Posted on 10/30/2002 12:34:06 PM PST by Junior
Just a point of clarification; if I understood the original article correctly, the star in question is NOT one of the original pre-galactic giants. It appears to be at least one generation removed from that, as it does have SOME "heavy" elements that could not have been synthesized within its own core; hence, there MUST have been a generation of stars prior to its formation that DID support synthesis, and these must be much larger than 0.8 solar masses to get synthesis of anything beyond helium.
I think the Bible is saying the same thing. -- In the beginning there wasn't anything out there and then later there was.
The Bible does not explain exactly how it was done. It is not important for us to know that, so science can continue to study and discover. It if it is true there is no conflict.
Seems to me this is additional evidence for the big bang theory, one of the necessary propositions of it has been found to be a fact.
Good question; according to the models for stellar energy production, a star such as the one in this article evolves very slowly compared to larger stars, and the production of haevier elements always occurs at the end of the star's life cycle, after its hydrogen fuel is used up. This star is still in its hydrogen burning phase, and has not had an opportunity to produce heavier nuclei than helium.
Additionally, at 0.8 solar masses, it might not be massive enough to EVER get hot enough to initiate helium fusion, and hence will never produce anything heavier than helium.
Lastly, truly heavy nuclei (>Fe) are never produced in normal stellar reactions; they are produced in supernovae.
So the Nature article states that this object "would allow the direct study of the pristine gas from the Big Bang". Does that actually seem likely? How "pristine" is the gas still going to be after all this time, even if it was kind of a ho-hum sun? What kind of information do they think they're going to find out?
Unless I've misundertood what's going on, this star should be characterized as a primordial galactic star, not a primordial pre-galactic star. The first pre-galactic stars are believed to have been enormous beasts that coalesced out of the detritus of the Big Bang (hydrogen, helium and a wee bit if lithium), and as such, would have burned fast and furiously for a brief period of time (in cosmological terms, at least). These would have produced small amounts of heavier nuclei that we see in the primordial galactic stars such as the one in this article, which explains why this star has SOME heavy nuclei, but far less than later generation galactic stars.
How "pristine" is the gas still going to be after all this time, even if it was kind of a ho-hum sun?
The observeable part is the surface, from which the photons are emitted that we see in our telescopes. This should be virtually the same composition as when it formed, as the fusion process and byproducts are confined to a central core region of the star. The energy produced there is transported by radiation and convection processes to the surface, but the byproducts remain within the core. Thus the material at the photosphere's surface should be pretty much the same as the original composition (i.e., 12 billion years ago), unless the star has been accreting matter from another stellar body, but at 0.8 solar masses, that would be rather unlikely.
Except helium doesn't seem to remain within the core. So the hydro-to-helium ratio would still be altered from what it was at the beginning.
I'm not aware of anything to suggest this. IF helium byproducts were distributed throughout a star, there would be no region of sufficient helium concentratiion in which helium fusion could take place in the later stages of suffciently large stars.
The only helium that should be present at the surface of the star should be that which was present in the primordial gas from which the star formed. Helium byproducts of hydrogen fusion remain in the central core of the star.
Excellent question!
Somewhat like a layer cake, stellar interiors are made of of zones in which convection takes place, and zones in which convection DOES NOT take place. Effectively, the non-convective zones prevent the escape material deeper within the stellar interior. Energy transport through the non-convective zones is achieved by radiative energy transport, and the governing factor that determines whether or not a particular zone is convective or non-convective is it's opacity. The opacity is directly related to the temperature gradient within a zone, and if the gradient is less than the adiabatic lapse rate, there's no convection.
This, BTW, explains why stars never utilize more than a tiny fraction of the total hydrogen present in nuclear fusion reactions: most of the hydrogen in the star exists outside the the zone in or near the core where conditions for hydrogen fusion are met, and have no way of getting into these zones because the hyrogen and fusion byproducts within the nuclear reaction zones have no way of getting out. Thus the the helium "ash" remains in the core region, and does not float to the surface of the star.
Thanks for the education!
Absent an infall of material from outside the sun, or a means to transport fusion byproducts from the hydrogen fusion region up to the surface, it would have to be so.
I'm relying on "The Stars: their structure and evolution" by R J. Tayler; Wykeham Publications (London) Ltd 1974. It's a bit long in the tooth by astrophysics standards, but absent so radical revision to the model for stellar dynamics and evolution, it should be reasonbly close to current understanding.
That said, it's always subject to revision based upon new observational evidence or better theoretical understanding of the phenomena.
Just as an aside, these barriers do evolve, and depending on the mass of the star, you can see processed materials later on in the development of the star. This is very apparent in the class of objects called Wolf-Rayet stars, which appear to be extremely high mass stars. High mass stars are easily the most luminous stars in the galaxy and they are all surrounded by a nebulae of gas. The class of stars is defined by anomolous spectral lines of doubly ionized Helium or triply ionized lines of Carbon. They have effective temperatures of about 150,000 degrees Kelvin (the Sun's is about 5800). The gas and dust of the inner nebulae has spectral line widths indicative of a large radial velocity of 1000 to 3000 km/sec. All of this evidence has led to the conclusion that Wolf-Rayet stars are high mass stars that are so bright that the radiation pressure of the photons created by the fusion reactions in the core are overcoming the infall due to gravity in the atmosphere. This pushes the atmospheric material away from the star (at a rate of 1 solar mass every 10,000 years or so). Over time, the processed core material begins to be exposed, and you begin to see the spectral lines indicative of a Wolf-Rayet star.
There are only 150 Wolf-Rayet stars known in the galaxy and about another 30 in the Large Magellanic Cloud. They are very rare.
Thanx for the summary of W-R stars. I assume my earlier comments are valid with respect to Main Sequence stars...
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