Starship Asterisk*

NGC 6888: The Crescent Nebula

Explanation: NGC 6888, also known as the Crescent Nebula, is a cosmic bubble about 25 light-years across, blown by winds from its central, bright, massive star. This sharp telescopic portrait uses narrow band image data that isolates light from hydrogen and oxygen atoms in the wind-blown nebula. The oxygen atoms produce the blue-green hue that seems to enshroud the detailed folds and filaments. Visible within the nebula, NGC 6888's central star is classified as a Wolf-Rayet star (WR 136). The star is shedding its outer envelope in a strong stellar wind, ejecting the equivalent of the Sun's mass every 10,000 years. The nebula's complex structures are likely the result of this strong wind interacting with material ejected in an earlier phase. Burning fuel at a prodigious rate and near the end of its stellar life this star should ultimately go out with a bang in a spectacular supernova explosion. Found in the nebula rich constellation Cygnus, NGC 6888 is about 5,000 light-years away.

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In the small thumbnail, it looks like....The Fetus Nebula...

It never fails to amaze me....Oxygen...

:---[===] *

What a joy it is to have neufer as a contributor. Your citations are great and well-researched. Thank you, Art.








art

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.
...
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.
...
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

History tends to repeat and so do stories... But sometimes with a twist. Today's lovely cosmic debris field could have once harbored an earth like planet inhabited by far advanced beings (from humans) capable of traversing the vast distances between stars. They would have of course evacuated their doomed home seeking refuge among more stable settings. These beings could be headed to our Earth for example. (We have a few billion years of relatively safe harbor left). It is logical to me that these space faring creatures of a much advanced state would likely have little or zero tolerance for crime. That makes me wonder whether they would come to Earth to blend with or replace our meager human complacency. On their approach, I expect they would be making that decision based on our social behavior which if they are using our broadcast news programs as a reference... our outlook is grim... as in reaper...

emc wrote:
History tends to repeat and so do stories... But sometimes with a twist. Today's lovely cosmic debris field could have once harbored an earth like planet inhabited by far advanced beings (from humans) capable of traversing the vast distances between stars. They would have of course evacuated their doomed home seeking refuge among more stable settings. These beings could be headed to our Earth for example. (We have a few billion years of relatively safe harbor left). It is logical to me that these space faring creatures of a much advanced state would likely have little or zero tolerance for crime. That makes me wonder whether they would come to Earth to blend with or replace our meager human complacency. On their approach, I expect they would be making that decision based on our social behavior which if they are using our broadcast news programs as a reference... our outlook is grim... as in reaper...

Wolf–Rayet stars are thousands of times brighter than the Sun and hence at least a thousand times younger. Today's lovely cosmic debris field could have once harbored an earth like planet inhabited by primitive bacteria capable of traversing the vast distances between stars. These germs could be headed to our Earth for example.

A Wolf-Rayet star – it'll huff and puff and blow itself in.

neufer wrote:

emc wrote:
History tends to repeat and so do stories... But sometimes with a twist. Today's lovely cosmic debris field could have once harbored an earth like planet inhabited by far advanced beings (from humans) capable of traversing the vast distances between stars. They would have of course evacuated their doomed home seeking refuge among more stable settings. These beings could be headed to our Earth for example. (We have a few billion years of relatively safe harbor left). It is logical to me that these space faring creatures of a much advanced state would likely have little or zero tolerance for crime. That makes me wonder whether they would come to Earth to blend with or replace our meager human complacency. On their approach, I expect they would be making that decision based on our social behavior which if they are using our broadcast news programs as a reference... our outlook is grim... as in reaper...

Wolf–Rayet stars are thousands of times brighter than the Sun and hence at least a thousand times younger. Today's lovely cosmic debris field could have once harbored an earth like planet inhabited by primitive bacteria capable of traversing the vast distances between stars. These germs could be headed to our Earth for example.

So Mister Art sir,

My superior space alien takeover from that particular region appears to be unfounded. This is good news!

And I optimistically expect the vast cosmos traversing space alien germs would be less prone to removing humans but rather want to party thru their lifecycle using earthlings as hosts. So maybe the invasion would be benign and their human body occupation would improve us somehow. Still, I've had some unpleasant experiences with germs so again, I could be out in left field.

This is one of my favourite Spring nebulae.

The bluish oxygen shroud is not visible, but with a narrow-band UHC filter, I can see all of the glowing hydrogen filaments (red in this photo.)

neufer wrote: Wolf–Rayet stars are thousands of times brighter than the Sun and hence at least a thousand times younger. Today's lovely cosmic debris field could have once harbored an earth like planet inhabited by primitive bacteria capable of traversing the vast distances between stars. These germs could be headed to our Earth for example.

neufer wrote:

emc wrote:
History tends to repeat and so do stories... But sometimes with a twist. Today's lovely cosmic debris field could have once harbored an earth like planet inhabited by far advanced beings (from humans) capable of traversing the vast distances between stars. They would have of course evacuated their doomed home seeking refuge among more stable settings. These beings could be headed to our Earth for example. (We have a few billion years of relatively safe harbor left). It is logical to me that these space faring creatures of a much advanced state would likely have little or zero tolerance for crime. That makes me wonder whether they would come to Earth to blend with or replace our meager human complacency. On their approach, I expect they would be making that decision based on our social behavior which if they are using our broadcast news programs as a reference... our outlook is grim... as in reaper...

Wolf–Rayet stars are thousands of times brighter than the Sun and hence at least a thousand times younger. Today's lovely cosmic debris field could have once harbored an earth like planet inhabited by primitive bacteria capable of traversing the vast distances between stars. These germs could be headed to our Earth for example.

Hitting the Earth from a distance of 2360 ± 790 light-years would be a cosmic hole-in-one like no other.

Ann

Ann wrote:
Hitting the Earth from a distance of 2360 ± 790 light-years would be a cosmic hole-in-one like no other.

• Still...I wouldn't recommend falling asleep.

Fred the Cat wrote:A Wolf-Rayet star – it'll huff and puff and blow itself in.

Okay, Fred! So maybe WR stars of type WN do blow themselves up in terrific explosions, then. Perhaps they go out with a real howl, blowing all little piggies and riding hoods to smithereens!

Ann

Ann wrote:

Fred the Cat wrote:A Wolf-Rayet star – it'll huff and puff and blow itself in.

Okay, Fred! So maybe WR stars of type WN do blow themselves up in terrific explosions, then. Perhaps they go out with a real howl, blowing all little piggies and riding hoods to smithereens!

Ann

You have to admit the Crescent Nebula looks like a puffer.
Although they can be lethal if not prepared correctly they may be useful too.

That puffer looks exactly like Donald Trump! But the APOD is truly beautiful -- absolutely gorgeous!

... ultimately go out with a bang in a spectacular supernova explosion.

Since this object has been studied, what type of supernova will be its end game?