A plasma refers to a collection of atoms in their gaseous state that have lost some or all of their electrons, usually by being heated or electrically excited. Stars are entirely composed of plasma, akin to the astrological element fire. The four states of matter (solid, liquid, gas, plasma) differ only in the degree to which their atoms vibrate.

The measure of the average vibrational motion of the atoms or molecules of a substance is called its temperature. Thus the lowest temperature possible is when there is no atomic vibration at all. This is called absolute zero: 459 degrees below zero on the Fahrenheit scale, and the beginning—zero degrees—of the Kelvin scale of temperature. Curiously enough, the universe has a very low and uniform temperature in all directions. Known as the "3 degree Kelvin background radiation," this 3° temperature everywhere in outer space is an afterglow of the Big Bang cooled to almost nothing after 13.7 billion years, and the strongest evidence we have that a Big Bang occurred at all.

In a solid—the counterpart of the astrological element earth—the energy holding the constituent atoms together—their "bonding energy"—is stronger than the atom’s vibrational energy. This is what makes a solid a solid and gives it a fixed geometry. As a solid is heated, a point is reached at which the average vibrational energy of its constituent atoms equals or exceeds the energy holding them together. At this point the solid melts and becomes a liquid. A liquid is that state of matter—the counterpart of the astrological element water - in which the average vibrational energy of the constituent particles exceeds their bonding energy. If a liquid is continuously heated, there comes a point at which the average vibrational energy of the constituent particles so exceeds their bonding energy that they repel each other. At this point the liquid boils and becomes a gas—the counterpart of the astrological element air.

If the gas is heated even more, the vibrational energy of its constituent particles eventually becomes so great that its atoms begin to permanently kick off their outer electrons. A gas having lost some or all of its outer electrons is said to be ionized, and is know as a plasma—the counterpart of the astrological element fire.