How old is the carbon in our bodies

The half-life of a radioactive isotope is the amount of time it takes for half of the atoms in a sample to decay. Carbon has a half-life of 5, years. That means that no matter how many carbon atoms were present when something died, after 5, years only half of them are left — the rest have decayed to nitrogen. And after 11, years two half-lives , only a quarter of the original carbon atoms are left. That's why radiocarbon dating is only reliable for samples up to 50, years old.

But old age isn't the only thing that affects the accuracy of carbon dating. The level of radiocarbon in the atmosphere has varied over time — it was about two per cent higher 3, years ago, possibly due to factors affecting cosmic rays like changes in solar cycles or the Earth's magnetic field.

And nuclear reactions have seen a leap in carbon activity since Luckily for us we have a record of atmospheric carbon levels for every one of the last 12, years. It's been painstakingly pieced together from the carbon content in living and long-dead tree rings.

Some trees grow a new layer each year. The exact age of an unknown sample can never be known for sure, so short of discovering a time machine, 95 per cent accuracy is as good as it gets. Radiocarbon may not be perfect, but as any single something can attest, no dating method is. Read more on calibration and accuracy of radiocarbon dating. Tags: archaeology , chemistry , physics.

Email ABC Science. By clicking 'Send to a friend' you agree ABC Online is not responsible for the content contained in your email message. Skip to navigation Skip to content. This site is being redeveloped. For all the latest ABC Science content click here. Site Navigation Video Audio Photos. By Bernie Hobbs From the moment we die the proportion of carbon compared to non-radioactive carbon in what's left of our bodies starts to drop, as it gradually turns to nitrogen.

Smart cookie preferences. Change cookie preferences Accept all cookies. Skip to content. Read later. You don't have any saved articles. By Kerry Lotzof. Planetary scientist and stardust expert Dr Ashley King explains. Most of the elements that make up the human body were formed in stars. The first generation of stars We think that the universe started 13 or 14 billion years ago, with the Big Bang.

The first stars that formed after the Big Bang were greater than 50 times the size of our Sun. Stargazing through time Large stars last for a few million years, while smaller stars more than 10 billion years. A single pre-solar grain viewed through an electron microscope. So are we really made of stardust? Meteorites Space What on Earth? Explore space Discover more about the natural world beyond Earth's stratosphere.

Blast off. What on Earth? Just how weird can the natural world be? Explore the unusual. Explore facts about the biggest and hottest object in the solar system. Meteorite impacts may have been key to life on Earth. Don't miss a thing. With iron, no other rearrangement of nuclei can generate any more energy. But stars do form elements heavier than iron, including cherished ones like silver and gold, dangerous ones like radon and uranium, and ones you've never heard of or could pronounce if you had like praseodymium and ytterbium.

Stars have one of two ways to produce these heavier-than-iron elements—and, not incidentally, to get them and all the other elements forged in their nuclear furnaces out into space so they can be incorporated into new stars, planets, and people. The first way occurs in red giants. These are stars that have burned up all the hydrogen in their centers. When that happens, the star becomes, as the astrophysicist Craig Wheeler has put it, somewhat schizophrenic: The core loses energy, contracts, and heats up even as the envelope—the rest of the star outside the core—gains energy, expands, and cools and appears redder.

The expansion is quite, well, expansive: When our sun becomes a red giant, it will grow so large that it will engulf and evaporate the inner planets, including the Earth. Some red giants last long enough to create elements in their cores heavier than iron through something called the s-process, for slow.

Over a time scale of thousands of years, the s-process can result in the manufacture of elements all the way up to bismuth These get pulled to the star's surface by convection and sloughed off into space via the star's stellar wind.

Some of that widely dispersed stardust is holding you up right now. This spectacular false-color image shows Cassiopeia A, the remnant of a supernova. At the center of the image lies the dead star, while surrounding it is the rapidly expanding shell of material blasted away from the star as it died. Krause Steward Observatory. Elements heavier than bismuth only arise through the r-process, for rapid.

How rapid? Seconds flat. The r-process is what happens when a star explodes in a supernova. It's easy for us to think of stars as lasting essentially forever, but the most massive stars survive only a few million years—a cosmic moment, really—and when they go, they go fast. What happens? When a red giant gets to the stage of having fused all its lighter elements and is left with an iron core, the star can no longer retain its equilibrium—heat energy pushing out as gravity pulls in.

Gravity suddenly gains the upper hand, collapsing the core all at once to billions of times the density of the Earth. The star then blows itself apart in an astronomical cataclysm. For a brief period, it shines as brightly as an entire galaxy and releases as much energy as our sun will in its billion-year lifetime.

In the first few seconds, protons in the atoms created during the star's life collide with highly energetic neutrons, fashioning in an instant all the naturally occurring elements heavier than bismuth up to uranium, and even a few short-lived still-heavier elements such as plutonium and californium. All these blast out into space at millions of miles an hour, seeding the interstellar medium with the atoms that eventually end up in new stars, new solar systems, and, in your case, you.

In this view of the Carina Nebula, the Hubble Space Telescope captured a tumult of star birth and death. In the image, green corresponds to hydrogen, blue to oxygen, and red to sulfur—three of the 92 naturally occurring elements that space has bequeathed to us. Over time, molecular clouds of gas and dust out in deep space develop from those strewn elements and begin to contract under their own gravity. Such clouds are almost all hydrogen and helium, but they've got a scatter of heavier elements, too.

And the most abundant elements begin to assemble into molecules, simple ones like water H 2 O and more complex ones like the sugar glycoaldehyde C 2 H 4 O 2.