At 300,000 years after the Big Bang, the Universe is primarily made up of three things : atoms of hydrogen, atoms of helium, and light. Of those three things, it was hydrogen that dominated and continues to dominate. When the Universe finally cooled enough for atoms to form, 73% of them were hydrogen and 26% of them were helium. And that was a good thing, as it allowed for the creation of all the other atoms in the Universe.
It did that by the same process of fusion that makes our Sun shine. Inside each star, atoms are compressed and heated to a white hot incandescent plasma; temperatures are so high that they strip the electrons from the atoms, effectively rolling the clock back to 300,000 years after the Big Bang . The naked nuclei would normally repel each other, fly apart, and cool allowing the electrons to rejoin the atoms, but they are forced together by their own gravity. Thus, it is the interaction of the different forces that allows fusion to happen – and that prevents it from happening everywhere.
This process can be thought of as being like skating over a hill into a valley. It is easy to roll into the valley, but first you have to have enough energy to get over the hill. Without that energy, you would not spontaneously fall into the valley. In fusion, the hill is the electromagnetic repulsion of the protons. Without it, fusion would occur spontaneously as gravity pulled neutrons together. However, pressure and temperature can supply energy to protons; the more extreme the conditions, the more protons that will have enough energy to make it “over the hill”. Because the necessary temperature and pressure is so high, fusion can only take place in the innermost 30% of the Sun. And, because the rate of cooling increases as things get smaller, pressures and temperatures are too low and fusion cannot happen in objects less than 1/10th the Sun’s mass .
Though the details of fusion in a star are fairly complex and only poorly understood, the broad outlines are well known and are based in the nuclear interactions of the protons and neutrons in the elements. In order to understand how fusion works, we must first understand how atomic nuclei are put together.
Inside of each nucleus are protons and neutrons. Each element is defined by the number of protons that it has in its atomic nucleus; it is the number of protons that determines the number of electrons that an atom can hold, and so determines how that atom with chemically react with other atoms. For example, every hydrogen atom has just one proton, every helium atom has two protons, and every oxygen atom has eight protons. But the number of neutrons in a given element can differ; each different type with a unique number of neutrons is called an isotope . For example, there are three types of hydrogen. One has no neutrons, and is commonly called “hydrogen”. One has one neutron, and is commonly called “deuterium”. And one variety of hydrogen has two neutrons, and is commonly called “tritium”. However, because it would be too complex to give each element a unique name for each isotope, scientists have simplified the system and refer to the isotopes by the total number of protons and neutrons, called the atomic weight, in the nucleus. Thus, hydrogen is hydrogen-1, deuterium is hydrogen-2, and tritium is hydrogen-3. And, because scientists get very tired of writing things like hydrogen-1 and uranium-235 all of the time, this gets further simplified to the element’s symbol with the atomic weight as a superscript on the left, so hydrogen-1 becomes 1 and uranium-235 becomes 1235U .
The number of protons and neutrons in a nucleus is important because that is what fusion changes. In the simplest kind of fusion, two 1 nuclei combine to create a 2H nucleus. This changes one of the protons into a neutron; because the neutron weighs slightly less than a proton, the extra weight is given off as a positron  and a neutrino . Next, two 2H nuclei fuse to create a 3He (helium-3) nucleus and a gamma ray. Finally, two 3He nuclei join together to make a 4He nucleus, two 1 nuclei, and just a little energy.
Though each individual fusion reaction only produces a microscopic amount of energy, a vast number of them take pace inside a typical star. In the Sun, about 4 x 1038 hydrogen nuclei are consumed each second, releasing the equivalent of one million times the total world consumption of energy. This process has been going on since the Solar System formed, 4.5 billion years ago and has in that time has converted forty times the Earth’s mass from hydrogen to helium. Obviously, this cannot go on forever. As the hydrogen in an area is used up, the reaction proceeds outward. Because the core of a star is the first part to reach the temperatures and pressures necessary for the reaction to proceed, hydrogen burning begins there and moves outward as a spherical shell. Fortunately, hydrogen is not the only element that releases energy when fused; however, those with more protons (and hence more electromagnetic repulsion) require higher temperatures to initiate fusion. For example, though hydrogen will fuse at a mere ten million degrees kelvin , temperatures must reach a billion degrees kelvin before carbon will fuse. In addition, elements with atomic numbers higher than iron do not produce energy when fused. Thus, there is a limit to the number of possible fusion reactions and to lifetime of stars.
Fusion inside a star happens in shells; as the fuel for one type of fusion runs out, the ashes become the fuel for the next type.
(Image from Wikipedia)
And it is the death of those stars that most interests the planetologists, because it is the death that creates the more massive elements and scatters them across the Universe. When a star that is less than half the size of the Sun runs out of hydrogen, it cannot reach the temperatures needed to fuse higher elements. So, like a campfire that has run out of fuel, it slowly dies and cools. The vast majority of stars in the Universe die this way. But when a start that is between half the size and ten times the size of the Sun runs out of hydrogen in a shell, it starts to burn higher and higher elements at higher and higher temperatures. The higher temperatures cause the star to expand, becoming a red giant and then finally collapsing. As it collapses, it warms the interior back up to the point where fusion is possible. As a result, these stars pulsate creating strong shock waves each time that fusion turns back on. Those shock waves propel the star’s outer shell outward as a planetary nebula . This nebula is rich in heavier elements up to iron and serves as a rich source of materials for new solar systems.
But it is the death of the largest stars, those that are more than ten times the size of the Sun, that most interest us. As the center of one of these massive stars runs out of fuel, it collapses suddenly and all at once, instead of in the herky-jerky manner of the smaller stars. Like adding gasoline to a fire, this sudden collapse ignites a flash of fusion that reaches temperatures and pressures high enough to create elements higher than iron on the periodic table. It also generates strong gamma rays and other high-energy photons; these will become important when we look at the creation of this Solar System. The force of the explosion is great enough to propel the newly created matter at immense speeds away from the star, enriching the interstellar medium and setting the stage for the next round of stellar evolution .
Because we know how long a star of a given size can burn before running out of fuel, we even know how many rounds of stellar evolution there have been since the Big Bang. After the Big Bang, the early stars formed. They were almost pure hydrogen, with some helium and just a trace of lithium. They formed somewhere between 150 million and 1,000 million years after the Big Bang; the uncertainty is a measure of just how difficult it is to see things that far back. Indeed, no images of the individual stars from that time have ever been recorded (though we do have images of early galaxies). From computer models, we know that these early stars were probably very large and so were very hot and burned very, very fast. These stars then exploded in supernovae that seeded the Universe with the elements needed for the next round of stellar formation, including carbon, iron, and the rest of the periodic table.
Because the second round of stars included those higher elements, they were smaller and burned more slowly. Though the early stars ran through their fuel in less than a hundred million years, the second set of stars lived for nearly ten billion years or a thousand times as long as their parents. But eventually, one by one, these stars also became supernovae and created yet more higher elements that were spread out to form the third set of stars and planets. That second population were the parents of our Sun and the planets in this Solar System. And the same elements that created the Solar System are in you, me, and every living thing on this planet; thus, we are all children of the stars – and the great grandchildren of the Big Bang.
 The discerning science fan will ask “What about dark matter and dark energy?” The honest cosmologist will reply “We’re not sure. Ask again after we’ve gathered some more data.” It is likely that the dark matter, which is the “missing mass” of the Universe that is required to make our observations of stellar motion match those predicted by Einstein and Newton’s theories of gravity, was created at the same time as all of the other matter in the Universe. However, some have suggested that the dark matter arises out of the dark energy in much the same way that matter was supposed to be created by the expansion of the Universe in the Steady State hypothesis. Without more observations, we simply cannot answer the question.
 Indeed, this is how all particle physics experiments work; we simply add enough energy to make conditions like those at some time after the Big Bang. But, because temperatures (and therefore energy levels) get exponentially higher as you get closer to the Big Bang, it takes exponentially more powerful (and therefore bigger and more expensive) “atom smashers” to simulate those conditions. The prototype of the van de Graff generator, an early particle accelerator, was built in the scientist’s kitchen using two aluminum bowls and his wife’s silk stockings; it could simulate conditions at about 100,000 years after the Big Bang. The Large Hadron Collider that has recently found evidence for the Higgs Boson, took ten years to build and cost $9 billion. It can simulate conditions less than a second after the Big Bang.
 The exceptions to this being the rare man-made cases where something else, such as an atomic bomb or fusion chamber, supplies the necessary pressure and temperature.
 “Isotope” means “in the same place” in Greek (Scientists are very fond of Greek), and refers to the fact that oxygen falls in the same place on the periodic chart and undergoes the same chemical reactions, no matter how many neutrons it has.
 To make things even more confusing, the name is pronounced as if the superscript were on the right; i.e., we say “U-235” and not “235-U” (which could sound very rude in some circles).
A positron is the antimatter equivalent of an electron. It has exactly the same mass as an electron but the opposite charge. Put a positron in the same room as an electron, and all you’ll see is a big flash of light.
 The “little neutral one”, or neutrino, was the subject of one of the greatest investigations in the history of science. We knew from the math exactly how many neutrinos should be generated from fusion in the Sun. But we detected only 1/3rd the amount that we should have. There were three possible answers to the conundrum Either our math was wrong, or the Sun was dying, or something unexpected was happening. After many experiments confirming the math, we finally discovered that there were actually three types of neutrino and our detectors could only measure one of them. The best part was that this discovery then led to more insight into how sub-atomic particles work.
 Kelvin degrees are the same as Celsius degrees, but start at absolute zero (-273.15°C) instead of at the freezing point of water. And Celsius degrees are exactly the same as centigrade degrees, only with a new name.
 These are called “planetary nebulae” because astronomers thought that they looked like the planet Uranus, not because planets form in them.
 Astronomers are probably out burning copies of this book even as we speak, as I have completely ignored complicating factors such as the star’s composition, the presence of binary companions, and the intricacies of fusion in stellar interiors. Then again, astronomers call everything that isn’t helium or hydrogen a metal, so we’re about even in the oversimplification race.