Expert Voices

The Stars Within Us: Why Everything in You is Stellar

Interstellar dust in our Galaxy's magnetic field
Interstellar dust interacts with the structure of our Galaxy's magnetic field. (Image credit: ESA/Planck Collaboration. Acknowledgment: M.-A. Miville-Deschênes, CNRS – Institut d’Astrophysique Spatiale, Université Paris-XI, Orsay, France)

Paul Sutter is a visiting scholar at The Ohio State University's Center for Cosmology and AstroParticle Physics (CCAPP). Sutter is also host of the podcasts "Ask a Spaceman" and "RealSpace," and the YouTube series "Space in Your Face." Sutter contributed this article to's Expert Voices: Op-Ed & Insights.

Take a deep breath. 

That air filling up your lungs, that oxygen pulled into your bloodstream, stoking your metabolic fire, making you possible, is old. Older than you, older than the Earth itself. That oxygen once lived in the heart of a star that is now long dead. That calcium in your bones? That iron in your blood? The same.  

Billions of years ago, there was no Earth, no sun, nor even a solar system. There was just a relatively featureless cloud of gas and dust, hundreds of light-years across. Pretty much in stable equilibrium, that cloud could persist for hundreds of thousands, or even millions of years. But given just a little nudge — perhaps by a nearby supernova going off, sending its blast wave echoing through the nascent cloud — it quickly fragmented, folding in on itself in a complicated tangle of knots and streams. 

Pockets of gas pinch off and catastrophically collapse, in some cases reaching such incredible densities and pressures that nuclear fusion begins deep in the heart of a young system: This is the birth of a star

The shattered remains of the cloud organize themselves into a disk. The disk spawns planets that cannibalize more material as they grow and compete for space around the new sun. In fits and starts and collisions, and in migrations and bursts of intense radiation, the leftover debris is cleared from the system, leaving a family: a star (maybe two), a few rocky planets, gas giants, asteroids and frozen leftovers in the outskirts. 

A solar system is born.

It's elemental

A few new elements breeze into the solar system over the millennia, but by and large, what the solar system was born with is all it has. 

The elemental mixture of that primordial gas cloud determines the fate of the system. Not enough silicon? No rocky planets. Just a hint of oxygen? No liquid water on those planets. A bare handful of carbon? Nothing to use to build little critters to swim around in that water. 

But how did this solar system's particular mixture of elements get in that gas cloud oh so many billions of years ago in the first place? To tell the truth, I already gave the answer: fusion. 

In that newborn sun, and in its heart today, a nuclear fire rages. The crushing weight of the sun's own gravity — layer after endless layer of gas trying to squeeze itself into the center — encourages atomic nuclei to overcome their natural repulsion for each other and fuse, like every bad rom-com you've ever seen.

The fusion process leaves a little bit of energy left over , and the countless fusion reactions are enough to power the sun's radiation for billions of years — and give Earth the warmth and light needed to make life possible. 

The process just needs to start with hydrogen, a simple proton. And there is plenty of that in the earliest moments of the universe. All else follows. Every star in the sky, including Earth's own sun, is a massive, sleepless factory for creating new elements. Hydrogen to helium. Onward to carbon, nitrogen and oxygen. In more massive stars, the chain pushes even further, to include calcium, magnesium, neon and argon. All the way to iron and nickel.

Getting heavy

But there, the party stops. After iron and nickel, fusion doesn't produce energy anymore — it takes it. Fusion still happens, but there's nothing to stop the relentless gravitational collapse, no energy production to re-flate the star and balance the contraction. The infalling material tries to jam onto the core, is stopped by the solid iron ball at the center, and quickly retreats. In other words: boom!

A supernova is one of the most fantastic displays of reckless energy seen across the universe . Billions of stars' worth of energy, wasted in a single flash lasting a few weeks. In that energetic inferno, anything is possible. Want to waste energy fusing some new heavy elements? Who cares — there's plenty to spare! Have some more! It's party time!

It's in that furious explosion that the rest of the periodic table is filled out. What wasn't fused in the heart of a star is birthed in the star's death throes, in complicated dances of nuclei and stray neutrons. 

This isn't just a cute bedtime story — this is science, after all, and it requires some evidence. The theory was sweated out in a landmark paper led by physicist William Fowler, who went on to receive a Nobel Prize for his efforts.

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Today, scientists can identify what elements stars are made of, as each element leaves a distinct thumbprint in the starlight. Physicists can observe the fusion happening in the core of Earth's sun by capturing stray neutrinos, a byproduct of the fusion reaction, in detectors here on Earth. Researchers can examine supernova remnants to identify trace elements drifting through the broken clouds. Scientists can explain why some elements are more abundant in the universe than are others: If a particular chain reaction is more common in stars, then the element produced by that reaction will make its mark on the cosmos.

And human beings can see how they really are made of stars — the ashes of long-dead stars, but stars nonetheless. 

Those points of light in the night sky are connected to people in a deep and meaningful way. Human blood and bones are a part of the natural cycle of formation, birth, life and death of humanity's heavenly cousins. People come from stars and will return to the stars; every star dies, and in some fashion spreads its material back from where it came. And when the light from Earth's sun finally snuffs out, it will carry humanity's ashes back into the darkness, to be reshaped again into new worlds, and possibly new life.

Learn more by listening to the episode "Are We Really Made of Stars?" on the "Ask a Spaceman" podcast, available on iTunes, and on the Web at Thanks to the City of Lima, Ohio, for the question that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and

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Paul Sutter Contributor

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.