Five billion years ago, there were no planets and no Sun. There was only an interstellar vapor, with nuclei of this and that element zipping one way and another, and rarer grains of dust. We think of space as cold, but the sparse atoms that "empty" space contains are moving very, very fast. Today, the temperature of our "local cloud" is several thousand kelvins, and the temperature of the "Local Bubble" in which our cloud is immersed is about a million kelvins. With extremely high relative speeds and rare encounters, the atoms in space are inclined to stay as a vapor. That vapor is the white area at upper left of the diagram: very high temperature, very low pressure.

Something happened that locally increased the density of the cloud just a bit, and gravity caused it to begin to collapse, increasing its density, pressure, and temperature. Our Sun formed, and it got hotter. It got hot enough that most of the preexisting dust also evaporated to vapor. We're still on the upper left area of the diagram.

Eventually and inevitably, things cooled, and wherever the density was high enough, mineral grains began to form. "There's always a sort of ethereal excitement surrounding the first solids of our solar system," Bramble wrote to me. "Examining the figure shows that at high temperatures and low pressures a vapor is all that exists, but as the temperature of the protoplanetary disk cools and the pressures increase, minerals begin to form." Start in the upper left area, and let your eyes follow a trajectory down and to the right, as pressure increases and temperature cools. Bramble went on: "The first mineral that should condense would be corundum, and hibonite next as the nebula cools and condenses. The highly refractory elements (Ca, Al, Ti) condense from the vapor first, then the moderately refractory (Si, Mg, Fe), and so on." It may help understanding to look at a simplified version of the diagram, generalizing the mineral names: