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In 2013 and 2020, the same telescope in Hawaii looked at the remnant of an exploded star — Cassiopeia A. The images were taken in the light of glowing iron, as if photographing invisible streams of metal flying through space.

Over seven years, astronomers found 263 dense clumps — debris ejected by the explosion. Some of them have barely moved: they likely collided with gas that the star shed before it blew up. Others are moving fast, but some of these fast clumps are slowing down, especially on the eastern side of the remnant.

This is not just cosmic dust: in these clumps travel iron, silicon, and other elements that will later form new stars and planets. By watching them slow down and disappear, scientists are refining how much material remains in the galaxy after a star dies.

Seven years is a blink of an eye by cosmic standards, but even in that time you can see the explosion still living: debris collides, fades, and changes course. Cassiopeia A is like a slow-motion fireworks display that has been going on for three hundred years.