Two windows should be open. This one in which the instructions are given and a smaller one which contains the actual applet that we will be using.

Tutorial:

Absorption lines in stellar atomspheres occur whenever a cooler gas (e.g. the atmosphere) is located between an observer or detector and a hotter source of continuous emission.

Absorption occurs when an incoming photon has an energy that is equal to the energy level difference between some lower energy level and a higher energy level. IF the background source (e.g. the star) emits photons of all energies, then this condition will always be satisfied if the electrons are in upper energy levels and are therefore ready for an absorption to occur.

Electrons can populate higher energy states in atoms due to collisions between atoms. The collisional rate and energy are both proportional to temperature and therefore there is a strong temperature dependence on the strength of the stellar absorption lines, depending on what element is involved.

For the case of Hydrogen Lines in the optical, the energies involved are fairly significant and hence one needs a relatively hot star to populate the energy levels so that hydrogen absorption can occur. If the star is too hot, of course, the hydrogen becomes ionized (not yet a feature of this applet).

This applet is designed to have the students measure the strength of the absorption lines as a function of temperature. There are no adjustable parameter tags for this applet.

Note, when the star is first "turned on" the detector in the applet will be in its "blank" state as no light has fallen on it. The applet will not be ready for absorption to occur until the whole detector is filled in with photons and the continuous spectrum is built up. At that point, the bar in the top middle of the applet labelled none will no longer be grayed out but will be black and active. Go there and select Hydrogen from the pull down menu (currently only hydrogen is there).

Suggested Activities:

• Have the student set the temperature of the star using the slider bar to 4000. Ask them if there are more red photons than blue photons on the screen. Ask them to notice if the red or blue ends of the detectors build up the fastest.

• Now have them set the temperature to 10000 and ask the same set of questions.

• At 10000 after the spectrum is built up ask them to select hydrogen. Ask them to record the wavelengths and intensities of the dark lines they will eventually see on the spectrum. This can be down by rolling the cursor over the dark line. Ask them to time how long it takes for say 100 counts to happen in the H-beta line.

• Have them repeat the exercises above for temperatures of 7500 and 5500 K.