# ⓘ Nuclear timescale. In astrophysics, the nuclear timescale is an estimate of the lifetime of a star based solely on its rate of fuel consumption. Along with the ..

## ⓘ Nuclear timescale

In astrophysics, the nuclear timescale is an estimate of the lifetime of a star based solely on its rate of fuel consumption. Along with the thermal and free-fall 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 nuclear time scale because as one fuel becomes scarce, another will generally take its place - hydrogen burning gives way to helium burning, etc. However, all the phases after hydrogen burning combined typically add up to less than 10% of the duration of hydrogen burning.

## 1. Stellar astrophysics

Hydrogen generally determines a stars nuclear lifetime because it is used as the main source of fuel in a main sequence star. Hydrogen becomes helium in the nuclear reaction that takes place within stars; when the hydrogen has been exhausted, the star moves on to another phase of its life and begins burning the helium.

τ n u c = total mass of fuel available rate of fuel consumption × fraction of star over which fuel is burned = M X L Q × F {\displaystyle \tau _{nuc}={\frac {\mbox{total mass of fuel available}}{\mbox{rate of fuel consumption}}}\times {\mbox{fraction of star over which fuel is burned}}={\frac {MX}{\frac {L}{Q}}}\times F}

where M is the mass of the star, X is the fraction of the star by mass that is composed of the fuel, L is the stars luminosity, Q is the energy released per mass of the fuel from nuclear fusion the chemical equation should be examined to get this value, and F is the fraction of the star where the fuel is burned F is generally equal to.1 or so. As an example, the Suns nuclear time scale is approximately 10 billion years.

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