Handout Authors: Ashley Thornburg Austin Ross Title of Lesson: The Life Cycle of Stars Date of Lesson: Wednesday March 14, 2012 Length of Lesson: 40-45 minutes Name/Level of Course: Science 8 th Source of the Lesson: http://www.middleschoolscience.com/earth.htm What type of stars are in our universe to make HR Diagram Concepts: Stars have a life cycle much like all living things. This life cycle can be seen on an HR Diagram. For this lesson students need to have a basic idea of what a star is and know that the sun is a star. Students also must be able to graph points on a chart. Objectives: SWBAT create a diagram of the life cycle of a star. SWBAT read an HR Diagram and interpret data from the chart. State Standards: 112.20. Science, Grade 8, Beginning with School Year 2010-2011. (b) Knowledge and skills. (8) Earth and space. The student knows characteristics of the universe. The student is expected to: (A) describe components of the universe, including stars, nebulae, and galaxies, and use models such as the Herztsprung-Russell diagram for classification; (B) recognize that the Sun is a medium-sized star near the edge of a discshaped galaxy of stars and that the Sun is many thousands of times closer to Earth than any other star; 2007 The University of Texas at Austin
Safety: There are no safety concerns with this lesson. Materials List and Advanced Preparations: Stars for HR Diagram (100) HR Diagram Graph (1) Tape (1 Roll) Red Index Card (35) Blue Index Card (35) Yellow Index Card (35) Email Mr. Armstrong slide for Life Cycle of Stars and have it put after the warm up. Vocabulary: Hertzsprung-Russell Diagram Main Sequence Dwarf Star Giant/Super Giant Equilibrium Supernova Protostar Neutron star Black Hole Page 2 of 9
ENGAGEMENT Allow students to come in and complete daily warm up. Good morning guys. I m Mr. Ross, and I m Ms. Thornburg and today we are here to talk to ya ll about stars. That s right, the constellations in the sky change as the Earth s position around the sun changes. But this is not the only thing that causes the night sky to change. Has everyone in here been outside at night and looked up at the sky and seen the stars? Can anyone tell me what a constellation is? Has anyone ever seen a constellation? If so which one? Do these constellations stay in the sky all year? What are some other things that you all think may make the night sky we see change over time? Time: 3 Minutes [Yes] [A group of stars that make a shape in the sky; Yes; the big dipper, the little dipper, ect.] [No] [The birth of new stars and the death of old ones] Page 3 of 9
EXPLORATION One of the tools astronomers use to categorize stars is called a Hertzsprung-Russell Diagram or HR Diagram. This diagram places stars on a graph based on their temperature and apparent luminosity or how bright the star appears. Today we are going to build our own HR Diagram. In this envelope I have about 100 stars that we are going to place onto the HR Diagram here on the board. Work in your groups of four to decide where your stars will go on the graph then once you are all in agreement go and place your stars on the graph. Before we get started though, let s take a quick look at the axis on this diagram. Kelvin is another scale of temperature and you convert to Kelvin by adding to a Celsius temperature. That s right. Typically when we number our axis we put the number closest to zero at the What is on the Y-axis? What are the units on the Y-axis? That is kind of a strange unit does anyone have any idea what that might mean? That s right. Now lets look at the X-axis. What is on the X-axis and what are its units? Has anyone ever heard of a Kelvin before? Does anyone notice anything else strange about the X-axis? Time: 20 Minutes [Luminosity] [Suns] [How bright the star is compared to the sun] [Temperature, Kelvins] [No][If yes ask if they can explain how it compares to Celsius] [The numbers are backwards] Page 4 of 9
origin but on the HR-Diagram it is backwards. This is because often times astronomers will also include the B-V color index of the star and if that is included then it is appropriate to label the axis backwards like it is here. So I am going to pass out your materials and then work with your group to decide where the stars need to go on the diagram. Can I get a thumbs up or thumbs down on whether or not you understand the instructions? Okay, you all have 15 minutes. Page 5 of 9
EXPLANATION Again what we just created is an HR Diagram. That dense diagonal of stars is called the main sequence. This is where stars spend most of their life time. Those are all there as well. The cluster in the very top right corner those are the super giants, these stars are very large but compared to other stars not very hot. The cluster right below that consists of giants. These stars are also large and not very hot comparatively. And the cluster at the bottom in the middle of the page consists of the white dwarfs. These stars are relatively hot but very small. That s right. Blue stars are the hottest stars on the spectrum and red stars are the coolest. Can I get a fist of five on whether or not you understand the trends in the HR Diagram? Where are most of the stars clustered? What other clusters of stars do you notice? There is one more pattern on this diagram. Does anyone see it? Time: 5 Minutes [Along a diagonal down the center] [Cluster in the top right corner, Cluster just below the one in the top right corner, Cluster below the main sequence in the center.] [The colors change with temperature.] Page 6 of 9
ELABORATION Bring up life cycle of star slide. As I mentioned in earlier the night sky changes over time and not just because we move around the sun. Stars are born and die just like humans do. Stars are created when space dust forms a cloud, this cloud is called a stellar nebula. Gravity begins to pull all of the particles together into a mass that we call a protostar. We do not see these on our HR Diagram. After the protostar phase, stars join the ranks of the stars on our HR Diagram in the main sequence. As stars get old they expand and grow to become giants and super giants. What happens next depends on the star s mass. The giants go on to become planetary nebula and then white dwarfs and finally they fade into black dwarfs. Supergiants on the other hand tend to have a more dramatic death. Those are both right. When a supergiant dies a supernova occurs and supernova is just a fancy word for the explosion of a star. After this explosion two things can happen. Either a neutron star can be formed or a black hole can be formed. A neutron star is a very dense star that has 3 times the mass of the sun but a diameter of only 20km. A black hole on the other hand is a point that has such a strong gravitational pull that nothing can escape it; not even light. These black holes distort Does anyone have any ideas of what might happen to a supergiant after it lives out its life? Time: 10 Minutes [It blows up, it makes a black hole] Page 7 of 9
space around them and suck in any matter near them including other stars. So now that we discussed the stages in a stars life, let s talk about the length of a stars life. The length of a stars life is dependent upon the mass of a star. If you said that a smaller star will live longer then you are right. The smaller stars can survive longer because they don t consume their fuel as quickly as the larger stars. That s right. So before we started making our HR Diagram, I said that an HR Diagram was one of the many tools that astronomers used when looking at stars and their stage in the life cycle of a star. So here is a quick animation of a star moving through its life cycle on an HR Diagram. Click on link on 2 nd slide and click through animation. Be sure students understand that the stars are not physically moving through the universe but just changing stages. How many people think a large star will live longer? Why do you think that? How many people think a small star will live longer? Why do you think that? Can anyone think of an example of this on earth? [Raise hands] [It s bigger so it will last longer] [Raise hands] [It is smaller so it doesn t have to consume as much energy to continue burning] [Bigger people eat more food, so if each person had limited food larger people would run out faster.] Page 8 of 9
EVALUATION To review today s concepts we are going to answer some questions on the screen. Each of you has three different colored cards. The pink card is for answer A, the yellow card is for answer B, and the blueish/purple card is for answer C. When the question is brought up on the board you will be given 30 seconds to read the question and the answer choices and select your card/choice, then I will ask you all to put your heads down on your desk and to hold up your answer card. Once everyone has made their selections we will allow you to bring your head back up and we will reveal the correct answer. Does anyone have any questions about the instructions? 1) Which of the following is the correct sequence for the life cycle of our sun? 2) If all stars were to be the same size of the sun then which of the following statements is true? 3) Along which line would you expect to find main sequence stars of the same size? 4) Which of the following conclusions can be drawn from the graph? 5) Based on the conclusions from the previous two questions which of the following is true? 6) The surface temperature and luminosity of a star change drastically over the course of a stars life time. 7) The surface temperature and luminosity of a star change drastically over the course of a stars life time. 8) Smaller mass stars have greater luminosity than larger mass stars. Time: 10 Minutes Page 9 of 9