Why do young supernova remnants expand faster than planetary nebulae?

Young supernova remnants expand significantly faster than planetary nebulae because they originate from cataclysmic, violent explosions of massive stars, whereas planetary nebulae are formed by the relatively gentle shedding of a red giant star's outer envelope.123

The differences in their expansion are rooted in their distinct origins and physical processes:

• Supernova Remnants: These are the result of a massive star's core collapse or thermonuclear detonation, which blasts material into space at speeds of hundreds or even thousands of kilometers per second. In their early stages, these remnants expand so violently that they sweep up interstellar material as they move.231
• Planetary Nebulae: These are created when a dying star, not massive enough to become a supernova, sheds its outer layers in a less-violent event. The gas in these nebulae typically expands at much slower speeds, generally measured in the tens of kilometers per second.245

In summary, the higher kinetic energy released during a supernova explosion drives a much more rapid expansion compared to the slower stellar winds that form planetary nebulae.26