Glossary -- E

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eccentricity

measures how far from a circular shape an ellipse is. Numerically, the eccentricity e = 1 -- (perihelion / semi-major axis). The eccentricity e = 0 for a circle and e = nearly one (1) for very long, skinny ellipses.

eclipsing binary

two stars orbiting each other in a plane that is along your line of sight so you see one star periodically pass in front of the other star. They are especially useful for determining the diameters and masses of stars.

ecliptic

great circle that is a projection of the Earth's orbit onto the sky, or the path the Sun takes through the stars in its annual motion. It is tilted by 23.5° with respect to the celestial equator.

electromagnetic radiation

a form of energy made of oscillating electric and magnetic fields. It is a fancy word for ``light'' and it includes (in order of increasing energy) radio, infrared, visible light (optical), ultraviolet, X-rays, and gamma rays.

electron

negatively-charged subatomic particle that moves around the atomic nucleus in specific energy levels. It has about 1800 times less mass than the proton and neutron.

electron degeneracy pressure

pressure exerted by a degenerate gas made of electrons. It is what prevents further collapse of a white dwarf.

element

a substance that cannot be decomposed by chemical means into simpler substances. All atoms of an element have the same number of protons in the nucleus.

ellipse

squashed circle that tapers at both ends. The total of the distance between any point on the ellipse and one focus + the distance from the point to the other focus = a constant. It is the shape of bound orbits.

elliptical galaxy

a galaxy with a smooth, rounded appearance. Early large burst of star formation long ago used up all of their original gas and dust. Star orbits are aligned in more random directions and have greater eccentricities than star orbits in spiral galaxies.

emission line spectrum

bright lines in a spectrum that are produced by hot, thin (low-pressure) gases. Made by electrons jumping down closer to the nucleus.

epicycle

a device in Ptolemy's Earth-centered model that makes a planet execute a small circular motion around a point that is itself in a circular orbit around the Earth. It was used to explain retrograde motion.

equation of state

the relation that describes the state or condition of a material as determined by how the temperature, density, and pressure depend on each other in the material.

Equation of Time

a relation that describes the difference in time between the meridian crossings of the mean Sun and the actual Sun.

equinox

point on the sky where the ecliptic and the celestial equator intercept. When the Sun is at the equinox point, it is on the celestial equator and we have 12 hours of daylight. Vernal (spring) equinox: March 21; autumnal equinox: September 22.

escape velocity

the initial speed an object needs to escape a massive body's gravitational influence and never return.

event horizon

the distance from a black hole's center at which the escape velocity equals the speed of light. No information of events occurring inside the event horizon can get to the outside.

extinction

reduction in the intensity of the light (the number of photons) from a celestial body as the light passes through a dust cloud. Dust clouds in space make stars behind the dust clouds appear dimmer than they would be if the dust was not there.

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last update: 06 August 1999