Thermal time scale

In astrophysics, the thermal time scale or Kelvin-Helmholtz time scale is the approximate time it takes for a star to radiate away its total kinetic energy content at its current luminosity rate.^[1] Along with the nuclear and dynamical time scales, it is used to estimate the length of time a particular star will remain in a certain phase of its life and its lifespan if hypothetical conditions are met. In reality, the lifespan of a star is greater than what is estimated by the thermal time scale because as one fuel becomes scarce, another will generally take its place--hydrogen burning gives way to helium burning, which is replaced by carbon burning.

Stellar astrophysics

The size of a star as well as its energy output generally determine a star's thermal lifetime because the measurement is independent of the type of fuel normally found at its center. Indeed, the thermal time scale assumes that there is no fuel at all inside the star and simply predicts the length of time it would take for the resulting change in outputted energy to reach the surface of the star and become visually apparent to an outside observer.

$\tau _{{th}}={\frac {{\mbox{total kinetic energy}}}{{\mbox{rate of energy loss}}}}={\cfrac {GM^{2}}{RL}}$

where G is the gravitational constant, M is the mass of the star, R is the radius of the star, and L is the star's luminosity. As an example, the Sun's thermal time scale is approximately 30 million years. ^[2]

^[3]

Time measurement and standards

Chronometry Orders of magnitude Metrology

International standards	UTC UTC offset UT ΔT DUT1 IERS ISO 31-1 ISO 8601 TAI 12-hour clock 24-hour clock Barycentric Coordinate Time Barycentric Dynamical Time Civil time Daylight saving time Geocentric Coordinate Time International Date Line Leap second Solar time Terrestrial Time Time zone

Obsolete standards	Ephemeris time Greenwich Mean Time Prime meridian

Time in physics	Absolute time and space Spacetime Chronon Continuous time Coordinate time Cosmological decade Discrete time Planck epoch Planck time Proper time Theory of relativity Time dilation Gravitational time dilation Time domain T-symmetry

Horology	Clock Astrarium Atomic clock Complication History of timekeeping devices Hourglass Marine chronometer Marine sandglass Radio clock Watch Water clock Sundial Dialing scales Equation of time History of sundials Sundial markup schema

Calendar	Astronomical Dominical letter Epact Equinox Gregorian Hebrew Hindu Intercalation Islamic Julian Leap year Lunar Lunisolar Solar Solstice Tropical year Weekday determination Weekday names

Archaeology and geology	Chronological dating Dating methodologies in archaeology Geologic time scale International Commission on Stratigraphy

Astronomical chronology	Galactic year Nuclear timescale Precession Sidereal time

Units of time	Century Day Decade Fortnight Hour Jiffy Lustrum Millennium Minute Moment Month Paksha Saeculum Second Shake Tide Week Year

Related topics	Chronology Duration music Mental chronometry Metric time System time Time value of money Timekeeper

References

↑ Bradt, Hale (2008). Astrophysics Processes. United States of America: Cambridge University Press.
↑ Mitalas, R.; Sills, K.R. (December 1992). "On the photon diffusion time scale for the sun". Astrophysical Journal. 401: 759–. Bibcode:1992ApJ...401..759M. doi:10.1086/172103.
↑ M. Walter, Frederick. "The Kelvin-Helmholtz Timescale". Stony Brook Astronomy Program. Stony Brook University. Retrieved 10 April 2015.

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