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:
-
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.
-
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.
-
Ask students the following after they finish the
activities.
-
Are all stars the same distance from our
planet? [Answer: No.]
-
How is apparent brightness different from real
brightness? [Apparent is what we observe on Earth.]
-
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 |
|
