temperature

Temperature is the measure of hotness or coldness of something. Its basic characteristic is that given two objects with different temperatures, heat consistently tends to shift from that with the higher temperature to that with the lower, raising the latter's temperature, and conversely, if they are at the same temperature, they do not shift heat from one to the other. This measurable temperature corresponds to the kinetic energy of the random motion of its constituent particles. When the motion is not random, a temperature may not be defined: thermodynamic equilibrium implies sufficient randomness, but sometimes a temperature is evident without it, e.g., local thermodynamic equilibrium. In daily life, temperature is fairly clear, but a cited temperature may need qualification for some extreme conditions, regarding how it is determined, or when describing something where temperature is varying in time or space. Among the means by which temperature is measured, particularly at astronomical distances:

• Excitation temperature is based upon the degree of excitation of the particles (atoms) and is defined by using the Boltzmann constant and Boltzmann equation that describes the relation of temperature to states of excitation, e.g., electrons above their ground state.
• Ionization temperature is similar for the degree of ionization, using the Saha equation.
• Effective temperature is the temperature of a black body that would produce the same total electromagnetic radiation (EMR) as a star or other object is producing.
• Color temperature (in astronomy) is the temperature of a black body that would match some color index of the star.
• Brightness temperature is the temperature indicated by a single band, being the temperature of a black body that would show the same spectral radiance over that band. For a true black body, such measurements of various bands would produce the same brightness temperature, but for other types of radiation, the source's measured brightness temperature depends upon the band observed. Given non-thermal generation of the EMR, a measured brightness temperature may be far from a "true temperature", merely serving as a description of the EMR, basically equivalent to a passband magnitude, or may be a description of a particular spectral feature.
• Kinetic temperature reflects the kinetic energy of a material's molecules, i.e., regarding the speed of the molecules but not their molecular rotation and vibration. It is defined as 2/3 the Boltzmann constant times the mean kinetic energy of the molecules.

Some of these are directly calculated from observational measurements, while others require some modeling. Some other "temperature" terms in astronomy:

• noise temperature (and antenna temperature): merely the equivalent temperature that would produce a certain amount of random noise in an electronic circuit: important concepts in radio astronomy.
• equilibrium temperature: a calculated temperature for a planet consisting of that which would balance the EMR received from the host star with the rate at which the planet would radiate heat. The term is often used taking only some basic factors into account, and contrasting it with the planet's actual temperature reveals additional factors at work.
• surface temperature: temperature specifically of the surface of an astronomical object.
• spectral temperature: may not be a common phrase, but I believe in astronomy it most often refers to temperature determined by the object's spectral type, essentially based upon temperature-dependent spectral signatures.