# Age of the Earth and the star that preceded the Sun

One of the great unheralded advances made in the history of science was the ability to determine the age of Earth based on the decay of isotopic uranium. Based on the apparent abundance of uranium in the early Earth, what conclusions can be drawn about the star that preceded the Sun?

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I don't understand what you mean by "the star that preceded the sun". There were lots of stars around before the sun, but there's no particular one to single out. –  Ted Bunn Jan 25 '11 at 15:01
Certainly thats possible, but I would seriosly question the amount of mixing that would occur. It seems more likely that a cosiderable amount of the dense matter in the Earth had a common origin. –  Humble Jan 26 '11 at 1:15
I think the OP is asking about the star from which our system is born. A star that exploded in a supernova and left behind a corpse of cosmic ashes, from which the sun and the planets formed. –  user12577 Sep 28 '12 at 0:46
In what sense is it "unheralded"? –  Keith Thompson Sep 28 '12 at 0:59
That assumes that there was one such star, something that's not at all obvious. –  Keith Thompson Sep 28 '12 at 1:00

Dear Humble, the cosmic nucleosynthesis (first three minutes) only produced hydrogen, helium, lithium, and beryllium. All heavier elements came from slow fusion inside living stars and, especially the heaviest ones, originate from dying stars.

Regular fusion-related processes inside stars (see the list at the bottom of the answer) only produce elements up to $Z=70$ or so: I included the periodic table for your convenience. The even heavier elements, especially gold and uranium, were produced by three extra processes

• s-process
• r-process
• rp-process

The s-process depends on the existence of elements in the iron group. An extra neutron may be absorbed (probably coming from reactions inside red giants), increasing $A$ by one, and if an unstable element is produced in this way, a neutron in the nucleus beta-decays by emitting an electron. Additional neutrons may be absorbed and the process may continue. Nuclei in the "valley of beta stability" can be produced in this way. "S" stands for "slow".

On the contrary, the r-process is "rapid". The neutrons are absorbed in a similar way but in the cores of supernovae. The seed nucleus is usually Ni-56. The rp-process, which may occur in the neutron stars and elsewhere, is also rapid, "r", but the particle is that absorbed is a proton, therefore "p" in "rp". Logically, unlike the previous two, it produces nuclei on the "proton-rich side" of the stable valley.

The uranium we observe on the Earth probably comes from all these processes - and from many stars - there is arguably no "the star" that preceded the Sun. In particular, our Earth hasn't orbited any other star before the Sun because it is as old as the Sun, at least this is what is believed. The hydrogen used by our Sun couldn't have been "recycled" and it began to burn soon after the sufficient collapse - it couldn't have been recycled from elsewhere. The heavier elements were recycled from many places. There was probably no "permanent region" that was inheriting the brand "Solar System". These issues were discussed yesterday:

How many times has the stuff of the Sun been recycled? How many times has the "stuff" in our solar system been recycled from previous stars?

Let me mention that it's not a problem for the heavy material to spread across large distances of the Cosmos. For example, an exploding supernova shoots most or all the matter by the speed 1% of the speed of light. So in 400 years, the material from the Sun - if it went supernova (it won't) - reaches Proxima Centauri and in less than 100 million years, it may reach almost any point in the Milky Way. Even the Solar System is moving at speed of 0.1% of the speed of light which is enough to move matter by light years in thousands of years. It's silly to imagine that the material had to wait on the same place from an "ancestor star", being saved for some humans on some Earth.

It may be useful to list all processes of stellar nucleosynethesis, not only those linked to uranium:

pp-chain / CNO cycle / α process / Triple-α / Carbon burning / Ne burning / O burning / Si burning / R-process / S-process / P-process / Rp-process

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"All heavier elements came from dying stars." Some clarification: After Berylium, the elements up to iron and Nickle are synthesised slowly over billions of years inside stars and spread around when it explodes. The other elements heavier than Nickle are sythesised only in the supernova explosion itself. This can happen in just a few seconds. –  Philip Gibbs Jan 25 '11 at 12:15
I was just going to correct this silly sentence of mine, thanks, Phil. –  Luboš Motl Jan 25 '11 at 12:18
the "Legend - click to find out more" indicates a html page somewhere on the web you took a picture of... please cite the source! (whoever it is, they deserve to be credited) –  Olivier Dulac Apr 15 '13 at 14:34

There is a consensus that several supernovas are the source of the heavy elements in our solar system. We can estimate approximately how long ago the heavy elements were created by looking at the isotope ratios of radioactive nuclei with long decay times ('nuclear cosmochronology'). Here are the data:

• $^{235}$U and $^{238}$U have half lifes of $7.0 \times 10^8$yrs and $4.5 \times 10^9$yrs respectively;
• detailed models of supernovae suggest that the inital production of $^{235}$U nuclei was approx. 1.3 times that of $^{238}$U nuclei.
• The $^{235}$U / $^{238}$U ratio is currently about 0.007.

with these ingredients one finds an age of $6.5 \times 10^9$years for 'our' supernova, compared to the age of the solar system, $4.57 \times 10^9$years. The calculation is in Bernard Schutz's 'Gravity from ground up', see also http://world-nuclear.org/info/inf78.html. However the isotope ratio $^{232}$Th / $^{238}$U gives a somewhat higher age, $\approx 7.5 \times 10^9$years. The current understanding is that there was an initial spike in activity, followed by more events (see references in J A Peacock, Cosmological Physics).

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Obviously we have a mix from multiple sources, dying stars dumping heavy elements into the interstellar medium, which over billions of years is turbulently partially mixed. I did see a paper a few years back that argued that even small bodies (asteroids) of only a few KM diameter melted. The implication being that there must have been enough Aluminum-27 (the principal radioactive decay product of supernova) to supply enough internal heating to melt them, and that implied these bodies had formed with a few hundred thousand years of the SN explosion. I don't know if that paper has stood the test of time, but there might be some properties of objects in the solar system that we think have been unchanged since the beginning, that will give us better information about the protosolar nebula.

The earth is more problematic, as it has been melted/differentiated (Iron Nickel sank to the core etc.), and blasted by numerous collisions with other bodies, so interesting information about shorter lived isotopes has probably been lost. The supposed collision with a Mars sized object forming the moon, would clearly have substantially rearranged stuff.

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The fact that we have Lithium and Beryllium, probably means some of the material in the gas cloud wasn't processed through stars, as otherwise these fragile elements would have been destroyed. –  Omega Centauri Jan 26 '11 at 0:09