Formation of Stellar Absorption Line Spectrum

Open up the Periodic Table of elements Applet

Tutorial:

Absorption lines in stellar atmospheres 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).

Interactive activity: Measure absorption line strength as a function of temperature. Select the element H, from the drop down list where it currently says "None:

Use the left click with the mouse on the "detector" part of the applet to measure counts in a dark line. Use the line at 4861 angstroms (where the blue merges into green). Click on the center of the dark line and a box appears. The top number is the wavelength and the bottom number is the number of "counts" in the line, which is a measure of its intensity.

Make these measurement after the applet stops generating virtual photons. Start at a temperature of 12,000 degrees and then increment down by 1000 degrees. Click on the Add Measurement tab in the measurements sub-window to record your data.

You should observe in your table that the number of counts in your line strongly decreases as the temperature decreases. In this way, the strength of certain absorption lines in a stellar atmosphere provides an indicator of temperature. Earlier we learned that stellar color also provides an indicator of temperature.

Finally note that in the case of Hydrogen, once you get below a certain temperature (about 6000), no absorption lines will be created, no matter how long you wait.




Fire up the applet here



Second interactive activity: Now you will try to sleuth out which chemical elements represent AX and BX in the virtual stellar atmosphere. Our spectrograph is limited in the sense that it can only detect 4 absorption lines. The strengths of those lines in a stellar atmosphere will be different than in the laboratory environment on the earth, but the wavelengths will be the same.

To successfully determine what AX and BX are you will have to measure the wavelengths of the 4 lines that appear on your detector and then compare those numbers to the Spectra of the Elements . The wavelengths of the lines you measured in the virtual atmosphere should be identical to the wavelengths of the lines in the elements (but of course there are more than just 4 spectral lines for the elements!). The only hint I will give you is that AX and BX have atomic numbers less than 20.

Also, make sure you use a temperature of 10,000 degrees. IF its too cold, there will be no line formation.