The Sun and stars are in a different state of matter than the solids, liquids and gases with which we are familiar.They consist of ionised particles that are in the plasma state.
Just as it takes energy to break chemical bonds to make a liquid from a solid (melting), and a gas from a liquid (evaporation), the addition of considerably more energy separates the electrons from their atomic nuclei, to form an electron-ion plasma.
Plasmas are the natural state of matter when the energy per particle – proportional to their temperature – is high enough to keep electrons separated from their parent atoms.
Most objects in astronomy bright enough to be seen are in the plasma state.
Along with charged particles, plasmas are flooded with light that is emitted and absorbed by the moving charges.
We can consider a plasma to consist of electrons, ions and photons, the latter being small quanta of light.
As we will see, the large energy densities required to make these astronomical plasmas arise naturally in astronomy from the inexorable effects of gravity, after matter condensed out of the raw energy of the Big Bang.
The defining property of plasma is that it is made up of free charges – electrons and ions.
Electrical charges were familiar to philosophers from 600 BCE. But its effects were first experimentally quantified far later, by Charles- Augustin de Coulomb (1736-1806).
He developed the concept of an “electric field”, in which each electrical charge endows the space around it with a force that acts on other charges. The lines of force begin and end at electrical charges.
The freedom of charged particles in a plasma to move between random collisions, means that the plasma is an excellent electrical conductor.
Just as the electrons easily move along household copper wires to form a current, an orderly relative motion of free electrons and ions in a plasma constitutes an electric current inside the plasma itself.
An image of the Sun showing solar flares erupting from its surface, the largest of which can be seen on the lower right. The image was captured by NASA’s Solar Dynamics Observatory on 26 October 2014. Credit: NASA/SDO
The Sun’s corona conducts electricity like copper, but plasma is more like a fluid than a wire. Mercury is familiar to us as a liquid on Earth’s surface, with a conductive effiency of about one fiftieth of copper, and indeed laboratory experiments with liquid metals have helped inform us about the workings of the Sun.
In contrast, seawater supports electrical currents carried by electrons removed from and attached to ions of sodium and chlorine in solution.
These heavier ions are far less mobile than electrons, and so seawater has about one ten-millionth of the conductive efficiency of copper.
Both magnetic and electric fields exist in the Earth’s atmosphere, allowing birds to navigate, and sparks to occur through lightning.
But in the Sun, the electrons are so mobile that any large-scale electric fields are almost immediately shorted out, just as in a copper wire.
Any magnetic fields survive intact against short-outs, because no-one has found evidence for any magnetic charges, called “magnetic monopoles”.
Based upon historic experiments by Coulomb, Biot, Savart, Ampére, and Michael Faraday (1791-1867), James Clerk Maxwell (1831-1879) “unified” electricity with magnetism.
Experiments show that magnetic fields are generated not by monopoles, but by electric currents, i.e. the relative motion of positive and negative charges.
In turn, magnetic fields exert forces only when charges and field are in relative motion.
In the absence of monopoles, magnetic field lines have no beginning, and no end. Only two kinds of magnetic field lines can satisfy this condition.
One is that the magnetic field lines make up complete loops, such as those threading through and around bar magnets.
The other kind is in an “ergodic” state where field lines go on forever throughout the universe. The former concept is most useful for discussing a finite object like the Sun.
An image of the Sun captured in ultraviolet light by NASA’s Solar and Heliospheric Observatory, 21 March 2016. Credit: SOHO (ESA/NASA)