Nimitha’s Reviews > Living with the Stars: How the Human Body is Connected to the Life Cycles of the Earth, the Planets, and the Stars > Status Update

Everything we are made of also moves through the depths of the Earth, in the geological cycles associated with continental drift. That deep-Earth cycle, as well as the use of fossil fuels, connects the materials that build us to extinct mammoths and dinosaurs, and even to the first life on Earth.

A smaller fraction of all the carbon around and inside us was manmade, resulting from atmospheric nuclear tests just a few decades ago. Other atoms started out on the Moon, on Mars, in asteroids and comets, or elsewhere in the solar system, and fell to the Earth sometime between the birth of the planet and days ago.

A little of the carbon in everything we eat was formed within only thousands of years, created somewhere in the Earth’s stratosphere when shards of atoms from far away, having bounced around the Galaxy for millions of years following an initial kick from a stellar explosion, finally ran into our planet.

The most common element in our bodies, hydrogen, is as old as the universe. Other elements were formed generally over 5 billion years ago, in the interior of stars. Some atoms were formed later, as radioactive stardust within the Earth, which decayed to create, for example, some of the calcium in our bones.

When we trace the pathways of our impermanence, we realize that the components of our bodies connect us to the plants and animals around us, bacteria within us, to volcanism, comets, cosmic rays, and to the Sun’s light, all the way to the birth and death throes of stars throughout the Galaxy and to the beginning of the universe itself.

Some of our brain cells do survive for long times, even if the materials out of which they are made are replaced, but it appears that we have to acknowledge that it is more likely that even memories are patterns that are exchanged rather than particular fixed atomic clusters that remain with us from the moment they are formed to the moment we forget, or die.

Planetary systems originate from ancient clouds of interstellar gas. If there is enough mass in a cloud for its gravitational pull to overcome the outward gas pressure, it begins to collapse toward what becomes its central star.

Once every few millennia, an object with a diameter of some 50 m collides with the Earth. One such collision happened in 1908, near the Podkamennaya Tunguska River in Russia. That “Tunguska event” had an estimated strength of several tens of millions of tons of TNT. The explosion is estimated to have knocked over approximately 80 million trees in the central Siberian region and would have killed all life in its path.

Exactly when and where a planet forms relative to the time when hydrogen and helium are blown out of the area of the disk likely determines whether the planet becomes a gas giant (such as Jupiter, Saturn, Uranus, or Neptune) or a rocky “terrestrial” planet (like Mercury, Venus, the Earth, or Mars).

Brown dwarfs do have some limited nuclear fusion going in their interior, but that involves deuterium rather than pure hydrogen. There is but little deuterium in stars, however, so the energy that is released by its fusion is limited. If no nuclear fusion occurs at all, which is the case when the object has a mass of less than approximately one-eightieth of that of the Sun, it is formally designated a planet.