Page "Supernova remnant" Paragraph 5

In either case, the resulting supernova explosion expels much or all of the stellar material with velocities as much as 10 % the speed of light, that is, about 30, 000 km / s.

These ejecta are highly supersonic: assuming a typical temperature of the interstellar medium of 10, 000 K, the Mach number can initially be > 1000.

Therefore, a strong shock wave forms ahead of the ejecta, that heats the upstream plasma up to temperatures well above millions of K. The shock continuously slows down over time as it sweeps up the ambient medium, but it can expand over hundreds of thousands of years and over tens of parsecs before its speed falls below the local sound speed.