Photoionization is the physical process in which an ion is formed from the interaction of a photon with an atom or molecule. Not every photon which encounters an atom or ion will photoionize it; the probability of photoionization is related to the photoionization cross-section, which depends on the energy of the photon and the target being considered. For photon energies below the ionization threshold, the photoionization cross-section is near zero, but with the development of pulsed lasers it has become possible to create intense, coherent light where multi-photon ionization may occur. At higher intensities, non-perturbative phenomena such as barrier suppression ionization and rescattering ionization are observed. Several photons of energy below the ionization threshold may combine their energies to ionize an atom; this probability decreases with the number of photons required, but the development of intense, pulsed lasers still makes it possible. In the perturbative regime, the probability of absorbing N photons depends on the laser-light intensity I as IN.
For higher intensities, this dependence becomes invalid due to the occurring AC Stark effect. Resonance-enhanced multiphoton ionization is a technique applied to the spectroscopy of atoms and small molecules in which a tunable laser can be used to access an excited intermediate state. Above-threshold ionization is an extension of multi-photon ionization where more photons are absorbed than would be necessary to ionize the atom; the excess energy gives the released electron higher kinetic energy than the usual case of just-above threshold ionization. More The system will have multiple peaks in its photoelectron spectrum which are separated by the photon energies, this indicates that the emitted electron has more kinetic energy than in the normal ionization case; the electrons released from the target will have an integer number of photon-energies more kinetic energy. When either the laser intensity is further increased or a longer wavelength is applied as compared with the regime in which multi-photon ionization takes place, a quasi-stationary approach can be used and results in the distortion of the atomic potential in such a way that only a low and narrow barrier between a bound state and the continuum states remains.
The electron can tunnel through or for larger distortions overcome this barrier. These phenomena are called over-the-barrier ionization, respectively. Ion source
California Institute of Technology
The California Institute of Technology is a private doctorate-granting research university in Pasadena, California. Known for its strength in natural science and engineering, Caltech is ranked as one of the world's top-ten universities. Although founded as a preparatory and vocational school by Amos G. Throop in 1891, the college attracted influential scientists such as George Ellery Hale, Arthur Amos Noyes and Robert Andrews Millikan in the early 20th century; the vocational and preparatory schools were disbanded and spun off in 1910 and the college assumed its present name in 1921. In 1934, Caltech was elected to the Association of American Universities and the antecedents of NASA's Jet Propulsion Laboratory, which Caltech continues to manage and operate, were established between 1936 and 1943 under Theodore von Kármán; the university is one among a small group of institutes of technology in the United States, devoted to the instruction of pure and applied sciences. Caltech has six academic divisions with strong emphasis on science and engineering, managing $332 million in 2011 in sponsored research.
Its 124-acre primary campus is located 11 mi northeast of downtown Los Angeles. First-year students are required to live on campus and 95% of undergraduates remain in the on-campus House System at Caltech. Although Caltech has a strong tradition of practical jokes and pranks, student life is governed by an honor code which allows faculty to assign take-home examinations; the Caltech Beavers compete in 13 intercollegiate sports in the NCAA Division III's Southern California Intercollegiate Athletic Conference. As of October 2018, Caltech alumni and researchers include 73 Nobel Laureates, 4 Fields Medalists, 6 Turing Award winners. In addition, there are 53 non-emeritus faculty members who have been elected to one of the United States National Academies, 4 Chief Scientists of the U. S. Air Force and 71 have won the United States National Medal of Technology. Numerous faculty members are associated with the Howard Hughes Medical Institute as well as NASA. According to a 2015 Pomona College study, Caltech ranked number one in the U.
S. for the percentage of its graduates who go on to earn a PhD. Caltech started as a vocational school founded in Pasadena in 1891 by local businessman and politician Amos G. Throop; the school was known successively as Throop University, Throop Polytechnic Institute and Throop College of Technology before acquiring its current name in 1920. The vocational school was disbanded and the preparatory program was split off to form an independent Polytechnic School in 1907. At a time when scientific research in the United States was still in its infancy, George Ellery Hale, a solar astronomer from the University of Chicago, founded the Mount Wilson Observatory in 1904, he joined Throop's board of trustees in 1907, soon began developing it and the whole of Pasadena into a major scientific and cultural destination. He engineered the appointment of James A. B. Scherer, a literary scholar untutored in science but a capable administrator and fund raiser, to Throop's presidency in 1908. Scherer persuaded retired businessman and trustee Charles W. Gates to donate $25,000 in seed money to build Gates Laboratory, the first science building on campus.
In 1910, Throop moved to its current site. Arthur Fleming donated the land for the permanent campus site. Theodore Roosevelt delivered an address at Throop Institute on March 21, 1911, he declared: I want to see institutions like Throop turn out ninety-nine of every hundred students as men who are to do given pieces of industrial work better than any one else can do them. In the same year, a bill was introduced in the California Legislature calling for the establishment of a publicly funded "California Institute of Technology", with an initial budget of a million dollars, ten times the budget of Throop at the time; the board of trustees offered to turn Throop over to the state, but the presidents of Stanford University and the University of California lobbied to defeat the bill, which allowed Throop to develop as the only scientific research-oriented education institute in southern California, public or private, until the onset of the World War II necessitated the broader development of research-based science education.
The promise of Throop attracted physical chemist Arthur Amos Noyes from MIT to develop the institution and assist in establishing it as a center for science and technology. With the onset of World War I, Hale organized the National Research Council to coordinate and support scientific work on military problems. While he supported the idea of federal appropriations for science, he took exception to a federal bill that would have funded engineering research at land-grant colleges, instead sought to raise a $1 million national research fund from private sources. To that end, as Hale wrote in The New York Times: Throop College of Technology, in Pasadena California has afforded a striking illustration of one way in which the Research Council can secure co-operation and advance scientific investigation; this institution, with its able investigators and excellent research laboratories, could be of great service in any broad scheme of cooperation. President S
Sauron is the title character and main antagonist of J. R. R. Tolkien's The Lord of the Rings. In the same work, he is identified as the Necromancer, mentioned in Tolkien's earlier novel The Hobbit. In Tolkien's The Silmarillion, he is described as the chief lieutenant of the first Dark Lord, Morgoth. Tolkien noted that the Ainur, the "angelic" powers of his constructed myth, "were capable of many degrees of error and failing", but by far the worst was "the absolute Satanic rebellion and evil of Morgoth and his satellite Sauron"; the Ainulindalë, the cosmological myth prefixed to The Silmarillion, explains how the supreme being Eru initiated his creation by bringing into being innumerable spirits, "the offspring of his thought", who were with him before anything else had been made. The being known as Sauron originated among these as an "immortal spirit". In his origin, Sauron therefore perceived the Creator directly; as Tolkien noted: "Sauron could not, of course, be a'sincere' atheist. Though one of the minor spirits created before the world, he knew Eru, according to his measure."In the terminology of Tolkien's invented language of Quenya, these angelic spirits were called Ainur.
Those who entered the physical world were called Valar the most powerful ones. The lesser Ainur who entered the world, of whom Sauron was one, were called Maiar. In Tolkien's letters, the author noted that Sauron "was of course a'divine' person". Tolkien noted that he was of a "far higher order" than the Maiar who came to Middle-earth as the Wizards Gandalf and Saruman; as created by Eru, the Ainur were all good and uncorrupt, as Elrond stated in The Lord of the Rings: "Nothing is evil in the beginning. Sauron was not so."Rebellion originated with the Vala Melkor. According to a story meant as a parable of events beyond Elvish comprehension, Eru let his spirit-children perform a great Music, the Music of the Ainur, developing a theme revealed by Eru himself. For a while the cosmic choir made wondrous music, but Melkor tried to increase his own glory by weaving into his song thoughts and ideas that were not in accordance with the original theme. "Straightway discord arose around him, many that sang nigh him grew despondent... but some began to attune their music to his rather than to the thought which they had at first."The discord Melkor created would have dire consequences, as this singing was a kind of template for the world: "The evils of the world were not at first in the great Theme, but entered with the discords of Melkor."
However, "Sauron was not a beginner of discord. Sauron was not one of the spirits that began to attune their music to that of Melkor, since it is noted elsewhere that his fall occurred later; the cosmic Music now represented the conflict between evil. Eru abruptly brought the Song of Creation to an end. To show the spirits, faithful or otherwise, what they had done, Eru gave independent being to the now-marred Music; this resulted in the manifestation of the material World, Eä, where the drama of good and evil would play out and be resolved. Entering Eä at the beginning of time, the Valar and Maiar tried to build and organize the world according to the will of Eru; each Maia was associated with one of the powerful Valar. As a result, Sauron came to possess great knowledge of the physical substances of the world and all manner of craftsmanship—emerging as "a great craftsman of the household of Aulë". Sauron would always retain the "scientific" knowledge he derived from the great Vala of Craft: "In his beginning he was of the Maiar of Aulë, he remained mighty in the lore of that people."
Sauron's original Elvish name in Valinor was Mairon, but this name was not used anymore after he joined Melkor. In Beleriand, he was called in Sindarin Gorthu "Mist of Fear" and Gorthaur "The Cruel". However, during the Second Age, Sauron continued to call himself Tar-Mairon. Melkor opposed the other Valar, who remained faithful to Eru and tried to carry out the Creator's designs. Within the larger universe, they focused on developing the world of Arda. Around this time, Sauron fell victim to Melkor's corrupting influence: "In the beginning of Arda, Melkor seduced him to his allegiance."As for Sauron's motives, Tolkien noted that "it had been his virtue that he loved order and coordination, disliked all confusion and wasteful friction". Thus, "it was the apparent will and power of Melkor to effect his designs and masterfully that had first attracted Sauron to him". For a while, Sauron kept up the pretence that he was a faithful servant of the Valar, all the while feeding Melkor information about their doings.
Thus, when the Valar made Almaren as their first physical abode in the world, "Melkor knew of all, done. They still did not perceive Sauron's treachery, for he too became "a being of Valinor". At some point, Sauron left the Blessed Realm and went to Middle-earth
An emission nebula is a nebula formed of ionized gases that emit light of various wavelengths. The most common source of ionization is high-energy photons emitted from a nearby hot star. Among the several different types of emission nebulae are H II regions, in which star formation is taking place and young, massive stars are the source of the ionizing photons. A young star will ionize part of the same cloud from which it was born although only massive, hot stars can release sufficient energy to ionize a significant part of a cloud. In many emission nebulae, an entire cluster of young stars is doing the work; the nebula's color depends on its chemical degree of ionization. Due to the prevalence of hydrogen in interstellar gas, its low energy of ionization, many emission nebulae appear red due to the strong emissions of the Balmer series. If more energy is available, other elements will be ionized and green and blue nebulae become possible. By examining the spectra of nebulae, astronomers infer their chemical content.
Most emission nebulae are about 90% hydrogen, with the remaining helium, oxygen and other elements. Some of the most prominent emission nebulae visible from the northern hemisphere are the North America Nebula and Veil Nebula NGC 6960/6992 in Cygnus, while in the south celestial hemisphere, the Lagoon Nebula M8 / NGC 6523 in Sagittarius and the Orion Nebula M42. Further in the southern hemisphere is the bright Carina Nebula NGC 3372. Emission nebulae have dark areas in them which result from clouds of dust which block the light. Many nebulae are made up of both emission components such as the Trifid Nebula. Herbig–Haro object
Cat's Eye Nebula
The Cat's Eye Nebula or NGC 6543, is a bright planetary nebula in the northern constellation of Draco, discovered by William Herschel on February 15, 1786. It was the first planetary nebula whose spectrum was investigated by the English amateur astronomer William Huggins, demonstrating that planetary nebulae were gaseous and not stellar in nature. Structurally, the object has had high-resolution images by the Hubble Space Telescope revealing knots, jets and complex arcs, being illuminated by the central hot planetary nebula nucleus, it is a well-studied object, observed from radio to X-ray wavelengths. NGC 6543 is a high northern declination deep-sky object, it has the combined magnitude of 8.1, with high surface brightness. Its small bright inner nebula subtends an average of 16.1 arcsec, with the outer prominent condensations about 25 arcsec. Deep images reveal an extended halo about 300 arcsec or 5 arcmin across, once ejected by the central progenitor star during its red giant phase. NGC 6543 is 4.4 minutes of arc from the current position of the North Ecliptic Pole, Less than 1⁄10 of the 45 arc minutes between Polaris and the current location of the Earth's northern axis of rotation.
It is a convenient and accurate marker for the axis of rotation of the Earth's ecliptic, around which the celestial North Pole rotates. It is a good marker for the nearby “invariable” axis of the solar system, the center of the circles which every planet's north pole, the north pole of every planet's orbit, make in the sky. Since motion in the sky of the ecliptic pole is slow compared to the motion of the Earth's north pole, its position as an ecliptic pole station marker is permanent on the time-scale of human history, as opposed to the Pole Star, which changes every few thousand years. Observations show the bright nebulosity has temperatures between 7000 and 9000 K, whose densities average of about 5000 particles per cubic centimetre, its outer halo is of much lower density. Velocity of the fast stellar wind is about 1900 km/s, where spectroscopic analysis shows the current rate of mass loss averages 3.2×10−7 solar masses per year, equivalent to twenty trillion tons per second. Surface temperature for the central PNN is about 80000 K.
Stellar classification is O7 + –type star Calculations suggest the PNN is over one solar mass, from a theoretical initial 5 solar masses. The central Wolf-Rayet star has a radius of 0.65 R☉. The Cat's Eye Nebula, given in some sources, lies about three thousand light-years from Earth; the Cat's Eye was the first planetary nebula to be observed with a spectroscope by William Huggins on August 29, 1864. Huggins' observations revealed that the nebula's spectrum was non-continuous and made of a few bright emission lines, first indication that planetary nebulae consist of tenuous ionised gas. Spectroscopic observations at these wavelengths are used in abundance determinations, while images at these wavelengths have been used to reveal the intricate structure of the nebula. Observations of NGC 6543 at far-infrared wavelengths reveal the presence of stellar dust at low temperatures; the dust is believed to have formed during the last phases of the progenitor star's life. It re-radiates it at infrared wavelengths.
The spectrum of the infrared dust emission implies that the dust temperature is about 85 K, while the mass of the dust is estimated at 6.4×10−4 solar masses. Infrared emission reveals the presence of un-ionised material such as molecular hydrogen and argon. In many planetary nebulae, molecular emission is greatest at larger distances from the star, where more material is un-ionised, but molecular hydrogen emission in NGC 6543 seems to be bright at the inner edge of its outer halo; this may be due to shock waves exciting. The overall appearance of the Cat's Eye Nebula in infrared is similar in visible light; the Hubble Space Telescope image produced here is in false colour, designed to highlight regions of high and low ionisation. Three images were taken, in filters isolating the light emitted by singly ionised hydrogen at 656.3 nm, singly ionised nitrogen at 658.4 nm and doubly ionised oxygen at 500.7 nm. The images were combined as red and blue channels although their true colours are red and green.
The image reveals two "caps" of less ionised material at the edge of the nebula. In 2001, observations at X-ray wavelengths by the Chandra X-ray Observatory revealed the presence of hot gas within NGC 6543 with the temperature of 1.7×106 K. It is thought that the hot gas results from the violent interaction of a fast stellar wind with material ejected; this interaction has hollowed out the inner bubble of the nebula. Chandra observations have revealed a point source at the position of the central star; the spectrum of this source extends to the hard part of the X-ray spectrum, to 0.5–1.0 keV. A star with the photospheric temperature of about 100000 K would not be expected to emit in hard X-rays, so their presence is something of a mystery, it may suggest the presence of a high temperature accretion disk within a binary star system. The hard X-ray data remain intriguing more than ten years later: the Cat's Eye was included in a 2012 Chandra survey of 21 central stars of planetary nebulae in the solar neighborhood, which found: "All but one of the X-ray point sources detected at CSPNe display X-ray spectra that are harder than expected from hot central star photospheres indicating a high frequency of binary companions to CSPNe.
Other potential ex
A white dwarf called a degenerate dwarf, is a stellar core remnant composed of electron-degenerate matter. A white dwarf is dense: its mass is comparable to that of the Sun, while its volume is comparable to that of Earth. A white dwarf's faint luminosity comes from the emission of stored thermal energy; the nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star. There are thought to be eight white dwarfs among the hundred star systems nearest the Sun; the unusual faintness of white dwarfs was first recognized in 1910. The name white dwarf was coined by Willem Luyten in 1922. White dwarfs are thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star, that of about 10 solar masses; this includes over 97% of the other stars in the Milky Way. § 1. After the hydrogen-fusing period of a main-sequence star of low or medium mass ends, such a star will expand to a red giant during which it fuses helium to carbon and oxygen in its core by the triple-alpha process.
If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, an inert mass of carbon and oxygen will build up at its center. After such a star sheds its outer layers and forms a planetary nebula, it will leave behind a core, the remnant white dwarf. White dwarfs are composed of carbon and oxygen. If the mass of the progenitor is between 8 and 10.5 solar masses, the core temperature will be sufficient to fuse carbon but not neon, in which case an oxygen–neon–magnesium white dwarf may form. Stars of low mass will not be able to fuse helium, hence, a helium white dwarf may form by mass loss in binary systems; the material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy. As a result, it cannot support itself by the heat generated by fusion against gravitational collapse, but is supported only by electron degeneracy pressure, causing it to be dense; the physics of degeneracy yields a maximum mass for a non-rotating white dwarf, the Chandrasekhar limit—approximately 1.44 times of M☉—beyond which it cannot be supported by electron degeneracy pressure.
A carbon-oxygen white dwarf that approaches this mass limit by mass transfer from a companion star, may explode as a type Ia supernova via a process known as carbon detonation. A white dwarf is hot when it forms, but because it has no source of energy, it will cool as it radiates its energy; this means that its radiation, which has a high color temperature, will lessen and redden with time. Over a long time, a white dwarf will cool and its material will begin to crystallize, starting with the core; the star's low temperature means it will no longer emit significant heat or light, it will become a cold black dwarf. Because the length of time it takes for a white dwarf to reach this state is calculated to be longer than the current age of the universe, it is thought that no black dwarfs yet exist; the oldest white dwarfs still radiate at temperatures of a few thousand kelvins. The first white dwarf discovered was in the triple star system of 40 Eridani, which contains the 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.
The pair 40 Eridani B/C was discovered by William Herschel on 31 January 1783. 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. In 1939, Russell looked back on the discovery:, p. 1 I was visiting my friend and generous benefactor, Prof. Edward C. Pickering. With characteristic kindness, he had volunteered to have the spectra observed for all the stars—including comparison stars—which had been observed in the observations for stellar parallax which Hinks and I made at Cambridge, I discussed; this piece of routine work proved fruitful—it led to the discovery that all the stars of faint absolute magnitude were of spectral class M. In conversation on this subject, I asked Pickering about certain other faint stars, not on my list, mentioning in particular 40 Eridani B. Characteristically, he sent a note to the Observatory office and before long the answer came that the spectrum of this star was A.
I knew enough about it in these paleozoic days, to realize at once that there was an extreme inconsistency between what we would have called "possible" values of the surface brightness and density. I must have shown that I was not only puzzled but crestfallen, at this exception to what looked like a pretty rule of stellar characteristics; the spectral type of 40 Eridani B was described in 1914 by Walter Adams. The white dwarf companion of Sirius, Sirius B, was next to be discovered. During the nineteenth century, positional measurements of some stars became precise enough to measure small changes in their location. Friedrich Bessel used position measurements to determine that the stars Sirius and Procyon were changing their positions periodically. In 1844 he predicted that both stars had unseen companions: If we were to regard Sirius and Procyon as double stars, the change of their motions would not surprise us.
New General Catalogue
The New General Catalogue of Nebulae and Clusters of Stars is a catalogue of deep-sky objects compiled by John Louis Emil Dreyer in 1888. It expands upon the cataloguing work of William and Caroline Herschel, John Herschel's General Catalogue of Nebulae and Clusters of Stars; the NGC contains 7,840 objects, known as the NGC objects. It is one of the largest comprehensive catalogues, as it includes all types of deep space objects, including galaxies, star clusters, emission nebulae and absorption nebulae. Dreyer published two supplements to the NGC in 1895 and 1908, known as the Index Catalogues, describing a further 5,386 astronomical objects. Objects in the sky of the southern hemisphere are catalogued somewhat less but many were observed by John Herschel or James Dunlop; the NGC had many errors, but an attempt to eliminate them was initiated by the NGC/IC Project in 1993, after partial attempts with the Revised New General Catalogue by Jack W. Sulentic and William G. Tifft in 1973, NGC2000.0 by Roger W. Sinnott in 1988.
The Revised New General Catalogue and Index Catalogue was compiled in 2009 by Wolfgang Steinicke. The original New General Catalogue was compiled during the 1880s by John Louis Emil Dreyer using observations from William Herschel and his son John, among others. Dreyer had published a supplement to Herschel's General Catalogue of Nebulae and Clusters, containing about 1,000 new objects. In 1886, he suggested building a second supplement to the General Catalogue, but the Royal Astronomical Society asked Dreyer to compile a new version instead; this led to the publication of the New General Catalogue in the Memoirs of the Royal Astronomical Society in 1888. Assembling the NGC was a challenge, as Dreyer had to deal with many contradicting and unclear reports, made with a variety of telescopes with apertures ranging from 2 to 72 inches. While he did check some himself, the sheer number of objects meant Dreyer had to accept them as published by others for the purpose of his compilation; the catalogue contained several errors relating to position and descriptions, but Dreyer referenced the catalogue, which allowed astronomers to review the original references and publish corrections to the original NGC.
The first major update to the NGC is the Index Catalogue of Nebulae and Clusters of Stars, published in two parts by Dreyer in 1895 and 1908. It serves as a supplement to the NGC, contains an additional 5,386 objects, collectively known as the IC objects, it summarizes the discoveries of galaxies and nebulae between 1888 and 1907, most of them made possible by photography. A list of corrections to the IC was published in 1912; the Revised New Catalogue of Nonstellar Astronomical Objects was compiled by Jack W. Sulentic and William G. Tifft in the early 1970s, was published in 1973, as an update to the NGC; the work did not incorporate several previously-published corrections to the NGC data, introduced some new errors. Nearly 800 objects are listed as "non-existent" in the RNGC; the designation is applied to objects which are duplicate catalogue entries, those which were not detected in subsequent observations, a number of objects catalogued as star clusters which in subsequent studies were regarded as coincidental groupings.
A 1993 monograph considered the 229 star clusters called non-existent in the RNGC. They had been "misidentified or have not been located since their discovery in the 18th and 19th centuries", it found that one of the 229—NGC 1498—was not in the sky. Five others were duplicates of other entries, 99 existed "in some form", the other 124 required additional research to resolve; as another example, reflection nebula NGC 2163 in Orion was classified "non-existent" due to a transcription error by Dreyer. Dreyer corrected his own mistake in the Index Catalogues, but the RNGC preserved the original error, additionally reversed the sign of the declination, resulting in NGC 2163 being classified as non-existent. NGC 2000.0 is a 1988 compilation of the NGC and IC made by Roger W. Sinnott, using the J2000.0 coordinates. It incorporates several errata made by astronomers over the years; the NGC/IC Project is a collaboration formed in 1993. It aims to identify all NGC and IC objects, collect images and basic astronomical data on them.
The Revised New General Catalogue and Index Catalogue is a compilation made by Wolfgang Steinicke in 2009. It is a authoritative treatment of the NGC and IC catalogues. Messier object Catalogue of Nebulae and Clusters of Stars Astronomical catalogue List of astronomical catalogues List of NGC objects The Interactive NGC Catalog Online Adventures in Deep Space: Challenging Observing Projects for Amateur Astronomers. Revised New General Catalogue