HR diagram-lab online-1.docx

NAME______________________ CLASS_________ Instructions: Go to web site http://astro.unl.edu . Click on the Nebraska astronomy ap- plet project and then go to NAAP Modules(at top of screen) and pick HR Diagram. Read the materials and complete the guide below and complete the exer- cises and complete the document below—the background materials will help you answer the questions—the flash demonstration will help you com- plete the rest. ON LINE LAB 10 Nebraska Astronomy Applet Project Student Guide to the HR DIAGRAM _____________________________ NAAP – HR Diagram Explorer 1/10

HR Diagram – Student Guide Pretest Score: Background Information Work through the background sections on Spectral Classification, Luminosity, and the Hertzsprung-Russell Diagram. Then complete the following questions related to the background information. Star Surface Tempera- ture K Spectral Type Color Betelguese 3,530 M2 Orange Arcturus 4,300 K5 Yellow The sun 5,830 G2 Yellow Procyon A 6,530 F5 Yellow-White Sirius A 9,145 A1 White Rigel A 11,000 B9 White-Blue Delta Orionis 33,200 O9 Blue 1.The table below summarizes the relationship between spectral type, temperature, and color for stars. Note that the surface temperature of the stars in the table increases. NAAP – HR Diagram Explorer 2/10

2.Complete the following table related to stellar luminosities in solar units using the equation . Radius (R ⊙ ) Temperature (T ⊙ ) Luminosity (L ⊙ ) 1 1 1 1 2 16.8 3.02 1 9 1 1/2 0.057 3.The mass luminosity relation describes the mathematical relationship between lumi- nosity and mass for main sequence stars. It describes how a star with a luminosity of 2 M ⊙ would have a luminosity of 11.31 L ⊙ while a star with luminosity of 3,160 L ⊙ would have an approximate mass of 10 M ⊙. HR Diagram Explorer Open the HR Diagram Explorer . Begin by familiarizing yourself with the capabilities of the Hertzsprung-Russell Diagram Explorer through experimentation. · An actual HR Diagram is provided in the upper right panel with an active location indi- cated by a red x. This active location can be dragged around the diagram. The options panel allows you control the variables plotted on the x-axis: (temperature, B-V, or spec- tral type) and those plotted on the y-axis (luminosity or absolute magnitude). One can also show the main sequence, luminosity classes, isoradius lines, or the instability strip. The Plotted Stars panel allows you to add various groups of stars to the diagram. · The Cursor Properties panel has sliders for the temperature and luminosity of the active location on the HR Diagram. These can control the values of the active location or move NAAP – HR Diagram Explorer 3/10

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in response to the active location begin dragged. The temperature and luminosity (in solar units) are used to solve for the radius of a star at the active location. · The Size Comparison panel in the upper left illustrates the star corresponding to the ac- tive location on the HR Diagram. Note that the size of the sun remains constant. NAAP – HR Diagram Explorer 4/10

Exercises · Drag the active location around on the HR Diagram. Note the resulting changes in the temperature and luminosity sliders. Now manipulate the temperature and luminosity slid- ers and note the corresponding change in the active location. Description Top Right Bottom Left Hot stars are found at the: x Faint stars are found at the: x Luminous stars are found at the: x Cool stars are found at the: x 4. Check the appropriate region of the HR diagram corresponding to each description below. · Drag the active location around on the HR Diagram once again. This time focus on the Size Comparison panel. 5.Check the appropriate region of the HR diagram corresponding to each description below. Description Upper Left Upper- Right Lower Right Lower- Left Large Blue stars are found at the: x Small Red stars are found at the: x Small Blue stars would be found at the: x NAAP – HR Diagram Explorer 5/10

Really Large Red stars are found at the: x · Check show isoradius lines. Note that at each point on a green line, stars have the same value of radius. Use these isoradius lines to check your answers in the table above. 6.The equation below describes the luminosity of a star in terms of its radius and temperature. Use this equation to explain the results you found in the table of the previous question. · In addition to the isoradius lines, check show luminosity classes . This green region (dwarfs V) is known as the main sequence and contains all stars that are fusing hydrogen into helium as their primary energy source. Over 90% of all stars fall in this region on the HR diagram. Move the active cursor up and down the main sequence and explore the different values of stellar radius. 7.Describe the sizes of stars along the main sequence. What are stars like near the top of the main sequence, the middle, and the bottom? At the top and left of it in the main sequence starts are going to be bigger and hotter. As the stars going down towards the lower right hand side it becomes colder and smaller/ · The background pages of this module talked about the mass-luminosity relationship for stars on the main sequence: NAAP – HR Diagram Explorer 6/10

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8. What can you conclude about the masses of stars along the main sequence? The higher the luminosity of the star tiger the mass is as well. 9.Use the results from the previous 5 questions to construct a “conceptual” HR Diagram. You simply want to draw arrows showing the direction in which variables are increasing. a) Draw in an arrow on the y axis showing the direction of increasing “intrinsic luminosity” of the stars. (This is complete for you.) b) Draw in an arrow on the x-axis showing the direction of increasing surface temperature of the stars. c) Draw in an arrow showing the direction of increasing radius on the diagram. (hint: thus must be perpendicular to the isoradius lines.) d) Draw in an arrow showing the direction of increasing mass form main sequence stars on the dia- gram. (Note that his arrow only applies to main sequence stars, but that is over 90% of stars.) NAAP – HR Diagram Explorer 7/10 L

Figure 1: Conceptual HR Diagram NAAP – HR Diagram Explorer 8/10

· Uncheck show luminosity classes and check show instability strip . Note that his region of the HR Diagram indicates where pulsating stars are found such as RR Lyrae stars and Cepheid variable stars. These stars vary in brightest because they are pulsating – alter- nately growing bigger and smaller – which changes their radii and surface temperatures and resulting their luminosities. 10.Describe the characteristics of stars that are found in the instability strip. You should cover their range of temperatures, colors, luminosities, and sizes. (Hint: Comparing them to the sun is useful.) Are variable stars necessarily on the main sequence? The stars found in the instability strip are larger, more luminous and hotter than the sun. · Check the plotted stars option the nearest stars . You should cover their range of tem- peratures, colors, luminosities, and sizes. 11.Describe the characteristics of the nearest stars. The nearest stars have a low temperature, low luminosity and are much smaller than the sun. 12.Do you think these stars are rare or very common among all of the stars of our galaxy? Ex- plain your reasoning. Are any assumptions involved in your reasoning? The stars are very common among all stars in our galaxy. There are many of them in a dense area, whereas the bigger stars seem to be fewer and more spaced out within the galaxy. · Uncheck the plotted stars option the nearest stars and check the brightest stars . Why are these stars are the brightest in the sky? Three students debate this issue: Student A: “I think it’s because these stars must be very close to us. That would make them ap- pear brighter to use in the sky.” Student B: “I think it’s because these stars are very luminous. They are putting out a tremen- dous amount of energy.” Student C: “ I think its because these stars are very close and very luminous.” 13.Use the tools of the HR Diagram to support the views of one of the three students. Why are the stars we perceive as bright in the night sky really bright?” (hint: You may find the options labeled both the nearest and brightest stars and the overlap useful.) The brightest stars are putting out a large amount of energy. By touting out a large amino of en- ergy the stars are able to shine brightly, more so than the common stars. NAAP – HR Diagram Explorer 9/10

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14.Do you think that these bright stars are very common (make up a large percentage of all stars in general)? Explain your reasoning. I do not believe that the brighter stars are very common, there are many other stars in the galaxy that far outnumber the bright stars. NAAP – HR Diagram Explorer 10/10