Student Manual PAGE 10 Because not all spectral types are represented in the Atlas, and because you want to get the classification precise to the nearest 1/10 of a spectral type (i.e. G2, not just G), you may have to do some interpolation.  Look at the relative strengths of the absorption lines to do this.  For your unknown star, for instance, you should note that it looks most like an A0 type star, but not quite.  When the top panel shows an A1 comparison star, the bottom panel will show a A5 star.  The strength of the lines in HD124320 lies somewhere between these two.  You can therefore make an educated guess that it is about A3. 7.  If you want to do this in a more quantitative fashion, click on the button labeled difference to the right of the spectrum display.  The bottom panel graph will now change, showing the digital difference between the intensity of the comparison spectrum at the top and the unknown spectrum in the center, with zero difference being a straight horizontal line running across the middle of the lower panel. Look at the dips and valleys on this bottom panel and think about them for a minute.  If an absorption line in the comparison star is shallower than the line at the same wavelength in the unknown star, then intensity at those wavelengths in the comparison star will be greater than those in the unknown.  So the difference between the two intensities will be greater than zero, and the difference display will show an upward bump.  If the top panel is showing an A0 spectra, for instance, and the middle panel HD124320, you should see a small bump at 3933 Å, indicating that the absorption line in the unknown is deeper than that in the A0. By the same reasoning, if an absorption line in the comparison spectrum is deeper than one in the unknown star, then the difference display will show a downward dip.  Click on the Standards down button to display an A5 comparison spectrum.  Note that the 3933 Å difference display now shows a dip, indicating that the absorption line in the unknown is shallower than that of an A5.  So it is somewhere in between A0 and A5. To use the difference display, page through the comparison spectra (using the Up and Down buttons) until the difference between the comparison and unknown star is as close to zero at all wavelengths as possible.  To estimate intermediate spectral types, watch to see when the display changes from bumps for some lines, to dips (Since some lines get stronger with temperature, and others get weaker, you will see some lines go from bumps to dips, and some from dips to bumps, as you change comparison spectra).  Try to estimate whether the amount of change places the unknown halfway between those two comparison types, or if it seems closer in strength to one of the two comparison types that it lies between. Your estimate of the spectral type of HD124320_________________. Give reasons for your answer.  ( For this example:  The strength of lines at 4340.4 Å and 4104 Å are almost exactly those of type A1 or A5, and the strength of the 3933 Å line lies somewhere between them.). 8.  Record your choice for HD 124320 and your reasons in the computer by dragging the menu RESULTS to the choice RECORD.  This opens up a window on which you can record your assigned spectral type and a brief note on your reasons.  As in the Login form, you can enter data by Tabbing to the proper box, or clicking the mouse to position the cursor in a box.   When you are done recording the classification, click OK.  You can choose RE- VIEW in this menu if you later want to edit or revise your entry. 9.  You have used one or two spectral lines for making a refined classification.  But what elements produced them? For reference, you will want to identify the source of the line you are looking at.  Select the SPECTRAL LINE TABLE from the LOAD pull-down menu.  You will see a window containing a list of spectral lines.  (see FIGURE 4) You can move this list over to either side of the display by pointing the cursor to the blue region of the list window and dragging it over .  Try moving it to the upper left.  Now, using the mouse, point the cursor at the center of any line in the spectrum (say the one at 4341) and double click the left-hand button.  A red line should appear across the screen and, if you’ve centered correctly,  a double dashed line on the line list to show you what the line is.