Stellar collision cross-section
= Stellar collision cross-section
{title2=$\sigma_{\rm coll}$}
For two approximately spherical <stars> approaching with speed $v_\infty$, contact at separation $R_1+R_2$ has gravitationally focused <cross section> $\pi(R_1+R_2)^2[1+2G(m_1+m_2)/((R_1+R_2)v_\infty^2)]$. This measures direct stellar encounters, not the probability of a collective encounter between their host <galaxies>.