A nonastronomical temperature scale in which 32 degrees is equivalent to 0 degrees Celsius and 212 degrees is equivalent to 100 degrees Celsius.
Fault:
A surface fracture where rock on one side has moved in relation to the rock on the other side. Fault movement may be vertical, horizontal, or oblique.
Fermion:
A particle of matter or antimatter; one of two classes of elementary particles .
Field:
The influence exerted by a force, such as electromagnetism or gravity, throughout a region of space. A field has a precise value at all points in space-time.
Field equation:
One of the complex equations used to describe the space-time contours of gravitational and other force fields.
Filament:
A threadlike structure of gases, stars, or galaxies.
Flat Universe:
A universe in which the average density of mass matches the critical density, so that gravity is just barely too weak to stop the universe's expansion. According to relativity theory, the geometry of such a universe is flat.see Critical Densit
y.
Flux ropes:
Lines of magnetic force found in the ionosphere of Venus.
Force:
A physical phenomenon that can change the momentum of an object the four accepted present-day forces are gravity, electromagnetism, the strong force, and the weak force.
Formaldehyde:
An organic compound of hydrogen, carbon, and oxygen, found both on Earth and in space.
Frame of reference:
A postion from which objects are viewed.
Frequency:
The number of oscillations per second of an electromagnetic wave.
Fullerene:
In theory, a large carbon-based molecule thought to occur in soot on Earth and in space.
Fusion:
The combining of two atomic nuclei to form a heavier nucleus, releasing great energy as a by-product.
The central plane of the Milky Way galaxy; also the central plane of any disk-shaped galaxy.
Galactic Rotation:
The movement of matter- including stars, gas ,and dust -around the center of a galxy.
Galaxy:
A system that contains stars numbering from millions to hundreds of billions as well as varying quantities of gas and dust.
Gamma Ray:
The most energetic form of electromagnetic radiation, with the highest frequency aqnd the shortest wave-length.
Gas:
Matter in its most diffuse state, having neither a definite shape nor volume. Gas accounts for a significant fraction of a galaxy's mass. Most gas in the universe is a form of hydrogen.
Gas Chromatograph:
A device for the analysis of chemical compounds or mixtures of compounds.
General Relativity:
A theoretical account of the effects of accelaration and gravity on the motion of bodies and the observed structure of space and time.
Giant Molecular Cloud:
A concentration of interstellar gas and dust up to several dozen light-years in diameter.
Globule:
A dense concentration of interstellar gas and dust, thought to be a protostar in the process of formation.
Geodesic:
The shortest path between two points. On a flat surface, a geodesic is a straight line; on a spherical surface, an arc.
Geometry:
A set of rules describing the structure of space in a given region. Classical Eucledean geometry applies to a flat surface or to "flat" space; non-Eucledean geometries apply to a curved sufaces and may include such unlikely phenomena as triangles whos
e vertices total less than 180 degrees.
Giant molecular cloud:
A concentration of interstellar gas and dust up to several dozen light-years in diameter.
Globular cluster:
A spherical system of up to a few million stars that normally orbits the center of a galaxy.
Gluon:
A boson, or force-carrying particle, that conveys the strong force and binds quarks together. There are eight types of gluons.
Grand unification theory (GUT):
Any of several competing but similar theories that unite electromagnetisms, the weak force, and the strong force into one electronuclear force.
Gravitational Lens:
An optical effect of general relativity in which the gravity of a very massive body bends the light of an object behind it, distorting its apparent image and often producing one or more duplicate images.
Gravitational Red Shift:
See Red Shift.
Graviton:
According to theory, a boson that transmits the force of gravity. Fravitons have not been experimentally verified.
Gravity:
The force responsible for the mutual attraction of separate masses.
Gravity Assist:
The use of a celestial's body's gravitational force to alter the trajectory of a spacecraft. Gravity Assist maneuvers allow speed and direction changes that are otherwise impossible for missions with limited fuel.
Gravity Wave:
A theoretical perturbation in an object's gravitational field that would travel at the speed of light. General relativity predicts that gravity waves may result from accelerating, oscillating, or violently disturbed masses, categories that include bla
ck holes and cosmic strings.
Gravity Well:
A local distortion in the fabric of space-time near a massive body, analogous to a well or depression in a two dimensional sheet.
Greenhouse effect:
A phenomenon in which heat is trapped near the surface of a planet by atmospheric gases and clouds. Short-wavelength solar radiation readily penetrates the atmosphere, but long-wavelength infrared radiation from the heated surface is absorbed, thereby
causing gradual warming.
Ground state:
The lowest possible energy level for a given electron.
Guide Star:
A star used by spacecraft navigation systems as a course marker; also a star tracked by a guide telescope during a long photographic exposure to ensure that the main telescope is accurately aligned.
A cycle in a planetary atmosphere in which convection pushes rising gas from warm areas of the atmosphere to cooler ones, where the gases sink , circulate back to the warm areas, and are again pushed upward.
Halo:
In a spiral galaxy, a spherical volume hundreds of thousands of light-years across, centered on the bulge, and defined by the orbits of extremely old stars and globular clusters.
H Higgs boson:
In theory, a massive boson capable of transforming the electroweak force into distinct electromagnetic and weak forces. H Higgs bosons would have existed onlyh from 10^-33 to 10^-12 second after Big Bang.
Heisenberg uncertainty principle
See Uncertainty principle.
Helium:
The second lightest chemical element and the second most abundant, with a nucleus that includes two protons and at least two neutrons.
Helium flash:
A brief stage in the evolution of stars with a mass of approximately two and a half solar masses or less, in which the fusion of degenerate helium to carbon spreads in a "flash" throughout the core.
Hertzsprung-Russell diagram:
A graph of stellar properties that charts absolute magnitude against spectral class.
Hubble constant:
An estimate of the rate at which the universe is expanding. Because values for the Hubble consant vry from 50 to 100 kilometers (30 to 60 miles) per second per megaparsec, astronomers usually specify which value they have used in converting a galaxy's
red shift to its distance from Earth.
Hydrocarbon:
A chemical compound made up of carbon and hydrogen atoms. Because carbon atoms readily form strong bonds, thousands of types of hydrocarbons are possible; seven have been found in interstellar material.
Hydrogen:
The most common detectable element in the universe, with a nucleus that includes one proton. In theory, hydrogen was the primary element produced just after the Big Bang.
Hydrogen Line:
see twenty-one-centimeter line.
Hydrometer:
An instrument that measures the specific gravity of a liquid, usually to help determine its composition.
Hydroxl:
A highly reactive, two atom molecule of hydrogen and oxygen; in interstellar space, it often operates as a maser.
Hyperbola:
A type of open loop that is sometimes followed by bodies with too high a velocity to travel a closed orbit.
The principle that for a given quantity of gas under normal conditions, the gas's pressure is directly proportional to the product of its density and temperature.
Imaging:
The techniques used to construct images from digital information produced by electronic sensors.
Inertia:
An object's tendency to stay in motion, if in motion, or at rest, if at rest; its resistance to acceleration.
Inferior Planet:
A planet whose orbit falls between the Sun and Earth's orbit, specifically Mercury or Venus.
Inflation:
According to theory, a sudden expansion in space that occurred 10^-35 second after the Big Bang.
Infrared:
A band of electromagnetic radiation with a lower frequency and longer wavelength than visible light.
Intensity:
The amound of radiation received from an object. Optical astronomers prefer the term brightness.
Interacting Galaxies:
Galaxies that are or have been at close enough range to affect each other gravitationally.
Interferometer:
In radio astronomy, an arrangement of two or more separated radio telescopes used as one to receive sources of radio emission. By studying the overlapping wave paterns produced from the emissions, scientists can determine the brightness and structure
of the emitting source.
Intermediate Vector Boson:
A Boson, or force carrier, that conveys the weak force. Charged intermediate vector bosons are also called W particles; neutral intermediate vector bosons are Z particles.
Interstellar absorption:
Te conversion of starlight into heat by interstellar dust, thus reducing the light received on Earth.
Interstellar absorption line:
An absorption line resulting from light's passage through interstellar dust and gas rather than from the outer layers of the radiating object.
Interstellar medium:
Clouds of dust and gas between the stars.
Inverse-square law:
the mathematical description of how the strength of some forces, including electromagnetism and gravity, changes in inverse proportion to the square of the distance from the source.
Ion:
An atom that has lost or gained one or more electrons. In comparison, the neutral atom has an equal number of electrons and protons, giveing it a zero net electrical charge. A positive ion has fewer electrons than the neutral atom; a negative ion has
more.
Ionopause:
The boundary area at the upper limit of an ionosphere.
Ionosphere:
An atmospheric layer dominated by charged, or ionized, atoms.
Irregular Galaxy:
A galaxy with an amorphous shape, neither spiral nor elliptical.
Isotope:
One of two or more forms of a chemical element that have the same number of protons but a different number of neutrons in the nucleus.
An early-twentieth-century model of the galaxy that placed the Sun near the center; named after its originator, Dutch astronomer Jacobus Kapteyn.
Kelvin:
An absolute temperature scale that uses Celsius degrees but sets 0 degrees at absolute zero, about minus 273 degrees Celsius.
Kepler's laws:
Three laws of planetary motion formulated by Johannes Kepler in the 1600s: that planets follow elliptical orbits, that the line between the Sun and a planet sweeps through equal areas in equal times (so that the closer a planet is to the Sun, the fast
er it moves) , and that a planet's orbital period is directly related to its average distance from the Sun.
