Light, Spectra and Stars - 1

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?

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?

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?

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