ASTROPHYSICS LAB 3: THE PROPERTIES OF THE SUN

Sep 5, 2007

GOAL: To measure the luminosity, radius, and temperature of the sun with a wax
                photometer, a meter stick, and standard luminosity source (for comparison).

The basic idea is to place a light bulb (of known wattage) and the sun on opposite sides of a
wax photometer (a flux-measuring device) and vary the distance from the photometer to the
bulb until the flux of the bulb at the photometer matches the flux of the sun at the photometer. See picture below.

light bulb

READING:  Universe  19(2); how a pinhole camera works (see web page)

EQUIPMENT: 200-watt OR 150-watt unfrosted light bulb

socket, cord, and clamp

wax photometer ( = 2 slabs of wax separated by opaque foil)

meter stick; fancy transparent rulers

pinhole camera

OBSERVATIONS

your lab book will be graded  -- for all observations made &

any relevant labeled diagrams that go with these -- before leaving the roof

unsafe behavior on the roof will result in a grade of 0 for the entire group

(We will assume that the sun-earth distance is known.)

1) Clamp the bulb onto something stable, such as the fence around the observing platform.

2) How should the bulb's filament be oriented to the photometer face? Why?
    How/where should the photometer be oriented relative to the bulb and sun? Why?
     I'm sure your well-labeled diagram will show all.... remember lab journal expectations....

3) a) One partner should be the flux judge and decide where the photometer should be held

such that the sun and bulb have equal fluxes on the two photometer faces. This person

should then hold the photometer steady at this distance while the other partners measure
     and record the appropriate data.

b) Repeat steps above until each partner in the group estimates the position of balanced
        flux (sun vs. light bulb) at least three times.

4) Individually (without consulting with any of your partners) record observations that you
     can make about the colors of the light observed in the two halves of the photometer.
     This  should of course be done with the bulb one and with the photometer at the distance
     where the fluxes of bulb and sun are approximately equal.

5) Use the pinhole camera to measure the size of the sun's image as accurately as possible

RESULTS

1) Using the average value of your group's measured distances (of balanced solar and bulb

flux), calculate the luminosity of the sun.   Make sure that you start with the condition that
     you made happen on the roof. 
    In any lab where you are measuring/determining something that has a standard known
    value, it is expected that you will compare (% difference) your value to the standard.

2) Explain your (individual) color observations using appropriate laws and
     standard temperatures of the light sources.
Both calculations and word explanations are appropriate here, right?

ANALYSIS

3) Give one excellent reason that would explain why your calculated value of the solar
     luminosity could be higher than the accepted value. Explain convincingly why your
     calculated value might be higher than the expected value. Be as quantitative as possible.
('Human errror' never counts as a good reason for why an experimental value disagrees
     with an accepted value.) Better answers always get much better credit.

4) Repeat question 3 with 'higher' replaced by 'lower.'

    Questions 3 and 4 will be graded according to 3 criteria:
a) importance of the reason offered (i.e., how likely your reason is to produce a significant
    discrepancy between the calculated and the expected answers);
b) correctness of the reason (does it actually result in 'higher' or 'lower' values that you said
     it did?);
c) a convincing explanation for why the effect results in 'higher' or 'lower' values.

5) Mars is about 1.5 times farther away from the sun than Earth. If you repeated this
    experiment on Mars, how many times farther would you have to hold the photometer
    away from your bulb (as compared to Earth) so that the two sides of the photometer
    were illuminated with equal flux?
    The answer should NOT depend on  the value of the luminosity of the sun! 
6) Use your measurements (& the distance of the sun) to determine the radius of the sun.
7) Determine the surface temperature of the sun from your measurements.
    (As a general guide, I would think maximum differences between your measurements
    and standard values would be 50%, 15%, and 10%, respectively for L, R, and T.