Hertzsprung-Russell Diagram

Activity:
Plotting near stars and bright stars on a color magnitude diagram

Science Standards:
Energy & Matter, Systems & Interactions, Stability & Equilibrium

Objectives:

  • The student will describe differences between Near and Bright stars visible.
  • The student will discern a relationship between temperature and brightness.
  • The student will correlate the evolution of stars to the HR Diagram plotted.
  • Materials:

  • Photocopies of Bright Stars Table
  • Photocopies of Near Stars Table
  • Photocopies of Student Answer Sheets
  • Graph Paper
  • Two different colored pencils/pens



  • Introduction:
    There are essentially two ways to describe the brightness of stars; apparent magnitude and absolute magnitude. Apparent magnitude is the brightness of a star as seen from Earth; absolute magnitude is how bright a star would be if it were 32.6 light years away. For historical reasons, astronomers still call the brightest stars magnitude 1 stars, the next brightest 2, then 3, 4, 5, and on down to the dimmest naked-eye stars, magnitude 6 stars. This backwards scale is now defined as mathematically as:

    brightness of star 1 = difference in magnitude
    brightness of star 2 (2.512)

    Using this sort of scale, we can actually describe the brightness of stars in real numbers. Really bright objects, like the full moon have a negative magnitude.

    Object apparent magnitude
    Sun -26.5
    Full Moon -12.5
    Venus -4.1
    Sirius -1.4
    North Star 2.0
    limit of naked eye 6
    Pluto 15
    limit of HST 28
    Astronomers also classify stars by their type of spectrum. Stars are easily classified by the amount of hydrogen which they contain as revealed to us by the intensity of their spectrum. Stars with the most hydrogen are called A stars, the remaining letters designate varying amounts of different chemical stellar compositions.
    If we consider the temperature of stars, as revealed to us by their color, we find that the O stars, are hottest, followed by B stars, A stars, F, G, K, and M stars. A star which has an intermediate temperature between an O star and a B star is called an O5 star. This layout of temperature is given from hottest to coolest as:

    O0, O1, O2, O3, O4, O5, O6, O7, O8, O9, B0, B2, B3, B4, B5, B6......

    Astronomers sometimes humorously teach their students to remember this sequence from hottest to coolest as:

    O h, b e a f ine g uy (or gal), k iss m e!

    Our Sun is G2 star, which means that it is a rather intermediate temperature star.

    Directions:
    1. Plot most or all of the Near Stars on the HR Diagram.
    2. Plot most or all of the Bright Stars on the HR Diagram using a different color.
    3. Compare the attributes of Near Stars and Bright Stars.
    4. Characterize the nature and relative number of stars in our galaxy.
    5. Summarize the evolution of a star.
    6. Complete the Student Answer Sheets.

    The Life-Line of the Stars - Student Answer Sheet

    Description: Using the concepts of stellar brightness and color, you will graphically analyze the relationships between bright stars, near stars, temperature. This activity will culminate in the a description of how stars change over time.

    Materials: graph paper (provided)
    2 different colored pencils, pens, or crayons
     

    Introduction: When you look up at the stars of the night-time sky, you will immediately notice that all of the stars do not have the same apparent brightness. What are several possible explanations for this observation?
     
     
     
     
     
     

    The brightness that a star has as seen from the Earth is called the apparent brightness . Stars which are very bright are called magnitude 1 stars . The next brightest are magnitude 2 stars. Then comes magnitude 3, 4, 5, and down to the very faintest stars visible with the naked eye, magnitude 6 stars.

    By observing stellar parallax, we know that all stars are not the same distance away. Obviously, a star which is far away will appear dimmer than a similar star which is closer. We adjust for the distance of stars by giving stars an absolute brightness . The absolute magnitude of a star is the apparent magnitude that a star would have at a distance of 32.6 light years (10 parsecs).

    Table 1 of this activity contains a list of 37 very bright stars. Table 2 of this activity contains a list of 30 stars near the Earth. Also provided on the table is the spectral class of each star (recall that the spectral class of each star tells us both the star's surface temperature and the amount of hydrogen it contains).

    Procedure:
    1. Using the list of Bright Stars in Table 1 and the list of Near Stars in Table 2, plot their absolute magnitude vs. spectral class on the attached graph. Use different colors for the near stars and for the bright stars. (Ask your instructor if you should plot all of the stars listed.)
     

    2. What general trends or concentrations do you see in the data? Are there generalizations you can make about bright stars? Any generalizations about near stars?
     
     
     
     
     
     

    3. This diagram was first published by Henry Norris Russell and Ejnar Hertzsprung in the early 1900's. It is sometimes called a color - magnitude diagram. Why is this ( or why is this not) an appropriate name it is a plot of magnitude versus spectral class?
     
     
     
     
     
     
     

    4. Our star, the Sun, is a G2 spectral class star with an absolute magnitude of 4.8 . How does it compare to the locations of the Near Stars on the diagram?
     
     

    How does it compare to the locations of the Bright Stars on the diagram?
     
     
     

    5. Which spectral class is most common?
     
     

    6. Which spectral class is the least common?
     

    7. In general, what is the relationship between the temperature of a star and its brightness?
     
     
     

    8. Most of the stars seem to be along a line from the upper left corner to the lower right corner of the HR Diagram. Stars which fall into this category of stars are called main sequence stars . Does our Sun fit into this category?
     
     
     

    9. Consider the rogue stars in the upper right hand corner and the lower left hand corner. What are the bright cool stars? What are the hot dim stars? (Compare your answer with the adjacent group.)
     
     
     
     
     
     

    10. Stars which are "on the main sequence" are generally very stable stars which are combining their hydrogen atoms into larger helium atoms (this reaction is called fusion and gives off energy). Where in the star do you think that this fusion reaction is most likely occurring? Why?
     
     
     
     

    Main sequence stars which are very bright are fusing hydrogen atoms into helium atoms at an enormous rate. Do you think that these bright stars will burn forever? How long do you think these stars will shine compared to the dimmer main sequence stars?
     
     

    11. Why is it that black holes do not appear on the HR Diagram?
     

    12. As a group and as a group, discuss what you think might occur when a star depletes its supply of hydrogen.
     
     

    Table 1 Bright Stars
    (Stars which appear very bright from the Earth)
    Star Name
    Spectral Class
    Absolute Magnitude
    Star Name
    Spectral Class
    Absolute Magnitude
    1. Sirius A
    A1
    +1.4
    19. Aldebaran A
    K5
    -0.2
    2. Sirius B
    B8
    +11.5
    20. Aldebaran B
    M2
    +12
    3. Canopus
    F0
    -3.1
    21. Crucis A
    B1
    -4.0
    4. Centaurus A
    G2
    +4.4
    22. Crucis B
    B3
    -3.5
    5. Centaurus B
    K5
    +5.8
    23. Antares A
    M1
    -4.5
    6. Arcturus 
    K2
    -0.3
    24. Antares B
    B4
    -0.3
    7. Vega
    A0
    +0.5
    25. Spica
    B1
    -3.6
    8. Capella A
    G0
    -0.7
    26. Pollux
    K0
    +.08
    9. Capella B
    M0
    +9.5
    27. Fomalhaut A
    A3
    +2.0
    10. Capella C
    M5
    +13.0
    28. Fomalhaut B
    K4
    +7.3
    11. Rigel A
    B8
    -6.8
    29. Deneb
    A2
    -6.9
    12. Rigel B
    B9
    -0.4
    30. Beta Crucis
    B0
    -4.6
    13. Procyon A
    F5
    +2.7
    31. Regulus
    B7
    -0.7
    14. Procyon B
    F0
    +13.0
    32. Adhara
    B2
    -5.0
    15. Achernar 
    B5
    -1.0
    33. Castor A
    A1
    +2.1
    16. Beta Centari
    B1
    -4.1
    34. Castor B
    A5
    +2.9
    17. Betelgeuse
    M2
    -5.5
    35. Castor C
    K6
    +8.8
    18. Altair
    A7
    +2.2
    36. Shaula 
    B1
    -3.3
    -
    -
    -
    37. Bellatrix 
    B2
    -4.2
     
    Table 2 Near Stars
    (Stars which are close to the Earth)
    Star Name
    Spectral Class
    Absolute Magnitude
    Star Name
    Spectral Class
    Absolute Magnitude
    1. Sun
    G2
    +4.8
    16. Procyon A
    F5
    +2.7
    2. Centari A
    G2
    +4.4
    17. Procyon B
    F0
    +13.0
    3. Centari B
    K5
    +5.8
    18. Struve 2398
    M4
    +11.1
    4. Centari C
    M5
    +15.0
    19. Struve 23948
    M5
    +11.9
    5. Lalande 21185
    M2
    +10.5
    20. Groom 34 A
    M1
    +10.5
    6. Sirius A
    A1
    +1.4
    21. Groom 34 B
    M6
    +13.2
    7. Sirius B
    B8
    +11.5
    22. Lacaille 9352
    M2
    +9.6
    8. Ross 154
    M4
    +13.3
    23. Tau Ceti 
    G8
    +5.7
    9. Ross 248
    M5
    +14.7
    24. BD +5 1668
    M4
    +11.9
    10 Epsilon Eridani
    K2
    +6.1
    25. Lacaille 8760
    M0
    +8.7
    11. Luyten
    M5
    +14.7
    26. Kapteyn's Star
    M0
    +8.7
    12. Ross 128
    M5
    +13.8
    27. Krueger 60 A
    M3
    +11.8
    13. 61 Cygnus A
    K5
    +7.5
    28. Krueger 60 B
    M4
    +13.4
    15 61 Cygnus B
    K7
    +8.3
    29. Ross 614
    M5
    +13.1
    15. Epsilon Indi
    K5
    +7.0
    30. BD -12 4523
    M4
    +12.0


    Note: Several of the star names appear twice followed by an A or a B. These are mostly double stars - two stars which are revolving around each other together in space. They seem to frequently have similar spectral types. Can you guess why they have similar spectral types?
     
     
     

    INSERT Hertzsprung Russell Graph

    FIGURE 107

    HERE