To determine the age of a star cluster by observing, measuring, and plotting astronomical data.
The stars have always fascinated human beings; they sparkle at us from above, reminding us of the vastness of the universe that surrounds us. Understanding how our universe and its components work gives us insight into our own existence on this planet. A study of the stars and the universe is also an important lesson in the history of human ideas and cultures. Throughout time, the patterned motion of the stars has been used to navigate boats, plan the planting of crops, and study the seasons. Stars have also been the inspiration for stories, myths, religions, philosophies, art, and beautiful poetry.
In addition to the cultural and aesthetic value of the stars, science is another lens through which we can view the sky. Science adds another dimension to the universe by allowing us the opportunity to determine what stars actually are, what they are made of, and how they form. The development of telescopes, computers, space probes, and photography brings the stars to us, despite the fact that they sit light-years beyond our grasp. Through such tools, the details of the universe begin to resolve themselves and the beauty of the universe becomes more apparent than ever before.
Moreover, studying the stars is a study into our existence and origin as well as the universal phenomena that bind us all as a universe. As stated in the Benchmarks for Science Literacy, “If being educated means having an informed sense of time and place, then it is essential for a person to be familiar with the scientific aspects of the universe and know something of its origin and structure.” (Benchmarks for Science Literacy, p.61.)
In this lesson, students examine the Jewelbox cluster, located within the southern constellation Crux, and determine its age using a relationship between temperature, color, and luminosity. Before beginning this lesson, students should have an understanding of what stars are composed of and their life cycle. Students should also understand the relationship between temperature and color.
For best results, the site recommends printing the Jewelbox Image and StarGauge on an ink-jet printer, using the highest resolution available and good quality paper designed for use on an ink-jet printer. Resolutions of 720 dpi should be acceptable, and available on most ink-jet printers. Resolutions up to 1440 are preferred if available, since students will be able to distinguish fine detail more easily. Color laser printers can also be used, but may not produce an image of as good quality as a good ink-jet printer.
The images are available at the NOAO Jewels of the Night site in three formats: TIFF, PDF, and JPEG. The PDF image can only be used with the free Adobe Acrobat program. Both the TIFF and JPEG images can be printed directly from the Web; the TIFF file provides the highest quality image for this exercise. The site also gives directions on how to print the image in each format. To allow multiple uses of the images, laminate the printouts and use a scissor to separate the Jewelbox image from the StarGauge.
- What do you know about stars?
Generate a list of students’ ideas about stars on the board. As students brainstorm, ask them the following guiding questions:
- Where are stars found?
- What are stars made of?
- How far away are stars from earth?
- Do stars move?
- How do stars move?
- How large are stars?
- How do stars produce light?
- How old are stars?
- How are stars produced?
Show students the overhead picture of the Southern Hemisphere constellations. Tell students that these constellations are visible all year to people living in the Southern Hemisphere (below the equator). If you choose, you can contrast this image with an overhead of the Northern Hemisphere constellations.
Point out the constellation Crux or Southern Cross. Tell students that this constellation is the smallest in the entire sky and is only visible from the Southern Hemisphere. Thus, it was used by explorers of the Southern Hemisphere to point south. Papua New Guinea, New Zealand, and Australia each have the Southern Cross across their flags.
Tell students that they will study a cluster of stars within Crux referred to as the Jewelbox Cluster. This cluster of about 100 stars is located at the south of Crux and is about 7,500 - 7,800 light years away (1 light year = 6 trillion miles or 10 trillion kilometers).
Point out some differences in the stars in this cluster (size, brightness, color). Ask students:
- Why do you think some stars are different colors? (Students should consider how color and temperature are related. Give students the example of charcoals in a grill. Red, glowing coal is cooler than white ones. If students do not understand this relationship between color and temperature, use a Bunsen burner flame. Ask students where the hottest part of the flame would be—the center. This center part is white, while the outer regions of the flame are red or orange.)
- Why are some stars brighter than others? (Students should suggest that the brighter a star is, the larger it will appear. As students consider why some stars are brighter than others, give an example of two flashlights—a small penlight and a regular-size flashlight. Ask students to consider in what ways the light emitted from the small penlight differs from that emitted by the regular-size flashlight. Students will likely say that the regular-size flashlight will give off more light because it is bigger. However, also ask students how distance affects how much light we see from each flashlight. If a person holding the regular-size flashlight is a mile away, the light from that flashlight might look as bright as the light from the small penlight that is being held right in front of you.)
Tell students that their objective is to determine the age of the Jewelbox cluster. Divide students into groups of two. Provide each group with a color image of the Jewelbox cluster and a StarGauge. Ask students:
- Where do you think the edge of the Jewelbox cluster lies in this image?
Have students outline the boundaries of the cluster using a marker. Using markers and a protractor (or a teacher-made cut out), have students draw 8 - 10 cm circles on their print within the marked boundaries of the cluster. Each group should make a circle in a different place although overlap will occur due to the small size of the print.
Provide each student with a Jewels of the Night graph worksheet. Using the StarGauge, students will measure the brightness of stars by comparing the size of the star in the image to the sizes of the dots on the StarGauge. They will also estimate the star’s color by using the color portion of the StarGauge. The results will be plotted on the graph worksheet. Do an example as a class to familiarize students with the use of the StarGauge.
In each pair, have one student measure the brightness of the stars and the other student record the star’s color. Both students should plot the information on their individual graph worksheets. Students should switch roles so that each member of the group has an opportunity to take both types of measurements. After recording the information for a star, have students place a dot with their marker and then proceed in some systematic fashion to measure the brightness and color of the next star.
Stars that are not within the Jewelbox cluster are called “field stars.” If time allows, provide students with another graph worksheet and ask them to determine the age of these field stars. To do this, have students draw a 4 cm square or circle near the edge of the print and measure the color and brightness of these stars using the StarGauge.
- Do the Jewelbox stars on your graph appear to be randomly scattered or do they fall in any kind of pattern?
- Compare the pattern of your Jewelbox cluster to those of the three sample graphs on your graph worksheet. Using these sample graphs, determine the age of the Jewelbox cluster. (The Jewelbox is a young cluster being only about 12 million years old.)
- Compare the pattern of the field stars to that of the Jewelbox cluster. Describe why you might see similarities or differences between the two patterns.
- Looking at the three sample graphs, what types of stars characterize a young cluster? (Newly formed stars occupy a band in your graph from the upper left corner to the lower right corner. The most massive stars are hot [blue] and bright. The least massive stars are cooler [red] and dim. This band of stars is called the "main sequence.")
- Using the sample graphs, describe trends that characterize stars as they get older. (The young stars [O/B], which are blue and bright, begin to cool down. As a result, they turn red although they remain bright. The most massive stars burn their fuel quickly and are the first stars in a cluster to leave the main sequence to become red giants. They expand and cool, to become brighter and redder, and move to the upper right corner of the graph. As the cluster ages, less and less massive stars leave the main sequence to become red giants. These stars are classified as K or M stars.)
Go over the life sequence of a star with the class, using the following points to guide you:
- Stars are born in a nebula, a huge globule of hydrogen gas and dust. Over time, gravity pulls the hydrogen gas together and it begins to spin. As the gas spins faster, it heats up and eventually the temperature climbs to 15,000,000° C. This extremely hot temperature causes the hydrogen atoms to combine together to form helium atoms and the cloud begins to glow brightly.
- At this point and temperature, the star becomes a little more stable and stops contracting. It now becomes a main sequence star and will remain in this stage, shining for millions or billions of years to come. (Refer to the “Young Cluster” sample graph.)
- As the main sequence star glows, the hydrogen at the core continues to go through fusion, combining with itself to form helium. At some point, the hydrogen at the core runs out. When this happens, the core becomes unstable and contracts. However, the outer shell of the star, which still has hydrogen, starts to expand.
- As it expands, it cools and glows red. The star has now reached the red giant phase. It is red because it is cooler than it was in the main sequence star stage and it is a giant because the outer shell has expanded outward. All stars evolve the same way up to the red giant phase.
- Throughout the red giant phase, the hydrogen gas in the outer shell continues to burn and the temperature in the core continues to increase. At 200,000,000 °C, the helium atoms in the core fuse to form carbon atoms.
- When the last of the helium atoms in the core are fused into carbon atoms, the medium-size star begins to die. Gravity causes the last of the star's matter to collapse inward and compact. This is the white dwarf stage. At this stage, the star's matter is extremely dense. White dwarfs shine with a white-hot light. But white dwarfs are so small (equal to the size of the earth, 12,600 km in diameter) and faint that they cannot be seen in this image of the Jewelbox Cluster. Once all of their energy is gone, they no longer emit light. The star has now reached the black dwarf phase in which it will forever remain.
Provide students with the Stars and the Hertzsprung-Russell Diagram student sheet. Instruct students to go the How Hot Is that Star? site. Students will read the information on this site and use it to answer questions. Answers to the questions can be found in the Stars and the Hertzsprung-Russell Diagram teacher answer sheet.
To summarize the ideas in the lesson, tell students that the sun is a G star, which means that it has already burned through half its lifetime. It will live for another five billion years approximately.
To assess student understanding, ask the following questions:
- What is the sun burning? (Hydrogen)
- As the sun continues burning as a Main Sequence star, what type of star will it become after it has moved beyond the G class? (The sun will become a K star, then an M star.)
- When the sun finally burns all of its hydrogen, will it become hotter or cooler? (The sun will cool down.)
- As the sun cools down, what color will it become? (Red)
- What type of star will the sun become after it has moved off the Main Sequence? (The sun will become a red giant.)
- As a red giant, what will happen to the size of the sun? (It will increase in size as it becomes more luminous.)
- What do you think will happen to the planets that are close to the sun when it becomes a red giant? (The sun will probably swallow Mercury and Venus and perhaps Earth.)
- After the red giant phase, what will be the last stage of the sun’s life as a star? (It will become a white dwarf.)
- Describe the sun’s temperature, color, size, and luminosity as a white dwarf. (The sun will become very hot, blue, small, and dim.)
Further detailed information on the stars can be found at the How Stuff Works site. This site provides a pretty clear introduction to stars and printable versions of the site are also available.
StarChild, a product of NASA’s Goddard Space Center, provides good background information on stars and stellar evolution.
Background teacher information on the size and age of the universe can be found in How do we measure the size and the age of the Universe? and at Astronomy Today.