Ionization is a special process that can occur in an atom when too much energy is given to the atom and the electrons are completely stripped out or removed from the atom. The ionization state of an atom (which is the fraction of that atomic species that is ionized in some environment) is critically dependent on temperature. This dependence will become apparent in the interactive simulation below.

The role of temperature:

The ionization state of atoms is controlled by temperature through two processes:

• Collisional Ionization: The kinetic energy of random motions between atoms can cause electrons to move from low energy states to high energy states. The amount of kinetic energy available per atom is directly proportional to the gas temperature. A gas that is twice as hot as another gas has twice as much kinetic energy per atom. If the average kinetic energy per atom is higher than the ionization energy for electrons in that atom, then most any collision will produce an ionizing event. This is the most common form of ionization for atoms in a stellar atmosphere.

• Radiative Ionization. If an incoming photon has more energy than the ionization energy of an atom, then that photon itself will be absorb by the atom. Some of that absorbed energy will produce an ionization event and the remaining will simply heat the atom (thus increasing the temperature of the gas). For most atoms, the incoming photons must have energies at least in the Ultraviolet part of the spectrum. Thus very hot stars might be expected to be surrounded by regions of ionized hydrogen and observations show that this is the case. These regions are called H II regions and they will be discussed further in the portion of this course that deals with the interstellar medium (ISM). Incoming photons with energy equal to the energy difference between any two atomic levels can cause and absorption to occur.

Ionization produces free electrons within the gas. So suppose we consider the simple case of hydrogen gas. A neutral hydrogen atom has one electron and one proton. If the electron is removed from the atom (through one of the above charges), it becomes a free electron and thus an ionized cloud of gas is also charged. Freely floating in that case are both negative charges (free electrons) and positive charges (the proton that the electron used to be bound with).

Well it turns out that electrons freed through the process of ionization due not stay free very long. Rather quickly a free electron will be attracted to a positively charge proton and recombine with that proton. This process of recombination causes the electron to cascade down energy levels to reach the ground state and with each transition a photon is emitted. This is called recombination emission.

Recombination emission is not continuous like blackbody emission, but instead is discrete emission at those energies that correspond to the energy levels in the atom. This is called an emission line spectrum.

Example the hydrogen emission line spectrum.

Hydrogen recombination emission

Interactive Procedure:

1. Lower the temperature to 10,000 degrees. You should observe nothing happening. The electron will not be ionized (it stays in the ground state). In this case the gas is too cold for any collision ionization of the hydrogen atom.

2. Now set the temperature to 20,000 degrees. YOu should start to observe a few ionization events followed by recombination events and the generation of an emission line spectrum (photon emission at discrete wavelengths). You may left click the cursor on any of the white "emission lines" to get their wavelengths and the number of emitted photons.

3. Let the simulation run until there area 100,000 total collisions (that event counter is labeled Total Events:) At the bottom right is another counter labeled Ionizing Events. Keep the temperature at 20,000 degrees At this temperature there should be 30-40 ionizing events so the overall fraction of ionized hydrogen in a 20,000 K gas would be very small.

4. Now raise the temperature of to 40,000 degrees to start the count. Since you have doubled the temperature, would you expect the number of ionizing events to also double (i.e. 60-80). Go ahead and find out.

   Key question: If you double the temperature do you see twice the ionization events? If so, why? If not, what might this tell you?