As this gusty front moves out, it confronts denser interstellar material and forms a visible shell (blue).

The surface of the shell is not uniform, researchers say, because as the bubble expands outward it encounters regions of cold gas, which are of different density and therefore slow the expansion at different rates and cause a rippled appearance. There is more interstellar material at the left of the image, so the stellar wind moves more slowly in that direction, causing the central star to be off-center in the bubble.

To the right of the central star is a ridge of much denser gas (yellowish). The lower left portion of this ridge is closest to the star and so is brightest.

Several loops and arcs, new to researchers, can be seen between this ridge and the central star. The origin of this bubble-within-a-bubble is unknown, but may be due to a collision of two distinct winds, researchers said.

At the top of the picture are dense clumps or fingers of molecular gas, which have not yet been tainted by the expanding shell. The clumps are illuminated by strong ultraviolet radiation from the Bubble Nebula's central star -- the light travels much faster than the shell.

The composite image, made with Hubble's Wide Field Planetary Camera 2, was produced by Donald Walter of South Carolina State University, along with Paul Scowen and Brian Moore of Arizona State University. It was released by NASA and the STScI.