Where Do We Come From?
Ever wondered where you came from? We are made of a complex mixture of elements, with a lot of carbon and water, some iron and other trace elements, but the Universe started out as just hydrogen and helium gas. So where did all the carbon and oxygen and all the other elements of which we are made come from? We'll explain, but first you need to understand a little about the way the universe works.
Everything we see going on around us in the Universe tells us that it is expanding, or at least that the average distance between galaxies is increasing with time. If we know that the universe is expanding it is quite logical to assume that at one time in the remote past, things were all much closer together than they are now, and if we do a few simple calculations we can predict that everything started moving apart about 13.6 billion years ago. The event which started this expansion is commonly known as the "Big Bang", and the universe has continued to expand ever since. In fact, latest measurements tell us that it will always continue to expand because there isn't enough matter in it to slow it down by way of gravity. (If there was more than a certain minimum amount of matter in the universe, gravitational effects would work to pull everything back together and generally slow down the expansion until it stopped, and eventually went into reverse).
As we've said, gravity is an interesting thing because it tends to make things clump together, and in particular it drives the gathering together of huge areas of gas and dust in the universe to form the galaxies, including our own Milky Way galaxy. Within these galaxies gravity also drives the accumulation of smaller clumps of gas and dust to form stars, and our own sun was formed in this way. A typical cloud from which stars are formed may produce hundreds or even thousands of stars, and as a fragment of the gas which is forming a star collapses to make that star, it tends to swirl into a disk of material surrounding the newly formed star. Out of this disk of material heavier elements condense and solidify, and this is how the planets are formed, with the rocky material able to solidify close to the star and the gassy material being driven further away by the heat and radiation from the new star. That's why the inner planets (like the Earth) have solid surfaces, and why the outer planets, from Jupiter onwards, are basically all made of gas. But, as you will recall from our opening statement, the simple fact that there is solid and rocky material available at all is where things start to get interesting!
Well, the Big Bang referred to above produced only the two most simple elements - Hydrogen and Helium. Nearly 100% of all the matter we can detect in the universe is one of these two elements, so where did the rocky stuff from which the Earth is made come from? How did the silicon and magnesium and nitrogen and oxygen get produced, and in particular - as we are mostly made of carbon - where did the carbon come from of which our bodies are made? The answer is astonishing - at least to me. Stars shine because there is a complicated process going on inside them called nuclear fusion. In this process hydrogen gas is "burned" to make helium gas, and at the same time a considerable quantity of energy is produced, which is what makes the stars shine. They shine in this way for most of their lives but as they age stars undergo other nuclear reactions, reactions which burn the helium to eventually produce additional elements such as carbon, oxygen, nitrogen and iron. Large stars and smaller stars age in different ways (you need to read the section called "The Life of a Star" but all of them create carbon, oxygen and nitrogen. The other, heavier elements are made inside the largest stars, but all stars eventually die and scatter their contents at great speed through space, including the new elements which the star made while it was still "together". Smaller stars like the sun eventually reach a period in their evolution where they grow very large and start to throw off "sooty rings", which is how they spread their material through space. Massive stars undergo supernova explosions, which scatters the material rather more effectively.
If you look at the images pages on this web site you will see pictures of "planetary nebulae" and "supernova remnants" which have been produced by stars going through exactly the processes we are describing here. On its way through interstellar space the expelled material mixes in with fresh hydrogen and helium gas and dust and eventually a time comes when these mixed gases again condense to form new stars - and that is how the heavier materials are available to condense into planets. This forming, aging and re-forming process has been going on since the formation of the universe and, incredible as it might sound, all the material from which our planet Earth is made was produced long, long ago inside a star. More significantly, since our bodies are based on carbon, it is true to say that the material from which we are made was once part of a star, or in the words of the famous astronomer Carl Sagan, who sadly died in 1996, "We, are Star Stuff".