Around eight thousand light years away, in the direction of the constellation Cassiopeia, there is a small intriguing object. This almost perfect sphere is the result of a violent explosion from an extremely hot star, forty five times as massive as the Sun, with catalogue number SaO 20575. It is a Wolf-Rayet star, the category of super massive stars that are rapidly depleting on their way to exploding as a supernova. The radiation from this same star causes the surrounding gas to glow brightly.

The peculiar shape of the Bubble Nebula marks the leading edge of the Wolf-Rayet wind front as it blasts into the denser stationary material of the interstellar medium. The stellar winds which are forcing it outwards move at an impressive 2,500 km/s.

The asymmetry of the Bubble is believed to be due to subtle differences in the density of the surrounding gaseous material. The bright arcs and small condensations which characterize the Bubble’s surface are also related to density variations in the swept up material forming the Bubble wall. The prominent inner bright knot projected along the right wall of the Bubble is actually the ionized edge of a larger cloud that physically lies outside the Bubble itself.

The Bubble is still expanding, but will not remain this regular for long. It is currently very young by cosmic standards at only around 300,000 years old. The Bubble Nebula sits in an area of sky dense with dust, measuring a total of some 35 light years across, from Earth a span of 15 arcminutes. However, the spherical Bubble itself is relatively small at only 2 arcminutes across, a width of about 5 light years.