timescale

The term timescale or time scale (typically indicated by the symbol t_something or τ_something) is used for the rough length of time something takes, sometimes meant to represent an upper or lower bound. For something that dwindles exponentially, the term may be used for such a rough estimate of (or limit regarding) its relaxation time; this may be a dwindling probability (e.g., the probability of something not yet occurring). In astrophysics and cosmology, timescales are modeled for various phenomena regarding stars, star systems, orbits, planets, galaxies, etc. They are generally worked out based upon just a single factor (or a few factors) that have most effect on the time over which something happens, a back-of-the-envelope calculation adequate for overall time comparisons with timescales of competing or concurrent factors/processes, with observations and demographics.

For example, in stellar theory, the nuclear timescale is the estimate of a star's lifetime assuming its luminosity is derived from fusion, while its thermal timescale (or Kelvin-Helmholtz timescale) is an estimate of its lifetime based upon the idea that its radiation was fueled solely by cooling. Free-fall time (or dynamical timescale) is the timescale for gravity to pull the mass of a body together, ignoring other forces. The fact that age-determinations of solar system objects (including Earth) match the nuclear timescale rather than the thermal timescale is basic evidence that main sequence stars, such as the Sun, run on nuclear energy. Similarly, for clouds, a cooling timescale (or radiative cooling timescale or just radiative timescale) is the timescale for a gas cloud to cool (based on its gas's cooling function), and the comparison of this and the free-fall timescale is a means to represent the results of the two tendencies. For an extra-solar planet's atmosphere, a comparison of the radiative timescale with an advection timescale timescale for transferring heated material away from the day portion, can affect the observed light curve of its secondary eclipse, yielding information. Various kinds of orbital decay have calculable timescales, and inferences can be carried out by comparing these to Hubble time.

The phrase time scale may also be used in a different sense, to refer to time quantification given by a specific time standard.