ASTR 103 - Astronomy

Glossary - E

eccentricity

In an ellipse, the numerical ratio of the distance of the focus from the center of the ellipse to the length of the semimajor axis.

eclipsing binary

Two generally close stars orbiting each other in a plane viewed edgewise, or nearly so, from Earth, resulting in the mutual eclipse of one star by the other.

ecliptic

Great circle extension of Earth's orbit on the celestial sphere. The apparent yearly path of the Sun in the sky.

ecoshell

Thermally habitable region around a star where life can flourish.

electric field

The field of force created by electrical charges, such as that about the electron.

electromagnetic energy

The energy in electric and magnetic fields, such as that transported by electromagnetic waves.

electromagnetic force

The force associated with electrical charges and their motion.

electromagnetic radiation

Energy transmitted with the velocity of light (290,800 km/s) in the form of gamma, X-ray, ultraviolet, visible, infrared, microwave, and radiowaves or portions thereof.

electromagnetic spectrum

The name given to the continuous variation of wavelength for electromagnetic waves, much as gamma rays, X-rays, ultraviolet, visible, infrared, microwaves and radio waves.

electron

Negatively charged particle that is the basic outer component of the atom.

electron shell

The grouping of electron orbits of similar, but distinct, energies in the atom.

electron volt (eV)

Kinetic energy gained by an electron moving across an electric potential of one volt.

element

 

ellipse

Closed path traced by a moving point whose distance form two fixed points (foci) remains constant.

elliptical galaxy

Spherical or oval-shaped galaxy lacking a spiral structure and containing little or no gas and dust.

elongation

The angular separation between the Sun and a planet measured in angular units.

emission line

 

emission nebula

 

emission spectrum

Spectrum containing bright lines or a set of discrete wavelengths produced in a rarefied incandescent gas.

energy

The ability of a physical system to do work when it changes from one describable state to another.

energy level (of a atom)

 

epicycle

Smaller circle in the Ptolemaic system along which the planet moves while the center of the circle revolves about its deferent.

equatorial mounting

Telescope arrangement that permits the instrument to be rotated about an axis parallel to Earth's axis of rotation in order to follow the rotation of the sky.

equation of time

 

equinox

Two points of intersection between the celestial equator and the ecliptic. These are the vernal equinox, through which the Sun passes on or about March 21, and the autumnal equinox, through which the Sun passes on about September 22.

erg

 

ergosphere

 

eruptive variable

Variable star characterized by sudden explosive or erratic light outbursts.

escape velocity

Minimum speed necessary to escape permanently from the gravitational field of a body.

Euclidean

Refers to the flat space geometry as perceived by our senses.

event

Occurrence specified by both time and place in the space time world of four dimensions in the theory of relativity.

event horizon

Surface in relativistic space around a black hole inside of which no photons of light can escape and will ever reach us.

evolutionary cosmological model

Time-dependent model of an expanding Universe that evolves from its superhot state of highly condensed matter originally contained in a small volume of space.

excitation

 

exobiology

Study of extraterrestrial life in space.

exosphere

Outermost fringe of Earth's atmosphere.

expanding cosmological model

 

exploding galaxy

Galaxy in which powerful explosions are occurring in its central region and which is emitting nonthermal radiation.

extinction

 

extragalactic