Ballistic streamline focusing by a moving star
= Ballistic streamline focusing by a moving star
{title2=$b=d^2v^2/(2GM)$}
In a cold pressureless stream of asymptotic speed $v$, conservation of <specific angular momentum> and the <eccentricity vector> makes a streamline at impact parameter $d$ cross the downstream axis at $b=d^2v^2/(2GM)$. Its axial velocity there remains $v$, while opposing transverse velocities collide in a shock.