APOD: Thor's Helmet (2021 Jul 20)

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APOD: Thor's Helmet (2021 Jul 20)

Post by APOD Robot » Tue Jul 20, 2021 4:06 am

Image Thor's Helmet

Explanation: Thor not only has his own day (Thursday), but a helmet in the heavens.  Popularly called Thor's Helmet, NGC 2359 is a hat-shaped cosmic cloud with wing-like appendages. Heroically sized even for a Norse god, Thor's Helmet is about 30 light-years across. In fact, the cosmic head-covering is more like an interstellar bubble, blown with a fast wind from the bright, massive star near the bubble's center. Known as a Wolf-Rayet star, the central star is an extremely hot giant thought to be in a brief, pre-supernova stage of evolution. NGC 2359 is located about 15,000 light-years away toward the constellation of the Great Overdog. This remarkably sharp image is a mixed cocktail of data from broadband and narrowband filters, capturing not only natural looking stars but details of the nebula's filamentary structures. The star in the center of Thor's Helmet is expected to explode in a spectacular supernova sometime within the next few thousand years.

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Re: APOD: Thor's Helmet (2021 Jul 20)

Post by VictorBorun » Tue Jul 20, 2021 4:53 am

the bubble's center?

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Re: APOD: Thor's Helmet (2021 Jul 20)

Post by shaileshs » 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.. ).

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Re: APOD: Thor's Helmet (2021 Jul 20)

Post by Ann » Tue Jul 20, 2021 6:42 am

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.

Wikipedia wrote:

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.

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Re: APOD: Thor's Helmet (2021 Jul 20)

Post by Ann » Tue Jul 20, 2021 6:53 am

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.. ).

There is certainly no (bright visible-light) nebula around the Sun today, but there will be in the future, when the Sun turns into a white dwarf, surrounded by a planetary nebula.

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Re: APOD: Thor's Helmet (2021 Jul 20)

Post by orin stepanek » Tue Jul 20, 2021 1:11 pm

ThorsHelmet_Miller_4096.jpg
I see Thor's Helmet; but to really look at the Nebula you can see a face and a dog! :mrgreen:
.jpg
Kitty with natural look? :lol2:
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Re: APOD: Thor's Helmet (2021 Jul 20)

Post by neufer » Tue Jul 20, 2021 10:27 pm

Ann wrote:
Tue Jul 20, 2021 6:42 am

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
  • Perhaps it should be called Valkyrie's Helmet rather than Thor's Helmet.
https://en.wikipedia.org/wiki/Valkyrie wrote:
<<In Norse mythology, a valkyrie (from Old Norse valkyrja "chooser of the slain") is one of a host of female figures who choose those who may die in battle and those who may live.

A prose introduction in the poem Völundarkviða relates that the brothers Slagfiðr, Egil and Völund dwelt in a house sited in a location called Úlfdalir ("wolf dales"). There, early one morning, the brothers find three women spinning linen on the shore of the lake Úlfsjár ("wolf lake"), and "near them were their swan's garments; they were valkyries". Two daughters of King Hlödvér are named Hlaðguðr svanhvít ("swan-white") and Hervör alvitr (possibly meaning "all-wise" or "strange creature"); the third, daughter of Kjárr of Valland, is named Ölrún (possibly meaning "beer rune"). The brothers take the three women back to their hall with them—Egil takes Ölrún, Slagfiðr takes Hlaðguðr svanhvít and Völund takes Hervör alvitr. They live together for seven winters, until the women fly off to go to a battle and do not return. Egil goes off in snow-shoes to look for Ölrún, Slagfiðr goes searching for Hlaðguðr svanhvít and Völund sits in Úlfdalir.>>
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Re: APOD: Thor's Helmet (2021 Jul 20)

Post by Ann » Wed Jul 21, 2021 4:52 am

neufer wrote:
Tue Jul 20, 2021 10:27 pm
  • Perhaps it should be called Valkyrie's Helmet rather than Thor's Helmet.
https://en.wikipedia.org/wiki/Valkyrie wrote:
<<In Norse mythology, a valkyrie (from Old Norse valkyrja "chooser of the slain") is one of a host of female figures who choose those who may die in battle and those who may live.

A prose introduction in the poem Völundarkviða relates that the brothers Slagfiðr, Egil and Völund dwelt in a house sited in a location called Úlfdalir ("wolf dales"). There, early one morning, the brothers find three women spinning linen on the shore of the lake Úlfsjár ("wolf lake"), and "near them were their swan's garments; they were valkyries". Two daughters of King Hlödvér are named Hlaðguðr svanhvít ("swan-white") and Hervör alvitr (possibly meaning "all-wise" or "strange creature"); the third, daughter of Kjárr of Valland, is named Ölrún (possibly meaning "beer rune"). The brothers take the three women back to their hall with them—Egil takes Ölrún, Slagfiðr takes Hlaðguðr svanhvít and Völund takes Hervör alvitr. They live together for seven winters, until the women fly off to go to a battle and do not return. Egil goes off in snow-shoes to look for Ölrún, Slagfiðr goes searching for Hlaðguðr svanhvít and Völund sits in Úlfdalir.>>

Nope, Ölrún does not mean "beer rune".

Ölrún is obviously the same word as the Swedish word "alruna".
Swedish Wikipedia wrote:

Al = all, runa = hemlighet (gotiska); dvs. alruna = allvetande.
Translation: Al = all, runa = secret, i.e. alruna = all-knowing, possessing knowledge of all secrets.

Swedish Wikipedia also informs us that the word alruna originally referred to a woman who knew all the secrets of the gods.



Nowadays the Swedish word refers to a plant, Mandragora officinarum, of the potato family, whose roots may look like humans.

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Der Ring des Siljan

Post by neufer » Wed Jul 21, 2021 5:01 pm

neufer wrote:
Tue Jul 20, 2021 10:27 pm
Ann wrote:
Tue Jul 20, 2021 6:42 am

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
  • Perhaps it should be called Valkyrie's Helmet rather than Thor's Helmet.
https://en.wikipedia.org/wiki/Siljan_Ring wrote: <<The Siljan Ring (Swedish: Siljansringen) is a prehistoric impact crater in Dalarna, central Sweden. It is one of the 15 largest known impact craters on Earth and the largest in Europe, with a diameter of about 52 kilometres. The impact that created the Siljan Ring occurred when a meteorite collided with the Earth's surface during the Devonian period, about 376.8 ± 1.7 Ma. This coincides around the first Devonian extinction, the Kellwasser Event or Late Frasnian extinction at 376.1 Ma ± 1.6 Ma. The effects of the impact can clearly be seen in the bedrock in the area. The Cambrian, Ordovician and Silurian sedimentary rocks deformed by the impact are rich in fossils.

The area around the Siljan Ring has been the site of recent prospecting for oil and natural gas, though none of the projects has so far been commercially viable. There are large deposits of lead and zinc near Boda at the eastern edge of the Ring.

There are several lakes in the vicinity, the largest of which is Siljan on the south-southwestern edge of the crater, with the smaller Orsa Lake to the west and Skattungen and Ore on the northeastern margin.

The Siljan Ring consists of an annular outcrop of Lower Paleozoic sedimentary rocks within Proterozoic granites of the Dala series. In Dalarna, the basement rocks consist of granites dated as 1.6 billion years old, putting them at the boundary between the Paleoproterozoic and the Mesoproterozoic. They were emplaced shortly after the Svecokarelian orogeny.

The oldest sedimentary rocks that outcrop in the Siljan area are of Ordovician age. The sequence is dominated by limestone formations with one prominent black shale, the Fjäcka Shale, which is bituminous and has generated petroleum, sourcing the oil found in limestone cavities in the same area. The Ordovician sequence is overlain by rocks of the Llandovery Series (Lower Silurian).

During the last ice age the area was covered by a thick icesheet. The bedrock was sculpted by the ice, with the softer Palaeozoic sedimentary rocks being preferentially eroded. These erosional hollows are now occupied by lakes Siljan, Orsa, Skattungen and Ore.

In accordance with theories about abiogenic petroleum (that hydrocarbons can be formed without involving material from dead plants and animals), astrophysicist Thomas Gold suggested that there might be major deposits of oil and natural gas in the area. Drilling was carried out in the late 1980s and early 1990s but proved inconclusive. Drilling for natural gas was resumed in the late 2000s and continues as of mid-2012. The scientific premise for prospecting of this sort is based on the work of physicist Vladimir Kutcherov, who is cooperating with Igrene, the company financing the drilling operations. In 2019, a study of gases and secondary minerals revealed that long-term microbial methanogenesis and methane oxidation have occurred deep within the fracture system of the crater (for at least 80 million years). and in 2021 a study revealed findings of fossilized anaerobic fungi that had lived in consortium with methanogens deep in the crater.>>
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Re: APOD: Thor's Helmet (2021 Jul 20)

Post by Eclectic Man » Thu Aug 12, 2021 6:53 pm

Ann wrote:
Tue Jul 20, 2021 6:42 am
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.

Ann
There is an irreverent take on what happened after Wagner's Ring Cycle in the book "Expecting Someone Taller" by Tom Holt