Module 3 – Lesson 7

Lesson 7: Stellar Types and Stellar Life Cycles

Overview

This lesson addresses the life cycle of a sun-like star and bigger, more massive, stars, from their birth to their death. Students go through the reading and related questions, and then get an opportunity to work with tactile diagrams of the H-R diagram and life-tracks of a sun-like star. We work through definitions, comparisons of humans with stars, and end with student comments about whether or not those comparisons are realistic. 

Learning Outcomes

• Summarize what is meant by “stellar evolution.”
• List the various stages of a star’s life. 
• Define nebula, red giant star, white dwarf, supernova, neutron star, pulsar, black hole. 
• List the different stellar types.
• Explain how the life track of a sun-like star differs from that of a much more massive star.

Materials

• Touch the Stars Braille book by Noreen Grice 
• Tactile H-R diagram (Figure 3.7.1)
• Tactile life-tracks of a sun-like star (Figure 3.7.2)
• Tactile stages of the Sun’s life (Figure 3.7.3)

Pre-assessment Questions and Discussion

Q. If we were to talk about the “life cycle” of a human, what would we say?
A. Humans are born; they eat and grow; they learn to walk and talk; they go to school; they grow into adults; they get married and have children; they get old; they die.

Q. What do you think about the lives of stars? Have they always been around? Do they grow old?
A. Accept all comments and exchange of ideas.

Text

• Students read The Life Cycle of a Star in Touch the Stars, pp. 74-77 
• Students read What Happens to Bigger Stars in Touch the Stars, pp. 77-79 

Follow-up Questions on Reading – Life Cycle of a Star 

1. What does stellar evolution mean? How does it compare to the life cycle of a human?
2. What is a nebula? Giant star? White dwarf? Binary star? 
3. What makes a nebula collapse (the material being brought closer and closer)?
4. What is nuclear fusion and why is it important to a star?
5. What happens when a star grows old and runs out of fuel in its core?
6. Return to page 19, Fig. 4, View 2, the image of the Ring Nebula. It was once a star like our sun. Describe its life story.
7. How might the evolution of a binary star system be different from that of a single star? 

Follow-up Questions on Reading – What Happens to Bigger Stars

1. The text says “bigger” star rather than “more massive” star. Which term do you think is more correct?
2. Would it be a good idea to live on a planet next to a massive star when it goes supernova? Why not?
3. When does a massive star become a neutron star?
4. When is a neutron star called a pulsar?
5. How can we detect black holes when, by definition, they are “black” and they are “holes”? 

Reinforcing Hands-On Activities

Predicting and exploring the H-R diagram: 

Read aloud. “The graph of how the temperatures and luminosities of stars are related is known as the Hertzsprung-Russell or H-R diagram. From this graph, we can also get an estimate of the size of a star, its radius. Astronomers worked with this graph long before they knew why stars varied in this way. You will be given a tactile H-R diagram, but before we study it, come up with some guesses as to how stars are.”

1. Why are some stars big (massive) while other stars are small (have masses only a fraction of the Sun’s)?
2. There exists massive hot stars (temperatures of 30,000 K and radii of about 10 times the Sun’s) AND small hot stars (temperatures of 100,000 K and radii about the size of the Earth). Would these different kinds of stars be putting out the same amount of energy? [HINT: Think about burners on a stovetop.]
3. Observations show that there are cool supergiant stars (temperatures of 3000 K and radii 1000 times that of the Sun’s) and cool red dwarf stars (temperatures of 3000 K but radii of 1/10 that of the Sun’s). Would these different kinds of stars be putting out the same amount of energy?

Figure 3.7.1. A tactile H-R diagram is made from foam and wood balls and beads attached to a foam board. All axes and objects are labeled in Braille.

Give students the tactile diagram of the H-R diagram (Figure 3.7.1) and have them explore the x- and y-axis to get a sense of what the graph is comparing. The O-stars are the most massive (60 times the mass of the sun) on the diagonal (called the main sequence) and the L stars are the least massive (barely 1/10 the mass of the Sun). Stars are born with a given mass and that determines how their lives go.

Exploration should also help answer questions 2 and 3. The amount of energy put out per second, that is, its luminosity or how many watts or power the star has depends on both the temperature and the size of the star. We say that the luminosity is proportional to the radius of the star squared and its temperature raised to the 4th power. It doesn’t take much of an increase in the temperature of a star to dramatically increase its luminosity. 

Students can find the giant star, a white dwarf, a sun-like star and approximate the relative sizes, temperatures, and luminosities. The diagonal line is where the stars are fusing hydrogen to helium in their cores, and once the “sun” is located on this line, the emphasis that the Sun will stay at its current size, temperature, and luminosity for a very long time.

Figure 3.7.2. Tactile diagram comparing the luminosity, temperature, and representative size of a sun-like star as it ages and dies. The life tracks are represented by stiff florist wire. The y- and x-axes are labeled in Braille.

Modeling the evolution of the Sun

It is difficult for some students to realize than an H-R diagram with life tracks given (Figure 3.7.2) is showing only how a star changes its luminosity and temperature as it goes through its life. Some think that the graph is figuratively showing how a star actually moves around in the sky. By emphasizing the changes in temperature, luminosity, and size for a star staying in one spot in the sky, the student will better understand what will happen to the Sun in the future.

The green, curvy diagonal line represents those stars that are fusing hydrogen to helium in their cores. The bead on that line represents the Sun. Students should work to follow the wire from the diagonal line through the change in luminosity and temperature to the top of the red giant branch. The Sun will be about 100 times its current radius. After the red giant stage, helium to carbon fusion will start in the Sun’s core, and the Sun will drop to a slightly lower luminosity and slightly higher temperature. When all of the helium in the core has been fused into carbon, the Sun will again go up to higher luminosities and lower temperatures (the second wire to follow), and be slightly larger that before. The Sun will expel about 30% of its mass in a planetary nebula and leave behind a white dwarf, its former core and interior. The third wire track shows the white dwarf decreasing in luminosity while staying hot for a while, and then going to cooler temperatures. This final process of cooling takes over 30 billion years. 

Bring the concepts into greater clarity by having the students look at the relative sizes of the Sun as it ages (Figure 3.7.3). The red giant sun – depicted with the largest Styrofoam ball with the netting – will be many times larger than what we are able to show here. If the tiny beads representing the Sun now are 5 millimeters in diameter, the Sun as a red giant would need a ball 500 mm, or half of a meter in diameter.

Figure 3.7.3. Tactile diagram comparing the relative sizes of the Sun during its lifetime. The scale along the bottom is time in billions of years, and indicates that the Sun spends most if its life just as it is today. Examination of this diagram could be part of an extension of this lesson. Use the tactile diagram of the sizes of the Sun as it evolves for additional insight into what our Sun will be like in the very distant future.

Summary and Post-Assessment Questions

1. 1. During the time a star grows old (millions or billions of years), is it correct to say they evolve? Explain.
   2. For the following pairs of stages of the lives of sun-like stars, state which one comes first:

a. main sequence or white dwarf

b. red giant or planetary nebula

c. white dwarf or red giant

0. 3. Where do the red giant, white dwarfs, and sun-like stars lie in the H-R diagram? 
   4. Where do the most massive and least massive stars lie in the H-R diagram? 
   5. Pick two of these terms and define them: nebula, red giant star, white dwarf, supernova, neutron star, pulsar, black hole
   6. Explain how the life track of a sun-like star differs from that of a much more massive star.

Relevant Information and Links 

• The Evolution of Stars — https://youtu.be/uCz-uXRf4rA
• Stars for Kids/Stellar Evolution for Kids/Evolution of a Star — https://youtu.be/fgqnh_6cCE4
• Death by Black Hole as explained by Dr. Neil deGrasse Tyson (YouTube) http://www.youtube.com/watch?v=h1iJXOUMJpg