By BBC News Online Science Editor Dr David Whitehouse

Astronomers using the Hubble Space Telescope have looked at the so-called "Bubble Nebula" with unprecedented clarity.

The nebula has long enchanted astronomers. It is 7,100 light-years distant and lies in the constellation of Cassiopeia. It is six light-years across.

Now, for the first time, astronomers are able to understand the geometry and dynamics of this very complicated system of moving gas clouds blown through space by the wind from stars.

The remarkably spherical "bubble" marks the boundary between an intense wind of particles coming from a powerful star and the more tranquil interior of the gas cloud.

It is the interface of the interaction between fast moving particles as they plough into opposition.

The central bright star of the gas cloud is 40 times more massive than our Sun and produces a stellar wind moving at 7 million km per hour (4 million miles per hour).

Cold gas

The surface of the bubble marks the leading edge of this wind's front, which is slowing as it ploughs into the denser surrounding material.

The surface of the bubble is not uniform because, as the shell expands outward, it encounters regions of cold gas, which are of different density. This slows the expansion by differing amounts, giving a rippled appearance.

Near the central star is a ridge of much denser gas. The lower left portion of this ridge is closest to the star and so is brightest.

It is experiencing the most intense ultraviolet radiation as well as the strong wind and is therefore being photo-evaporated the fastest.

The region between the star and ridge reveals several loops and arcs that have never been seen before.

The high-resolution capabilities of Hubble make it possible to examine these features in detail in a way that is not possible from the ground.

The origin of this "bubble-within-a-bubble" is unknown but may be due to a collision of two different stellar winds.