Ionospheres of dead stars and living galaxies

It is unlikely that, on putting his `idle pole¿ to work in probing gas discharges, William Crookes could have realised, in the latter half of the last century, that he was studying the commonest form of matter in the Universe. It is equally improbable that, in proposing electric currents in the upper atmosphere as the cause of the daily magnetic variations, Balfour Stewart would have thought of plasmas surrounding stars or even galaxies. Indeed the concept of a galaxy other than our Milky Way was yet to come, and Irving Langmuir was yet to apply the term plasma to the subtle glowing gas of the low-pressure arc. Today, however plasma is sometimes referred to as the fourth state of matter; its technological importance is great and astrophysicists recognise that by far the greater part of the Universe exists in this highly ionised condition