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funestis
2006-Jan-18, 02:30 PM
First, I am sorry if I made some spelling errors as English is not my prime language

Well the question is, what would happened if an large cloud of large helium content would collapse to form a star like object?

In some parts of our and other galaxies exist such regions in witch hydrogen is in low ratio with helium.

In those regions a star like object, after condensation would have more helium in its core thus normal fusion process of a young star would not be possible.

Would this star skip a main sequence period and undergo a red giant period immediately or there is another process which would take place?

:confused: :confused: :confused:

Nereid
2006-Jan-18, 02:40 PM
Welcome to BAUT funestis! :)

I'm unsure why you posted here in BAUT's ATM section, your question seems well-suited for our Q&A section (so I've moved it).

funestis
2006-Jan-18, 02:49 PM
Well thanks for moving the thread, only after I posted I saw the specialized forum for questions.

sorry

Romanus
2006-Jan-18, 04:29 PM
Though I'm no astrophysicist, I'm guessing that if a hypothetical cloud of helium collapsed under its own gravity, it simply wouldn't begin fusion, not unless it were many, many times more massive than even the progenitor nebula for a high-mass, regular star.

If fusion never got started, I'm guessing the hypothetical star would collapse until it reached a density where it's held up by degenerate pressure, like a brown dwarf or white dwarf. If fusion got started though, it would probably shine like a regular star, but its life would be very short compared to a normal star of the same mass.

Grey
2006-Jan-18, 04:52 PM
Well the question is, what would happened if an large cloud of large helium content would collapse to form a star like object?

In some parts of our and other galaxies exist such regions in witch hydrogen is in low ratio with helium.

In those regions a star like object, after condensation would have more helium in its core thus normal fusion process of a young star would not be possible.

Would this star skip a main sequence period and undergo a red giant period immediately or there is another process which would take place?First, I expect that even in regions where there's a higher Helium to Hydrogen ratio than normal, there's still significantly more Hydrogen. I doubt we could find a mostly pure cloud of Helium occurring naturally.

That said, I expect that it would collapse until it reached a temperature sufficient for Helium fusion (so it would have an especially hot core), and achieve stability similar to that of a normal star. I'm not sure whether that would mean that you'd end up with something looking like a red giant, though. You'd have the higher temperature core, but the material the star was made from would have a different density than in a normal star. You'd probably have to work out the dynamics carefully.

Hmm, Tim Thompson was doing some discussion of standard solar models recently; I wonder if he'd have some useful input.

korjik
2006-Jan-18, 07:20 PM
If there is still a large amount of hydrogen, say 25-50%, then it would probably evolve to look like an unususally aged star (it would look older than it is). It would then evolve just like any other star then.

If it was essentially pure He (>90%) it is much harder to figure. if the cloud is large enough where triple alpha would start, it might just blow up if the infall was too quick and it created a powerful helium flash. If it was too small to fuse He, it would prolly look like an odd cross between a brown dwarf and a white dwarf, with hot nondegenerate He atmosphere around a degenerate He core

Tim Thompson
2006-Jan-20, 01:31 AM
First, I expect that even in regions where there's a higher Helium to Hydrogen ratio than normal, there's still significantly more Hydrogen. I doubt we could find a mostly pure cloud of Helium occurring naturally.
I have to agree with this, I don't know of any environment where helium exceeds hydrogen in abundance, except for some regions in the interiors of evolved stars.

However, in the event of such an exceptional environment, and in the event that a helium cloud collapsed to form a star composed mostly of helium from the beginning, its evolution would be very different. Helium does not fuse at all like hydrogen. The fusion of hydrogen is slow to start, and really rather ponderous except in really hot, really massive stars. Even in our own sun, it takes billions of years for the average roaming hydrogen atom to meet another hydrogen atom, with enough energy to fuse. It is a relatively slow process. But helium fusion will start suddenly, once the temperature threshold is reached (about 100,000,000 Kelvins, as compared to about 10,000,000 Kelvins for hydrogen). It is a fast & explosive process, hence the term "helium flash (http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/990409a.html)", to describe the event when a normal main sequence star switches from core hydrogen fusion to core helium fusion. It should be no different for our hypothetical "main sequence helium star". It will begin with a flash.

But the peculiarity of this thing does not stop there. The envelope of a normal star is made mostly of hydrogen, even if it is helium that the core is fusing. So a normal star expands into a red giant (http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/971016.html), so the hydrogen envelope can stay in thermal equilibrium; as much energy flows through the surface of the star and into space, as flows from the core into the envelope. But if the envelope is mostly helium, there is no reason to believe it will behave in the same way. Helium transfers heat very differently than does hydrogen. My guess is that the helium envelope would expand to a smaller & hotter surface, not a red giant, but maybe a yellow giant (of course, there are yellow giant stars (http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980105a.html), but none of the them have helium envelopes). I don't know that, because I really don't know the opacity of helium well, but that's my guess.

George
2006-Jan-23, 01:59 AM
... But if the envelope is mostly helium, there is no reason to believe it will behave in the same way. Helium transfers heat very differently than does hydrogen. Would the convective zone transfer differently, or are you refering to the radiatiative zone?

funestis
2006-Mar-10, 10:19 AM
This article from UT is interesting and explains much about formation of helium stars.

And of course, my error was assuming that in our (or any other) galaxy exist such regions of helium rich clouds.

It is actually impossible.

This late article explains formation of such exotic objects as helium stars.

http://www.universetoday.com/am/publish/extreme_helium_stars_origins.html?932006

TKS everybody for help.

:D :D :D :D