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Cocoons found enveloping Giant Stars

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Cocoons found enveloping Giant Stars was created by JohnMurphy

Here is an interesting article on new developments in the Variable Star arena.

www.space.com/scienceastronomy/060313_mystery_monday.html
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John Murphy
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16 years 10 months ago #25326

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Replied by albertw on topic Re: Cocoons found enveloping Giant Stars

Hi,

Sounds like a cross between carbon stars and Cepheid/RRLyra variables?
If they are speaking just about supergiants, its been understood for a while that they basicaly spew off material which blocks optical light, but they still glow brightly in IR.

Now astronomers are baffled by why the newly discovered cocoons, or envelopes, don’t burn up despite their proximity to these exceptionally hot-burning stars.


:?: Burn up? Lack of oxygen perhaps. Though where there is oxygen, as generarally there is with carbon stars, you get carbon monoxide.

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~Al
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16 years 10 months ago #25328

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Replied by JohnMurphy on topic Re: Cocoons found enveloping Giant Stars

I think "burn up" is a bit of a misnomer here.
I don't think they mean burning as we interpret it i.e. as a combination of an element with oxygen. Rather they mean nuclear burning, conversion of hydrogen to helium etc.
There may well be nuclear burning going on here with helium ash falling back onto the star, this could contribute to the pulsating effect. They simply aren't giving us enough data here. It is tantalizing by the very lack of data and hence the speculation.
And what do they really mean by cocoon? - presumably a layer of gas blown off by the underlying star............ - just not enough data/info - cannot compute...
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16 years 10 months ago #25332

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Replied by albertw on topic Re: Cocoons found enveloping Giant Stars

IRather they mean nuclear burning, conversion of hydrogen to helium etc.


I thought that initially too, but they seem to be talking about burning in this cocoon, and nuclear burning only takes place in the core.

As you say, Need more data!
Albert White MSc FRAS
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16 years 10 months ago #25334

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Replied by JohnMurphy on topic Re: Cocoons found enveloping Giant Stars

We are talking about supermassive red giants here so presumably core burning is already at 2nd or higher cycle, i.e. Helium fusion, or carbon fusion.
Stars entering and leaving these stages can create conditions in their interiors that trap their radiated energy in their outer layers. The outward thermal pressure increases enough to expand the outer layers of the star. The trapped energy is able to escape when the outer layers are expanded and the thermal pressure drops. Gravity takes over and the star shrinks, but it shrinks beyond the equilibrium point. The energy becomes trapped again and the cycle continues. This is what makes it a variable.
They don't tell us whats different about the envelopes or "cocoons" this time round.
Presumably there is something different going on here as all the above is part of the accepted normal cycle and unless something new is being observed they wouldn't bother mentioning it. - Think I'll off and do a trawl of t'net and see if I can find out more.
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16 years 10 months ago #25339

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Replied by Maddad on topic Re: Cocoons found enveloping Giant Stars

Some supergiants are larger than our entire solar system.

The author gives some additional inaccurate information. Extremely large stars exist, but are rare. The largest one I ever heard of was about the size of Saturn's orbit, substantially less than our entire solar system. When I went looking, the largest star was described as more than the orbit of Mars, which is a whole lot less than Saturn. Wish I remember where I got that description from.

curious.astro.cornell.edu/question.php?number=377
www.eso.org/outreach/press-rel/pr-1997/pr-05-97.html

Numbers given in Wikipedia appear to be exaggerated as compared to other sources.
en.wikipedia.org/wiki/List_of_largest_stars
16 years 4 months ago #32801

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Replied by johnflannery on topic Re: Cocoons found enveloping Giant Stars

Hi Maddad,

Welcome to the IFAS forums!

You are right about the over-exaggerated diameter of the largest stars known. The press release at www.lowell.edu/press_room/releases/recen...argest_star_rls.html says that the current candidates would just engulf Jupiter's orbit. Of course the outer layers of these stars would be extremely tenuous and not far off the general density of the inter-planetary medium.

All the best,

John
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16 years 4 months ago #32804

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Replied by Maddad on topic Re: Cocoons found enveloping Giant Stars

Thank you skynotes. I read the page and then added it to my favorites. It got me thinking about why these stars are so large. They're not the most massive, the coolest, or the brightest. It's sort of hitting the sweet spot, and that got me wondering if a very massive star might go through a very short super-large phase. Because they would be so rare, and this size would last such a short time, it may be that we don't find any super-biggies because there are none. At the moment.
16 years 4 months ago #32848

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