Lab 1: Space Science

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Lab 1: Space Science
The goals of this space-oriented lab:
(1) Use spectroscopes to identify the elements in various light sources.
(2) Indirectly calculate the distance to an object outside or in the classroom in order to model
how astronomers calculate the distance to an object in space.
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I. Spectroscopy
A. The electromagnetic spectrum: A quick review
Stars such as the Sun produce radiation over a range of wavelengths. We see visible light in a
narrow range of this spectrum, as shown on Plate 1.
1. Does red light or blue light have a shorter wavelength? ___________
2. Do radio waves or microwaves have a shorter wavelength? __________________
3. Which has a shorter wavelength: ultraviolet radiation or infrared radiation? (circle one)
Scientists use the full electromagnetic spectrum to learn more about the Sun, stars, and universe.
For example, radio waves help them understand magnetic fields in space, and ultraviolet waves
help them study hot gases that aren’t apparent in visible light. The most accurate information is
collected by satellites (e.g, the Hubble Space Telescope) that lie above the interfering effects of
Earth’s atmosphere.
Astronomical studies also frequently use visible light, and that’s what we’ll focus on in this lab.
We’ll use spectroscopes to identify chemical elements in various light sources. Each element radiates
light in a specific wavelength combination, known as a spectrum (plural = spectra) that is
as distinctive as human fingerprints. Astronomers use these “spectral fingerprints” to remotely
identify the elements in the Sun, distant stars, gas clouds, and other features.

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B. Three types of spectra
Radiation can be displayed in three types of spectra (Plate 1).
A continuous spectrum looks like a continuous band of color (red to violet).
An absorption spectrum consists of dark lines on a continuous band of color. Absorption spectra
are produced when light from a star passes through a gas cloud before it reaches Earth;
atoms in the gas cloud absorb particular wavelengths of light, depending on what gases are
present. The absorbed wavelengths show up as black lines in otherwise continuous spectra.
An emission spectrum consists of bright colored lines against a black background. In space,
emission spectra are produced when a gas cloud absorbs and re-radiates specific wavelengths.
On Earth, emission spectra are produced by neon lights and other single-gas sources.
C. Spectroscopic Analysis of Unknowns
The scale within the spectroscope ranges from 400 to 700 nanometers
(abbreviated nm), which are very small units of length.
1. Use your spectroscope to observe the spectra produced by the light sources provided by your
instructor, and use the form on p. 3 to show your observations (i.e., place vertical lines at the appropriate
wavelength; use shading to indicate a continuous spectrum).
2. At the top right of each record, indicate whether the spectrum you observed was a continuous,
absorption, or emission spectrum.
3. Using the known spectra of selected elements (Plate 2), try to identify each mystery element,
and name it at the top left of each record.
4. Based on your results, and on an examination of Plate 2, what element probably is used inside
the fluorescent ceiling lamps?
____________________________
5. Some or all of you probably identified lines at positions that are 5 nm, maybe up to 10 nm,
away from their expected/formally determined values. Why do you think this is so?

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D. Spectral Analysis, the Milky Way Galaxy, and the Universe
Astronomers measure how fast an object is approaching or receding by using the Doppler effect
to analyze spectral lines. You’re familiar with the Doppler effect for sound waves if you’ve heard
the change in frequency (or “pitch”) of a
sound as the source of the sound approaches
and then passes you. The waves emitted by
the approaching object (e.g., a whistle on a
train) have a shorter wavelength (they’re
“higher-pitched”) than the waves emitted by
a receding object (which sound “lowerpitched”).
This diagram shows how this
might work.
For light, the Doppler “shift” of a spectral line from its normal position is proportional to the
speed of the object relative to the speed of light.
An example: One of hydrogen’s spectral lines normally has a wavelength of precisely 650 nm.
Observations of Star X show that this hydrogen line is shifted to 715 nm.
Calculate the percentage shift:
Difference (Δ) between measured
and expected wavelength λ Δ λ 715 nm – 650 nm! 65 nm
————————–————! = ––– = ———————! = ——— = 0.1 = 10%
Expected wavelength! λ! 650 nm! 650 nm
In other words, the wavelength of Star X’s hydrogen spectral line is 10% greater than expected.
To determine the velocity of Star X, simply multiple the speed of light (300,000 km/sec) by this
percentage:
(300,000 km/sec) x 10/100 = 30,000 km/sec
So Star X is moving 30,000 km/sec with respect to Earth.
1. Is Star X moving towards or away from Earth? _____________
Don’t proceed until you’re sure you understand the answer to this question. If you’re stuck, reread
the first paragraph of this section.

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2. What is the motion (velocity AND direction!) relative to Earth of each of the following stars?
Star A: a spectral line normally seen at 471 nm is now at 462 nm.
Star B: a spectral line normally seen at 668 nm is now at 691 nm.
Star C: a spectral line normally seen at 405 nm is now at 412 nm.
Star D: a spectral line normally seen at 589 nm is now at 570 nm.
3. The Sun is one of billions of
stars in the Milky Way galaxy,
which itself is one of billions
of galaxies in the universe.
This stunning photograph from
the Hubble Space Telescope
shows thousands of galaxies in
a tiny corner of space.
Extrapolating to the entire universe,
astronomers conservatively
estimate 100 billion galaxies,
with an average of at
least 200 billion stars in each
galaxy.

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Astronomers have used spectral “fingerprints” to determine that the Sun consists chiefly of the
gases hydrogen and helium. The spectra for these (and other) gases have been verified and reverified
by countless thousands of experiments here on Earth.
Hydrogen and helium spectral lines from the Sun are also present in the spectra from other stars
in our Milky Way Galaxy.
Astronomers have found that the spectral lines of objects outside of our galaxy all are shifted
from their “normal” pattern to longer wavelengths, i.e., closer to the red end of the visible light
spectrum.
4. Does this “red shift” indicate that these objects are moving towards or away from the Milky
Way galaxy? __________________
In contrast, the spectral lines of stars within our Milky Way galaxy are a mixed bag; some are
where we expect them based on measurements of the Sun, but others have varying amounts of
red shift or blue shift.
5. Which of stars A, B, C, and D (preceding page) could lie outside the Milky Way galaxy?
Explain your reasoning.
You may have heard of the Big Bang hypothesis, which proposes that all matter in the universe
was confined to an incomprehensibly tiny volume until the universe began to expand (incomprehensibly
quickly) about 13.7 billion years ago.
6. How is the Big Bang hypothesis supported by the information about spectral lines in Part D of
the lab?

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II. Measuring “Unmeasurable” Distances
Even before NASA sent probes to the Moon and planets, scientists knew the distances to these
places. In fact, scientists can determine the distances to objects that we’ve still never visited. In
this section we’ll model the technique they use.
A. Measuring Distance
First, though, brainstorm with classmates about how humans determine these distances:
1. Width of this classroom
2. Depth of the ocean floor
3. Distance to Earth’s center
4. Your instructor will lead a class discussion to ensure that you understand the techniques used
in each of the three cases above. Could any of the techniques be used to determine the distances
to astronomical bodies like the Sun, planets, stars, and galaxies? Explain.
B. Parallax
1. Hold your thumb in front of you at arm’s length. Close your left eye, and observe your thumb
and the background on which it appears. Now open left eye, close your right eye, and observe
your thumb and its background. Describe what you saw.

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2. Why does this happen?
The sketch below (sizes and distances are not scaled accurately) shows the lines of sight to a
nearby star from the Earth when it is on opposite sides of the Sun during its year-long orbit.
If this sketch was scaled accurately, the parallax angle “p” formed by the Earth, nearby star, and
Sun would be a fraction of a degree—in fact, impossible to see on this page. Nevertheless, modern
astronomical equipment is precise enough to measure such small angles.
3. What simple trigonometric equation could astronomers use to calculate the distance “d” from
the Sun to the star? They already know the length of the Earth-Sun leg of the triangle, and could
determine the parallax angle “p” from precise measurements. Just set up the equation; don’t try
to solve it (you can’t without knowing p). [Hint: it’s a right triangle...]

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C. Procedure
This experiment can be done outdoors or indoors; your instructor will decide based on the
weather, time, and other factors. Your instructor will identify the object whose distance you will
determine, and the baseline you will use.
1. Measure the length of your baseline, in feet to the nearest tenth of a foot (e.g., 7.4 feet).
baseline length: _______________
2. On the attached piece of graph paper, draw a scaled-down version of your baseline parallel to
and about one inch above the bottom edge of the paper. Choose your scale so that you use
about two-thirds of the available horizontal length.
3. Lightly tape the graph paper to a piece of backing board.
4. Stand at one end of your baseline. Stick a pushpin into your paper to mark your position.
5. Orient your graph paper so that the baseline on paper is exactly parallel to the real baseline.
6. Visually line up the pushpin and the object-to-be-measured. Stick a second pushpin along the
same line of sight. In other words, the object and both pins should be exactly aligned. Be sure
your pins are vertical.
7. Remove the pins. Label the pinhole on the baseline A, and the other pinhole A’.
8. Stand at the other end of the baseline, and repeat steps 4-6.
9. Remove the pins. Label the pinhole on the baseline B, and the other pinhole B’.
10. Using a ruler, draw a straight line from A through A’ all the way to the top of the graph. Draw
a second line from B through B’ to the top of the graph.
11. Using the same scale you used for your baseline, determine the straight-line distance from the
object to your baseline. Record it here: ___________________________
12. Now use the tape measure to determine the actual distance from your object to your baseline
(measured at a right angle to the baseline). Record it here: _____________________
13. The values recorded in #11 and #12 may not be (and probably aren’t) the same. Why not?
Describe at least one source of error or imprecision involved in using this technique.

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Hand in this graph along with the rest of this lab.