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accretion  The growth in the mass of a body by the in fall of matter gravitationally attracted to the body.
accretional heating  The heating of a body by the impacts that occuras it grows by adding infalling material.
albedo  The ratio of the light reflected in all directions by a surface to the light incident on it. A perfectly reflecting surface has an albedo of 1, a perfectly absorbing surface has an albedo of 0.
atomic number  The number of protons in the nucleus of an atom.
blackbody  An object that is a perfect absorber of radiation falling on it.
blackbody radiation  The electromagnetic radiation emitted by a blackbody. The spectrum and intensity of blackbody radiation are controlled by the temperature of the blackbody. Many stars and other celestial bodies approximate blackbodies.
conduction  The transfer of heat by means of direct collisions between adjacent atoms, molecules, or ions.
convection  The process of energy transport in which heat is carried by hot, rising and cool, falling currents or bubbles of liquid or gas.
density  The mass of a body divided by its volume.
deuterium  An isotope of hydrogen. The nucleus of a deuterium atom is a deuteron.
electron  A low-mass, negatively charged particle that can either orbit a nucleus as part of an atom or exist independently as part of a plasma.
fission  (A) A nuclear reaction in which a nucleus splits to produce two less massive nuclei. (B) A possible explanation for theorigin of a close binary pair of stars in which a star splits into two pieces, each of which becomes a star.
fusion  A nuclear reaction in which two nuclei merge to form a more massive nucleus.
half-life  The time required for half of the atoms of a radioactive substance to disintegrate.
hydrostatic equilibrium  The balance between the inward directed gravitational force and the outward directed pressure force within acelestial body.
isotope  Nuclei with the same number of protons but different numbers of neutrons.
mass number  A measure of the mass of a nucleus given by the total number of protons and neutrons in the nucleus.
neutral gas  A gas containing atoms and molecules but essentially no ions or free electrons.
neutron  A nuclear particle with no electric charge.
nuclei  consist of protons and electrons. (B) An irregularly shaped, loosely packed lump of dirty ice several kilometers across that is the permanent part of a comet.
nucleus  The massive, positively charged core of an atom. The nucleus of an atom is surrounded by one or more electrons. A nucleus missing one or more accompanying electrons is called an ion.
number density  The number of particles in a given volume of space.
planetology  The comparative study of the properties of planets.
plasma  A fully or partially ionized gas.
pressure  The force exerted per unit area.
proton  A positively charged nuclear particle.
radiative transfer  The transport of energy by electromagnetic radiation.
radioactivity  The spontaneous disintegration of an unstable nucleus of an atom.
solar constant  The solar energy received by a square meter of surface oriented at right angles to the direction to the Sun at the Earth’s average distance (1 AU) from the Sun. The value of the solar constant is 1372 watts per square meter.
Stefan-Boltzmann law  The relationship between the temperature of a blackbody and the rate at which it emits radiant energy.
thermal equilibrium  The condition in which a body or a portion of a body gains energy (by generating it or absorbing it) at the same rate at which energy is transported away from it.
Wien’s law  The relationship between the temperature of a blackbody and the wavelength at which its emission is brightest.

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