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:

The spectrum of a galaxy is the sum of the light of all the individual stars. In this exercise there is a galaxy spectrum as the bottom spectrum The user can select 4 stars to add to together, from the library of stars that we have already been working with. As the spectra are added, they are normalized by their V-band luminosity. The percentage contribution to the integrated spectra by the selected stellar type is given as the bottom number in the rightmost green box. A multiplier lets you add as many objects of that stellar spectral type as you want.

Here is an example:

The default spectrum is for NGC 1357. How can that be reproduced?

Let's try using the following 4 spectral types:

• A1-3V
• F8-9V
• G6-8 V
• K7 III

So go ahead and set the 4 types to be those stars and leave the multiplier as 1.

You will see that's a reasonably good fit (qualitatively to the galaxy spectrum at the bottom - you are comparing the 5th and 6th spectra now; the fifth is the result of adding the 4 above in the correct normalized proportion).

Notice however, that stellar type 2 (F8-9V) contributes only 2% of the light and the next stellar type (G6-8V) only contributes 1%.

Let's multiply the contribution of the F stars by 10 by changing the multiplier from 1 to 10. Now you can see that their contribution to the integrated light has risen to 19%.

So this applet is meant to allow you to add together different stellar types in different proportions to try and reproduce the features of available galaxy spectra (Rob Kennicutt's data).

One final feature:

If you go to the bottom spectra and click on the mouse and drag the mouse you will see a line appear on that spectra and the composite spectra above. In this way you can roughly ty to see if the slopes of the continuum are the same between the two.

Also the left most of the pair of green boxes with the green vertical line - that can be dragged to other wavelengths and the bottom of the two numbers records the actual counts of the individual stars fkuxed spectra if you wan to say compare contributions of the individual stars at blue vs red wavelengths. Also Ignore the number in the middle gray panel as well as the vertical blue line on the left side. (these are used if you want to measure equivalent widths of the synthetic vs real spectra in the same way that you already measured equivalent width for the stellar spectra)