Instructions
Access the simulator & provide the answers to the following
questions.
Typed work needed
HR Diagram Explorer:
https://astro.unl.edu/naap/hr/animations/hr.html/n Lab 11: HR Diagram
Background Information
Work through the background sections on Spectral Classification, Luminosity, and the
Hertzsprung-Russell Diagram. Then complete the following tables.
Table 1: The table below summarizes the relationship between spectral type, temperature, and color
for stars.
Surface
Temperature K
Spectral Type
Color
M1
M5
4560 K
G2
6100 K
F5
8590 K
13000 K
09
40000K
Table 2: Complete the following table related to stellar luminosities in solar units using the
equation L × R²T4. Note that Ro represents the radius of the Sun, To its temperature and Lo
its luminosity.
Radius (R)
Temperature (To)
Luminosity (LO)
1
1
1
2
2
1
1
1/3
10
1
1
10 Table 3: Complete the following table relating luminosity and mass in solar units for main
sequence stars using the equation L × M³.5
Mass (Mo)
Luminosity (Lo)
1
2
5
3160
0.1
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 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.
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 sliders
and note the corresponding change in the active location. Table 4: Check the appropriate region of the HR diagram corresponding to each description
below.
Description
Hot stars are found at the:
Cool stars are found at the:
Luminous stars are found at the:
Dim stars are found at the:
Top Right Bottom Left
Drag the active location around on the HR Diagram once again. This time focus on the Size
Comparison panel.
Table 5: Check the appropriate region of the HR diagram corresponding to each description
below.
Description
Upper
Left
Upper Lower
Right Right
Lower
Left
Large Blue stars are found at the:
Small Red stars are found at the:
Small Blue stars would be found at the:
Really Large Red stars are found at the:
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. Table 6: Fill the table with brief explanation of the location of the described stars referring to the
equation L × R²T4.
Description
Large Blue stars
Small Red stars
Small Blue stars
Really Large Red stars
Reason for location
For the next exercise you will have to change the luminosity or the temperature of the star in order
to keep it on the main sequence, an identify the star's approximate radius.
Table 7: Fill the table with the correct values to keep the star on the main sequence. Use To=5800K
Temperature (To) Luminosity (L)
Radius (R)
0.5
0.7
1
1
1
2
15
6.5 For the last exercise explore the luminosity classification by checking show luminosity classes. The
green region (labeled 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. The other highlighted regions are giants, supergiants, white dwarfs.
Move the active cursor up and down each region and explore the different values of, luminosity,
temperature and stellar radius.
Table 8: Fill the table with statements describing the stars that belong to each class. (Example:
Stars in this class have high/low luminosity, high/low temperature, big/small radius)
Description
Giant
Supergiant
White dwarf
Luminosity, temperature and radius.
Final Questions
1. 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?
2. Describe the masses of stars along the main sequence. What can you conclude?
3. Can a star with a radius equal 1000 Ro be on the main sequence? Explain your answer.
4. Which is larger a blue supergiant or a red supergiant? Explain.
5. Which is larger a white dwarf or a red dwarf? Explain.
