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Carl Sagan famously observed that we are all made of “star stuff.” But what does that mean? And how much of you is really made of dead stars? SciShow Space explains!

Hosted by Reid Reimers
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It was one of the most beautiful things Carl Sagan ever said.   And, for a man who so famously could find beauty in the universe, that’s saying something.   In his 1973 book, The Cosmic Connection, Sagan wrote:   “All of the rocky and metallic material we stand on, the iron in our blood, the calcium in our teeth, the carbon in our genes were produced billions of years ago in the interior of a red giant star.   We are made of star-stuff.”   It’s a poetic way of viewing the universe that Sagan later made even more popular in the original Cosmos series.   But what did he mean? And how much of you really is star stuff?   Well, the idea goes back to a truism that we’ve talked about before: In addition to providing light and heat, stars are also element factories.   The smallest and simplest of all the elements is hydrogen, just a single proton and an electron.    It’s the most basic form of the “stuff” Sagan was talking about, and it -- along with much of the helium in the universe, which is four hydrogen nuclei fused together -- are the result of the Big Bang.   But everything that’s in the universe that’s heavier than hydrogen -- and that ancient helium -- was made by a star.   Stars spend most of their lives fusing hydrogen into helium. This is called the main sequence of a star’s development, and it includes stars as diverse as our sun -- a so-called yellow dwarf -- to our nearest celestial neighbor, Proxima Centauri, a red dwarf.   But some stars become massive enough to fuse more than hydrogen. The enormous heat and pressure generated by stars even a little bit bigger than the sun can go on to smash three helium nuclei to make carbon, and then add even more hydrogen nuclei to create nitrogen, and oxygen.     On and on these reactions go, as the stars fuse elements into more, heavier elements, all the way up to iron, element 26.    These element-makers -- the sources of all carbon, calcium and oxygen and other precious things -- are the red giants Sagan was referring to.   But it doesn’t stop there. We know that, because the universe -- and you -- is full of stuff that’s heavier than iron.   In some red giants, during their final death throes, their cores will collapse and heat up even more, generating enough heat to fuse even iron nuclei, forming elements as heavy as bismuth -- element 83.   But most of the heaviest elements come from something even more fantastic: supernovae. These elements can only be fused into being with the energy that comes with the final collapse and explosion of a dying star.    Gold, silver, lead, uranium -- they were all created by stars that had run out of nuclear fuel, their enormous masses collapsing in on themselves by the force of gravity, and then flinging out their remains into the universe as clouds of gas and dust.    And these clouds went on to form other stars, and other planets, and ultimately, you.   So how much of you is made up of dead stars?   Well, you’ve probably heard that you’re mostly made of water, which is true. The average human is about 53% H2O, each molecule of which contains two atoms of hydrogen dating back to the big bang.   There are lots of other compounds in your body, of course, but scientists estimate that, mainly because there’s so much water inside you, that hydrogen is by far the most common atom in your body -- accounting for 62% of all the seven billion billion billion atoms that make you you.   So. Counting atom by atom, you are 38% star stuff -- that’s everything that’s not hydrogen: oxygen, carbon, nitrogen, calcium -- all products that were manufactured by red giant stars and supernovae billions of years ago.   But hydrogen is also the smallest, least massive atom, so if you wanted to measure your body’s chemistry by mass, you’re only about 10% hydrogen.   So by mass, then, you’re 90% star stuff.   But no matter how you size it up, all the rest of you -- that hydrogen? -- is 13 billion years old.   So you’re not all star stuff.   But the way I like to think of it, and I think Sagan would agree, is that there’s no part of you that isn’t -- literally -- astronomical.   Thanks for joining me for SciShow Space. If you want to help us keep exploring the universe, just go to And don’t forget to go to and subscribe!