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Vocabulary:

The development of the H-R Diagram began with Danish astronomer Ejnar Hertzsprung who began plotting the stars around 1911.  American astronomer Henry Norris Russell independently developed his own diagram.  These two scientists independently discovered that comparing magnitudes and spectral class (color) of stars yielded a lot of information about them.  Together, they created a diagram on which they mapped stars by magnitude and spectral class.  The result is the picture below.

(click for larger image)

After the astronomers had completed graphing the stars, they noticed that several patterns appeared.  First, they noticed that ninety per cent of the stars fell along a diagonal line from the top-left corner to the bottom-right corner.  These are called main sequence stars, of which our Sun is a member.  Another pattern they noticed was that the Cepheid’s (class of variable stars that brighten and dim in a regular fashion); giants; super-giants and dwarfs fell into groupings quite separate from the main sequence stars.  The white dwarfs were on the bottom-left; the red super-giants were in the upper-right; red giants were on the diagonal that those two made; blue giants were slightly to the right of the start of the main sequence; Cepheid’s were in the upper middle.

Laboratory Activity

Objective:

Investigate the relationship between stars temperature, brightness and diameter.

Background:

The H-R diagram is a graph of star brightness versus star temperature.  When many stars are plotted on an H-R diagram, it is found that they fall into groups.  These groupings indicate star sizes and are clues to how the stars change during their lifetime.  The measure of star brightness used in the H-R diagram is termed absolute magnitude.  A star’s absolute magnitude is not affected by its distance from Earth.  The smaller the absolute magnitude, the brighter the star.  The very brightest stars have negative magnitudes.

Materials:

Pencil, graph paper

Procedure:

1. Using the graph below, plot the stars from Group 1.
2. Once you have plotted the stars from Group 1, answer the following questions.  Label this group of questions as “Group 1 Questions.”
   1. What would you tell someone who thinks that all stars are very similar (be sure to discuss temperature and brightness)?
   2. How does our sun compare to other stars in brightness and temperature?
   3. Are the stars scattered randomly on the graph, or is there a pattern?  Explain.
   4. Would you expect hotter stars to be dim or bright?  Does the graph agree with this answer?
3. Using the same graph, plot the stars from Group 2.
4. Once you have plotted the stars from Group 2, answer the following questions.  Label this group of questions as “Group 2 Questions.”
   1. Do the Group 2 stars follow the same pattern as the Group 1 stars that you plotted?  Explain.
   2. Overall, are the stars in Group 2 very bright or very dim?
   3. Are these stars hot or cool compared to other stars?
   4. Is the relationship of brightness to temperature for these stars puzzling, or does it make sense?  Explain.
5. Using the same graph, plot the stars from Group 3.
6. Once you have plotted the stars from Group 3, answer the following questions.  Label this group of questions as “Group 3 Questions.”
   1. Compare the areas of the graph where the Group 2 and Group 3 stars are plotted. How are they different?
   2. Overall, are the stars in Group 3 very bright or very dim?
   3. Are these stars hot or cool compared to other stars?
   4. Is the relationship of brightness to temperature for these stars puzzling, or does it make sense?  Explain.
7. Conclusion – you may wish to consult your textbook and use the internet to assist in answering the following questions.
   1. As you can see from the Group 1 stars, the cooler or hotter a star is, the brighter the star.  The Group 2 and Group 3 stars do not follow this pattern.  Hence, there must be something besides temperature that can affect the brightness of stars.  Describe your own theory about these stars (Group 2 and Group 3).  Why would their brightness not be strictly related to their temperature?
   2. What is the "Main Sequence?"
   3. Label the Main Sequence on your H-R Diagram.
   4. What percent of all stars are on the Main Sequence?
   5. Label “dwarfs" and "giants" on your H-R Diagram.
   6. Explain the process of Nuclear Fusion.
   7. Why is the process of nuclear fusion important?
   8. Summarize the history and probable future of our sun (a main sequence star).  How did it begin and how will it end its life cycle? Be sure to include the following terms in your discussion: nebula; fusion; gravity; giant; white dwarf.
   9. Define the following terms: super-giant; supernova; neutron star; black hole.
   10. What determines if a star will end its life as a white dwarf, a neutron star or a black hole?
   11. At the beginning of the universe, scientists believe it contained only what two elements?
   12. Where were all of the other elements formed?
   13. Why aren’t the Group 2 and Group 3 stars not on the Main Sequence?

   

   You may wish to print this graph for your final copy of your lab report or you may use regular graph paper.  First, click on it to enlarge.  Then simply right click on it; save it and then print.  Remember to use the proper format when writing up this lab.

   <=====  Graph used for lab.

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