Ionization

Ionization is the process that occurs when an electron is completely stripped from an atom and is no longer bound to it.

In the case of hydrogen, the ionization potential is 13.6 electron volts (eV). This is just a unit of energy, don't worry about it and don't memorize it.

This means that if the hydrogen atom receives more than 13.6 eV of energy from some source (e.g. an incoming photon), the electron will be removed from the hydrogen atom. The resulting hydrogen atom is now just a proton and has a positive charge. So an ion has been created.

Temperature is very important in the ionization process as we will interactively discover in a moment.

The energy level populations of electrons in atoms are controlled by temperature through two processes:

• Collisional excitation: The kinetic energy of random motions between atoms can cause electrons to move from low energy states to high energy states. This is directly proportional to temperature. A gas that is twice as hot as another gas has twice as much kinetic energy per atom. If the kinetic energy of the collision exceeds the ionization potential of the atom, the atom will be come ionized (i.e. an electron will be lost).

• Radiative excitation: Incoming photons with energy equal to the energy difference between any two atomic levels can cause and absorption to occur. If the incoming photon has more energy than the ionization potential of the atom, the atom will be come ionized (i.e. an electron will be lost).

Recombination Emission:

Recall our graphic that shows the 3 types of spectra:

So far we have dealt with the continuous spectrum (that's blackbody radiation) and the absorption spectrum (that's what we did last week and again below) but not yet the "emission spectrum".

An emission spectrum is generated by recombination emission What the hell does that mean?

Well it means that the free electron can created by the ionization process can then "recombine" with the atom and cascasde down energy levels to reach the ground state. The subsequent spectrum that is produced is an emission line spectrum

We can illustrate this principle in the following simulation.

In class activity: In the above applet, set the temperature to 40,000 degrees and run until the total event rate is 20,000. Use this to record the number of ionizing events Now do the same think for T = 20,000 degrees (make aure you run til the total event rate is 20,000). When done, publish your results. Question: you decreased the temperature by a factor of 2, did the number of ionizing events decrease by a factor of 2 or did it decrease more rapidly?

Now repeat this procedure for the atom below and note the difference in ionization rates. Look carefully at the energy levels for this atom. What is different?

Moving on now to Stellar Spectra and Classification. This will be an essential step in being able to understand and characterize stellar evolution.

Classifying the Spectra of Stars:

Overview of Spectral Classification

Today we will measure actual stellar spectra and determine the ratio of hydrogen line strengths to Calcium and Sodium line strengths.

Procedure:

Open the applet in a new window

We will be measuring the strengths of the following lines:

• (H) Hydrogen = 4340 angstrom line
• (He) Helium = 4472 Angstrom line
• (Ca) Calcium = 3934 Angstrom Line
• (Na) Sodium = 5885 Angstroms

Select the following stars in which to make the measurment:

• O7-BOV
• B3-4V
• B6V
• A5-7V
• F6-7V
• G1-2V
• K4V
• M4V

Finally, for each type selected, measure the approximate wavelength at which most of the energy is emitted. In the example below that would be 4080 angstroms. The bottom number in the green readout box is the counts on the Y-axis, so where that is a maximum (e.g. 120) this is about the maximum wavelength.

Record results here and publish them when your done

The following is a guide to how to use the applet simulation correctly. Please read through it. Again, these are real stellar spectra and your measuring them in a real scientific way and producing real results. Hints: To select a line, bracket it with the green lines and then use the blue line as a reference point. The strength of the line is shown in the readout box on the lower right of the applet. In this case the answer is 6.18 for the selected star B3-4V. Note the blue reference line may be a bit hard to see, sorry about that, will change the color later.

Its important that the reference point, the blue line, be close to the green boundaries. You will need to move the blue reference line to other side, as shown here. In this case, the answer is 4.64. Average the two measurements together for the final result. Again, there is no need to be anal about measurement precision, just round your numbers up or down to the nearest 1/2 so 4.64 is 4.5, etc).

Other options with this applet including the following.

If you want to expand the range, then type in the wavelength limits of the plot and hit the enter button. In the case shown below we not plot from 3600 to 4800 angstroms.

You might want to therefore expand the plot as its easier to see sometimes what you are measuring.

To assist in identifying the lines, you can load in the relevant element background (H,He,Ca, or Na). Shown below is the example for K4 star and the sodium background which clearly shows the 5885 feature to be measured.