Enthalpy is the total energy of a system. Plasma has more energy than a gas state does. Unlike solids, liquids and gases, plasma is a mixture of electrons and ions rather than atoms. You might be surprised to learn that plasma is by far the most common physical state of matter in the universe, by mass and by volume. All the stars and both intergalactic and interplanetary space exist as plasma, with the exception of molecular clouds.
The diagram below compares solids, liquids, gases and plasma.
To ionize into plasma, one or more electrons in an atom must have enough energy to overcome the electrostatic force binding it to the nucleus. The negative charge of the electron is attracted to the positive charge of the protons in the nucleus. If the energy of the system falls, ions and electrons will recombine into neutrally charged atoms. An atom may be partially (lose some electrons) or completely ionized (lose all electrons). The diagram below shows the difference between partially and completely ionized atoms using hydrogen and helium as examples.
Hydrogen can only be completely ionized but helium can be partly or completely ionized because it has more than one electron. Larger atoms have even more partial ionization possibilities, by losing one, two, three or more electrons.
What Is Plasma?
Plasmas vary widely. The ionized gases in a plasma TV or a neon light, for example, are many magnitudes less energetic than the plasma in the core of the Sun. The TV kind of plasma is called cold plasma. Plasmas such as this can be created by applying an electrical current to a gas that is at very low density. When electrical energy is applied to the gas, electrons, which have much less mass than the nuclear ions, will have much higher kinetic energy. Particles are far enough apart that infrequent collisions between electrons and ions do not transfer significant energy between them. The electrons are "hot," they have a lot of kinetic energy, but the ions, containing the bulk of the mass of the atoms, are not. Their kinetic energy is much lower. This means that the average kinetic energy, or temperature, of the system is relatively low. In cold plasma, there is no appreciable pressure exerted by the thermal (average kinetic) motion of the particles, and magnetic forces can also be ignored. Earth's ionosphere is an example of cold plasma. It is sparsely populated by ionized gas as well as excited gas atoms if there is a solar storm. Excited atoms are atoms with one or more electrons that have enough energy to climb to a higher-energy orbital but not enough to leave the atom altogether. In the ionosphere, excited atoms glow, creating an aurora. Many cold plasmas may contain excited neutral atoms as well as atoms at various degrees of ionization. The diagram below compares hydrogen atoms, excited hydrogen atoms and hydrogen plasma.
many excited states, each one associated with a photon of a particular wavelength. I explore excited hydrogen in detail in my article Atoms and Light.
You can even create ultracold plasma, which may exist at less than 1°C above absolute zero, by laser-ionizing super-cooled atoms, giving only the outermost electrons just enough energy to escape the nucleus.
It doesn't take much ionization for a gas to begin to exhibit plasma behaviours such as electrical conductivity. Plasmas are electrically conductive because they contain freely mobile electrons that can flow in the direction of an electrical potential, creating an electrical current. Many cold plasmas become electrically conductive when as few as one in every thousand atoms loses one outermost electron.
In order to be called plasma, the collection of electrons and ions must be quasi-neutral. This means it must have more or less the same number of electrons as protons in it. Any collection of (neutral) atoms will ionize into neutral plasma. A beam of electrons or other charged particles, however, is not plasma. Plasma must also exhibit collective behaviour, but this definition can sometimes be fuzzy. It means it must have electrical and magnetic properties that describe it as a whole. Most (but not all) researchers, for example, don't consider a candle flame to be plasma. Though it contains very weakly ionized gases, it doesn't exhibit significant collective behaviour.
Hot, or thermal, plasma is created from gas under high pressure. Ions and electrons are forced close together so collisions are frequent, and energy is continuously transferred between them. Unlike cold plasma, electrons and ions exist in thermal equilibrium with each other. As you increase the temperature of thermal plasma you increase the degree of ionization. It takes a great deal of energy to completely ionize all the atoms in most gases. Under extremely high pressure, you can have very hot plasma with 100% ionization. The helium/hydrogen plasma in the core of the Sun is completely ionized.
The energy required to ionize gas into plasma comes in different forms. Heat, electromagnetic radiation and electrical charge can all supply the energy to ionize atoms into plasma. In the TV, electricity does the job. In ultracold plasma, electromagnetic radiation (a laser) does the job. In Earth's ionosphere, collisions with fast electrons ionize atmospheric gases. The gas in the Sun is fully ionized by heat and pressure.
The Sun's plasma is not the only kind of star plasma. If we explore plasma inside different kinds of stars, we can glimpse what happens when we continue to add energy to plasma, and we can end up at the extreme edge of matter itself.
With the exception of the Sun, most of what I have described so far can be called common plasma. The electron orbitals are partly deteriorated but the atoms hold onto some of their (inner orbital) electrons. The Sun's core, however, contains thermonuclear plasma. Here, the electron orbitals are gone. The atoms are completely ionized into ions and free electrons. The plasma in an exploding supernova is far hotter and under much more pressure than the core of the Sun. Some researchers call this nucleon plasma. The nuclei themselves are shattered, so it is a mixture of free electrons, protons and neutrons. This plasma is an intensely creative zone where all the heavy elements in the universe are created. If this plasma is pressurized further, for example inside the star remnant leftover from the supernova explosion, the free protons will absorb the free electrons and what remains is an ultra-dense star filled with tightly squeezed neutrons, a neutron star. Under even higher energy conditions, such as when the universe was under a millisecond old or perhaps inside quark stars, which are especially massive neutron stars, researchers expect even the neutrons themselves to be shattered. This plasma, called quark-gluon plasma, contains only free quarks and gluons. At energies beyond this, researchers speculate that quarks may break down into even smaller units called preons, so a kind of preon plasma may exist, perhaps in the core of a maximum mass neutron star that is not quite massive enough to form a black hole, where matter has collapsed all together.
When you increase the energy of an already extremely energetic system, you may begin to see a trend where building blocks of matter degenerate into more and more fundamental particles, a process called matter degeneracy. Reversing this trend gives you an idea of how matter originally formed in the universe. We'll revisit degenerate matter in the next article.
Stellar Plasma Dynamics Is a Complex Field
Heat and pressure are not the only factors that make the Sun behave the way it does. Stellar plasma is a very complex physical state. Radiation, pressure, gravity, electrical charge and magnetism all play critical roles in the Sun's activity. Even when all the atoms in a gas mixture are completely ionized, as they are in the Sun's core, plasma is complex and difficult to model. How does it transfer heat? How do electrical currents and magnetic flux that originate from drifting or rotating plasma further influence its motion? How do changes in pressure and temperature change it? Later on in the Sun series, we'll explore some of the models of Sun plasma behaviour, which are especially important for understanding and predicting solar weather.
A Closer Look at the Hydrogen and Helium in Solar Plasma
The Sun's core is basically a mixture of two completely ionized gases - helium and hydrogen. Let's look at what it takes to ionize these atoms. Hydrogen and helium have very different ionization energies. In fact, every atom has its own unique ionization energies. Hydrogen, for example has only one ionization energy, 1312 kJ/mol (although it has many excitation energies as we saw earlier). 1312 kJ/mol is the energy required to remove its single electron. Helium has two ionization energies, 2372 kJ/mol and 5250 kJ/mol. Compared to hydrogen, it takes almost twice as much energy to remove helium's first electron and almost twice as much again to remove both electrons and completely ionize the atom. In fact, the second number, 5250 kJ/mol, is the highest ionization energy of any element. Why is helium so different?
A high number generally means a strong attraction to the nucleus, but as we'll see, the picture can be a bit more complex. Element first ionization energies follow a general wavelike pattern, shown below.
The pattern hints at how electrons bind to atoms. First ionization energy is the energy required to remove one of the most loosely bound outermost electrons from an atom. The energy depends on the charge (how many protons) of the nucleus, the distance of the electron's orbital from the nucleus, the number of electrons between the outermost electron and the nucleus (inner electrons "screen" the positive charge from the outermost electron), and whether the electron is paired or not. When electrons are paired in the outermost orbital, they experience some repulsion from each other. This offsets some of the attraction to the nucleus so they are removed more easily.
Helium is an especially interesting atom. It exhibits unique symmetry. The charge cloud of its nucleus, an alpha particle with two neutrons and two protons, and the charge cloud of its two electrons both occupy spherical 1s orbitals. To review what a 1s orbital is, try my article Atomic Orbitals and Bonding. Both quantum mechanical clouds are perfect spheres centered on the center of the atom, as shown below right.
Neither of these orbitals possesses any orbital angular momentum, and each pair of particles cancels out each other's intrinsic spin. This puts the atom in a uniquely low potential energy state that makes it very stable. Adding one more electron, neutron or proton would drastically increase potential energy by introducing angular momentum. This is why there is no stable atom with five nucleons. That arrangement would have too much potential energy to be stable. The stable arrangement of helium means that it takes tremendous energy to remove electrons from, or ionize, it (and it also explains why helium exists only as an atomic gas and why it won't react with any other atoms to form any compounds).
The Sun's core has enough energy to completely ionize helium and hydrogen. On Earth, these elements exist in very small concentrations as gases in the atmosphere (although while helium is always found as a gas, hydrogen is also, very abundantly, bound up in various compounds). Inside the Sun's core, they exist as plasma under intense pressure. To compare, hydrogen gas on Earth has a density of 0.00009 g/cm3 at 0°C at sea level. In the core of the Sun, it is compressed to 150 g/cm3, a density almost two million times higher.
The Sun's Core Is Evolving
Helium nuclei are heavier than hydrogen nuclei so helium sinks into the center of the Sun's core. A ball of dense helium plasma is currently building up there. For now, it sits inert as the Sun maintains a state of equilibrium where fusion pressure balances gravitational pressure. The Sun is in the main sequence stage of its life, where equilibrium is maintained, and it will remain in this phase for another 4.5 billion years, contracting slowly all the while to compensate for energy lost through nuclear fusion and radiation. The temperature and pressure in the core, meanwhile, slowly increase as well.
The rate of nuclear fusion will eventually slow down as hydrogen is consumed. The Sun will evolve into a red giant star and then into a white dwarf, as shown below.
The Sun will last as a red giant for another billion years. The core hydrogen is used up, so the helium inner core of the Sun becomes unstable as gravity starts to overcome waning fusion pressure. The core will shrink even more and the inner helium plasma sphere will be compressed further, into an even more bizarre state, one that will set off a series of events that ultimately seal the Sun's fate, next, in The Sun Part 4.