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There are a number of fascinating stellar phenomenon on Wikipedia. The one that immediately comes to mind is V838. The short of it is that a star randomly decided to increase its luminosity significantly. The shell around the star isn't ejected matter, what you're seeing is a light echo. Because light moves at different speeds depending on the medium, light travelling through gas travels slower than light in a vacuum. You're literally seeing the progression of light through space. Additionally, what you're seeing is actually the backside of the illuminated space because the light travelling towards earth had less distance to travel and already arrived, while the light you're seeing was reflected off the gas behind the star (or to put it another way, it's like you're looking at a 3d model of a sphere, but the normals were inverted).