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Kelvin Helmholtz time scale is an estimate of the time it takes a star to collapse after nuclear burning ceases. This time scale is obtained by taking the ratio of the gravitational binding energy and the luminosity

$ t_{KH} = \frac{GM^2}{RL} $

where $ G $ is the gravitation constant, $ M $ is the mass of the star, $ R $ is its radius and $ L $ its luminosity. For example, the Kelvin Helmholtz time of our sun is about 30 million years.