In astronomy, the interstellar medium is the matter and radiation that exists in the space between the star systems in a galaxy. This matter includes gas in ionic and molecular form, as well as dust and cosmic rays, it fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field; the interstellar medium is composed of multiple phases, distinguished by whether matter is ionic, atomic, or molecular, the temperature and density of the matter. The interstellar medium is composed of hydrogen followed by helium with trace amounts of carbon and nitrogen comparatively to hydrogen; the thermal pressures of these phases are in rough equilibrium with one another. Magnetic fields and turbulent motions provide pressure in the ISM, are more important dynamically than the thermal pressure is. In all phases, the interstellar medium is tenuous by terrestrial standards.
In cool, dense regions of the ISM, matter is in molecular form, reaches number densities of 106 molecules per cm3. In hot, diffuse regions of the ISM, matter is ionized, the density may be as low as 10−4 ions per cm3. Compare this with a number density of 1019 molecules per cm3 for air at sea level, 1010 molecules per cm3 for a laboratory high-vacuum chamber. By mass, 99% of the ISM is gas in any form, 1% is dust. Of the gas in the ISM, by number 91% of atoms are hydrogen and 8.9% are helium, with 0.1% being atoms of elements heavier than hydrogen or helium, known as "metals" in astronomical parlance. By mass this amounts to 70% hydrogen, 28% helium, 1.5% heavier elements. The hydrogen and helium are a result of primordial nucleosynthesis, while the heavier elements in the ISM are a result of enrichment in the process of stellar evolution; the ISM plays a crucial role in astrophysics because of its intermediate role between stellar and galactic scales. Stars form within the densest regions of the ISM, which contributes to molecular clouds and replenishes the ISM with matter and energy through planetary nebulae, stellar winds, supernovae.
This interplay between stars and the ISM helps determine the rate at which a galaxy depletes its gaseous content, therefore its lifespan of active star formation. Voyager 1 reached the ISM on August 25, 2012, making it the first artificial object from Earth to do so. Interstellar plasma and dust will be studied until the mission's end in 2025, its twin, Voyager 2 entered the ISM in November 2018. Table 1 shows a breakdown of the properties of the components of the ISM of the Milky Way. Field, Goldsmith & Habing put forward the static two phase equilibrium model to explain the observed properties of the ISM, their modeled ISM consisted of a cold dense phase, consisting of clouds of neutral and molecular hydrogen, a warm intercloud phase, consisting of rarefied neutral and ionized gas. McKee & Ostriker added a dynamic third phase that represented the hot gas, shock heated by supernovae and constituted most of the volume of the ISM; these phases are the temperatures where cooling can reach a stable equilibrium.
Their paper formed the basis for further study over the past three decades. However, the relative proportions of the phases and their subdivisions are still not well known; this model takes into account only atomic hydrogen: Temperature larger than 3000 K breaks molecules, lower than 50 000 K leaves atoms in their ground state. It is assumed. Pressure is assumed low, so that durations of free paths of atoms are larger than the ~ 1 nanosecond duration of light pulses which make ordinary, temporally incoherent light. In this collisionless gas, Einstein’s theory of coherent light-matter interactions applies, all gas-light interactions are spatially coherent. Suppose that a monochromatic light is pulsed scattered by molecules having a quadrupole resonance frequency. If “length of light pulses is shorter than all involved time constants”, an “impulsive stimulated Raman scattering ” works: While light generated by incoherent Raman at a shifted frequency has a phase independent on phase of exciting light, thus generates a new spectral line, coherence between incident and scattered light allows their interference into a single frequency, thus shifts incident frequency.
Assume that a star radiates a continuous light spectrum up to X rays. Lyman frequencies are absorbed in this light and pump atoms to first excited state. In this state, hyperfine periods are longer than 1 ns, so that an ISRS “may” redshift light frequency, populating high hyperfine levels. An other ISRS “may” transfer energy from hyperfine levels to thermal electromagnetic waves, so that redshift is permanent. Temperature of a light beam is defined from spectral radiance by Planck's formula; as entropy must increase, “may” becomes “does”. However, where a absorbed line reaches Lyman alpha frequency, redshifting process stops and all hydrogen lines are absorbed, but the stop is not perfect if there is energy at frequency shifted to Lyman beta frequency, which produces a slow redshift. Successive redshifts separated by Lyman absorptions generate many absorption lines, frequencies of which, deduced from absorption process, obey a law more dependable than Karlsson’s formula; the previous process excites more and more atoms because a de-excitation obeys Einstein’s law of coherent interactions: Variation dI of radiance
A molecular cloud, sometimes called a stellar nursery, is a type of interstellar cloud, the density and size of which permit the formation of molecules, most molecular hydrogen. This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas. Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most used to determine the presence of H2 is carbon monoxide; the ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies. Within molecular clouds are regions with higher density, where lots of dust and gas cores reside, called clumps; these clumps are the beginning of star formation, if gravity can overcome the high density and force the dust and gas to collapse. Within the Milky Way, molecular gas clouds account for less than one percent of the volume of the interstellar medium, yet it is the densest part of the medium, comprising half of the total gas mass interior to the Sun's galactic orbit.
The bulk of the molecular gas is contained in a ring between 3.5 and 7.5 kiloparsecs from the center of the Milky Way. Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy; that molecular gas occurs predominantly in the spiral arms suggests that molecular clouds must form and dissociate on a timescale shorter than 10 million years—the time it takes for material to pass through the arm region. Vertically to the plane of the galaxy, the molecular gas inhabits the narrow midplane of the galactic disc with a characteristic scale height, Z, of 50 to 75 parsecs, much thinner than the warm atomic and warm ionized gaseous components of the ISM; the exception to the ionized-gas distribution are H II regions, which are bubbles of hot ionized gas created in molecular clouds by the intense radiation given off by young massive stars and as such they have the same vertical distribution as the molecular gas. This distribution of molecular gas is averaged out over large distances.
A vast assemblage of molecular gas with a mass of 103 to 107 times the mass of the Sun is called a giant molecular cloud. GMCs are around 15 to 600 light-years in diameter. Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average density of a GMC is a hundred to a thousand times as great. Although the Sun is much more dense than a GMC, the volume of a GMC is so great that it contains much more mass than the Sun; the substructure of a GMC is a complex pattern of filaments, sheets and irregular clumps. The densest parts of the filaments and clumps are called "molecular cores", while the densest molecular cores are called "dense molecular cores" and have densities in excess of 104 to 106 particles per cubic centimeter. Observationally, typical molecular cores are traced with CO and dense molecular cores are traced with ammonia; the concentration of dust within molecular cores is sufficient to block light from background stars so that they appear in silhouette as dark nebulae.
GMCs are so large. These local GMCs are arrayed in a ring in the neighborhood of the Sun coinciding with the Gould Belt; the most massive collection of molecular clouds in the galaxy forms an asymmetrical ring about the galactic center at a radius of 120 parsecs. The Sagittarius region is chemically rich and is used as an exemplar by astronomers searching for new molecules in interstellar space. Isolated gravitationally-bound small molecular clouds with masses less than a few hundred times that of the Sun are called Bok globules; the densest parts of small molecular clouds are equivalent to the molecular cores found in GMCs and are included in the same studies. In 1984 IRAS identified a new type of diffuse molecular cloud; these were diffuse filamentary clouds. These clouds have a typical density of 30 particles per cubic centimeter; the formation of stars occurs within molecular clouds. This is a natural consequence of their low temperatures and high densities, because the gravitational force acting to collapse the cloud must exceed the internal pressures that are acting "outward" to prevent a collapse.
There is observed evidence that the large, star-forming clouds are confined to a large degree by their own gravity rather than by external pressure. The evidence comes from the fact that the "turbulent" velocities inferred from CO linewidth scale in the same manner as the orbital velocity; the physics of molecular clouds is poorly much debated. Their internal motions are governed by turbulence in a cold, magnetized gas, for which the turbulent motions are supersonic but comparable to the speeds of magnetic disturbances; this state is thought to lose energy requiring either an overall collapse or a steady reinjection of energy. At the same time, the clouds are known to be disrupted by some process—most the effects of massive stars—before a significant fraction of their mass has become stars. Molecular clouds, GMCs, are
In atomic physics, the fine structure describes the splitting of the spectral lines of atoms due to electron spin and relativistic corrections to the non-relativistic Schrödinger equation. It was first measured for the hydrogen atom by Albert A. Michelson and Edward W. Morley in 1887 laying the basis for the theoretical treatment by Arnold Sommerfeld, introducing the fine-structure constant; the gross structure of line spectra is the line spectra predicted by the quantum mechanics of non-relativistic electrons with no spin. For a hydrogenic atom, the gross structure energy levels only depend on the principal quantum number n. However, a more accurate model takes into account relativistic and spin effects, which break the degeneracy of the energy levels and split the spectral lines; the scale of the fine structure splitting relative to the gross structure energies is on the order of 2, where Z is the atomic number and α is the fine-structure constant, a dimensionless number equal to 1/137. The fine structure energy corrections can be obtained by using perturbation theory.
To perform this calculation one must add the three corrective terms to the Hamiltonian: the leading order relativistic correction to the kinetic energy, the correction due to the spin-orbit coupling, the Darwin term coming from the quantum fluctuating motion or zitterbewegung of the electron. These corrections can be obtained from the non-relativistic limit of the Dirac equation, since Dirac's theory incorporates relativity and spin interactions; this section discusses the analytical solutions for the hydrogen atom as the problem is analytically solvable and is the base model for energy level calculations in more complex atoms. The gross structure assumes the kinetic energy term of the Hamiltonian takes the same form as in classical mechanics, which for a single electron means H 0 = p 2 2 m e + V where V is the potential energy, p is the momentum, m e is the electron rest mass. However, when considering a more accurate theory of nature via special relativity, we must use a relativistic form of the kinetic energy, T = p 2 c 2 + m e 2 c 4 − m e c 2 where the first term is the total relativistic energy, the second term is the rest energy of the electron.
Expanding this in a Taylor series, we find T = p 2 2 m e − p 4 8 m e 3 c 2 + ⋯ Since the first term above is part of the classical Hamiltonian, the first order correction to the Hamiltonian is H ′ = − p 4 8 m e 3 c 2 Using this as a perturbation, we can calculate the first order energy corrections due to relativistic effects. E n = ⟨ ψ 0 | H ′ | ψ 0 ⟩ = − 1 8 m e 3 c 2 ⟨ ψ 0 | p 4 | ψ 0 ⟩ = − 1 8 m e 3 c 2 ⟨ ψ 0 | p 2 p 2 | ψ 0 ⟩ where ψ 0 is the unperturbed wave function. Recalling the unperturbed Hamiltonian, we see H 0 | ψ 0 ⟩ = E n | ψ 0 ⟩ | ψ 0 ⟩ = E n | ψ 0 ⟩ p 2 | ψ
The Jmol applet, among other abilities, offers an alternative to the Chime plug-in, no longer under active development. While Jmol has many features that Chime lacks, it does not claim to reproduce all Chime functions, most notably, the Sculpt mode. Chime requires plug-in installation and Internet Explorer 6.0 or Firefox 2.0 on Microsoft Windows, or Netscape Communicator 4.8 on Mac OS 9. Jmol operates on a wide variety of platforms. For example, Jmol is functional in Mozilla Firefox, Internet Explorer, Google Chrome, Safari. Chemistry Development Kit Comparison of software for molecular mechanics modeling Jmol extension for MediaWiki List of molecular graphics systems Molecular graphics Molecule editor Proteopedia PyMOL SAMSON Official website Wiki with listings of websites and moodles Willighagen, Egon. "Fast and Scriptable Molecular Graphics in Web Browsers without Java3D". Doi:10.1038/npre.2007.50.1
National Radio Astronomy Observatory
The National Radio Astronomy Observatory is a Federally Funded Research and Development Center of the United States National Science Foundation operated under cooperative agreement by Associated Universities, Inc for the purpose of radio astronomy. NRAO designs and operates its own high sensitivity radio telescopes for use by scientists around the world; the NRAO headquarters is located on the campus of the University of Virginia in Charlottesville, Virginia. The North American ALMA Science Center and the NRAO Technology Center and Central Development Laboratory are located in Charlottesville, Virginia. NRAO was, until October 2016, the operator of the world's largest steerable radio telescope, the Robert C. Byrd Green Bank Telescope, which stands near Green Bank, West Virginia; the observatory contains several other telescopes, among them the 140-foot telescope that utilizes an equatorial mount uncommon for radio telescopes, three 85-foot telescopes forming the Green Bank Interferometer, a 40-foot telescope used by school groups and organizations for small scale research, a fixed radio'horn' built to observe the radio source Cassiopeia A, as well as a reproduction of the original antenna built by Karl Jansky while he worked for Bell Labs to detect the interference, discovered to be unknown natural radio waves emitted by the universe.
Green Bank is in the United States National Radio Quiet Zone, coordinated by NRAO for protection of the Green Bank site as well as the Sugar Grove, West Virginia monitoring site operated by the NSA. The zone consists of a 13,000-square-mile piece of land where fixed transmitters must coordinate their emissions before a license is granted; the land was set aside by the Federal Communications Commission in 1958. No fixed radio transmitters are allowed within the area closest to the telescope. All other fixed radio transmitters including TV and radio towers inside the zone are required to transmit such that interference at the antennas is minimized by methods including limited power and using directional antennas. With the advent of wireless technology and microprocessors in everything from cameras to cars, it is difficult to keep the sites free of radio interference. To aid in limiting outside interference, the area surrounding the Green Bank observatory was at one time planted with pines characterized by needles of a certain length to block electromagnetic interference at the wavelengths used by the observatory.
At one point, the observatory faced the problem of North American flying squirrels tagged with US Fish & Wildlife Service telemetry transmitters. Electric fences, electric blankets, faulty automobile electronics, other radio wave emitters have caused great trouble for the astronomers in Green Bank. All vehicles on the premises are powered by diesel motors to minimize interference by ignition systems; the NRAO's facility in Socorro is the Pete V. Domenici Array Operations Center. Located on the New Mexico Tech university campus, the AOC serves as the headquarters for the Very Large Array, the setting for the 1997 movie Contact, is the control center for the Very Long Baseline Array; the ten VLBA telescopes are located in Hawaii, the U. S. Virgin Islands, eight other sites across the continental United States. Offices are located on the University of Arizona campus. NRAO operated the 12 Meter Telescope on Kitt Peak. NRAO suspended operations at this telescope and funding was rerouted to the Atacama Large Millimeter Array instead.
The Arizona Radio Observatory now operates the 12 Meter Telescope. The Atacama Large Millimeter Array site in Chile is at ~5000 m altitude near Cerro Chajnantor in northern Chile; this is about 40 km east of the historic village of San Pedro de Atacama, 130 km southeast of the mining town of Calama, about 275 km east-northeast of the coastal port of Antofagasta. The Karl G. Jansky Lectureship is a prestigious Lecture awarded by the Board of Trustees of the NRAO; the Lectureship is awarded "to recognize outstanding contributions to the advancement of radio astronomy." Recipients have included Fred Hoyle, Charles Townes, Edward M. Purcell, Subrahmanyan Chandrasekhar, Philip Morrison, Vera Rubin, Jocelyn Bell Burnell, Frank J. Low and Mark Reid; the lecture is delivered in Socorro. Official website
A glow discharge is a plasma formed by the passage of electric current through a gas. It is created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas; when the voltage exceeds a value called the striking voltage, the gas ionization becomes self-sustaining, the tube glows with a colored light. The color depends on the gas used. Glow discharges are used as a source of light in devices such as neon lights, fluorescent lamps, plasma-screen televisions. Analyzing the light produced with spectroscopy can reveal information about the atomic interactions in the gas, so glow discharges are used in plasma physics and analytical chemistry, they are used in the surface treatment technique called sputtering. Conduction in a gas requires charge carriers, which can be either ions. Charge carriers come from ionizing some of the gas molecules. In terms of current flow, glow discharge falls between dark arc discharge. In a dark discharge, the gas is ionized by a radiation source such as ultraviolet light or Cosmic rays.
At higher voltages across the anode and cathode, the freed carriers can gain enough energy so that additional carriers are freed during collisions. In a glow discharge, the carrier generation process reaches a point where the average electron leaving the cathode allows another electron to leave the cathode. For example, the average electron may cause dozens of ionizing collisions via the Townsend avalanche. In an arc discharge, electrons leave the cathode by thermionic emission and field emission, the gas is ionized by thermal means. Below the breakdown voltage there is no or little glow and the electric field is uniform; when the electric field increases enough to cause ionization, the Townsend discharge starts. When a glow discharge develops, the electric field is modified by the presence of positive ions; the glow discharge starts as a normal glow. As the current is increased, more of the cathode surface is involved in the glow; when the current is increased above the level where the entire cathode surface is involved, the discharge is known as an abnormal glow.
If the current is increased still further, other factors come into play and an arc discharge begins. The simplest type of glow discharge is a direct-current glow discharge. In its simplest form, it consists of two electrodes in a cell held at low pressure. A low pressure is used to increase the mean free path; the cell is filled with neon, but other gases can be used. An electric potential of several hundred volts is applied between the two electrodes. A small fraction of the population of atoms within the cell is ionized through random processes, such as thermal collisions between atoms or by gamma rays; the positive ions are driven towards the cathode by the electric potential, the electrons are driven towards the anode by the same potential. The initial population of ions and electrons collides with other atoms, exciting or ionizing them; as long as the potential is maintained, a population of ions and electrons remains. Some of the ions' kinetic energy is transferred to the cathode; this happens through the ions striking the cathode directly.
The primary mechanism, however, is less direct. Ions strike the more numerous neutral gas atoms, transferring a portion of their energy to them; these neutral atoms strike the cathode. Whichever species strike the cathode, collisions within the cathode redistribute this energy resulting in electrons ejected from the cathode; this process is known as secondary electron emission. Once free of the cathode, the electric field accelerates electrons into the bulk of the glow discharge. Atoms can be excited by collisions with ions, electrons, or other atoms that have been excited by collisions. Once excited, atoms will lose their energy quickly. Of the various ways that this energy can be lost, the most important is radiatively, meaning that a photon is released to carry the energy away. In optical atomic spectroscopy, the wavelength of this photon can be used to determine the identity of the atom and the number of photons is directly proportional to the concentration of that element in the sample; some collisions will cause ionization.
In atomic mass spectrometry, these ions are detected. Their mass identifies the type of atoms and their quantity reveals the amount of that element in the sample; the illustrations to the right shows the main regions. Regions described; as the discharge becomes more extended, the positive column may become striated. That is, alternating dark and bright regions may form. Compressing the discharge horizontally will result in fewer regions; the positive column will be compressed while the negative glow will remain the same size, with small enough gaps, the positive column will disappear altogether. In an analytical glow discharge, the discharge is a negative glow with dark region above and below it; the cathode layer begins with the Aston dark space, ends with the negative glow region. The cathode layer shortens with i
Microwaves are a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter. Different sources define different frequency ranges as microwaves. A more common definition in radio engineering is the range between 100 GHz. In all cases, microwaves include the entire SHF band at minimum. Frequencies in the microwave range are referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations; the prefix micro- in microwave is not meant to suggest a wavelength in the micrometer range. Rather, it indicates that microwaves are "small", compared to the radio waves used prior to microwave technology; the boundaries between far infrared, terahertz radiation and ultra-high-frequency radio waves are arbitrary and are used variously between different fields of study. Microwaves travel by line-of-sight. At the high end of the band they are absorbed by gases in the atmosphere, limiting practical communication distances to around a kilometer.
Microwaves are used in modern technology, for example in point-to-point communication links, wireless networks, microwave radio relay networks, radar and spacecraft communication, medical diathermy and cancer treatment, remote sensing, radio astronomy, particle accelerators, industrial heating, collision avoidance systems, garage door openers and keyless entry systems, for cooking food in microwave ovens. Microwaves occupy a place in the electromagnetic spectrum with frequency above ordinary radio waves, below infrared light: In descriptions of the electromagnetic spectrum, some sources classify microwaves as radio waves, a subset of the radio wave band; this is an arbitrary distinction. Microwaves travel by line-of-sight paths. Although at the low end of the band they can pass through building walls enough for useful reception rights of way cleared to the first Fresnel zone are required. Therefore, on the surface of the Earth, microwave communication links are limited by the visual horizon to about 30–40 miles.
Microwaves are absorbed by moisture in the atmosphere, the attenuation increases with frequency, becoming a significant factor at the high end of the band. Beginning at about 40 GHz, atmospheric gases begin to absorb microwaves, so above this frequency microwave transmission is limited to a few kilometers. A spectral band structure causes absorption peaks at specific frequencies. Above 100 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is in effect opaque, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges. In a microwave beam directed at an angle into the sky, a small amount of the power will be randomly scattered as the beam passes through the troposphere. A sensitive receiver beyond the horizon with a high gain antenna focused on that area of the troposphere can pick up the signal; this technique has been used at frequencies between 0.45 and 5 GHz in tropospheric scatter communication systems to communicate beyond the horizon, at distances up to 300 km.
The short wavelengths of microwaves allow omnidirectional antennas for portable devices to be made small, from 1 to 20 centimeters long, so microwave frequencies are used for wireless devices such as cell phones, cordless phones, wireless LANs access for laptops, Bluetooth earphones. Antennas used include short whip antennas, rubber ducky antennas, sleeve dipoles, patch antennas, the printed circuit inverted F antenna used in cell phones, their short wavelength allows narrow beams of microwaves to be produced by conveniently small high gain antennas from a half meter to 5 meters in diameter. Therefore, beams of microwaves are used for point-to-point communication links, for radar. An advantage of narrow beams is that they don't interfere with nearby equipment using the same frequency, allowing frequency reuse by nearby transmitters. Parabolic antennas are the most used directive antennas at microwave frequencies, but horn antennas, slot antennas and dielectric lens antennas are used. Flat microstrip antennas are being used in consumer devices.
Another directive antenna practical at microwave frequencies is the phased array, a computer-controlled array of antennas which produces a beam which can be electronically steered in different directions. At microwave frequencies, the transmission lines which are used to carry lower frequency radio waves to and from antennas, such as coaxial cable and parallel wire lines, have excessive power losses, so when low attenuation is required microwaves are carried by metal pipes called waveguides. Due to the high cost and maintenance requirements of waveguide runs, in many microwave antennas the output stage of the transmitter or the RF front end of the receiver is located at the antenna; the term microwave has a more technical meaning in electromagnetics and circuit theory. Apparatus and techniques may