PSC 153 – Earth/Space Science Lab
Lab #11
Spectral Classification
Group worksheet:
group members ___________________
___________________
___________________
___________________
Introductory Information:
This lab is composed of four parts. The first two parts must be done in a dark room. The
remaining sections will be done with the lights on. If your group finishes before other
groups are ready, you will need to wait. You can always read over the material for the
latter sections in the hallway while waiting.
Terms:
 spectrum – the distribution of energy from a source
 diffraction grating – an optical component used to separate light into its visible
spectrum
 ROYGBIV – the initials for red, orange, yellow, green, blue, indigo, violet
 continuous spectrum– a spectrum which contains a continuous range of visible colors
with no apparent breaks
 emission spectrum - a spectrum which consists of narrow bands of light at specific
wavelengths, which appears as thin bright lines of color on a dark background
 absorption spectrum – a spectrum which has a continuous background with narrow
bands of fainter or missing light at specific wavelengths
At the front of the lab, get the sheet of paper with three color photographs showing you
examples of the three spectrum types. Use this sheet throughout the lab to help you
understand the differences between these types.
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Section 1: White light and temperature
Goals:
observe the spectrum of a light bulb at two temperatures
determine how the intensity changes with temperature
determine how the color changes with temperature
Procedure:
At each table, there is a light bulb in a special base and some diffraction gratings. You
will need to share the diffraction gratings among the group members.
Setup the equipment:
 The base has a knob on the front that will allow you to adjust the voltage going
through the bulb.
 Make sure the base is facing the partially open door enough to see the numbers around
the knob.
 Turn the knob to position 6.
1) How would you describe the color of the light coming directly from the bulb?
__________________
Look at the spectrum of the bulb using a diffraction grating. It will work best if you hold
the grating up to your eye and then look through the grating to the side of the light bulb.
2) What type of spectrum does the light bulb have?
(continuous) (emission) (absorption)
3) Can you see the entire spectrum of ROYGBIV? __________________
4) How does the brightness of the red end of the spectrum compare to the brightness of
the blue end of the spectrum? (blue is brighter) (red is brighter) (about the same)
Slowly turn the knob down to the 2 position, which represents a cooler temperature. If the
bulb goes out, turn it up past 5 and then slowly turn down again.
5) How has the overall brightness changed from the previous setting? _______________
6) What color does the light coming directly from the bulb appear to be? ____________
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Look carefully at the spectrum of the bulb using a diffraction grating.
7) Can you see the entire spectrum of ROYGBIV? __________________
8) How does the brightness of the red end of the spectrum compare to the brightness of
the blue end of the spectrum? (blue is brighter) (red is brighter) (about the same)
Based on all of your observations:
9) Does brightness increase or decrease with increasing temperature? _____________
10) Which of the two temperatures, cooler or hotter, was redder in color? ___________
11) Would a yellow glow represent a warmer or cooler temperature than a red glow?
________________
12) Would a bluish glow represent a warmer or cooler temperature than a yellow glow?
________________
13) Give an example of a situation where the color of light coming from an object might
tell you if it is hotter or cooler.
_____________________________________________________________
_____________________________________________________________
Section 2: Gas Identification
Goals:
view emission line spectra of unknown gases
describe emission line spectra
identify unknown gases by their emission lines
Procedure:
On three sides of the darkened room, there is a gas discharge lamp each containing an
unknown gas (A, B, C). From your lab table, you should be able to see each lamp. You
may move to one side or another to get a better view, but do not go right up to the lamp or
you will block the view for other groups.
You do not have to look at the lamps in order, but make sure that you record your
observations in the correct line of the table.
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Repeat the following steps for each lamp, recording your answers in Table 1 below.
1) First, look directly at the glowing gas without using the diffraction grating. What color
is the light coming from the lamp? Record this value in the table.
2) Looking through the diffraction grating, you should see distinct lines of color to each
side of the lamp. Try rotating the grating to see the lines clearly and to confirm that
they are coming from the discharge lamp you are interested in. Note the brightest lines
and record the color of each bright emission line in the spectrum in the table.
3) Looking through the diffraction grating, record the color of any faint lines you can see.
It is recommended that you do this part in pencil and have multiple group members
discuss their observations.
(Table 1)
lamp color of gas
bright lines
fainter lines or background
A
B
C
4) Table 2, below, lists the known features for each of the gases. Compare the features
you observed and recorded in Table 1 with the descriptions in Table 2.
5) Determine which lamp corresponds to each gas. If you need to, go back and look at the
lamps again.
6) Record your determinations in Table 3.
(Table 2)
gas
bright lines
fainter lines
helium
yellow, green
red, blue (maybe green)
hydrogen
red, blue
violet
neon
many red, orange, yellow lines
Other red, yellow, orange, and green
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(Table 3)
lamp
Lab #11
gas
A
B
C
Methods very similar to this are used in practical applications to determine the
compositions of unknown samples under many circumstances.
Think of one circumstance that a scientist or technician might need to determine the
composition of an unknown substance. ___________________________________
Section 3: Stellar spectra
Goals:
identify key features in the spectrum of stars
understand the temperature sequence of the classes
Terms:
 spectral class – a categorization of stars based on the features in the spectra
 peak wavelength – the wavelength of light in the spectrum which has the highest
intensity
 review the 3 spectrum types from the front page
Procedure:
Get a spectral packet from the front of the room. The top sheet shows color images of the
spectra for each spectral class. The colors depicted here are fairly accurate with the
exception of violet (printers tend to print violet as more of a purple color which contains
too much red pigment).
1) The seven spectra on sheet one are examples of what type of spectrum?
(continuum) (emission) (absorption)
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2) Temperature in spectra is often measured by comparing the red intensity to the
violet/blue intensity. For each spectral class, indicate with a check mark which side of
the spectrum (red or violet) is brighter or if the two sides are roughly equal.
Class
red brightest
equal red and violet violet/blue brightest
O
B
A
F
G
K
M
3) Which spectral class (O, B, A, F, G, K, or M) is the hottest? ______
(Hint, look at the caption on sheet 1)
4) Do the hottest spectral classes have more red light or more violet/blue light?
_______________________________
In addition to the difference in the continuum brightness, the spectral classes show dark
lines at different wavelengths (colors) and the lines may be very dark or just slightly
fainter than the continuum.
5) Pick one spectra class _____. Describe the lines that are obvious in the spectrum.
(What color are they, are the lines dark, faint, wide, narrow...)
________________________________________________________________
________________________________________________________________
6) Pick a spectral class that is immediately above or below the one you described in
question 5 _____. Describe what features make it different from the spectral class you
described above. ___________________________________________________
__________________________________________________________________
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Now look at the second sheet, which has the spectrum of the sun.
7) Compare the features in the sun's spectrum to the different spectral classes.
8) Which spectral class is the sun most like? _____
9) List the spectral class that is the second closest to the sun's spectrum. _____
10) Describe what features helped you make your decisions above? (list specific
continuum features, and absorption lines that you notice in the Sun's spectrum and the
spectral class you choose as the closest match) _____________________________
___________________________________________________________________
___________________________________________________________________
Now look at the third sheet in the packet. This page shows a single spectrum in two
different ways. The top spectrum is a color photograph as you have seen on sheets one
and two. The second spectrum is a plot of the intensity verses wavelength (color). Notice
the labeling of the continuum and absorption lines.
11) Does red correspond to small wavelengths or large wavelengths? ___________
12) What do absorption lines look like on the plot? _________________________
Now look at the spectral plots labeled spectra #1 – spectra #7
These spectra represent the 7 spectral classes but in a random order. They are the
graphical versions of the color pictures you viewed on sheet #1.
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The x-axis lists the wavelength of the light in a unit of Angstrums (1 A = 1 x 10 m).
The y-axis lists the intensity of light.
Notice the the continuum is not the same intensity at all wavelengths. In each plot, the
continuum has a peak value where the intensity is higher than any other point on the plot.
The wavelength that corresponds to the peak intensity is called the peak wavelength.
13) Record your estimate of the peak wavelength for each plot in the table below.
plot #
1
2
3
4
5
6
7
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plot #
1
2
Lab #11
3
4
5
6
7
peak wavelength
The peak wavelength corresponds to the temperature of the star by a mathematical
formula called Wein's Law.
T
=
3 x 107
wavelength
where T is the temperature in degrees K, and wavelength is measured in Angstrums
14) Use Wein's Law to calculate the temperature of the star in each plot.
plot #
1
2
3
4
5
6
7
Temperature
15) If the peak wavelength is at a short wavelength, is the star hot or cool? _________
16) Using the temperatures for each spectra to place the plots into the correct order for
the spectral classes.
Spectral Class
O
B
A
F
G
K
M
Plot #
_____
_____
_____
_____
_____
_____
_____
17) Is it easier to judge the temperature on the color photos or the plots of each spectral
class? Why? __________________________________________________________
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Section 4: The H-R diagram
Goals:
understand an H-R diagram
plot stars on the H-R diagram
determine the category of a star based on its location on the H-R diagram
Terms:
 H-R diagram – a diagram used to plot the temperature and luminosity of stars
 Luminosity – the energy output of a star
 Main sequence – the diagonal band across the H-R diagram which contains most of the
stars (running from upper left to lower right)
 Giants – stars which are larger and more luminous than typical stars of the same
temperature (above the main sequence)
 Dwarfs – stars which are smaller and less luminous than typical stars of the same
temperature (below the main sequence)
Background:
As you saw in the previous section not all stars have the same properties, like temperature
and intensity. Even though these properties vary, there are some distinct relationships
between their values related to the size of the star. The H-R diagram is used to understand
the properties of stars and classify them based on which size group the star falls into.
Procedure:
Look at the H-R diagram provided to you, and answer the following questions.
1)
2)
3)
4)
What variable is plotted along the x-axis? ____________________
Do values increase or decrease towards the right on the x-axis? ____________
What variable is plotted along the y-axis? ___________________
Do values increase or decrease towards the top of the y-axis? _______________
Look carefully at the scales on both the x and y axes. Notice that the values are not evenly
spaced. These axes have “non-linear” scales specifically setup to show the broad ranges
of values needed in the most efficient manor.
Consider the stars already plotted on the H-R diagram and the definitions listed in the
terms section.
5) Label each of the following on the diagram using colored pencils
 Draw a line along the approximate path of the Main-sequence.
 Circle the region containing Giant stars and label the area with the word Giants.
 Circle the region containing Dwarf stars and label the area with the word Dwarfs.
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6) For each star in the table below, plot it on the H-R diagram with a colored pencil and
label your point with the star #.
7) Based on its position on the H-R diagram, determine which category each star belongs
to (main-sequence, giants, or dwarfs) and list this in the table.
Star #
Temperature
Luminosity
1
25000
2500
2
3000
0.00002
3
7500
0.0008
4
11000
60
5
12000
0.005
6
6000
4300
7
6800
5
8
4000
800
Category
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