MadSci Network: Astronomy
Query:

Re: Heavy element distribution and concentration in the Universe

Date: Thu Nov 11 17:14:08 1999
Posted By: Ian Lyon, Faculty, Department of Earth Sciences, University of Manchester
Area of science: Astronomy
ID: 942333936.As
Message:

In most light mass stars (like our sun), the normal nucleosynthetic reaction is the conversion of hydrogen to helium. This is by far the most efficient nuclear reaction releasing the most energy. Other reactions can occur basically by sticking together helium nuclei (look at which are the most abundant elements and isotopes, 12C, 16O, 20Ne, 24Mg, 28Si, 32S, you’ll see that they all increase by 4 mass units). Each of these fusion reactions building heavier and heavier elements can occur in more massive stars, especially as they start to exhaust their hydrogen fuel. As they run out of hydrogen the inner nuclear fusion source which powers the star starts to fail, and gravity, which is always trying to collapse the star, starts to win, compressing it into a smaller volume. As that happens the internal temperature rises to the point where these higher fusion reactions building heavier and heavier elements can work, providing enough energy to stop the gravitational collapse and giving the star a new lease of life. But only up to a point! The amount of energy released is less and less efficient as the atoms get heavier and heavier until when the process reaches iron, it actually takes more energy to make heavier elements than is released.

At this point in very massive stars, like 25 times the size of our sun, very significant quantities of all these elements at the lighter end of the periodic table have been produced. (This is why iron is relatively abundant in the galaxy and everything heavier than iron is quite rare). For complicated reasons, what can happen next depends upon the exact mass of the star but for the more massive ones usually results in a supernova explosion. There is so much energy released in this explosion that nuclear reactions building up elements heavier than iron can happen, the explosion simultaneously distributing this material out into interstellar space to seed hydrogen clouds with these heavier elements.

The lifetime of a star, how fast it uses up its nuclear fuel, depends very much on its mass. A star like the sun is expected to last for approximately 10 billion years whereas a star 25 times the mass of the sun uses its fuel so quickly that it may go through its whole life cycle in only 100 million years. Thus, since the Universe has lasted approximately 10--15 billion years already, there have been many generations of these massive stars being formed in the galaxy from the collapse of hydrogen clouds and using their fuel at a profligate rate to form the heavier elements and blow up distributing these elements back out into the interstellar environment. There are other nuclear reactions called s-, r- and p processes which form the heavier elements in stars like red giants and what are called asymptotic giant branch stars which make different isotopes that we see today but this would be getting to be the size of a text book to describe all the ins and outs here!.

When a proto-solar system like our forms from the collapse of a cloud of hydrogen and dust, it contains nearly all hydrogen and these bits of heavier elements that have been made in supernovae. As the centre of the cloud collapses to form the sun, the central regions get hotter and hotter, until finally the hydrogen-burning nuclear reactions turn on in the new sun blasting away all volatile material from near the sun. Thus, what we see in the planets of the solar system is not so much a concentration of these elements but what is left if you take away the hydrogen and helium. There is much more hydrogen and helium if you go out into the colder reaches of the solar system, look at Jupiter and Saturn. The sun contains all the heavier elements, too. The ratios between all elements in the sun (apart from hydrogen and helium!) matches well the abundances of all elements in primitive meteorites so what we see in the solar system is mainly the result of hydrogen and helium and some other volatile elements being removed and we live on what is left. This will probably be true of any other planetary systems, too.

But it is true that the atoms which go to make up you, me, the Earth have been processed through many stars and probably several supernovae so that in the famous words of the song, "we are all stardust." (Quite a thought!) This sort of information is quite well known. Have you tried some Web searches or looking at some books on nucleosynthesis? A well known text is by D. D. Clayton of Clemson University which I’m sure you will be able to search it out.


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