Sun middle-aged?

[thx4answers]

If the sun is middle aged ( 4.5 of 10 billion years ), and the Hydrogen is continually being converted to Helium, why is the ratio of Hydrogen to Helium 70:28 instead of approx. 49:49?

[firefly]

You're obviously usually the term middle-age to mean at the centre, rather than what it is usually used, above the central age. Isn't it something to do with Hydrogen being the most common element in the Universe?

[darth vader]

surprised no one has had a go at answering this. I don't know the right answer but can hypothesise two possible reasons.
1) 2 hydrogen molecules react by nuclear fusion to make 1 helium molecule - helium has two electrons, hydrogen 1 so possibly two hydrogen atoms fuse together to make a helium atom? OR
2)The rate of conversion is not linear but in fact exponential (which as the reaction is nuclear i would expect the reaction rate to follow the rate of radioactive decay, and therefore this to be the more feasible answer)

[darth vader]

ok so just to make me look a ninny someone else finally has a go. BTW i think on 2nd thoughts it would be an inverse exponential curve i.e. the reaction rate is slow at the start and speeds up, but the rate of acceleration increases also. Hence more H then He at halfway, but eventually will be the other way round

[darth vader]

....and i didn't explain properly in case you're all looking confused. me and firefly posted simlutaneously. I'll get me coat..........

[wildwood]

Pure speculation - I imagine that in 5bill. odd years the sun would not be pure helium. Perhaps it will be around half/half at the time when it is going to fry our butts.

Merry Christmas.

[incitatus]

Its complicated. the sun consists of several zones, some more compact than others. in order to fuse the hydrogen must be compressed, so it tends to do this either below the surface or in surface anomalies. the most dense bits sink to the middle. helium is more dense than hydrogen and so theis tends to be more prevalent in the core. Once the core has gone beyond a certain percentage of helium it kicks over into making carbon from helium (as i remember it) and then through a variety of other stages until it hits iron. each stage is quicker than the previous one. the star only makes iron for about 20 minutes before it blows up. Consequently a star will burn hydrogen until old age sets in because everything else is much quicker

[thx4answers]

I guess my mistake was looking at things in a linear way, and astronomy can't always be explained that way! Incitatus seems to know what's happening out there, and it sounds like the right answer, but some great replies from everyone. Thanks again