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Dictionary of Geophysics, Astrophysic, and Astronomy.pdf

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Stark effect

spect to normal galaxies of the same morphological type. Associated features include often high ultraviolet emission, galactic superwinds, dominant photoionization lines in the far infrared spectra, evidence for active periods in the recent star formation history, etc. Spectral features of Wolf Rayet stars (which are very bright and have broad, easily-identiﬁed emission lines) imply a recent burst of massive star formation, since these last only 3 to 4 Myr before they explode. Red supergiants, that last ≈ 10 Myr, signify a somewhat older burst. Starburst galaxies often show evidence of some sort of interaction, harassment or merger in their recent past. Prototype Starburst galaxies are the edge-on spiral NGC 253 and the dwarf irregular galaxy Messier 82. FIR-strong galaxies, blue galaxies, UV excess galaxies, or HII galaxies are often systems where star formation is enhanced with respect to normal galaxies, and can be synonyms of Starburst galaxies. The different names reﬂect the different technique and the different spectral range of observation and discovery.

Stardust A spacecraft launched on February 7, 1999, that is the ﬁrst space mission that will ﬂy close to a comet and bring cometary material back to Earth for analysis. It was developed under NASA’s Discovery Program of low-cost solar system exploration missions. Its primary goal is to collect comet dust and volatile samples during a planned close encounter with comet Wild 2 in January of 2004. The spacecraft will also bring back samples of interstellar dust including the recently discovered dust streaming into the solar system from the direction of Sagittarius. A unique substance called aerogel is the medium that will be used to catch and preserve comet samples. When Stardust swings by Earth in January 2006, the samples encased in a reentry capsule will be jettisoned and parachuted to a pre-selected site in the Utah desert. Groundbased analysis of these samples should yield important insights into the evolution of the sun and planets, and possibly into the origin of life itself.

Stardust is a collaborative effort between NASA, university and industry partners.

star formation The process by which diffuse gas collapses to form a star or system of stars. This follows the concentration of the atomic gas

within galaxies into dense, opaque, molecular clouds, and the further condensation of regions within these clouds into dense cores that become unstable to collapse. During collapse, a star and its surrounding protoplanetary accretion disk grow from infalling matter.

Stars are born with vastly less magnetic ﬂux and angular momentum than their parent clouds. The loss of magnetic ﬂux is attributed to ambipolar diffusion, the motion of neutral material across magnetic ﬁeld lines, that operates in molecular clouds. Magnetic ﬂux may be also lost by reconnection. Angular momentum is lost by magnetic braking before collapse, and then by the emission of a powerful, collimated wind powered by accretion through the disk. Such winds strongly affect the star-forming cloud by adding turbulent motion and by casting away material.

star formation rate (SFR) The rate at which new stars are being formed in a galaxy, or in any star forming region, often measured in units of solar masses per year. The star formation rate can be estimated from measurement of the luminosity of the Hydrogen Hα spectral line, or from the luminosity emitted in the far infrared, if the initial mass function of stars is assumed. Galaxies of late morphological types, on the contrary, have yet to exhaust their molecular gas, and are still forming stars. The star formation rate in galaxies varies widely along the Hubble sequence: For galaxies in the local universe, typical values are close to zero for elliptical galaxies and 10 solar masses per year for Sc spirals.

Stark effect The sub-division of energy levels within an atom caused by the external application of an electric ﬁeld. For hydrogenic atoms, the energy shift is directly proportional to the applied ﬁeld strength; this is known as the linear Stark effect. For other atoms and ions, the displacement of the energy levels is proportional to the square of the applied electric ﬁeld strength, giving rise to the quadratic Stark effect. The latter results from the additional condition that the induced dipole moment is also proportional to the intensity of the applied electric ﬁeld. For hydrogen atoms, electric ﬁelds of intensity > 10 V/cm are required to produce an

star spots

observable Stark effect, with greater ﬁelds being required to produce the quadratic Stark effect.

star spots The analog of sunspots on other stars. They are usually darker (cooler) than their surroundings (though bright spots also occur) and are associated with concentrations of magnetic ﬁeld in their vicinity, as are sunspots. The ﬁeld apparently inhibits energy transport by convection in the atmospheres of the relatively cool stars that have spots, leading to cooler, darker regions. Star spots are particularly conspicuous (and sometimes found near the poles rather than near the equator as in the sun) in young, rapidly rotating stars, and in close binary pairs, where the rotation period is synchronized to the short orbital period. They show up as changes in the brightness and spectrum of the star that approximately repeats at the rotation period.

statcoulomb Unit of electric charge, 3.3356 ×10−10 coulomb. Previously deﬁned as equal to the charge that exerts a force of 1 dyne on an equal charge at a distance of 1 centimeter in vacuum.

state parameter See electric regime (cosmic string).

steady-state model The cosmological model of Bondi, Gold, Hoyle and Narlikar in which matter is continuously created to ﬁll the voids left as the universe expands. Consequently, such a universe has no beginning and no end and always maintains the same density. This model of the universe is thus derived from a stronger version of the cosmological principle, that the universe is not only homogeneous and isotropic (see homogeneity, isotropy), but also unchanging in time. This stronger version, the “perfect cosmological principle,” was postulated as a universal law of nature. The rate at which new matter would have to appear to offset the dilution caused by expansion is one hydrogen atom in one liter of volume once in 109 years. There is no experimental evidence against this assumption since the rate is undetectably small. The steady state models were motivated by the discrepancy between the age of the universe and the age of the Earth — the latter was apparently longer than the former; the steady-state model

removed this discrepancy by making the universe eternally-lived. The discrepancy was later resolved as an error in estimating the value of the Hubble parameter that led to a drastic underestimation of the age of the universe. The steady-state model was deﬁnitively proven false when the microwave background radiation was discovered in 1965. This radiation ﬁnds a simple explanation in models with a Big Bang, and no natural explanation in the steady-state model.

steepness (wave) A term used in the study of water waves, deﬁned as the ratio of wave height, H , to wavelength, L, or H/L. A common rule of thumb is that the wave steepness cannot exceed 1/7 in deep water before wave breaking will occur.

Stefan–Boltzmann constant (σ ) Constant in the equation for the radiant emittance M (radiant energy ﬂux per unit area) from a black body at thermodynamic temperature T : M = σ T 4.

σ = 2π5k4 = 5.66956 × 10−5 15c2h3

erg cm−2 deg−4 sec−1

where k is Boltzmann’s constant, h is Planck’s constant, and c is the speed of light.

stellar activity The complex of X-ray, visible light, and radio wave phenomena associated with rapid rotation, strong magnetic ﬁelds, and the presence of a chromosphere and corona on a star (generally one whose photospheric temperature is less than about 6,500 K). These phenomena include emission lines of hydrogen, calcium, and sodium emitted by the chromosphere, X-rays and occasionally radio waves emitted by the corona, starspots, with magnetic ﬁelds up to several thousand gauss (generally detectable only on the sun), and ﬂares.

stellar classiﬁcation A classiﬁcation of stars according to the observed surface temperature; see below.

There is a further subdivision within each range with the numbers from 0 to 9, eg G5, A0.

The stellar classiﬁcation is often called the Henry Draper system. It was ﬁrst published (1918–1924) by Annie Jump Cannon of Harvard

				stick-slip

Type	Temperature	Color	Spectral Lines	Example Star
O	35000 K	blue	singly ionized helium	spiral arm blue stars
B	11000 to 28000 K	blue-white	hydrogen, oxygen, neutral Helium in absorption	Rigel, Spica
A	10000 K	white	hydrogen Balmer	Vega, Sirius
F	6000 to 7500 K	yellow white	Hydrogen, singly ionized Calcium	Canopus, Procyon
G	5000 to 6000 K	yellow	Calcium, neutral and ionized metals	sun, Capella
K	3500 to 5000 K	orange	Strong H and K lines of Calcium	Arcturus, Aldebaran
M	less than 3500 K	red	molecular	Betelgeuse and Antares

College Observatory with the title Henry Draper Catalog because Draper’s widow sponsored the project in his honor.

stellar evolution The set of processes that gradually transform a newly-formed, chemically homogeneous (main sequence) star into sequential phases like red giant or supergiant, horizontal branch star, Cepheid variable, and so forth onward to death as a white dwarf, neutron star, or black hole. The star changes its brightness and surface temperature (and so can be followed on an HR Diagram). It also burns a sequence of nuclear fuels from hydrogen and helium burning (for low mass stars) on up to silicon burning (for stars of more than about 10 solar masses). It is the changes of chemical composition resulting from these nuclear reactions that are primarily responsible for the changes in luminosity and temperature that we see. Detailed calculations of stellar evolution are done by solving a set of four coupled, non-linear differential equations and allowing for these composition changes.

stellar population The observed color of stars depends on the surface temperature. More massive stars have higher surface temperatures and are therefore bluer than lower mass stars. The surface temperature also depends on the chemical composition of a star. Many stars have a composition similar to the sun. Such stars are about 23% Helium and have traces of elements more massive than He (see metallicity). However, the majority of the mass is Hydrogen. Such stars are called Population I stars. Population II stars are those with a greatly reduced concentration of elements with an atomic number greater than 2 (i.e., Helium). Population II stars are deﬁcient in these elements by factors up to 10,000 relative to Population I stars. This difference in composition causes the star to be less opaque to

light and results in a much bluer star, for a given mass.

stellar winds Gas blowing off the surface of a star, driven by some combination of magnetic energy and acoustic waves. The solar wind is a mild one, carrying only about 10−19 solar masses per year. Stars that are very bright for their mass or very rapidly rotating can have winds up to 10−4 solar masses per year, for brief periods of time. Stellar winds are one source of mass transfer in cataclysmic variables and X-ray binaries and a short-lived superwind phase at the end of the life on an asymptotic giant branch star is probably the origin of planetary nebulae.

stepped leader In lightning strokes, the initiating process is that charge is forced down a broken path from the cloud, in steps of about 30 m in length taking about 1 ms for each step, with about 50 ms of pause between steps. This is called the stepped leader. See return stroke, dart leader.

steric height An oceanographic quantity describing the depth difference between two surfaces of constant pressure. The steric height h is deﬁned by

h(z1, z2) = δ(T , S, p)ρ0 dz

where z1 and z2 are the depths of the pressure surfaces, δ is the speciﬁc volume anomaly, T is the temperature, S is the salinity, p is the pressure, and ρ0 is a reference density. It has the dimension of height and is expressed in meters.

stick-slip The behavior of a fault that generates earthquakes. When the tectonic stress is low a fault “sticks”. When the tectonic stress exceeds the rupture strength of the fault, “slip” occurs on the fault generating an earthquake.

stiff ﬂuid

stiff ﬂuid A ﬂuid in which the energy-density ρ equals p/c2 at every point (where p is the pressure and c is the speed of light), so that the velocity of sound is equal to the velocity of light. This is a theoretical construct (no known physical system has such a large pressure). The equality between the velocity of sound and the velocity of light greatly simpliﬁes the dynamical properties of such a medium, and several problems that would be difﬁcult to solve in realistic matter become solvable in a stiff ﬂuid. One example is the propagation of soliton waves through such a medium. Also, solutions of Einstein’s equations in which a stiff ﬂuid is a source are easier to ﬁnd than solutions with more realistic sources.

still water level The level at which the water in an ocean or lake would be if all waves were absent. Differs from the mean water level because of the inﬂuence of wave setup and setdown.

stishovite A high pressure polymorph of quartz having a density of about 1.6 times that of ordinary quartz thought to be naturally stable only in the Earth’s mantle. Stishovite was ﬁrst synthesized in the laboratory by Sergei Stishov in 1960, but later small crystals were discovered in nature at the site of the Barringer Meteor Crater in Arizona. Stishovite has the rutile structure and is one of the very few known materials in which Si occurs in octahedral coordination with oxygen.

stochastic acceleration The energization of a population of particles in which the spread in momenta of the accelerated particles increases at the same time as the average energy per particle. Physically, stochastic acceleration of a particle occurs via a series of random kicks from the interaction with ambient particles.

Stokes’ law The statement, valid when viscous forces dominate inertial effects, that the viscous force F experienced by a sphere of radius a moving at velocity v in a medium of viscosity η is given by F = −6πηav.

Stokes parameters Four parameters used to describe polarized radiation; one component describes the radiance, two components describe

the states of linear polarization, and one component describes the state of circular polarization.

Stokes parameters Four quantities obeying s02 = s12 + s22 + s32, describing the polarization state of a beam of light. s0 is the total intensity, s1 is the difference in intensity of the two polarizations. The other two parameters describe the relative phases of the two polarization states. Stokes parameters can be deﬁned in terms of linear, circular polarizations. (The resulting parameters differ in the two bases, for the same beam of light.)

Stokes polarimetry The means by which to measure the amount of polarization in light from the sun in order to yield information about the direction of the magnetic ﬁeld in threedimensions. An accurate determination of the vector magnetic ﬁeld, using the Zeeman effect, needs a precise measurement of the four Stokes parameters.

Stokes’ Theorem In three dimensions, the surface integral over a topological disk, of the dot product of the curl of a vector with the normal to the disk (n · × A ) equals the line integral around the boundary of the surface of A · dl, where dl is the directed line element in the boundary. In general, the integral of an exact differential form dω in a region equals the integral of its potential ω in the boundary of the region.

Stokes wave theory A theory for description of surface water waves. Generally refers to ﬁrst-order (linear) wave theory, but higher-order terms may be retained during the derivation to yield an nth-order Stokes wave theory.

stony-iron meteorite A meteorite that shows a combination of stone and nickel-iron. This type of meteorite may have been formed from the core-mantle boundary of a parent body that broke up to produce the meteorite, or may have been formed in an energetic collision that partly melted the colliding objects.

storativity (S) The volume of water produced from a conﬁned aquifer per unit area per unit decline in the potentiometric surface is a di-

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