shaileshs wrote: ↑Tue Jul 20, 2021 5:09 am
I'm a bit confused (and yes, I'm going to read up relevant things online). I thought nebulae were mostly remnants after supernova explosion.. And here, the supernova hasn't happened yet (expected in next few thousand years) and the nebula is formed out of just "winds" blown from star's atmosphere.. Hmmmmm..
And, if so, similarly, would aliens be seeing a nebula around our Sun (now or in future ? Yes/No ? Possible/Not possible ? What are the chances (how hot and active the star/atmosphere has to be for blowing gas out to result in a nebula ?)..
As usual, thanks to experts who provide their thoughts (especially Ann, Chris, Neufer.. ).
Very massive stars often shed huge amounts of mass towards the end of their brief lives.
Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon.
The surface temperatures of known Wolf-Rayet stars range from 20,000 K to around 210,000 K, hotter than almost all other kinds of stars.
Classic (or Population I) Wolf–Rayet stars are evolved, massive stars that have completely lost their outer hydrogen and are fusing helium or heavier elements in the core.
A separate group of stars with WR spectra are the central stars of planetary nebulae (CSPNe), post-asymptotic giant branch stars that were similar to the Sun while on the main sequence, but have now ceased fusion and shed their atmospheres to reveal a bare carbon-oxygen core.
Basically, Wolf-Rayet stars are extremely hot stars that have shed much of their stellar atmospheres and revealed deeper, hotter layers of themselves. These stars are evolved and approaching the ends of their lives. This is the very reason why they have shed so much of their outer atmospheres. They are becoming unstable, because the the energy production of fusion that have so far sustained them no longer function well, and have begun to sputter like the dodgy engine of a car.
In asymtotic giant branch stars, the end stage of stars like the Sun before they turn into white dwarfs with planetary nebulas, the swollen red star has an inert core and two shells where fusion occurs: an inner shell where helium is fusing carbon and oxygen, and an outer shell where hydrogen is fusing helium. The helium and hydrogen shells interact, so that one shell drives fusion while the other is turned off (read about it here
. This on-and-off-fusion leads to stronger and stronger pulsations that eventually cause the star to shed its outer layers altogether, leaving a bare, blisteringly hot core.
The phenomenon of stars blowing off (parts of) their outer layers is something we see quite often. We don't call Eta Carina a Wolf-Rayet star (it is too bright for that), but obviously it has blown off much of its outer layers, which now form a dense nebula around the star. The well-known Crescent Nebula was created by, and is centered on, a Wolf-Rayet star. The progenitor star of supernova SN 1987A in the Large Magellanic Cloud was a blue supergiant, and the reason for its unexpected color was that it had previously blown off much of its swollen atmosphere.
There are many other examples. So the star inside Thor's Helmet is just another Wolf-Rayet star (these stars are certainly uncommon, don't get me wrong) that has blown off a lot of its atmosphere.
The blown-off atmosphere in NGC 2359 just happens to look like the unrealistic Viking helmets that Richard Wagner made his Viking-impersonating singers wear for his four German-language epic music dramas, Der Ring des Nibelungen.