Which phenomenon causes the edges of a shell-type supernova remnant to appear brighter than the center when observed?

Answer

Limb brightening, due to viewing more material along the line of sight toward the edge.

In shell-type remnants, viewing the edge of the three-dimensional shell means observing more material along the line of sight compared to looking through the center, resulting in the phenomenon called limb brightening.

Which phenomenon causes the edges of a shell-type supernova remnant to appear brighter than the center when observed?
astronomysupernovaremnants