Stellar Spectra (1)
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Dec 6, 2023
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Stellar Spectra and Blackbody Radiation
Part I: Stellar Spectra
Last week’s lab materials included a supplemental two-minute video called “How Big is a
Star?” This week, you will find the spectral type and temperature of many of the stars
used in the comparison. Begin by opening the
Stellarium
program. Use the magnifying
glass icon from the left screen menu (or hit the F3 key to open the search window).
Search for each star in the list, and then look at the information listed in the info pane to
find the Spectral Type. You only need the first letter and number of the spectral type; the
other letter/numbers give the classification of the star on the HR Diagram.
Once you have found the spectral type of a star, it is easy to know the temperature. A
supplemental sheet is posted giving temperatures for different spectral types.
•
For each star in the following list, give the spectral type (first letter and number only)
as listed in Stellarium. Then give the temperature for that spectral type as listed in
the “Spectral Class & Temperature” Supplement in this week’s module. Estimate
temperatures for numbers that are between spectral types. Proxima Centauri has
already been completed as an example.
Star
Spectral Type
Temperature
Proxima Centauri
M5.5
2675 K
Sirius
A1
9750 K
Pollux
K0
5000 K
Arcturus
K1
4850 K
Rigel
B8
14000 K
Deneb
A2
9500 K
Antares
M0
3500 K
Betelgeuse
M1
3350 K
VY Car
F7
6450 K
This is the only section of the lab which uses Stellarium. You may close the program and
answer the questions.
Use the following table to answer questions 2-4:
Mass
Radius
Luminosity
Star A : K0 III
1.25 M
ʘ
25 R
ʘ
200 L
ʘ
Star B : K0 V
0.75 M
ʘ
0.125 R
ʘ
0.5 L
ʘ
•
Looking at the stellar properties in the above table, what are the
two major
differences
between Star A and Star B?
The two major differences between Star A and Star B are their luminosity and their
mass. Star A has more mass and is more luminous than Star B.
•
Both of these stars have one (unlisted) property which is equal for both stars. (1)
What property is it?
(2) What is its value?
(3) How do you know this?
The property that is equal for both stars is their temperature. The temperature of
Star A and Star B is around 5000 K. You can identify that based on the fact that they
are both K type stars. K type stars have a temperature around 5000 K.
•
Make an educated guess at what type of star each is. This is what is represented by
the
III
and
V
. (Hint: Think of an HR diagram and what two properties are plotted)
Star A has a luminosity class of 3 so it is a giant star. Star B has a luminosity class of 5
so it is a main sequence star.
Part II: Blackbody Radiation
White light is made up of all colors of light. We can see the individual colors when white
light is passed through a prism or when we look at a rainbow. Light can come in an array
of types or forms, which we call a spectrum. A spectral curve (like the one shown below)
is a graph that displays the amount of energy given off by an object each second versus
the different wavelengths (or colors) of light. For a specific color of light on the
horizontal axis, the height of the curve will indicate how much energy is being given off
at that particular wavelength.
1. Which color of light has the greatest energy
output in Fig. 1? Red because it has the greatest energy.
2. Imagine that the blue light and orange
light from the source were blocked. Which
colors would now be present in the spectrum
of light observed? The colors that would be present would be violet, indigo, green,
yellow, and red.
3. Which of the following is the most accurate spectral curve for the spectrum described
in question 2?
Graph A would best represent the scenario because there is no blue or orange present.
4. Which two colors of light are present in 3b above? blue and orange
5. What colors are present in 3c above? Would this object appear reddish or bluish?
There is violet and indigo the most present. Then blue, green, yellow, and orange have
some presence, but less. It would appear bluish since red isn't even one of the colors.
Different colors of light are manifestations of the same phenomenon but have different
wavelengths. For example, red light has a wavelength between 650 nm and 750 nm,
whereas violet light has a shorter wavelength between 350 nm and 450 nm. Stars also
give off light at wavelengths outside the visible part of the spectrum.
The two most important features of a star's spectral curve (or blackbody curve) are:
•
its maximum height or peak (where the energy output is greatest); and
•
the corresponding wavelength at which this peak occurs (which indicates the
star's temperature). Peaks at longer wavelengths indicate cooler stars.
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For example, two stars with identical size and temperature will have identical blackbody
curves. However, if the two stars are the same size, but one is hotter - the hotter star will
peak at a shorter wavelength and have greater output at ALL wavelengths.
For the figure to the right, compare stars E and F for questions 6-8.
6. Which of these stars gives off more red light? Explain.
Star F gives off more red light. The curve of star F is at red light wavelength.
7. Which of these stars gives off more blue light? Explain. Star E gives off more blue light.
Star E curves at the blue light wavelength.
8. Which star looks redder? Explain. Star F looks redder. Star F has more red light so it
will look more red compared to Star E.
For the figure to the right, referencing the blackbody
curves for Star E and Star C (right). Answer question 9.
9. How must Star C be different from Star E to
account for their difference in energy output? Star E has a higher energy output than
Star C. They share the same temperature and radiation peaks.
Consider the blackbody curves for Stars E and D
shown (right) when answering questions 10-12.
10. For each star,
describe the color as either reddish or bluish.
Star E:
bluish
Star D: reddish
11.
Which star has a greater surface temperature? Explain. Star E has a greater surface
temperature. Its peak was at a lower wavelength, so it has a greater surface
temperature.
12. Which star is larger? Use the luminosity relationship to explain (L = R
2
T
4
). Star D is
larger. Star D peaked at a higher wavelength but has the same peak energy output.