Download Wien`s Law and Temperature

Stellar Classification
Name
Period
Data
One way to measure a star's temperature is to use Wien's law. Cool stars will have the peak of their
continuous spectrum at longer (redder) wavelengths than hotter stars. As the temperature of a star
increases, the peak of its continuous spectrum shifts to shorter (bluer) wavelengths. The final way to
measure a star's temperature is more accurate than the previous two methods. It uses the strength of
different absorption lines in a star's spectrum. We will look at this approach later.
Sample graph demonstrating Wien’s Law
Hot star with peak λ in the blue (short λ)
Intensity
Sun-like star with peak λ in the green/yellow
Cool star with peak λ in
the Red (long λ)
UV
V
B
G
Y
O
R
IR
Wavelength λ
Wien’s Law relates temperature in Kelvin (K) to peak wavelength in (Angstrom Å) as
2.9  10 7
shown by the equation T 
.
 PEAK
1. Go to the website http://astro.unl.edu/naap/blackbody/animations/blackbody.html. This site
generates graphs simulating the blackbody spectra of stars of varying temperatures. It has a
default of a star that is 6,000 K which is just slightly hotter than our sun. What is the peak
wavelength for a star with a surface temperature of 6,000 K?
What type/color of light is
this?
What would be the spectral class of this star?
2. Now decrease the temperature of the star by sliding the temperature bar to the left. Set it to 3,500 K.
You may need to type in this value. What did you notice about the changes in the graph?
What is the peak wave-length of a star with a surface temperature of 3,500 K?
of light is this?
What would be the spectral class of this star?
What type
3. Now increase the temperature of the star by sliding the temperature bar to the right. Set the
temperature to 10,000 K. You may have to type this in. What did you notice about the changes in
the graph this time?
What is the peak wavelength of a star with a surface temperature of 10,000 K?
type of light is this?
What would be the spectral class of this star?
What
4. Lock this curve in by clicking on the add curve button. This locks the vertical scale in place. Type
in the temperature 6,000K for the new curve. How do the two curves compare?
5. Now add another curve for the 3,500 K star. How does this new curve compare to the others?
What does this tell you about the energy production in stars? (This is related to the area under the
curve)
Notice that a blue star with a peak wavelength in the UV is much less luminous in the red, but is
many times more luminous in the red than a red star!
6. Analyze the spectra of each of the eight stars provided. Use Wien’s Law T 
2.9  10 7
 PEAK
to calculate
the surface temperature of each of these stars. Be careful, real stars don’t have the perfect curves
that were shown on the blackbody simulation. Fill in the table below for the temperature estimate
for each star. If the peak wavelength is not on the spectra graph for the star, state whether it is on the
infrared (IR) or the ultraviolet (UV) side of the spectrum. Use the temperature determined from
Wien’s Law to estimate the spectral class of the star. You can either estimate the peak wavelength
from the hand out or open the program Graphical Analysis and then open the file for the star located
in the Astronomy folder on the Z:drive. The star files must be opened from the Graphical Analysis
program.
Star
Estimated
Peak
Wavelength
(A)
Calculated
Surface Temp
(K)
Estimated
Spectral Class
HD 1069
HD 249240
HD 191010
HD 17971
HD 21110
HD 13136
SAO 11344
SAO 62808
7. Wein’s Law has limitations. In the space below, list a few of its limitations.
8. A more accurate way to determine the spectral class of a star is by looking at the absorption lines in the
star and the general shape of the curve. Don’t panic, this is almost more of an art and it takes practice,
so here we go. Open the program Graphical Analysis and then open the file for star HD 1069 located
in the Astronomy folder. Determine the wavelengths of the strongest absorption lines for this star. List
them in the table below from shortest wavelength to longest and then identify the element associated
with each absorption line. Repeat this process for each of the other stars on the list.
9. A couple of sentences about how you decided on the spectral type and luminosity class. Things like,
“The hydrogen lines were stronger in the star than in the sample A3V star and weaker than in the
A4V star,” would be appropriate.
Star
HD 1069
HD 249240
Wavelengths
of the
strongest
absorption
lines (A)
Element
responsible for
the absorption
line
Estimated
Spectral Class
based on the
absorption
lines and
curve shape
Luminosity
Class
(I, II, III, IV, V)
Star
HD 191010
HD 17971
HD 21110
Wavelengths
of the
strongest
absorption
lines (A)
Element
responsible for
the absorption
line
Estimated
Spectral Class
based on the
absorption
lines and
curve shape
Luminosity
Class
(I, II, III, IV, V)
Star
HD 13136
SAO 11344
SAO 62808
Wavelengths
of the
strongest
absorption
lines (A)
Element
responsible for
the absorption
line
Estimated
Spectral Class
based on the
absorption
lines and
curve shape
Luminosity
Class
(I, II, III, IV, V)