Sunday 1 October 2017

NGC7635: The Bubble

When I showed you Thor's Helmet, I told you about the rare, extraordinary Wolf-Rayet stars. These are super-massive stars, easily containing 20 solar masses or more, which at the end of their very short lives shed a part of their atmosphere and thereby regain a certain stability, re-igniting fusion in their cores. Their surfaces become extremely hot with temperatures reaching 200,000°C and their immense radiation blows the previously ejected matter away, which appears to form a sort of bubble. Eventually these stars will explode as supernovae.

In Cassiopeia we find another beautiful example within NGC7635 or the Bubble Nebula. It's one of every photographer's favourites, but unlike Thor's Helmet it doesn't do as well for visual observers. They both lie at approximately the same distance (around 11,000 light-years), but the Bubble Nebula appears decisively fainter and you need a sizeable telescope in order to see the bubble itself within the faint, nebulous patch. Then again, the bright central star really leaps out at you, even from its incredible distance. Don't be confused by the brightest star in the field of view, by the way, which lies ten times closer to us and doesn't have anything to do with the nebula.

SAO20575, the central star's scientific name, coincidentally lies near a big cloud of gas and dust and the bubble, consisting of ejected stellar matter, is currently ploughing through it. The surrounding cloud itself is also excited by the fierce radiation of SAO20575, up to the point that it's beginning to glow, offering us an unforgettable spectacle...

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