TWENTY-SIX years ago, on April 24, 1990, the Hubble Space Telescope (HST) was launched into orbit as the first space telescope of its kind. Every year, to commemorate this momentous day in space history, the HST spends a modest portion of its observing time capturing a spectacular view of a specially chosen astronomical object. This year’s anniversary object is the Bubble Nebula, a cloud of gas and dust illuminated by the brilliant star within it. William Herschel discovered the nebula in 1787, and this is not the first time it has caught the HST’s eye. However, previous Hubble images have only shown small sections of the nebula. Now, a mosaic of four images from the HST’s Wide Field Camera 3 allows one to see the whole object in one picture for the first time and fully appreciate the almost perfectly symmetrical shell that gives the nebula its name.
This shell is the result of a powerful flow of gas, known as a stellar wind, from the bright star visible just to the left of centre in this image. The pressure created by its stellar wind forces the surrounding interstellar material outwards into this bubble-like form. The giant molecular cloud that surrounds the star tries to stop the expansion of the bubble. Although the sphere already measures around 10 light years in diameter, it is still growing, owing to the constant pressure of the stellar wind, currently at more than 100,000 km/hr!