nuclear time scale
marpel-e zamâni-ye haste-yi
Fr.: échelle de temps nucléaire
The time required for a star to exhaust its hydrogen (H) supply in → nuclear fusion. The nuclear time scale is given by the relation t = E/L, where E is the total nuclear energy that can be generated by a star and L is the stellar → luminosity. Assuming that the end point of fusion is → iron (Fe), the → atomic mass difference between H and Fe is Δm = 0.008 mH. Therefore, the maximum amount of energy a star with a hydrogen mass M can release is Δ M = 0.008 Mc2. The nuclear time scale is then: t = 0.008 c2M/L. However, stars use up only a fraction of their hydrogen supply, because only the inner part of the star is hot enough for fusion. For example, the Sun will spend only about 10% of its hydrogen supply before evolving into a → red giant. In other words, the solar life time on the → main sequence is about 1010 years.