The Universe sprang into being about 13.8 billion years ago, expanding exponentially from a point of infinite density and infinite temperature. Such a point is called a singularity, and this event is popularly known as the ‘Big Bang’. Before this point, time and space did not exist, so the word ‘before’ itself ceases to hold meaning. Within the space of a second, the first elementary particles were formed, and the universe shifted from a state of pure energy to one containing matter as well as energy. Within 20 minutes, the universe had cooled down enough to allow protons and neutrons to combine through nuclear fusion, and the nuclei of the first atoms graced the universe. These were the nuclei of hydrogen (simply a proton), helium, and a tiny bit of lithium. The first true atoms, with positively charged nuclei surrounded by negatively charged electrons, would not arise till about 300,000 years later.
The first stars were created about 100 – 200 million years after the big bang, when areas of large matter densities began to cool and undergo gravitational collapse. These first stars were composed primarily of hydrogen and helium, the first elements in the universe, with perhaps a trace of lithium. These stars were probably massive, many hundreds of times the mass of our sun, significantly hotter, and relatively-short lived. Higher molecular weight elements (rather confusingly called ‘metals’ by astronomers) were formed within these stars as a result of thermonuclear reactions, and when these stars exploded as supernovae, these metals were flung far and wide, finding their way into clouds from which the next generation of stars took birth. As a result, newer stars (our own sun being one of these) are much more metal-rich than those formed during the dawn of the universe.