You are made of old material
You may have heard that we are “made of stardust”. Like many popular phrases, it is a simplification, but it points to something accurate and remarkable. Most of the atoms in a human body were not made on Earth. They were produced in much earlier events in the universe, and the story of how they got here connects the smallest scales to the largest.
What the body is made of
By number of atoms, the body is mostly hydrogen, with large shares of oxygen and carbon, and smaller amounts of nitrogen, calcium, phosphorus and many other elements. Much of the hydrogen and oxygen is in the form of water. Carbon is the backbone of the molecules of life, such as proteins, fats and DNA. Trace elements, such as iron in blood and iodine in certain hormones, are present in tiny amounts but are essential.
The exact proportions vary from person to person and depend on how they are counted, so it is better to think in broad terms than to memorize figures.
Where the hydrogen came from
The hydrogen is the oldest. In the first few minutes after the hot, dense early phase of the universe, conditions were right for the lightest nuclei to form. Calculations and observations indicate that this process produced mainly hydrogen and helium, with tiny traces of a few other light elements. Hydrogen atoms in your body, including those in water, are thus thought to date from around that time, roughly 13.8 billion years ago. This is one reason the measured amounts of light elements are treated as part of the Big Bang evidence.
At that stage, nothing heavier was produced in meaningful quantities. The universe had no carbon to speak of, no oxygen, no iron, and no rocky planets.
Where heavier elements were made
The heavier elements came later, mostly from stars. A star shines because nuclear fusion in its core combines lighter nuclei into heavier ones, releasing energy. Stars like the Sun fuse hydrogen into helium for most of their lives. More massive stars can continue fusing heavier elements, building up carbon, oxygen and others, in stages. Elements up to around iron can be built up through this kind of fusion in massive stars.
Getting these atoms out of the stars is just as important as making them. Stars lose material through winds, and at the end of life, they release their contents in different ways. Some stars shed outer layers gently, while massive ones end in supernova explosions that spread newly made elements into space. The path of a star from birth to death is covered by the stellar life cycle, and in a real sense that cycle is the factory and delivery system for the chemistry of life.
Elements heavier than iron need other processes. Some are thought to form when neutrons are captured by existing nuclei in particular stellar environments. Observations of a merger of two neutron stars suggested that such events can make some of the heaviest elements, such as gold, although scientists continue to study how much of each element comes from which source. That is a good example of a topic where the details are actively debated.
From clouds to planets to people
The enriched gas and dust mixed into interstellar clouds. Some of those clouds collapsed to form new stars, with leftover material forming disks that became planets. Our Sun and the solar system formed this way, from gas that included heavier elements made by earlier generations of stars. Rocky planets like Earth could exist because of that enrichment.
On Earth, chemistry took over. Atoms bonded into molecules, and in at least one place those molecules became living things. Over time, the same atoms get recycled: you take in carbon, hydrogen, oxygen and other elements through food, water and air, and they eventually return to the environment. The atoms in your body today have been in other living things, rocks and oceans before.
Scale and perspective
This story also illustrates how large the universe is compared with us. A body is a few feet tall, a planet thousands of miles across, a star far larger. The distances between stars are measured in light-years. Yet the same physical rules apply throughout, which is why laboratory results on Earth help us understand distant stars. Spectra, the patterns of light that elements absorb and emit, reveal what distant stars are made of.
Keeping the claim in proportion
The phrase “we are star stuff” is poetic, but a careful version is: most elements in our bodies heavier than hydrogen were made in stars or stellar events, while the hydrogen dates to the early universe. There are uncertainties in the details, especially for the heaviest elements. And “stardust” is a metaphor, not a literal claim that we are made of dust from any particular star.
What the idea does offer is a connection. The history of the universe is not only something out there. It is written in the chemistry of the body, and the same processes that built stars made the ingredients for life.