This activity is in chapter 9 of the HOA manual and comes right after an explanation of stellar classification and the Hertzsprung-Russell Diagram.

Plotting an H-R Diagram

The Hertzsprung-Russell diagram is a graph that plots a star's absolute magnitude versus its temperature (spectral class). Below are a list of 25 of the brightest stars and a list of 25 of the nearest stars. Set up a graph of an H-R diagram with an appropriate scale and plot the 50 stars, using different symbols or colors to differentiate the two lists on the diagram. Then answer the questions about the stars that you have plotted.

* Absolute magnitudes of all stars except the Sun were calculated from parallax measurements and apparent magnitude (Hp) measurements taken from The European Space Agency, et al., The Hipparcos and Tycho Catalogues (17 Vols.), Noordwijk, The Netherlands: ESA Publications Division, 1997. ISBN 92-9092-399-7 (Vols. 1-17).

* Absolute magnitudes of all stars except the Sun were calculated from parallax measurements and apparent magnitude (Hp) measurements taken from The European Space Agency, et al., The Hipparcos and Tycho Catalogues (17 Vols.), Noordwijk, The Netherlands: ESA Publications Division, 1997. ISBN 92-9092-399-7 (Vols. 1-17).

QUESTIONS ABOUT THE H-R DIAGRAM GRAPH:

1. Label the following branches of the H-R diagram: Main Sequence, Giant, Supergiant, White Dwarf. Calculate the percentage of stars that occupy each branch, and briefly describe, from the information on the two axes, the types of stars that occupy each branch.

2. Which stars are similar in magnitude and spectral class to the Sun?

3. Which list (brightest or nearest) gives the more typical example of the star population of the Milky Way Galaxy? Explain your reasoning.

4. Write a brief statement describing the relationship between absolute magnitude and stellar classification (temperature) of main sequence stars.

5. Does the above relationship hold for stars occupying the other branches of the H-R diagram?

6. Can you describe any relationships between absolute magnitude and stellar classification for any of the non-main sequence branches? Explain why you can or cannot.

7. Would you expect the same percentage of stars to occupy each branch of the diagram if all stars within the Milky Way Galaxy were plotted? Explain why or why not.

8. Why are there places on the diagram where no stars exist?

9. The next activity involves plotting variable stars on the same H-R diagram that you have constructed. Where do you think stars that vary in magnitude will end up on the diagram? Will they occupy one or more of the existing branches? Some of the empty places? Both? Write down your prediction, along with your reasoning. How would you plot a star that changes in brightness? What exactly is changing in these stars besides brightness?