E ring A faint ring, well outside the main ring system of Saturn, which was discovered by Voyager and is believed to be associated with volcanism on the moon Enceladus.

Earth-crossing asteroid An asteroid whose orbit crosses that of the Earth. Earth-crossing asteroids are also called Apollo asteroids, after the first of the type discovered.

earthquake A sudden dislocation of rocky material near the Earth's surface.

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.

eclipse season Times of the year when the Moon lies in the same plane as the Earth and Sun, so that eclipses are possible.

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

ecliptic The apparent path of the Sun, relative to the stars on the celestial sphere, over the course of a year.

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 Another term for light, electromagnetic radiation transfers energy and information from one place to another, even through the vacuum of empty space.

electromagnetic spectrum The complete range of electromagnetic radiation, from radio waves to gamma rays, including the visible spectrum. All types of electromagnetic radiation are basically the same phenomenon, differing only by wavelength, and all move at the speed of light.

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, one of the components of the atom.

electron degeneracy pressure The pressure produced by the resistance of electrons to compression once they are squeezed to the point of contact.

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

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. A heated gas in a glass container produces emission lines in its spectrum.

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 the spectrum, because of a dominant hydrogen emission line.

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

Encke Gap A small gap in Saturn's A ring.

epicycle A construct of the geocentric model of the solar system which was necessary to explain observed planetary motions. Each planet rides on a small epicycle whose center in turn rides on a larger circle (the deferent).

equinox See autumnal equinox and vernal equinox.

escape speed 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 speed will never return.

event horizon Imaginary spherical surface surrounding a collapsing star, with radius equal to the Schwarzschild radius, within which no event can be seen, heard, or known about by an outside observer.

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

evolutionary track A graphical representation of a star's life as a path on the Hertzsprung&151;Russell diagram.

excited state 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.

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