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Goldreich-Kylafis Effect
The Goldreich-Kylafis (GK) effect is a quantum mechanical effect with applications in Astrophysics. The theoretical background of the work was published by Peter Goldreich and - his postdoc at the time - Nick Kylafis in a series of two papers in The Astrophysical Journal. The GK effect predicts that, under special conditions, the spectral lines emitted by interstellar molecules should be linearly polarized and the linear polarization vector should reveal the magnetic field direction in the molecular cloud. Even a μG magnetic field is enough for this effect. The lines arise from rotational transitions of molecules, say J=1 to J=0, where J is the rotational quantum number. If the magnetic sublevels of the J=1 level are equally populated, as it is usually the case, then the line is unpolarized. However, if the magnetic sublevels are unequally populated, then the line is polarized. Goldreich & Kylafis (1981) showed that, if the radiation field (their own plus external) in wh ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Astrophysics
Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of the founders of the discipline said, Astrophysics "seeks to ascertain the nature of the heavenly bodies, rather than their positions or motions in space–''what'' they are, rather than ''where'' they are." Among the subjects studied are the Sun, other stars, galaxies, extrasolar planets, the interstellar medium and the cosmic microwave background. Emissions from these objects are examined across all parts of the electromagnetic spectrum, and the properties examined include luminosity, density, temperature, and chemical composition. Because astrophysics is a very broad subject, ''astrophysicists'' apply concepts and methods from many disciplines of physics, including classical mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Anisotropy
Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties ( absorbance, refractive index, conductivity, tensile strength, etc.). An example of anisotropy is light coming through a polarizer. Another is wood, which is easier to split along its grain than across it. Fields of interest Computer graphics In the field of computer graphics, an anisotropic surface changes in appearance as it rotates about its geometric normal, as is the case with velvet. Anisotropic filtering (AF) is a method of enhancing the image quality of textures on surfaces that are far away and steeply angled with respect to the point of view. Older techniques, such as bilinear and trilinear filtering, do not take into account the angle a surface is viewed from, which can result in alias ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Astronomy
Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest include planets, natural satellite, moons, stars, nebulae, galaxy, galaxies, and comets. Relevant phenomena include supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, astronomy studies everything that originates beyond atmosphere of Earth, Earth's atmosphere. Cosmology is a branch of astronomy that studies the universe as a whole. Astronomy is one of the oldest natural sciences. The early civilizations in recorded history made methodical observations of the night sky. These include the Babylonian astronomy, Babylonians, Greek astronomy, Greeks, Indian astronomy, Indians, Egyptian astronomy, Egyptians, Chinese astronomy, Chinese, Maya civilization, Maya, and many anc ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
TW Hydrae
TW Hydrae is a T Tauri star approximately 196 light-years away in the constellation of Hydra (the Sea Serpent). TW Hydrae is about 80% of the mass of the Sun, but is only about 5-10 million years old. The star appears to be accreting from a face-on protoplanetary disk of dust and gas, which has been resolved in images from the ALMA observatory. TW Hydrae is accompanied by about twenty other low-mass stars with similar ages and spatial motions, comprising the " TW Hydrae association" or TWA, one of the closest regions of recent "fossil" star-formation to the Sun. Stellar characteristics TW Hydrae is a pre-main-sequence star that is approximately 80% the mass of and 111% the radius of the Sun. It has a temperature of 4000 K and is about 8 million years old. In comparison, the Sun is about 4.6 billion years old and has a temperature of 5778 K. The star's luminosity is 28% (0.28x) that of the Sun, equivalent to that of a main-sequence star of spectral type ~ K2. Howeve ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
AGB Stars
The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) late in their lives. Observationally, an asymptotic-giant-branch star will appear as a bright red giant with a luminosity ranging up to thousands of times greater than the Sun. Its interior structure is characterized by a central and largely inert core of carbon and oxygen, a shell where helium is undergoing fusion to form carbon (known as helium burning), another shell where hydrogen is undergoing fusion forming helium (known as hydrogen burning), and a very large envelope of material of composition similar to main-sequence stars (except in the case of carbon stars). Stellar evolution When a star exhausts the supply of hydrogen by nuclear fusion processes in its core, the core contracts and its temperature increases, causing the outer lay ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Collisional Excitation
Collisional excitation is a process in which the kinetic energy of a collision partner is converted into the internal energy of a reactant species. Astronomy In astronomy, collisional excitation gives rise to spectral lines in the spectra of astronomical objects such as planetary nebulae and H II regions. In these objects, most atoms are ionised by photons from hot stars embedded within the nebular gas, stripping away electrons. The emitted electrons, (called photoelectrons), may collide with atoms or ions within the gas, and excite them. When these excited atoms or ions revert to their ground state, they will emit a photon. The spectral lines formed by these photons are called ''collisionally excited lines'' (often abbreviated to CELs). CELs are only seen in gases at very low densities (typically less than a few thousand particles per cm³) for forbidden transitions. For allowed transitions, the gas density can be substantially higher. At higher densities, the reverse process ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Spontaneous Emission
Spontaneous emission is the process in which a quantum mechanical system (such as a molecule, an atom or a subatomic particle) transits from an excited energy state to a lower energy state (e.g., its ground state) and emits a quantized amount of energy in the form of a photon. Spontaneous emission is ultimately responsible for most of the light we see all around us; it is so ubiquitous that there are many names given to what is essentially the same process. If atoms (or molecules) are excited by some means other than heating, the spontaneous emission is called luminescence. For example, fireflies are luminescent. And there are different forms of luminescence depending on how excited atoms are produced (electroluminescence, chemiluminescence etc.). If the excitation is affected by the absorption of radiation the spontaneous emission is called fluorescence. Sometimes molecules have a metastable level and continue to fluoresce long after the exciting radiation is turned off; thi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Optical Depth
In physics, optical depth or optical thickness is the natural logarithm of the ratio of incident to ''transmitted'' radiant power through a material. Thus, the larger the optical depth, the smaller the amount of transmitted radiant power through the material. Spectral optical depth or spectral optical thickness is the natural logarithm of the ratio of incident to transmitted spectral radiant power through a material. Optical depth is dimensionless, and in particular is not a length, though it is a monotonically increasing function of optical path length, and approaches zero as the path length approaches zero. The use of the term "optical density" for optical depth is discouraged. In chemistry, a closely related quantity called " absorbance" or "decadic absorbance" is used instead of optical depth: the common logarithm of the ratio of incident to transmitted radiant power through a material, that is the optical depth divided by ln 10. Mathematical definitions Optical de ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Isotropic Radiation
Isotropic radiation is radiation that has the same intensity regardless of the direction of measurement, such as would be found in a thermal cavity. The radiation may be electromagnetic, sound or may be composed of elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include electrons, the fundamental fermions (quarks, leptons, antiq ...s. Radiation {{Physics-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Rotational Transition
In quantum mechanics, a rotational transition is an abrupt change in angular momentum. Like all other properties of a quantum particle, angular momentum is quantized, meaning it can only equal certain discrete values, which correspond to different rotational energy states. When a particle loses angular momentum, it is said to have transitioned to a lower rotational energy state. Likewise, when a particle gains angular momentum, a positive rotational transition is said to have occurred. Rotational transitions are important in physics due to the unique spectral lines that result. Because there is a net gain or loss of energy during a transition, electromagnetic radiation of a particular frequency must be absorbed or emitted. This forms spectral lines at that frequency which can be detected with a spectrometer, as in rotational spectroscopy or Raman spectroscopy. Diatomic molecules Molecules have rotational energy owing to rotational motion of the nuclei about their center of ma ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Peter Goldreich
Peter Goldreich (born July 14, 1939) is an American astrophysicist whose research focuses on celestial mechanics, planetary rings, helioseismology and neutron stars. He is the Lee DuBridge Professor of Astrophysics and Planetary Physics at California Institute of Technology. Since 2005 he has also been a professor at the Institute for Advanced Study in Princeton, New Jersey. Asteroid 3805 Goldreich is named after him. Career Goldreich received a bachelor of science in engineering physics from Cornell University in 1960, and obtained a Ph.D. from Cornell in 1963 under the supervision of Thomas Gold. In 1963 and 1964 Goldreich was a postdoctoral fellow at Cambridge University. From 1964 to 1966 he was an assistant professor of astronomy and geophysics at UCLA. Goldreich joined the faculty at Caltech in 1966 as an associate professor. He later became a full professor in 1969 while remaining at Caltech, and in 1981 he became the Lee A. DuBridge Professor of Astrophysics & P ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Molecular Cloud
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, H2), and the formation of H II regions. 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 often used to determine the presence of H2 is carbon monoxide (CO). 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 much dust and many gas cores reside, called clumps. These clumps are the beginning of star formation if gravitational forces are sufficient to cause the dust and gas to collapse. History The form of molecul ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
