Free Astronomy Magazine July-August 2016

SPACE CHRONICLES images have only shown small sections of the nebula, providing a much less spectacular overall effect. Now, a mosaic of four im- ages from Hubble’s Wide Field Camera 3 (WFC3) allows us to see the whole object in one picture for the first time. This com- plete view of the Bubble Nebula allows us to fully appre- ciate the al- most perfectly s ymme t r i c a l shell which gives the neb- ula its name. This shell is the result of a powerful flow of gas — known as a stellar wind — from the b r i g h t s t a r visible just to the left of centre in this im- age. The star, SAO 20575, is between ten and twenty times the mass of the Sun and the pressure created by its stellar wind forces the surround- ing inter- s t e l l a r ma t e r i a l outwards into this b u b b l e - like form. The giant molecular cloud that surrounds the star — glowing in the star’s in- tense ultra- violet radia- tion — tries to stop the expan- sion of the bub- ble. However, al- though the sphere already measures around ten light-years in diameter, it is still growing, owing to the con- stant pressure of the stellar wind — currently at more than 100,000 ki- lometres per hour! Aside from the symmetry of the bubble itself, one of the more strik- ing features is that the star is not located at the centre. Astronomers are still discussing why this is the case and how the perfectly round bubble is created nonetheless. The star causing the spectacular colour- ful bubble is also notable for some- thing less obvious. It is surrounded by a complex system of cometary knots, which can be seen most clear- ly in this image just to the right of the star. The individual knots, which are gen- erally larger in size than the Solar System and have masses compara- ble to Earth’s, consist of crescent shaped globules of dust with large trailing tails illuminated and ion- ised by the star. Observations of these knots, and of the nebula as a whole, help astron- omers to better understand the geometry and dynamics of these very complicated systems. As always, and twenty six years on, Hubble gives us much more than a pretty picture. T his animation, using the observation from the NASA/ESA Hubble Space Telescope, gives an impression on the 3D structure of the Bubble Nebula. [ESA/Hubble, M. Kornmesser] n JULY-AUGUST 2016