Free Astronomy Magazine November-December 2015

the faint red hy- drogen filaments. In this image, one such filament can be seen as it me- anders through the middle of the brighter features that dominate the image. Astronomers sus- pect that before the Veil Nebula’s source star ex- ploded it expel- led a strong stel- lar wind. This wind blew a large cavity into the surrounding interstellar gas. As the shock wave from the super- nova expands out- wards, it encoun- ters the walls of this cavity — and forms the neb- ula’s distinctive structures. Bright filaments are pro- duced as the shock wave interacts with a relatively dense cavity wall, whilst fainter structures are gener- ated by regions nearly devoid of material. The Veil Nebula’s colour- ful appearance is generated by var- iations in the temperatures and densities of the chemical elements present. The blue coloured features — outlining the cavity wall — ap- pear smooth and curved in compar- ison to the fluffy green and red coloured ones. This is because the gas traced by the blue filter has more recently encountered the neb- ula’s shock wave, thus still maintain the original shape of the shock front. These features also contain hotter gas than the red and green coloured ones. The latter excited longer ago and have subsequently diffused into more chaotic struc- tures. The colours in the image have been chosen to help identify- ing the three different species of gas; they do not represent the real colours of the nebula. Hidden amongst these bright, cha- otic structures lie a few thin, sharp- ly edged, red coloured filaments. These faint hydrogen emission fea- tures are created through a total- ly different mechanism than that which generates their fluffy red com- panions, and they provide scientists with a snapshot of the shock front. The red colour arises after gas is swept into the shock wave — which is moving at almost 1.5 million kilo- metres per hour! — and the hydro- gen within the gas is excited by particle collisions right at the shock front itself. Despite utilising six full Hubble fields of view, these new WFC3 images cover just a tiny frac- tion of the nebula’s outer limb. Lo- cated on the west side of the su- pernova remnant, this section of the outer shell is in a region known as NGC 6960 or — more colloquially — the Witch’s Broom Nebula. T his image shows the Veil supernova remnant and the surrounding sky. Due to the size of the nebula the NASA/ESA Hubble Space Telescope was able to only observe a small part of it in detail. [NASA, ESA, Digitized Sky Survey 2] n