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eccentricity A measure of the flatness of an ellipse, equal to the distance between the two foci divided by the length of the major axis.

eclipse Event during which one body passes in front of another, so that the light from the occulted body is blocked.

eclipsing binary Rare binary-star system that is aligned in such a way from Earth we observe one star pass in front of the other, eclipsing the other star.

Eddington limit The limit beyond which the radiation force on matter is greater than the gravitational force. [More Info: Field Guide]

electric field A field extending outward in all directions from a charged particle, such as a proton or an electron. The electric field determines the electric force exerted by the particle on all other charged particles in the universe; the strength of the electric field decreases with increasing distance from the charge according to an inverse-square law.

electromagnetic radiation Consists of massless packets of pure energy called photons produced by changes in the energy of charged particles, usually electrons. Photons travel through space at the speed of light. When the changes of energy are small, streams of photons can be described as waves of changing electric and magnetic fields, called electromagnetic waves. The most familiar type of electromagnetic radiation is visible light, but the full spectrum of electromagnetic radiation includes radio, microwave, infrared, ultraviolet, X rays and gamma rays. [More Info]

electromagnetic spectrum The entire range of electromagnetic waves, named in order of increasing frequency or energy, ranges from radio waves, to microwave, to infrared, to visible or optical, to ultraviolet, to X rays, to gamma rays. [More Info]

electromagnetism The union of electricity and magnetism, which do not exist as independent quantities, but are in reality two aspects of a single physical phenomenon.

electron An elementary particle with a negative electric charge. Electrons orbit the nucleus of an atom. They can be torn away from an atom by collisions with other particles or photons.

electron degeneracy pressure The pressure produced by the resistance of electrons to compression once they are squeezed to the point where quantum effects become important.

electron volt (eV) The energy gained by an electron accelerated by a potential of 1 volt.

element Matter made up of one particular atom. The number of protons in the nucleus of the atom determines which element it represents.

elementary particle One of the basic particles of matter. The most familiar of the elementary particles are the proton, neutron, and electron.

ellipse Geometric figure resembling an elongated circle. An ellipse is characterized by its degree of flatness, or eccentricity, and the length of its long axis. In general, bound orbits of objects moving under gravity are elliptical.

elliptical galaxy Category of galaxy in which the stars are distributed in an elliptical shape on the sky, ranging from highly elongated to nearly circular in appearance.

emission line Bright line in a specific location of the spectrum of radiating material, corresponding to emission of light at a certain frequency.

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emission nebula A glowing cloud of hot interstellar gas. The gas glows as a result of a nearby young star which is ionizing the gas. Since this gas is mostly hydrogen, the emitted radiation falls predominantly in the red region of a spectrum, because of a dominant hydrogen emission.

emission spectrum The pattern of spectral emission lines, produced by an element. Each element has its own unique emission spectrum.

equilibrium A condition of balance between forces, or competing processes, such as emission and absorption of radiation.

escape velocity The speed necessary for an object to escape the gravitational pull of an object. Anything that moves away from the object with more than the escape velocity will never return.

event horizon Imaginary spherical surface surrounding a black hole, with radius equal to the Schwarzschild radius, within which no event can be seen heard, or known about by an outside observer. [More Info: Field Guide]

evolutionary theory A theory which explains observations in a series of gradual steps, explainable in terms of well-established physical principles.

excited state The state of an atom when one of its electrons is in a higher energy orbital than the ground state. Atoms can become excited by absorbing a photon of a specific energy, or by colliding with a nearby atom.

exoplanet An exoplanet is any planet beyond our Solar System. Most orbit other stars, but free-floating exoplanets, called rogue planets, orbit the Galactic Center and are untethered to any star.

extinction The dimming of starlight as it passes through the interstellar medium.

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