"Now entertain conjecture of a time When creeping murmur and the pouring dark Fills the wide vessel of the Universe" — Shakespeare (Henry IV, АсЦ)
Nowhere in the universe can we find a perfect vacuum. Around ordinary stars, a hot magnetized plasma seethes and bubbles outward in a thermally-powered wind. Within the disks of spiral galaxies, dusty clouds of molecular gas continually coalesce and collapse under their self gravity to form new stars. The light from these new-born stars heats and ionizes their placental cloud before finally dispersing it back into the galactic disk, ready to repeat the cycle. Wherever there are stars, some reach the end of their lives and explode as supernovae, hurling out the gas that transformed in their thermonuclear furnaces to heavy elements such as iron. The shocks they produce rumble their way through space, heating the interstellar gas anew. Some is thrown up and far away from the galactic plane, while other parts are crushed into dense sheets and filaments which shine briefly as their shock energy is radiated away. In clusters of galaxies, the gaseous shells ejected by old stars collide one with another, heating the gas so that it glows softly in X-rays, cooling over billions of years before finally falling back into the bright galaxy cores to feed the massive monster black holes that lurk at their centres. In intergalactic space jets of relativistic plasma shot from the cores of active galaxies emit radio waves as charged particles circle and shed their energy in the magnetic fields. Even in the vast reaches of space between the clusters of galaxies, hot plasma can still be found, even though it is so tenuous that it can never cool again, and a hydrogen nucleus could travel a distance equal to the width of our galaxy before encountering another of its kind.
This then is the broad canvas of diffuse matter in the universe, displaying a rich range of thermal plasma phenomena and covering a remarkable variety of conditions and chemical compositions. We work to understand the physics of such plasmas because, by gaining insight into their physics, we can hope to understand and interpret the observed phenomena, measure their physical parameters and determine their chemical composition. Since all the stars in all the galaxies have been ultimately formed from this gas, our study provides insight into the structure and evolution of the universe we live in.
