CALCULATE BOLOMETRIC MAGNITUDE (BIG "M")

Now we calculate the absolute bolometric magnitudes (M) from the luminosities (L) and distances (D). The formula is given in Lesson 4. The bolometric magnitude includes radiation from the entire spectrum. It is a better way to compare stars' total energy output than visible magnitudes for the following reason: a cool (redder) star emits proportionately more radiation in the IR than a hotter star like the sun. In the visible, the hotter star would seem brighter in relation to the cooler star than if you compared the two in the infrared. Bolometric magnitude adds up the contributions from infrared, visible, UV, etc.

We will do our calculation by entering the appropriate equation into the cell in the spreadsheet where we want to see the answer. You can find tips on entering formulas here. Calculate the absolute bolometric magnitude for both the brightest and nearby stars (upper and lower sections of the spreadsheet.) Refer to Excel's pretty good online documentation to see how to do a log function in a formula¹. If you are feeling confident use the "round" function to truncate the results to just 1 or 2 numbers after the decimal point.

As you enter formulas, it is very important that you have saved your file in a spreadsheet format besides "Delimited text", otherwise your formulas may vanish everytime you hit the save button, just leaving the numbers that the formula calculates behind. (this is a problem if you need to go bakc later and change the formula!) You can do this with the "Save as . . ." command in the pull-down File menu and saving as a "worksheet" or "workbook" or "spreadsheet." Avoid formats that contain the words "text" or "delimited" in their description. Once you have saved the file this way once, you can just use the regular save command from then on.

MAKE AN H-R DIAGRAM

Now make an HR diagram plotting Absolute Magnitude along the y-axis and temperature along the x axis.Use different symbols for the nearby stars and the brightest stars. For information on how to make a chart and customize it click here. Use the scatter chart format and don't connect the dots and a sensible scaling and limits on the axes. Also reverse the values on the axes to standardize your HR diagram. If the layout of the stars does not look something like the layout in the HR diagrams in the book, scrutinize youre axis labels to see what is different.

On a hard copy of your chart, circle and label the following:

1. The brightest stars
2. The nearest stars
3. Red Giants
4. White Dwarfs
5. Asymptotic Giant Branch (AGB) stars
6. Solar type stars

Making charts come out "just right" in Excel can be a pain so don't kill yourself doing so! If the only problem with your chart is you can't get the labels or axes just so and you are plenty fed up with the process, write next to the chart how you would have fixed it if only you could have figured out Excel.