Once upon time, there
was a universe but no Earth. We know this from geology, which tells us that Earth
is made up of "heavy" elements - that is, elements with relatively high
atomic masses. Only a supernova, or exploding star, could have produced these elements.
Therefore, such a star must have existed before Earth did.The core of this argument, the heavy-element evidence, requires a little chemistry and some astrophysics to understand. We'll begin with the basic unit of matter, the atom, and its component parts: protons, electrons, and neutrons. Electrons, which have a negative charge, orbit the nucleus of the atom, where the protons and neutrons reside. Because protons have a positive charge, they would repel one another when grouped together unless buffered by neutrons, which have no charge and act as glue.
The simplest atom, consisting of a single electron orbiting a single proton, is elemental hydrogen. The next step up, elemental helium, consists of two electrons orbiting a nucleus with two protons and two neutrons. Elemental carbon has six electrons, six protons, and six neutrons.
In the early universe, the first and most abundant element was hydrogen. Initially, the hydrogen was probably evenly distributed, but over time it began to clump, the result of random motion and the attractive force of gravity. Eventually, some of these clumps became dense enough for nuclear fusion to begin. (This is the process by which stars "burn" matter to produce energy.)
The simplest nuclear fusion involves two hydrogen atoms being forced together to form a single helium atom. For most stars, fusion ends here, but within the cores of very large stars, fusion continues. Helium is fused into carbon, carbon is fused into oxygen, and so on. The last element in this chain is iron, which cannot be fused.
Once a star uses up all of its nuclear fuel, fusion ceases, and the star begins to contract. The awesome gravitational forces involved eventually compress the core beyond the limit matter will allow, and a supernova results. The energy of the supernova forces more atoms together, producing even heavier elements. Thus, without supernovas, there would be no elements heavier than iron and no Earth. No doubt the late astronomer Carl Sagan had this conclusion in mind when he reflected, "We are all star stuff."
The fact that Earth is composed of heavy elements created by the explosion of a star demonstrates that, at one point in the early Universe, there was no Earth. (The Bedside Baccalaureate, by David Rubel)