Sci - Lab

PHYS 105 Unit 6 Lab

Observation – H-R Diagrams

Open the HR Diagram Explorer at https://astro.unl.edu/naap/hr/animations/hr.html. This is a Flash based program, so make sure you can run it – tips on getting it to work here.

Part 1 - HR Diagram Explorer An actual HR Diagram is provided in the upper right panel with an active location indicated by a red x. This active location can be dragged around the diagram. The options panel allows you to control the variables plotted on the x-axis: (temperature, B-V, or spectral 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 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 active location on the HR Diagram. Note that the size of the sun remains constant.

• Question 1 - 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 sliders and note the corresponding change in the active location.

• Question 2 - 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.

• Question 3 and 4 - Drag the active location around on the HR Diagram once again. This time focus on the Size Comparison panel.

• Question 6 - Uncheck show luminosity classes and check show instability strip. Note that this region of the HR Diagram indicates where pulsating stars are found such as RR Lyrae stars and Cepheid variable stars. These stars vary in brightness because they are pulsating – alternately growing bigger and smaller – which changes their radii and surface temperatures and resulting their luminosities.

• Question 7 - Check the plotted stars option the nearest stars. You should cover their range of temperatures, colors, luminosities, and sizes.

Questions 1. Use the HR diagram to describe which part of diagram you would find each of these types of

stars: a. Hot stars are found _______ b. Faint stars are found ________ c. Luminous stars are found ________ d. Cool stars are found _______

2. Use the HR diagram to describe which part of diagram you would find each of these types of stars:

a. Large Blue stars are found ______ b. Small red stars are found ______ c. Small blue stars are found ______ d. Really huge red stars are found _______

3. 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?

4. What can you conclude about the masses of stars along the main sequence? 5. Where does our Sun lie on the main sequence relative to the other stars? 6. 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?

7. Describe the characteristics of the nearest stars in 50 words in terms of mass, luminosity and spectral class.

8. Do you think these stars are rare or very common among all of the stars of our galaxy? Explain your reasoning. Are any assumptions involved in your reasoning?

9. 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.)

a. Student A: “I think it’s because these stars must be very close to us. That would make them appear brighter to us in the sky.”

b. Student B: “I think it’s because these stars are very luminous. They are putting out a tremendous amount of energy.”

c. Student C: “I think it’s because these stars are very close and very luminous.” 10. Do you think that these bright stars are very common (make up a large percentage of all stars

in general)? Explain your reasoning.