This gas, with temperatures from 1 million to 100 million degrees Kelvin, is spread throughout the Universe, from regions close to stars, throughout the interstellar medium of our own Galaxy, and nearby galaxies, through to vast accumulations of hot gas around clusters of galaxies. Determining the density and pressure of such regions is an important part of the XMM mission, and to our understanding of the cosmos. The pressure exerted by a gas is, at its simplest, the simple ratio of the force exerted on a surface by the gas, divided by the area that the force is exerted over. Consequently, the units of pressure are then in units of N m -2 .
Gas contains molecules that are in motion.
From the motion of the molecules we can see that several things will affect the forces exerted by the gas. Consequently, when they collide with a surface the force they exert is greater than if the gas was cooler. If the density is higher the more particles are exerting a force then clearly the pressure is higher. Some other units of pressure To complicate matters, physicists and astrophysicists often use different units for pressure. In weather forecasts you will often have reference to a pressure of 1000 millibar.