ECCENTRICITY - Ratio of the distance between the foci and the major axis of an ellipse (planetary orbit).
ECLIPSING BINARY - Binary star system where one of the two stars passes between its companion and Earth, yielding systematic changes in its brightness.
ECLIPTIC - Plane of Earth's orbit around the sun; all the planets except Mercury and Pluto have orbits in nearly the same plane. The Sun's axis is tilted at an angle of 7°15' to the ecliptic, which has been proposed as evidence the Sun was originally part of a binary star system.
EJECTA - Fractured and/or molten rocky debris thrown out of a crater during an impact event, or, alternatively, material, including ash, lapilli, and bombs, erupted from a volcano.
EJECTA BLANKET - Generally symmetrical apron of ejecta surrounding a crater; it is thick at the crater's rim and thin to discontinuous at the blanket's outer edge.
ELECTROMAGNETIC FORCE - Force between charged particles, which accounts for electricity and magnetism. One of the four fundamental forces, the electromagnetic force is conveyed between real particles by virtual photons (photons which come into existence in accordance with the Heisenberg uncertainty principle).
ELECTROMAGNETIC RADIATION - Radiation, consisting of an electric and a magnetic disturbance, which travels in a vacuum at the velocity of light (c). Usually electromagnetic radiation is considered wave motion with the distance between successive crests being the wavelength (λ). The number of wavecrests passing a fixed point in one second is the frequency (ν):
Short wavelength corresponds to high frequency. In our universe the shortest possible wavelength is likely to be the Planck length, and the longest possible wavelength, the size of the universe itself (although in principle the spectrum is of wavelengths is infinite). Electromagnetic radiation is divided, in order of increasing frequency and decreasing wavelength into: radio, microwave, infrared, visible, ultraviolet, x-rays, and γ-rays. Electromagnetic radiation may also be described as particles (quanta or photons = "packets" of energy). The energy associated with electromagnetic radiation is inversely proportional to wavelength and directly proportional to frequency:
Thus, shorter wavelengths correspond to higher energy photons.
ELECTRON - Elementary particle (of the type known as a lepton) with a mass of 9.1091 x 10-31 kg (0.511 MeV) and a negative charge of 1.602 x 10-19 coulombs. Electrons can exist either as free particles or as the negatively charged components of atoms. Electrons surround the atomic nucleus and their distribution and number, specified by their quantum numbers, determine the chemical properties of an element. A flow of electrons constitutes an electric current. For historical reasons, electrons are sometimes referred to as β particles.
ELECTRON CAPTURE DECAY - Nuclear decay by capture of an atomic electron. If the decay energy is greater than 1.022 MeV, β+ emission can also occur in competition with electron capture.
ELECTRON DEGENERACY PRESSURE - The force that supports white dwarf stars against gravity. Quantum mechanics restricts the number of electrons that can have low energy. When electrons are packed together, the number of available low energy states is too small and many electrons are forced into high energy states. When this happens the electrons are said to be degenerate and make a significant contribution to pressure. This electron degeneracy pressure arises from quantum mechanical effects and is insensitive to temperature, i.e., the pressure doesn't go down as the star cools.
ELECTRON MICROPROBE - Instrument that analyzes the chemistry of very small spots by bombarding the sample with electrons and measuring the X-rays produced.
ELECTRON SHELL - Collection of energy states with the same energy (in the absence of electromagnetic fields) for electrons in atoms. According to the Pauli Exclusion Principle, electrons can never occupy identical states specified by quantum numbers.
ELECTRON VOLT - Unit of energy used to describe the energies of subatomic particles and photons, or energy levels of atoms. An electron volt is defined as the kinetic energy acquired when an electron is accelerated through a potential difference of 1 volt. 1 eV = 1.60217646 x 10-17 erg = 1.60217646 x 10-19 Joule. Particle masses are commonly given by their "mass-equivalent" energy (E = mc2); the mass of the proton is 938.27231 MeV. Commonly used multiples are keV (103 eV), MeV (106 eV), and GeV (109 eV).
ELECTRONEGATIVITY - Measure of the ability of atom in a crystal structure to attract electrons to its outer shell. Elements with low electronegativities (metals and semimetals) tend to lose electrons and become cations. Elements with high electronegativities (nonmetals) tend to gain electrons and become anions.
ELECTRONIC CONFIGURATION - Summary of the distribution of electrons about a nucleus, specifying where these electrons are most likely to be found.
ELECTROWEAK FORCE - Unified force combining the electromagnetic force and weak nuclear forces. The unification of these two forces was proposed in the 1960s by Sheldon Glashow, Steven Weinberg and Abdus Salam. According to their theory, the electromagnetic force and the weak nuclear interaction will behave in exactly the same way at high energy. The electromagnetic force is conveyed between real particles by virtual photons. whereas, the weak nuclear force is conveyed (or "mediated") by very massive virtual particles, the W+, W-, and Z0 bosons. Electroweak theory predicts that at energies >100 GeV, W and Z bosons are produced as frequently as photons and the two forces behave as a unified force. The discovery of W and Z particles at CERN in 1983 by Rubbia and van der Meer provided strong confirmation of electroweak theory.
ELEMENT - Substance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of orbiting electrons is equal to the number of protons in its nucleus. Each chemical element is denoted by a symbol, which may be amplified by the addition of the atomic mass (A) as a superscript and the atomic number (Z) as a subscript, e.g., 42He.
ELLIPTICAL GALAXY - Galaxy with an elliptical overall shape. It was once thought that the shape of ellipticals varied from spherical to highly elongate. The Hubble classification of elliptical galaxies ranges from E0 for those that are most spherical, to E7, which are long and thin. It is now recognized that most ellipticals are of middling thinness, and that the Hubble classifications result from the angle with which the galaxy was observed. Stars in an elliptical galaxy have random motion, unlike those in spiral galaxies, which have no random motion and are dominated by rotation. They contain an inconsiderable amount of interstellar matter, no young stars (consisting of old, Population II stars), and no open star clusters. There is a wide range in size and mass for elliptical galaxies: as small as 100 pc to over 100 kpc, and from 107 to nearly 1013 solar masses. The smallest, dwarf elliptical galaxies, may be no larger than a typical globular cluster, but contain a considerable amount of dark matter not present in clusters. Elliptical galaxies probably are the result of a long process where two galaxies of comparable mass, of any type, collide and merge.
EMISSION LINE - More or less narrow range of wavelengths in a spectrum that is brighter than neighboring wavelengths, corresponding to emission of light at a certain frequency. A heated gas in a glass container produces emission lines in its spectrum.
ENDOTHERMIC - Situation in which a reaction causes heat to flow from the surroundings into the system. Since we observe the "surroundings," endothermic reactions are often detected by noting a decrease in temperature.
ENERGY - Physical quantity that, traditionally, is defined as the ability to do work, where "work" is the application of a force over a distance. Energy can never be created or destroyed; it can merely change from one form to another. Commonly used units are ergs, Joules (1 J = 107 erg), and electron volts. Energy per unit time is power (1 Watt = 1 J/s).
ENERGY DIAGRAM - Convenient way of depicting the thermodynamics of reactions. Typically, the y-axis is given in units of energy, and the x-axis is defined as the reaction coordinate (see left). The reaction coordinate is a very general entity which you can think of as the progress of the reaction. It is similar to time, but, the reaction coordinate has no units and no scale. We simply use it to show different stages of the reaction. Usually, the reactants are drawn at a certain level on the left side of the reaction coordinate, and the products are given on the right. A curved line connects the reactants and the products and shows the energies of any intermediates and/or transition states present in the reaction.
ENERGY LEVEL - Essentially synonymous with electron shell.
ENERGY STATE - Complete set of variable values that a particle can exhibit. For example, an electron in a hydrogen atom can be described by its location, its energy and its angular momentum. The collection of these properties constitutes the "state" of the electron. An electron in an atom can take only certain values of energy and angular momentum, as defined by their quantum numbers, so the number of states in an atom is limited.
ENERGY-WAVELENGTH RELATIONSHIP - Inverse relationship between the wavelength of electromagnetic radiation, λ, (Å) and the photon energy, E, (Joules):
Conversion to Ångstroms and electron volts (1 eV = 1.6021 x 10-19 Joule) yields Duane-Hunt equation:
ENSTATITE - Mg-rich pyroxene, MgSiO3.
ENSTATITE CHONDRITE - Rare and unusual type of meteorite unusual in that they are high in the mineral enstatite. Although they contain substantial amounts of Fe, it is in the form of Ni-Fe metal or sulfide rather than as oxides in silicates. This indicates that E chondrites formed in an area of the solar nebula that was very poor in oxygen, possibly inside the orbit of Mercury. Enstatite chondrites are further classified by H and L based on Fe content and by petrologic types 3 to 6. EH chondrites contain ~30% Fe, whereas EL chondrites have ~25% Fe.
ENTROPY - Quantitative measure of the disorder of a system. The greater the disorder, the higher the entropy.
EQUILIBRIUM - Term used to describe physical or chemical stasis. Physical equilibrium may be divided into two types: static and dynamic. Static equilibrium occurs when the components of forces and torques acting in one direction are balanced by components of forces and torques acting in the opposite direction. A system in static equilibrium will have a constant translational and angular velocity. Dynamic equilibrium occurs whenever a change in the statistical behavior of a large group of particles is balanced by an opposite change in the statistical behavior of a similarly large group of different particles. Dynamic equilibrium applies to one component chemical systems, where the changes are exclusively in state (solid, liquid, gas) or are polymorphic (internal structure). Chemical equilibrium describes chemical systems in which no reactions will occur: two or more phases coexist indefinitely with no changes in their proportions.
EROSION - Removal of weathered rocks by moving water, wind, or ice.
ESCAPE VELOCITY - Velocity that an object needs to escape the gravitational well of a more massive object:
where, m = the object's mass, r = distance from object's center, and G = gravitational constant.
EUCRITE (EUC) - Most common type of achondrite meteorite and a member of the HED group. Eucrites are basalts composed primarily of pigeonite and anorthite (An60-98). Eucrites have been placed into three subgroups based on mineralogical and chemical differences. Non-cumulate eucrites represent the upper crust that solidified on a magma ocean after the core and the mantle had already been formed. Rare cumulate eucrites are the products of gravitational settling of crystallized minerals, mainly pyroxene and plagioclase, within magma chambers trapped below the early crust. Finally, polymict eucrites are breccias that contain over 90% eucritic material and less than 10% diogenitic clasts. This 9:1 ratio is an arbitrary criterion to discriminate between polymict eucrites and the closely related howardites, which show a more even distribution of eucritic and diogenitic clasts.
EVAPORATING GAS GLOBULES (EGGs) - Small dense clumps of gas and dust (sometimes with newborn stars inside) part of a larger molecular cloud that are being overrun by a photoevaporating ionization front.
EVENT HORIZON - The surface surround the region out of which light itself cannot escape a black hole. No signal or information from within the event horizon can reach the outside universe. For a nonrotating black hole, the horizon is located at the Schwarzschild radius.
EXCITED STATE - Any energy state of an atom, or of a solid, above its ground state. For atoms and solids, excited states refers to energy levels of electrons higher than the ground-state energy. Excited states are not indefinitely stable, so the atom will eventually return to its ground state.
EXCLUSION PRINCIPLE - Property that fermions of the same type that can interact with each other can not simultaneously occupy the same quantum state.
EXOTHERMIC - Situation in chemical or nuclear reactions in which heat flows from the system to the surroundings. Exothermic reactions cause an increase in temperature.
EXSOLUTION - Segregation, during cooling, of a homogeneous solid solution into two or more different minerals.
EXTINCTION - Dimming of starlight as it passes through the interstellar medium. Dust scatters some of the light, causing the total intensity of the light to diminish. It is important to take this effect into account when measuring the apparent brightness of stars. The dark bands running across portions of the Milky Way in the sky are due to extinction by copious amounts of dust in the plane of our galaxy.
EXTRINSIC SEMICONDUCTOR - Semiconductor material intentionally doped with impurity atoms to create vacancies for electrons known as holes or add electrons to material.
EXTRUSIVE - Refers to igneous rocks erupted on a planetary body's surface.