Light, Spectra and Stars - 1
Name:________________________________
Go to website for Black Body Radiation. This will start up a little program that shows the energy
output of solid objects (black bodies) at various temperatures. The sliding scale at the bottom
changes the temperature of the black body and the resulting energy output for a range of
wavelengths is shown in the graph. The x-axis is the wavelength of the light emitted in
Ångstroms, while the y-axis measures the amount of energy given off in Joules at that particular
wavelength. The total energy of the black body would be the total area under the curve. This
value (the total energy emitted) is displayed in the upper right corner. If you click on the graph
you will get the value of energy and the corresponding wavelength at that location. Since the
size of the curve can change drastically, you’ll notice the y-axis scale will change as the curve
gets larger or smaller so that it remains in view.
Let’s look at the properties of black bodies. First you’ll examine the characteristics of a set of
specific temperatures to determine any trends. For each temperature determine the peak of the
curve, the total energy output (that’s the number in the upper right corner), and the color/type of
light that the object emits mainly.
Temperature
Peak wavelength
Total energy output
Type/color of peak light
4000
6000
8,000
10,000
12,000
a. As the temperature increases, how would you describe the change in the value for the peak
wavelength?
b. If an object has a temperature two times that of another object, how many times does the peak
wavelength change?
Light, Spectra and Stars - 2
c. How would you describe the relationship between the peak wavelength and the temperature
for a black body in a formula?
d. Now let’s look at the energy output – how would you describe the change in the value for the
total energy output as you increase the temperature?
e. If an object has a temperature three times that of another object, how many times does the total
energy output change?
f. How would you describe the relationship between the total energy and the temperature for a
black body in a formula?
g. What would the peak wavelength be for a black body with a temperature of 40,000 K?
h. What would the total energy output be for a black body with a temperature of 40,000 K?
i. What type of light (x-ray, infrared, etc.) would be emitted by black body with a temperature of
40,000 K?
Light, Spectra and Stars - 3
Follow the link to the Spectra program. There you will see the spectra of various stars displayed.
The amount of light that is given out at each wavelength is displayed on the vertical axis –
though it isn’t numbered and the scale is different for each type of star. Absorption features
would look like valleys in this type of display. If you click on the “Color” button, a typical
visible light spectrum that you’d see with a prism is displayed. In this view, the absorption
features are the dark areas – you can see how they also correspond to the valleys. Since the color
spectrum shown is only for visible light, the spectral features at longer and shorter wavelengths
can only be seen in the lines. The spectra that are shown in black/white are the “comparison”
spectra that you’ll use to identify some unknown spectra – but we’ll get to that later. The spectra
are arranged in the spectral sequence going from O5 to M5. Use the right and left arrow keys on
your computer keyboard to change from one spectral type to another.
j. Let’s determine the range of our vision. Humans can see light typically between a range of
4000 and 7000 Å. What temperature would a black body have for it to emit most of its energy at
a wavelength of 4000 Å?
k. What temperature would a black body have for it to emit most of its energy at a wavelength of
7000 Å?
l. Follow the Spectral Type Characteristics Link to determine the spectral types that correspond
to the temperatures you determined in the two previous questions. Approximately what spectral
types do these temperatures correspond to?
m. Since our eyes are limited to a rather narrow range of light, how is it possible for us to see
stars outside of the range of spectral types that you discovered in the previous question?
Light, Spectra and Stars - 4
n. The first spectrum that is displayed is that of an O5 type star (surface temperature = 54,000
K). A great deal of light in this star’s spectrum is absorbed, particularly at wavelengths shorter
than 3000 Ångstroms. What type of light is absorbed and why is it being absorbed?
o. The spectra for the coolest stars, like the M0 and M5, have very wide absorption features
(wide valleys). What aspect of these stars’ atmospheres would cause such wide features?
If you click on the buttons number 1 – 5 you’ll see an unknown spectra displayed in red.
Determine the stellar spectral type for the unknown spectra. Your designation should not only
include the letter (OBAFGKM – no LT types are here), but also a number. So your answer could
be B4, K2, A9, etc. Don’t just write B, K or A.
p. Unknown #1
q. Unknown #2
r. Unknown #3
s. Unknown #4
t. Unknown #5