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. In astrophysics and cosmology, timescales are modeled for various phenomena regarding stars, star systems, orbits, planets, galaxies, etc. They are often worked out based upon just a single factor or a few factors, simplified for theoretical use. Timescales of competing or concurrent factors/processes are compared to see which match observation.

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 or KH timescale, or for short, Kelvin time) 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 stars' observed actual lifetimes match the nuclear timescale rather than the thermal timescale was evidence convincing astronomers that main sequence stars 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.

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