Names: _______________________________
_______________________________
INTERSTELLAR MEDIUM LABORATORY
The purpose of this lab is to familiarize you with the many different forms of the material between the
stars. This material is referred to as the “interstellar medium” and is the source of many of the most
beautiful objects in the sky.
INTRODUCTION
When astronomers turned telescopes to the sky, they noticed fuzzy patchy regions between the stars.
These regions were called nebulae, which means “clouds.” As study of the interstellar medium
progressed, these nebulae were divided into categories:
Emission – a gas cloud that receives energy from a hot star(s), allowing the nebula to give off radiation in
emission.
Reflection – dust clouds that reflect (scatter) a star’s light to us
Dark – high densities of dust and gas that redden or extinct the light from the stars located behind the
cloud. These are also where molecules are likely to be found.
During the course of this laboratory exercise, you will study the interstellar medium – where stars are
formed and into which the stellar material is returned at the end of a star’s life.
QUESTIONS
Because emission (bright) nebulae, reflection nebulae, and dark nebulae are frequently associated with
each other, there can be difficulty in classifying them. We can classify a specific nebula based on the
predominant factor causing it to appear visible. Keep this in mind when answering the questions below.
1. Figure 1a shows the North American Nebula. Is this nebula an emission, reflection, or dark
nebula?
2. Figure 1b shows the Cone Nebula. Is this nebula an emission, reflection, or dark nebula?
3. Figure 1c shows the Horsehead Nebula. Is this nebula an emission, reflection, or dark nebula?
The next two images (Figures 2a and 2b) show bright nebulosities. Look at the images to answer the
following questions.
4. Which figure (2a or 2b) shows a reflection nebula? Explain your choice.
5. Which figure (2a or 2b) shows an emission nebula? Justify your answer.
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6. The following are sample spectra: each set contains one spectrum representing the nebula and the
other represents the spectrum of an associated star. Label each set of spectra as being
representative of a reflection nebula or emission nebula and defend your selection.
Set 1: star
nebula
type of nebula =
_____________
Set 2: star
nebula
type of nebula =
_____________
The brightness (intensity) of an object depends on the distance to the object (inverse square law). If stars
all had equal luminosities and were uniformly distributed throughout space, we would calculate that the
total number of stars through to the next fainter magnitude would be about 3 times greater than the total
number in the previous magnitude interval. That is, if 100 stars are counted up through apparent
magnitude 6, the approximately 300 stars should be counted up through magnitude 7. You should see
more faint stars than bright stars. Although not all stars have equal luminosities, the factor of 3 is an
adequate assumption if we only claim to be dealing with average values.
Use the information given above for the following 3 questions.
7. Complete the table below assuming the star counts out through magnitude 10 represent a section
of the sky where interstellar absorption is non-existent.
Magnitude 5
100
# of stars
6
300
7
900
8
9
10
8. Based on the data for regions A, B, and C given in the table below, which region shows no
evidence of an obscuring cloud? Explain.
Region
A
B
C
Magnitude
# of stars
# of stars
# of stars
6
200
300
300
7
600
900
900
8
1800
1200
2700
9
2000
3600
8100
10
6000
10800
24300
11
18000
32400
72900
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9. Using the table given in question 8, which region indicates the presence of an obscuring cloud at
the greatest distance? Explain.
10. Figure 3 shows a region towards the center of our galaxy. Describe what you see in this image.
Be sure to include any pertinent interstellar processes.
Figure 4 shows the constellation Orion and an enlargement of the Great Nebula in Orion (M42 – also
referred to as the Orion Nebula). In the middle of the constellation are three bright stars in a diagonal
straight line. This line of stars is generally referred to as Orion’s belt. Below the belt of Orion is the
sword, consisting of several bright stars and a bright patch, which is the Orion Nebula.
11. Based on figure 4a, make a sketch of the constellation Orion and clearly mark the location of the
nebula.
12. If the apparent magnitude of the nebula is about 4, can the nebula be seen with the unaided eye
under good conditions?
13. HOMEWORK - Go outside and look at Orion , can you see the nebula? If so, what does it look
like?
14. The scale of the enlarged image of the Orion Nebula is 1 cm = 2.0 light-years. Determine the size
of the nebula based on the longest dimension (show your work). Please express your answer in
light-years.
15. The distance to the Orion Nebula is known to be approximately 1500 light-years. Using this
information and your answer to question 14, determine the angular size of the Orion Nebula on the
sky.
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Figure 5a is a photograph of the Lagoon Nebula (M8). The scale on the image is approximately 1 cm = 7
light-years. Use this image to answer the following questions.
16. What kind of nebula is the Lagoon Nebula?
17. Using the scale given above, measure the longest dimension of this nebula in light-years (show
your work).
18. Count the number of dark globules present in the Lagoon Nebula and record that number here.
19. Using the scale established above, estimate the size of the 3 largest globules you see.
20. A certain dark globule in a nebula is measured to have a diameter of 1 light-year (9.5 x 1015 m)
and an average density of 2 x 1010 atoms/m3. Based on this information, complete parts a – d of
this question.
a. Determine the volume of this globule in m3, assuming the globule is spherical (V = (4/3)*
r3).
b. Using the density and volume of this globule, what is the total number of atoms in the
globule?
c. If we assume that the total number of atoms in the Sun is approximately 1057 atoms, how
many stars equal to the mass of the Sun could be formed from this globule?
d. How do you think the actual number of stars formed from this globule would compare to
your calculated number? Explain.
21. Figure 5b is a close-up image of some dark globules. From this image, state whether or not
approximating the volume of a globule as the volume of a sphere is valid.
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22. The characteristic absorption spectra of two stars are shown. Indicated in each is a pair of
absorption lines due to interstellar material between the stars and us. Using this information and
the spectral drawings shown below, complete the following questions.
Interstellar absorption lines
Star A
Star B
a. Which spectrum contains stronger interstellar absorption lines?
b. What does having stronger interstellar absorption lines imply about the number of absorbing
atoms along the line of sight to that star?
c. If we assume that the interstellar material is uniformly distributed along the direction to each
of these stars, which star is probably farther away? Explain.
d. Lets say that these two stars are at the same distance away from us, explain what could cause
the difference in the strength of the interstellar absorption lines.
e. Because scattering by dust leaves the colors of stars reddened, we can recognize the existence
of interstellar material in the direction of any particular star by comparing its color to what we
expect for its particular spectral class. Describe how the amount of “reddening” may depend
on the distance to the star and/or the amount of material between the star and us.
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23. Figure 6 shows two different images of the Crab Nebula (M1), located in the constellation Taurus.
This nebula is a remnant of a supernova, which is a star that explodes. This explosion was
observed in 1054 AD. If the scale on image 6a is 1 cm = 1.3 light-years. Assuming a constant
expansion rate, calculate how fast the material is traveling away from the center of the explosion.
Express your answer in km/sec.
24. Using your knowledge of the various structures in the interstellar medium, is it appropriate to
assume a constant expansion rate? Explain.
25. The image in Figure 6b was made from four plates taken on the Palomar 5m telescope using a
filter that excluded the emission lines. Each of the four images was taken with a Polarorizing filter
rotated 45 degrees (i.e. 0 deg, 45, 90, 135, 180). One of these plates was contact copied to make a
positive film copy, and this positive aligned in turn with each of the three remaining images,
revealing the difference between them. These three difference images were used as the red, green
and blue channels to make a color negative that made the print. All the work was done
photographically. These colors would not be visible through a telescope.
a. From this image, state whether or not the radiation from the Crab Nebula is polarized?
b. Radiation can be polarized by the presence of dust grains aligned in a magnetic field.
From this image, state whether or not the Crab Nebula contains a magnetic field. Explain
your reasoning.
26. Figure 7 shows the Helix Nebula. Ever since its discovery in the 18th century, this type of nebula
is traditionally called a planetary nebula. Why do you think it earned that name?
27. A planetary nebula occurs when a red giant star expels its outer atmosphere. This will happen
eventually to our Sun. The scale on Figure 7 is 1 cm = 0.25 light-years. Does this planetary
nebula extend farther from its central star than Pluto orbits from our Sun? (Recall: Pluto is 40 AU
from the Sun.)
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28. Figure 8 shows a famous part of the sky around the star Rho Ophiucus. The large yellowish object
is the star Antares, from the constellation Scorpius. Located in near the bottom right of the image
is a globular cluster in the background, far from the other stars you can see. This entire image is a
beautiful region with many of the processes that occur in the interstellar medium showing up
prominently. Write a paragraph discussing the effects of the interstellar medium in the Rho
Ophiucus region.
29. Figure 9 shows two views of the famous Eagle Nebula (M16). Figure 9a is representative of the
true color of the nebula. Ionized hydrogen causes the overwhelmingly red color of the nebula.
The Eagle contains other elements, all emitting at characteristic wavelengths (colors). Why do we
see mostly red?
30. Figure 9b was taken with the Hubble Space Telescope. This is not a true color image. The colors
are coded as follows: Oxygen (500 nm) – blue, Hydrogen (656 nm) – green, and sulfur (672 nm)
– red.
a. Which of the colors represents the highest energy?
b. Which of the colors represents the lowest energy?
c. Draw a sketch of the three pillars and label where you think the group of hot, young stars,
which are ionizing the nebula, is located.
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