As usual, I find the central star more interesting than the nebula. Like the caption says, the star, HD 192163, is a Wolf-Rayet star. The most famous and visually brightest such star in our skies is one of the components of the binary star Gamma-2 Velorum, also known as Regor.
Jim Kaler wrote in his entry about Regor:
Wolf-Rayet stars, named after the astronomers who discovered them, are very rare and in an extremely advanced state of age. Enormously windy, they produce powerful emissions of radiation at particular colors. They have stripped off most of their mass, their outer hydrogen envelopes, and have exposed deep helium-rich layers heavily contaminated with the by-products of nuclear fusion. WR stars come in two flavors, nitrogen-rich and carbon-rich, Regor's belonging to the latter class.
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Estimating from other stars, the O7 dwarf should have a luminosity of around 180,000 times that of the Sun, a temperature of 32,500 Kelvin, a radius of 13 solar, and a mass around 30 solar. The WR star is much hotter (57,000 to 70,000 Kelvin, most of the light coming out in the ultraviolet part of the spectrum) with a luminosity of 100,000 solar.
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The windy WR star probably started with somewhere around 40 solar masses and has now stripped itself down by an unknown amount, perhaps to under 10. Only a few million years old, the visually fainter Wolf-Rayet component is almost certainly in the last stages of preparing to blow up as a supernova.
I am now going to carefully contradict Professor Jim Kaler, and because I am an utter amateur, you have to take my objection for what it is worth. Professor Kaler said that the WR component of Gamma-2 Velorum
is almost certainly in the last stages of preparing to blow up as a supernova. Just before that, however, he said that the WR star has lost so much mass through its furious wind that now it may contain less than ten solar masses.
My careful objection is that it is the state of the core that makes a massive star go supernova. The reason why a massive star can build up heavier and heavier elements in its core is that the outer layers of the star are so massive that they keep pressing down on the core and compressing it, and thereby they keep driving up the core temperature. Each new fusion of an element heavier than the one before requires a higher core temperature than the one before. A massive star explodes as a supernova because it has driven its core temperature to the level needed to fuse silicon into iron. But since iron will not give off any energy through fusion, a star that has built up an iron core becomes catastrophically unbalanced until it explodes as a supernova. My question is whether a 10-solar-mass star that is furiously shredding mass can still compress its core enough and give its core a sufficiently high temperature to get silicon fusion going.
My careful conclusion is that we can't be absolutely sure that furiously windy once-massive stars will retain enough mass to create the sort of core that will go supernova. So therefore we can't be absolutely certain that HD 192163 will ever go supernova. Perhaps HD 192163, even though it is a nitrogen-rich WN star instead of a carbon-rich WC star like the WR component of Gamma-2 Velorum, has lost so much mass that it will not be able to build up an iron core and explode.
Or, alternatively, if HD 192163 does explode in the future, maybe it will produce an underluminous supernova, like
SN 1987A. The progenitor of SN 1987A is known since it was located in the Large Magellanic Cloud and had been photographed before it exploded and had been labeled Sanduleak -69° 202. This star was a blue supergiant. It was much smaller than a typical red supergiant, and it was probably less massive, too. It had almost certainly passed through a red giant phase and lost mass there.
Sanduleak -69° 202 did retain enough mass to build up an iron core, but much of its outer mass had been lost and scattered, and its bang, for a supernova, was more like a whimper.
Ann
NGC 6888: The Crescent Nebula
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