# 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 - inactive 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
Just a note that contrary to the view expressed by @Philip Gibbs, the galactic ISM is enriched very quickly, mostly within a billion years, by alpha elements like oxygen from massive stars. Carbon, Nitrogen and iron enrichment is slower, because they and also many elements heavier than nickel can be produced in lower mass AGB stars and dispersed by winds into the ISM (or type Ia supernovae for iron and nickel). – Rob Jeffries Jul 13 '15 at 19:30
@Rob Jeffries, nothing you have said is in contradiction to what I said. My statement that elements up to iron and Nickle are synthesised over billions of years does not imply that none of them are made in under a billion years. You are reading more into what I said than was there. My point was to distinguish between these elements are produced by different much slower processes than the heavier elements. – Philip Gibbs - inactive Jul 13 '15 at 20:55

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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Good answer, but the resulting age is just some average. There is no serious suggestion that all the U came from one, or even a few supernovae is there? The sheer number of SNe (about 1 per 50 years) and effective mixing in the ISM surely mean that many, many more events contributed. – Rob Jeffries Jul 13 '15 at 19:43

We really can't tell very much at all about the presolar epoch by looking at Uranium in the Earth. Uranium is produced by the r-process in supernovae. The isotopic ratios of U on Earth tell us this was produced at a weighted mean time of about 7 billion years ago. However, given that mixing in the ISM is highly efficient, that supernovae explode about once every 50-100 years in our Galaxy, but that the rate was probably greater in the early Galaxy about 10 billion years ago, and that we're not sure where the Sun was born, it is rather hard to invert this single number to tell us that much about the stars that formed the protosolar nebula.

However much more is known by looking at more "pristine" materials from the early solar system.

The Sun formed in an interstellar medium containing contributions from lots of different sources and lots of different stars. We know this from the study of presolar grains and the decay products of short-lived radionuclides found in meteorites. Condensed grains of refractory materials like diamond or silicon carbide are found inside meteorites and because they were incorporated into the bodies of meteorites we know that they were around in the nebula of the protosun, since meteorites were among the first solid bodies formed in the solar nebula. They are thought to have condensed in the cooling ejecta from supernovae and in the dense winds of AGB stars.

The compositions of these grains has been examined in exquisite detail. The isotopic ratio of elements like Al, Ca, Titanium, Oxygen, Silicon and Noble gases combined with our models for stellar nucleosynthesis allow some sort of estimate of the mixture of materials that ended up in the solar nebula. Radio isotope dating using Iridium and Xenon revealed that many grains are a lot older than the Sun. The discovery of the products of short-lived radionuclides like 26Al and 60Fe in meteorites have been explained as arising from a supernova that exploded very close to the forming protosun. Though there is controversy over how much of these short-lived radioisotopes might have been produced by irradiation from the early Sun.

There are too many details to discuss in a SE answer and it is a very mature field; and I am no expert. A reasonable review of techniques is given by Clayton (2010). The consensus seems to be that the stuff of the solar nebula came from many sources - certainly not just from one or even a few stars, though there is the possibility that a relatively nearby supernova injected short-lived radioactive isotopes into the solar nebula just as the first solids were forming.

More or less at random I picked out a figure from a paper by Zinner at al. (2005) from Lunar and Planetary Science that illustrates the type of study you can do with the isotopic abundance distributions from many presolar grains. You compare the distribution of isotopic ratios with the predictions of stellar evolution calculations and draw conclusions.

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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
Star formation is a 1-10% efficient process. Most gas in a giant molecular cloud does not get turned into stars. Furthermore, the Li content of the present day ISM is 10 times higher than it was when the first stars in our Galaxy was born. Li is produced in AGB stars. – Rob Jeffries Nov 25 '14 at 22:27

In my belief, before our sun existed there was a huge gas cloud building up all the ' local group stars, and one two of the biggest shortest lived stars exploded creating the two periods of early bombardments into the primitive earth, these asteroids meteorites carried with them water from the planets that were created around the supernova stars before they exploded and also materials of life, I wouldn't be surprised if proxima centauri had also received some of these asteroids comets holding lots of water. Therefore if any small planets like earth are there, they might have oceans too.

So you never know another possibility is that a proto planet that crashed into early earth might of had oceans and maybe a lot of bacteria/ life forms with DNA , and when the conditions became correct for life to evolve it took root on early earth and later on flourished.

Another speculation is that mercury and mars both had collisions with proto planets which could have caused the ' heavy bombardments' and mars before it died could and probably did have water, did Mars have life forms with DNA in it we don't know yet?, but if it did, then that DNA must have come from somewhere.

It's seems to me seeding planets with DNA for life to evolve, might be common place if the object delivered to like the earth had the right mix of habitat for the DNA to evolve into life firms.

So we can expect in the near future to find more planets with the right mix of enviroents for DNA life to evolve.

Like bees seeding flowers, asteroids comets and supernovas seed life to other solar systems and planets, so it's niece to think we are alone as the process that happened here for life to exist must have started somewhere before it came to earth.

That's just my fun way of thinking about the gas cloud we were in with our local group of stars and how life evolved on this planet.

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Proxima Centauri could have been on the opposite side of the Galaxy when the Sun was born. I really can't see what this answer has to do with the question posed. About the only bit with any factual basis is that there may have been a nearby supervnova as the Sun was forming. – Rob Jeffries Nov 25 '14 at 23:39
I'm not sure how this garnered an upvote. I could be missing something, but I don't see any facts here. – HDE 226868 Nov 25 '14 at 23:44
Science is more than barroom guessing. For serious work on where the Sun's siblings are, see Quest for finding the lost siblings of the Sun and Elemental abundances of solar sibling candidates. – David Hammen Jul 13 '15 at 23:08

## protected by Qmechanic♦Jul 13 '15 at 18:11

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