Our bodies are made of many kinds of atoms. The hydrogen in our bodies was made just after the Big Bang, about 13.8 billion years ago. Carbon and oxygen were made inside stars. When big stars exploded, these atoms spread into space.

Later, they became part of the Earth and of us. Much of the gold on Earth may come from crashes between dead stars. We really are made of stardust.

By mass, the human body is about 65 percent oxygen, 18 percent carbon, 10 percent hydrogen and 3 percent nitrogen. These elements were born in different places. Soon after the Big Bang about 13.8 billion years ago, the universe contained almost nothing but hydrogen and helium. The hydrogen in the water in our bodies dates from that time.

Heavier elements such as carbon and oxygen were made by nuclear fusion in the hot centers of stars. When massive stars exploded as supernovae, these elements were scattered into space, and smaller stars like the Sun also blow out carbon late in their lives. This material later formed new stars, planets and, eventually, us. A famous 1957 paper explained this process in great detail.

The phrase "we are made of stardust" sounds like poetry, but it is science. Most of the hydrogen in our bodies formed shortly after the Big Bang, when the universe contained almost nothing but hydrogen and helium. Heavier elements came later. Inside stars, nuclear fusion builds carbon, oxygen and other elements, which are released when massive stars explode as supernovae or when aging stars like the Sun shed their outer layers.

A landmark 1957 paper by Burbidge, Burbidge, Fowler and Hoyle laid out how stars produce many of these elements. The heaviest ones, such as gold and platinum, are hard to make through ordinary fusion. In 2017, astronomers observed two neutron stars merging, through both gravitational waves and telescopes, and a study of the resulting light indicated that large amounts of heavy elements had been created.

The popular version of the story leaves out some details. Much of our carbon and nitrogen probably came from aging low- and medium-mass stars rather than supernovae. Neutron star mergers may also be too rare or too slow to explain the heavy elements found in very old stars, so astronomers are looking at certain unusual supernovae as another source.