field of view: The angular expanse viewable.
filament: A feature of the solar surface seen in Hα as a dark wavy line; a prominence projected on the solar disk.
fireball: An exceptionally bright meteor.
fission, nuclear: The splitting of an atomic nucleus.
flare: An extremely rapid brightening of a small area of the surface of the Sun, usually observed in Hα and other strong spectral lines and accompanied by x-ray and radio emission.
flash spectrum: The solar chromospheric spectrum seen in the few seconds before or after totality at a solar eclipse.
flat (critical) Universe: A Universe in which Euclidís parallel postulate holds, one that is barely expanding forever.† It has infinite volume and its age is 2/3 the Hubble time.
flatness problem: One of the problems solved by the inflationary theory, that the Universe is exceedingly close to being flat for no obvious reason.
flavors: A way of distinguishing quark; up down, strange, charmed, truth (top), beauty (bottom).
fluorescence: The transformation of photons of relatively high energy to photons of lower energy through interactions with atoms, and the resulting radiation.
flux: The amount of something (such as energy) passing through a surface per unit time.
flux tube: A torus through which charged particles circulate.
focal length: The distance from a lens or mirror to the point to which rays from an object at infinity are focused.
focus (plural: foci): (a) A point to which radiation is made to converge; (b) of an ellipse, one of the two points the sum of the distances to which remains constant.
force: In physics, something that can or does cause change of momentum, measured by the rate of change of momentum with time.
fraunhofer lines: The absorption lines of a solar or other stellar spectrum.
frequency: The rate at which waves pass a given point.
full moon: The phase of the Moon when the side facing the Earth is fully illuminated by sunlight.
fusion: The amalgamation of nuclei into heavier nuclei.
fuzz: The faint light detectable around nearby quasars.
galactic cannibalism: The incorporation of one galaxy into another.
galactic year: The length of time the Sun takes to complete an orbit of our galactic center.
Galilean satellites: Io, Europa, Ganymede, and Callisto: The four major satellites of Jupiter, discovered by Galileo in 1610.
Galileo: (a) The Italian scientist Galileo Galilei (1564-1642); (b) the NASA spacecraft in orbit around the planet Jupiter that sent a probe into Jupiterís clouds on December 7, 1995.
gamma rays: Electromagnetic radiation with wavelengths shorter than approximately 0.1 Ň.
gamma-ray burst: A brief buster of gamma rays, studied especially by the Compton Gamma-ray Observatory during 1991-2000, which found thousands of such bursts uniformly distributed in the sky; discovered to come, at least in many cases, from exceedingly distant and thus extremely powerful sources of energy.
gas tail: The puffs of ionized gas trailing a comet.
general theory of relativity: Einsteinís 1916 theory of gravity.
geology: The study of the Earth, or of other solid bodies.
geothermal energy: Energy from the Earthís surface.
giant: A star that is larger and brighter than main-sequence stars of the same color.
giant ellipticals: Elliptical galaxies that are very large.
giant molecular cloud: A basic building block of our galaxy, containing dust, which shields the molecules present.
giant planets: Jupiter, Saturn, Uranus, and Neptune; or even larger extra solar planets.
giant star: A star more luminous than a main-sequence star of its spectral type; a late stage in stellar evolution.
gibbous moon: The phases between half moon and full moon.
globular cluster: A spherically symmetric type of collection of stars that shared a common origin.
gluon: The particle that carries the color force (and thus the strong nuclear force).
grand unified theories (GUTs): Theories unifying the electroweak force and the strong force.
Granulation: Convection cells on the Sun about 1 arc sec across.
grating: A surface ruled with closely spaced lines that, through diffraction, breaks up light into its spectrum.
gravitational force: One of the four fundamental forces of nature, the force by which two masses attract each other.
gravitational instability: A situation that tends to break up under the force of gravity.
gravitational lens: In the gravitational lens phenomenon, a massive body changes the path of electromagnetic radiation passing near it so as to make more than one image (or a brightening) of an object.† A double quasar was the first example to be discovered.
gravitationally: Controlled by the force of gravity.
gravitational redshift: A redshift of light caused by the presence of mass, according to the general theory of relativity.
gravitational waves: Waves that many scientists consider to be a consequence, according to the general theory of relativity, of changing distributions of mass.
gravity: The tendency for all masses to attract each other; described in a formula by Newton and more recently described by Einstein as a result of a warping of space and time by the presence of a mass.
gravity assist: Using the gravity of one celestial body to change a spacecraftís energy.
grazing incidence: Striking at a low angle.
great circle: The intersection of a plane that passes through the center of a sphere with the surface of that sphere; the largest possible circle that can be drawn on the surface of a sphere.
Great Dark Spot: A giant circulating region on Neptune seen by Voyager 2 in 1989; it has since disappeared.
Great Red Spot: A giant circulating region on Jupiter.
greenhouse effect: The effect by which the atmosphere of a planet heats up above its equilibrium temperature because it is transparent to incoming visible radiation but opaque to the infrared radiation that is emitted by the surface of the planet.
Gregorian calendar: The calendar in current use, with normal years that are 365 days long, with leap years every fourth year except for years that are divisible by 100 but not by 400.
ground level: An atomís lowest possible energy level.
ground state: See ground level.
GUTs: See grand unified theories.