Timescales and basic equations of stellar structure

2 Timescales and basic equations of stellar structure 5.2.1 Timescales 

Using the self-evident relation (in spherical symmetry) 

It is often convenient to treat all the internal variables as functions of the included 

The first term on the left of Eq. (5.12) vanishes in the absence of external pressure, which leads to the Virial Theorem 

In the absence of nuclear energy sources, a star contracts on a thermal timescale and radiates energy at the expense of gravitational potential energy. Since, by the Virial Theorem, the total energy 

---