operated on the principle of electron-positron pair production from high energy photons interacting in the detector. The tracks of the high-energy electron and positron created were measured within the detector volume,and the axis of the V of the two emerging particles projected to the sky. Finally, their total energy was measured in a large calorimeter scintillation detector at the rear of the instrument.
In March 2010 it was announced that active galactic nuclei are not responsible for most gamma-ray background radiation. Though active galactic nuclei do produce some of the gamma-ray radiation detected here on Earth, less than 30% originates from these sources. The search now is to locate the sources for the remaining 70% or so of all gamma-rays detected. Possibilities include star forming galaxies, galactic mergers, and yet-to-be explained dark matter interactions.
Sensitivity to celestial sources by Vela 5A and 5B was severely limited by the high intrinsic detector background, equivalent to about 80% of the signal from the Crab Nebula, one of the brightest sources in the sky at these wavelengths.
Kosmos 60 measured the gamma-ray background flux density to be 1.7×104 quanta/(m2·s). As was seen by Ranger 3 and Lunas 10 & 12, the spectrum fell sharply up to 1.5 MeV and was flat for higher energies. Several peaks were observed in the spectra which were attributed to the inelastic interaction of cosmic protons with the materials in the satellite body.
X-rays are electromagnetic radiation from a portion of the wavelength spectrum of about 5 to 8 nanometers (nm)s down to approximately 5 to 8 picometers (pm)s. As the figure at the left indicates with respect to surface of the Earth measurements, they do not penetrate the atmosphere. Laboratory measurements with X-ray generating sources are used to determine atmospheric penetration.
There is an “extensive 1/4 keV emission in the Galactic halo”, an “observed 1/4 keV [X-ray emission originating] in a Local Hot Bubble (LHB) that surrounds the Sun. ... and an isotropic extragalactic component.” In addition to this “distribution of emission responsible for the soft X-ray diffuse background (SXRB) ... there are the distinct enhancements of supernova remnants, superbubbles, and clusters of galaxies.”
The ROSAT soft X-ray diffuse background (SXRB) image shows the general increase in intensity from the Galactic plane to the poles. At the lowest energies, 0.1 - 0.3 keV, nearly all of the observed soft X-ray background (SXRB) is thermal emission from ~106 K plasma.
Generally, a coronal cloud, a cloud composed of plasma, is usually associated with a star or other celestial or astronomical body, extending sometimes millions of kilometers into space, or thousands of light-years, depending on the associated body. The high temperature of the coronal cloud gives it unusual spectral features. These features have been traced to highly ionized atoms of elements such as iron which indicate a plasma's temperature in excess of 106 K (MK) and associated emission of X-rays.
A spectral distribution is often a plot or intensity, brightness, flux density, or other characteristic of a spectrum versus the spectral property such as wavelength, frequency, energy, particle speed, refractive or reflective index, for example.
The first three dozen or so astronomical X-ray objects detected other than the Sun "represent a brightness range of about a thousandfold from the most intense source, Sco XR-1, ca. 5 x 10-10 J m-2 s-1, to the weakest sources at a few times 10-13 J m-2 s-1."
A temporal distribution is a distribution over time. Also known as a time distribution. A temporal distribution usually has the independent variable 'Time' on the abscissa and other variables viewed approximately orthogonal to it. The time distribution can move forward in time, for example, from the present into the future, or backward in time, from the present into the past. Usually, the abscissa is plotted forward in time with the earlier time at the intersection with the ordinate variable at left. Geologic time is often plotted on the abscissa versus phenomena on the ordinate or as a twenty-four hour clock analogy.
"An X-ray source was observed in the constellation Vela from an attitude-controlled Aerobee 150 rocket launched from the White Sands Missile Range on February 2, 1968. The object, which may be the previously reported Vel XR-1 (Chodil et al. 1967), lies close to the galactic plane; we designate it as GX263+3."
The image on the upper right "shows the counting rates plotted against time during the maneuver in which the new source was observed. The peaks labeled 1, 2, 3, anf 4 all refer to this source. Peaks 1 and 2 determine a location that is the same within experimental error as the location determined by peaks 3 and 4. The reduction in background level from 130 to 150 sec after launch occurs when a large portion of the field of view falls below the horizon."
The second image down on the right shows the most "probable celestial locations defined by the peaks in [the upper image on the right] and counting-rate ratios are shown as line segments. Separations between intersections are consistent with a single X-ray source and statistical errors in determination of times of peak counting rates. Shaded area around intersections, and enlargement, show the region of uncertainty of the source. Lines labeled 1st pass and 2d pass refer to the center of the field of view during scan. Area inclosed by dashed lines is the region of uncertainty [well within Vela and the Chodil polygon] of Vel XR-1 as reported by Chodil et al. (1967)."
"The location of the source as α(1950) = 8h57m, δ(1950) = -41°15', with a region of uncertainty of about 3 square degrees, as shown in the [second figure down on the right]. The associated galactic coordinates are lII = 263.3°, bII = 2.9°. [...] This source lies about 3° from the position of the source Vel XR-1 as reported by Chodil et al. (1967) and is within its region of uncertainty. The two objects are probably coincident, since we see no other sources in the vicinity."
Actually, according to NASA's Universal coordinate converter , the X-ray source at lII = 263.3°, bII = 2.9° is 5.26° from Vela XR-1 at lII = 259° 08' 33.8", bII = 00 19' 35.7" not about 3°.
The spectrum displayed on the right uses short wavelength ultraviolet to capture the strong Fe XVIII lines at 9.372 nm and 10.376 nm, Fe XIX at 10.820 nm and another Fe XVIII line at 13.266 nm.
"The spectra were acquired during 1992 December 10-13. Capella was at phase 0.82-0.86 (Barlow et al. 1993, where phase 0.0 corresponds to orbital quadrature with the more massive [cooler] primary star [spectral class K0III] receding with maximum positive velocity)."
"A continuous distribution of temperatures (105 - 107.8) is present in the Capella system."
In the image on the right, "Asymmetric Betelgeuse and its environment [are] imaged in visible light (top) and polarized visible light (bottom). Each column is a different filter. The red dashed circle indicates Betelgeuse’s infrared photospheric radius. The light dashed circle is three times this." Both are specialized spatial distributions vs. intensity.
"As you can see in the images [on the right], Betelgeuse is not symmetric, and neither is its circumstellar material. The top row shows brightness in different visible-light filters while the bottom row shows degree of polarization (light colors are more polarized than dark)."
"Most of the imaged polarized light is far from the star’s photosphere, and is probably polarized due to dust scattering. However, bits of this dust are close to the star, too! It’s well known that red supergiants like Betelgeuse lose significant amounts of mass. Mass loss seems to be connected to the huge convective cells inside supergiants, because they too are not spherically symmetric, but we don’t know precisely how. We do know the lost mass forms a circumstellar envelope around the star and and provides the material from which dust can form. It follows that if the dust was all far away or all close-in, that would tell us something about how it got there. Instead, at any single distance away from the star, we find different amounts of dust and gas in a range of different temperatures and densities."
"A careful look to our visibility curves [on the right, full line is our data and broken line is speckle data from Aime et al. 1985] reveals a small periodic modulation that we have interpreted as possibly due to a stellar companion (Roddier, Roddier, and Karovska 1984). The estimated position angle is 85° ± 5° (mod. 180°). The period of modulation corresponds to an angular distance of 0.4"-0.5" and the depth of modulation to a magnitude difference of the order of 3.5-4."
The image on the right is an interferometric image of the photosphere of Betelgeuse at 1.64 microns.
The image on the left is the unsmoothed infrared interferometry of the surface (photosphere) of Betelgeuse at 1.64 µm.
"The image reveals the presence of two giant bright spots, whose size is equivalent to the distance Earth-Sun : they cover a large fraction of the surface. It is a first strong and direct indication of the presence of phenomena of convection, transport of heat by the moving matter, in a star other than the Sun."
"The analysis of the brightness of the spots shows a variation of 500 degrees compared to the average temperature of the star (3 600 Kelvins). The largest of the two structures has a dimension equivalent to the quarter of the star diametre (or one and a half the distance Earth-Sun). This marks a clear difference with the Sun where the cells of convection are much finer and reach hardly 1/20th of the solar radius (a few Earth radius). These characteristics are compatible with the idea of luminous spots produced by the convection."
"This is one of the early spectra obtained with the SPIRE fourier transform spectrometer on Herschel. Shown here is a portion of the SPIRE spectrum of VY Canis Majoris (VY CMa), a red supergiant star near the end of its life, which is ejecting huge quantities of gas and dust into interstellar space. The inset is a SPIRE camera map of VY CMa, in which it appears as a bright compact source near the edge of a large extended cloud."
"The VY CMa spectrum is amazingly rich, with prominent features from carbon monoxide (CO) and water (H2O). More than 200 other spectral features have been identified so far in the full spectrum, and several unidentified features are being investigated. Many of the features are due to water, showing that the star is surrounded by large quantities of hot steam. Observations like these will help to establish a detailed picture of the mass loss from stars and the complex chemistry occurring in their extended envelopes. As in all of the SPIRE spectra, the underlying emission increases towards shorter wavelengths, and is due to the emission from dust grains. The shape of the dust spectrum provides information on the properties of the dust."
"VY Canis Majoris (VY CMa) is a red supergiant star located about 4900 light years from Earth in the constellation Canis Major. It is the largest known star, with a size of 2600 solar radii, and also one of the most luminous, with a luminosity in excess of 100 000 times that of the Sun. The mass of VY CMa lies in the range 30-40 solar masses, and it has a mass-loss rate of 2 x 10-4 solar masses per year."
"The shell of gas it has ejected displays a complex structure; the circumstellar envelope is among the most remarkable chemical laboratories known in the Universe, creating a rich set of organic and inorganic molecules and dust species. Through stellar winds, these inorganic and organic compounds are injected into the interstellar medium, from which new stars orbited by new planets may form. Most of the carbon supporting life on Earth was forged by this kind of evolved star. VY CMa truly is a spectacular object, it is close to the end of its life and could explode as a supernova at any time."
The figure at the right "shows the three-year WMAP spectrum compared to a set of recent balloon and ground-based measurements that were selected to most complement the WMAP data in terms of frequency coverage and l range. The non-WMAP data points are plotted with errors that include both measurement uncertainty and cosmic variance, while the WMAP data in this l range are largely noise dominated, so the effective error is comparable. When the WMAP data are combined with these higher resolution CMB measurements, the existence of a third acoustic peak is well established, as is the onset of Silk damping beyond the 3rd peak."
The image on the right is of Betelgeuse at 7 mm (45 Gz) radio waves.
"Close to the star, we find that the atmosphere has an irregular structure, and a temperature (3,450 +/- 850K) consistent with the photospheric temperature but much lower than that of gas in the same region probed by optical and ultraviolet observations. This cooler gas decreases steadily in temperature with radius, reaching 1,370 +/- 330K by seven stellar radii. The cool gas coexists with the hot chromospheric gas, but must be much more abundant as it dominates the radio emission."
The asymmetric structure of Betelgeuse in radio waves shown in the image on the left is likely due to activity in the outer atmosphere of the star.
"Two of the filters used to image Betelgeuse are sensitive to the familiar red hydrogen alpha spectral feature. Because one filter is broader than the other, subtracting the light in the narrow filter from the light seen with the broad filter yields a map of where hydrogen gas is emitting or absorbing light. It also turns out to be highly asymmetric. Most of the hydrogen emission is confined within a distance of three times Betelgeuse’s near-infrared radius. It’s a similar distance from the star as most of the polarized dust, but the spatial distributions are different."
In the image on the right, "Left: A map of hydrogen emission (red) and absorption (blue) in the vicinity of Betelgeuse, with the same dashed lines [The red dashed circle indicates Betelgeuse’s infrared photospheric radius. The light dashed circle is three times this.] for reference. Right: Color composite of three of the filters from the first figure (the narrow hydrogen alpha filter is excluded)."
"Betelgeuse’s asymmetries persist in both in dust and gas, with a major interface between the two located around three times the near-infrared stellar radius. These asymmetries agree with different types of past observations and also strongly point toward a connection between supergiant mass loss and vigorous convection."
"[I]on lines of Fe II, Al II, and C II [have been observed] out to 1′′ in the upper chromosphere. [The set of graphs on the right] shows (scaled) emission lines of Fe II λ2716 (UV 62), Al II λ2669 (UV 1), and C II λ2327 (UV 1)."
The set of graphs on the right "shows the detailed profiles of the Mg II h & k lines observed up to 1000 mas. The emission line intensities decrease by a factor of ∼700 from chromospheric disk center (TP 0) to 1′′. These optically thick chromospheric lines show remarkable changes of their detailed shapes when scanning off-limb. The full width across both emission components at half intensity maximum decreases by ∼20%, while the broad and saturated central absorption core narrows by more than 50%. Beyond 600 mas the central core assumes a constant width which results from absorption contributions by the local interstellar medium (d∗≃132 pc). We observe a strong increase of the (relative) intensity of the long-wavelength emission component in both lines beyond 200 mas. It signals fast wind acceleration beyond this radius. Note that the short-wavelength emission components of the k and h lines are blended with chomospheric Mn I lines (decreasing the k- and increasing the h-component), but that become much weaker in the outer chromosphere."
Above is a spectrum of Betelgeuse "taken with a SkyWatcher 200p / EQ5 / SynScan Tracking Star Analyser 100."
The above spectrum of Betelgeuse has magnesium (Mg I) lines and strong titanium oxide (TiO2) bands. Its temperature is between 2,000 to 3,600 K.
The spectrum of Betelgeuse on the right ranges from about 520.0 nm (blue-green) to about 640.0 nm (yellow) on the right end.
Analysis "of the photometry, the radial velocity and the rotational velocities shows that the stars are concentrated in a small range of U-B colors, that they generally exhibit variable radial velocity and slow rotation."
A "large fraction exhibits evidence of [temporal] changes in the spectrum."
- Lee Armus, James Houck, Vassilis Charmandaris, Henrik Spoon, Harry Teplitz, Daniel Devost, Patrick Morris, Phil Appleton, Carl Grillmair (18 December 2003). Galaxy IRAS F00183-7111. Pasadena, California USA: Caltech. http://www.spitzer.caltech.edu/images/1098-ssc2003-06h-Galaxy-IRAS-F00183-7111. Retrieved 2017-05-25.
- Crouch, Stanley; Skoog, Douglas A. (2007). Principles of instrumental analysis. Australia: Thomson Brooks/Cole. ISBN 0-495-01201-7.
- . doi:10.1351/pac198658121737.
- SemperBlotto (7 April 2005). spectroscopy. San Francisco, California: Wikimedia Foundation, Inc. https://en.wiktionary.org/wiki/spectroscopy. Retrieved 2014-06-04.
- A. Lobel, J. Aufdenberg, A. K. Dupree, R. L. Kurucz, R. P. Stefanik, and G. Torres (January 2004). A.K. Dupree and A.O. Benz. ed. Spatially Resolved STIS Spectroscopy of Betelgeuse’s Outer Atmosphere, In: Stars as Suns: Activity, Evolution and Planets. 219. San Francisco, CA USA: Astronomical Society of the Pacific. pp. 641-5. Bibcode: 2004IAUS..219..641L. http://arxiv.org/pdf/astro-ph/0312076v1. Retrieved 2015-12-28.
- NASA. NASA’s Fermi Probes “Dragons” of the Gamma-ray Sky. http://www.nasa.gov/mission_pages/GLAST/news/gamma-ray-dragons.html.
- Priedhorsky WC, Holt SS (1987). "Long-term cycles in cosmic X-ray sources". Space Science Review 45 (3–4): 291–348. doi:10.1007/BF00171997.
- S. L. Snowden, R. Egger, D. P. Finkbiner, M. J. Freyberg, and P. P. Plucinsky (February 1, 1998). "Progress on Establishing the Spatial Distribution of Material Responsible for the 1/4 keV Soft X-Ray Diffuse Background Local and Halo Components". The Astrophysical Journal 493 (1): 715-29. doi:10.1086/305135. http://iopscience.iop.org/0004-637X/493/2/715/fulltext/. Retrieved 2012-06-14.
- Friedman H (November 1969). "Cosmic X-ray observations". Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences 313 (1514): 301-15. http://www.jstor.org/pss/2416439. Retrieved 2011-11-25.
- H. Gursky, E. M. Kellogg, and P. Gorenstein (November 1968). "The Location of the X-ray Source in Vela". The Astrophysical Journal 154 (11): 71-4. http://adsabs.harvard.edu//abs/1968ApJ...154L..71G. Retrieved 2015-12-15.
- A. K. Dupree, N. S. Brickhouse, G. A. Doschek, J. C. Green, and J. C. Raymond (November 1993). "The Extreme Ultraviolet Spectrum of Alpha Aurigae (Capella)". The Astrophysical Journal Letters 418 (11): L42-44. doi:10.1086/187111. http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993ApJ...418L..41D&link_type=ARTICLE&db_key=AST&high=. Retrieved 2017-05-26.
- Meredith Rawls (24 November 2015). Zooming in on Betelgeuse. Astrobites. http://astrobites.org/2015/11/24/zooming-in-on-betelgeuse/. Retrieved 2016-01-01.
- F. Roddier, C. Roddier, and, R. Petrov, F. Martin, G. Ricort, and C. Aime (15 June 1986). "New Observations of Alpha Orionis with a Rotation Shearing Interferometer". The Astrophysical Journal 305: L77-L80. https://www.researchgate.net/profile/Claude_Aime/publication/234281653_New_observations_of_Alpha_Orionis_with_a_rotation_shearing_interferometer/links/55bf400408aed621de122b40.pdf. Retrieved 2017-01-03.
- Xavier Haubois and Guy Perrin (1 January 2010). Unprecedented details on the surface of the Betelgeuse star. Paris, France: Observatoire de Paris, LESIA, et CNRS. https://www.obspm.fr/spip.php?page=imprimer&id_article=1948&lang=fr. Retrieved 2015-12-31.
- M. Groenewegen (November 27, 2009). SPIRE spectrum of VY Canis Majoris. Pasadena, California USA: Caltech. http://www.herschel.caltech.edu/image/nhsc2009-021a. Retrieved 2014-03-12.
- G. Hinshaw, M. R. Nolta, C. L. Bennett, R. Bean, O. Doré, M. R. Greason, M. Halpern, R. S. Hill, N. Jarosik, A. Kogut, E. Komatsu, M. Limon, N. Odegard, S. S. Meyer, L. Page, H. V. Peiris, D. N. Spergel, G. S. Tucker, L. Verde, J. L. Weiland, E. Wollack, and E. L. Wright (5 January 2007). "Three-Year Wilkinson Microwave Anisotropy Probe (WMAP1) Observations: Temperature Analysis". The Astrophysical Journal (Supplement Series) 170 (2): 288-334. doi:10.1086/513698. http://arxiv.org/pdf/astro-ph/0603451.pdf. Retrieved 2014-10-19.
- Jeremy Lim, Chris L. Carilli, Stephen M. White, Anthony J. Beasley, and Ralph G. Marson (April 1998). "Large convection cells as the source of Betelgeuse's extended atmosphere". Nature 392 (676): 575-7. doi:10.1038/33352. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1998Natur.392..575L. Retrieved 2015-12-28.
- Alex Lobel, Andrea Dupree, and Roland Gilliland (13 January 2000). LIKE A HUMAN HEART: BETELGEUSE'S CHROMOSPHERE BEATS ASYMMETRICALLY. 60 Garden Street, Cambridge, Massachusetts 02138 USA: Harvard-Smithsonian Center for Astrophysics. https://www.cfa.harvard.edu/news/archive/alobel0100.html. Retrieved 2015-12-28.
- Raymond Gilchrist (23 January 2013). Betelgeuse Graph and Spectrum. Flickr.com. https://www.flickr.com/photos/raygil/8409572424. Retrieved 2016-01-03.
- Mercedes Jaschek, Carlos Jaschek (December 1974). "The CNO stars". Astronomy and Astrophysics 36 (3): 401-8. http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1974A%26A....36..401J&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf. Retrieved 2016-10-18.