F ring Faint narrow outer ring of Saturn, discovered by Pioneer in 1979. The F ring lies just inside the Roche limit of Saturn, and was shown by Voyager to be made up of several ring strands apparently braided together.

flare Explosive event occurring in or near an active region on the Sun.

flatness problem One of two conceptual problems with the Standard Big Bang model, which is that there is no natural way to explain why the density of the universe is so close to the critical density.

fluidized ejecta The ejecta blankets around some Martian craters, which apparently indicate that the ejected material was liquid at the time the crater formed.

focus One of two special points within an ellipse, whose separation from each other indicate the eccentricity. In a bound orbit, objects move in ellipses about one focus.

forbidden line A spectral line seen in emission nebulae but not seen in laboratory experiments, because under laboratory conditions, collisions kick the electron in question into some other state before emission can occur.

force Action on an object that causes its momentum to change. The rate at which the momentum changes is numerically equal to the force.

fragmentation The breaking up of a large object into many smaller pieces (for example, as the result of high-speed collisions between planetesimals and protoplanets in the early solar system).

Fraunhofer lines The collection of over 600 absorption lines in the spectrum of the Sun, first categorized by Joseph Fraunhofer in 1812.

frequency The number of wave crests passing any given point per unit of time.

full Moon Phase of the Moon in which it appears as a complete circular disk in the sky.

fusion Mechanism of energy generation in the core of the Sun, in which light nuclei are combined, or fused, into heavier ones, releasing energy in the process.