Momentum-conserving supernova-remnant expansion
= Momentum-conserving supernova-remnant expansion
{title2=$R\propto t^{1/4}$}
In the ideal cooled-shell limit with negligible continuing interior pressure, swept-up mass in a uniform medium scales as $R^3$. Constant radial shell <momentum> then gives $R^3\dot R={\rm const}$ and $R\propto t^{1/4}$. This differs from an energy-conserving blast wave or a pressure-driven snowplough.