Universe Cycle - Universe (3)
Post Lab 

   
OBJECTIVES:
  • Discovering how light moves.
  • Comparing absolute and relative brightness of stars.
VOCABULARY:
  • brightness
  • star
MATERIALS:

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

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