What is the initial, high-velocity phase of supernova remnant formation called before significant mixing with ambient ISM?

Answer

free expansion phase

The initial stage immediately following the massive stellar explosion is termed the free expansion phase. During this brief cosmic timescale, the ejected stellar material moves outward at extremely high velocities, often reaching tens of thousands of kilometers per second, without yet having encountered substantial resistance or having significantly mixed with the surrounding interstellar medium (ISM). It is characterized by the blast wave advancing supersonically into what is effectively a near-vacuum region, driven purely by the inertia of the ejected ejecta. This phase precedes the interaction with denser interstellar gas pockets that triggers the subsequent Sedov phase.

What is the initial, high-velocity phase of supernova remnant formation called before significant mixing with ambient ISM?

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What Is A Supernova Remnant (SNR)? - Physics Frontier - YouTube

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