These are some suggested answers for the questions asked on the student sheet.
- What variables does the HR Diagram compare?
Luminosity and temperature
- Describe why it is possible to relate the temperature of a star to its luminosity, or brightness.
Stars that are hotter should be brighter than stars that are cooler.
- Why is measuring the luminosity of stars difficult?
The distance of stars relative to earth makes it difficult to measure brightness since a bright star that is more distant might appear to be as bright as a more dim star closer to earth.
- What star serves as a standard of comparison against which the luminosity of other
stars is measured?
- Sketch and label the HR Diagram. Make sure to label the axes, the main sequence, white dwarves, giants, and super giants.
Students should sketch the HR Diagram, from the website: http://cse.ssl.berkeley.edu/segwayed/lessons/startemp/l6a.htm
- What do the letters at the bottom axis of the HR Diagram represent?
The class of star that corresponds to the color we observe from earth.
- What are the four important things to note about the HR Diagram?
Most of the stars in the solar neighborhood fall on a well defined “Main Sequence”; there are very few “red giants”; there are very few “blue supergiants”; and there are a few faints stars near the bottom left of the diagram, which are white dwarfs.
- Are blue stars hotter or cooler than red stars?
They are hotter.
- If a star has a luminosity of ten thousand (10,000), how many times brighter is it than the sun?
- How does the brightness of white dwarfs relate to that of the sun?
White dwarfs are less bright than the sun.
- Describe the general trend between temperature and luminosity that the Main Sequence shows.
A hotter star should be brighter than a cooler star.
- Why do giants differ from stars in the Main Sequence?
Although they are cool [red], they are very luminous, and therefore bright. In the Main Sequence, stars that are cool are not as luminous.
- How do white dwarf stars differ from stars in the Main Sequence?
White dwarf stars are very hot [blue], but dim because they are so small.
- Describe stars A, B, C, and D in terms of their brightness and temperature.
Star A is red and therefore, cool. Its luminosity is 1/1000 of that of the sun; therefore, it is dim. Star B is a hot, blue star and very luminous. Both A and B are on the Main Sequence. Star C is also a hot, blue star. However, it is very dim and therefore off the Main Sequence. Star D is a cool, red star but very luminous; it is also off the Main Sequence.
- The sun is a Main Sequence star. The HR Diagram tells us the luminosity of the sun. Using this information and the HR Diagram, determine what the sun's class might be as a star.
The sun is a G star.
This teacher sheet is a part of the How Old Are the Stars?