Universe Cycle - Universe (3) Post Lab
 OBJECTIVES: Discovering how light moves. Comparing absolute and relative brightness of stars. VOCABULARY: brightness star MATERIALS: penlight flashlight mirror Students experiment with light.
BACKGROUND:

Students have learned that stars emit light. Light, for all practical purposes, moves in a straight line in space, unless it hits an object and then the light is reflected from that object. Albert Einstein in his theory of relativity, state that light is "curved" by gravitational forces. This has been proven. For the purposes of this lab, light effectively travels in a straight line on a scale that humans can perceive.

Stars are different distances from the Earth. This means we see the relative brightness or magnitude of stars, not their real, or absolute magnitude. The light from stars travels to us in essentially straight lines. In contrast, within the Solar System, sunlight is also reflected from the surface of a moon, planet, or other objects. In this activity, students will experiment with absolute magnitudes, relative magnitudes, and reflection.

PROCEDURE:

1. Have the students work in pairs. Assign one student to hold the penlight. Have the second student gradually move away from the penlight, trying to find the distance at which the light appears significantly dimmer. Have them measure the distance in footsteps. If the students do this assignment as homework, have them record what kind of penlight they are using. This will allow for comparisons of different strengths of flashlights, which are analogous to the different magnitudes of stars.

2. Have the students devise an experiment with a flashlight that makes light" bend". Do not give the students too many hints, but suggest using a mirror or other reflective material. Discuss what groups did to make light bend.

3. Ask students the following after they finish the activities.

1. Are all stars the same distance from our planet? [Answer: No.]

2. How is apparent brightness different from real brightness? [Apparent is what we observe on Earth.]

4. Discuss the star classification chart with the students, so they realize that stars have different elements of color and temperature. This chart is not for students to memorize but to get a sense of the variety of stars. The absolute brightness data chart shows students that different stars like supergiants are very bright, emitting large amounts of light energy.

 STAR CLASSIFICATION elements found color temperature range in centigrade example O H, He,O,N BLUE 40,000-25,000 Zeta Puppis B He, H BLUE 25,000-11,000 Spica Regulus Rigel A H, Ca, metals BLUE-WHITE 11,000-7,500 Vega Sirius Daneb F Ca, metals, Fe WHITE 7,500-6,000 Canopus Procyon Polaris G Fe, Ca YELLOW-WHITE 6,000-5,000 Sun Alpha Centauri K H ORANGE 5,000-3,500 Arcturus M TiO RED 3,500-3,000 faint stars N,R,S,I (unknown) TiO not visible 1,000 (?) unknown

 ABSOLUTE BRIGHTNESS (LUMINOSITY) I supergiant II bright giant III giant IV subgiant V main sequence (like our Sun) VI subdwarf VII white dwarf