A wall of gas at four thousand km/s
A forward shock races ahead of the debris into near-empty interstellar space. Where it slams into denser knots it flares into filaments — the bright lace around the rim. It has barely slowed in three centuries, and will coast for tens of thousands of years before it stalls.
The near face blue, the far face red
The shell is a sphere seen flat. Its near face rushes toward us and its light crowds to blue; the far face recedes and reddens. Only the limb — moving sideways — keeps its true colour, which is why the edge burns brightest and whitest.
Thirty million kelvin, and falling
Freshly shocked gas glows near thirty million kelvin — hot enough to shine in X-rays. As it expands it thins and cools, and the colour slides from blue-white toward magenta and, in the end, to nothing. In ten thousand years there will be no shell to see: only enriched dust, seeding the next stars.