<P> The reaction cross section σ is a measure of the probability of a fusion reaction as a function of the relative velocity of the two reactant nuclei . If the reactants have a distribution of velocities, e.g. a thermal distribution, then it is useful to perform an average over the distributions of the product of cross section and velocity . This average is called the' reactivity', denoted <σv>. The reaction rate (fusions per volume per time) is <σv> times the product of the reactant number densities: </P> <Dl> <Dd> f = n 1 n 2 ⟨ σ v ⟩ . (\ displaystyle f = n_ (1) n_ (2) \ langle \ sigma v \ rangle .) </Dd> </Dl> <Dd> f = n 1 n 2 ⟨ σ v ⟩ . (\ displaystyle f = n_ (1) n_ (2) \ langle \ sigma v \ rangle .) </Dd> <P> If a species of nuclei is reacting with a nucleus like itself, such as the DD reaction, then the product n 1 n 2 (\ displaystyle n_ (1) n_ (2)) must be replaced by (1 / 2) n 2 (\ displaystyle (1 / 2) n ^ (2)). </P>

Where does nuclear fusion happen in a star