FROZEN IN TIME?
Last week you learned about the electromagnetic spectrum. Now let's put your knowledge to use! Using light as data is called "spectroscopy". On to the lab...
In this interactive laboratory, you'll investigate for yourself how fast several galaxies are moving. Here's what you'll need:
This set of instructions to guide you.
The Virtual Spectroscope. This device allows you to observe and compare the patterns of light -- called spectra -- produced by different sources of light. Open the Virtual Spectroscope in a separate window by clicking here.
A notebook to record your measurements
STEP 1. GETTING A FEEL FOR THE SPECTROSCOPE
Take a few minutes to explore what the spectroscope actually shows you.
Start by selecting the Sun on the pull-down menu labeled Source of light. The pattern you see is produced by passing light from the Sun through a glass prism, or similar device, which separates the light into its component colors.
The pattern is the familiar rainbow of colors. Note that the pattern extends past the red, into a region called the infra-red. Infra-red is not visible to our eyes, but is detectable by photographic film or special instruments. It is colored grey in this image.
STEP 2. WHAT DO THE PATTERNS TELL US?
Now select the Fluorescent Lamp from the Source menu. Instead of a rainbow, we see only certains colors of light. We don't see a rainbow, because rainbows are produced only by light sources that are very hot. The pattern of lines that we see is a kind of "fingerprint" that is unique to the particular types of molecules in the lamp.
STEP 3. "FINGERPRINTING" AN ELEMENT
Now select Hydrogen from the Source menu. Hydrogen is the simplest chemical element. The pattern you see was produced by taking the light from a glowing tube of hydrogen gas, and passing the light through a prism.
There is one bright red line, a fainter blue line, and several other very faint lines. This pattern is characteristic of the element hydrogen. If you see this unique pattern in the light from an unknown source, then you can conclude that the source must contain the element hydrogen.
For each color of light in the pattern, it's easy to read off the wavelength of that color: Just move the cursor along the Emission Graph and center the vertical line on the corresponding peak on the graph. The wavelength appears as number at the upper right of the graph. Note that the red line for hydrogen has a wavelength of 656 nanometers. (A nanometer is one-billionth of a meter, or about one-thousandth the width of a single bacterium.)
The element hydrogen is the most common element in the universe, and it is plentiful in galaxies. That will help us as we investigate the speeds of galaxies.
STEP 4. EXPLORING THE DOPPLER EFFECT
What happens when a source of waves moves? You can investigate for yourself using this interactive. First see what happens when the source is not moving. Then experiment with clicking the "Observer Moves" and "Source Moves" buttons. What happens to the wavelength of the waves in each case?
Your prediction: Based on your observations, what do you predict you will observe if a source of waves move towards you: Will the wavelength of the waves appear shorter, longer, or the same as when the source is stationary? How about for a source that is moving away from you?
This phenomenon is called the "Doppler effect." It applies to all kinds of waves, such as light waves, sound waves, and water waves. Why don't you observe this effect when you ride a bicycle down the street, for example? (Hint: For the effect to be noticeable, how fast should the source be moving, relative to the speed of the waves themselves?)
STEP 5. UNDERSTANDING "REDSHIFT"
Now select Galaxy 1 from the Source menu. This is the pattern produced when the light from this distant galaxy was passed through a prism.
Note that the spectrum includes a faint rainbow. What do you think is the source of this rainbow? (Hint: What's in a galaxy?)
In addition to the rainbow, there is a bright red line. You may also be able to make out a fainter blue line as well. These lines should be familiar from Step 3: They come from the element hydrogen, which is the most common element in the universe. Hydrogen is present in huge clouds of gas that fill some of the space between the stars in a galaxy.
But there's something unusual about these lines. Use the cursor to determine the wavelength of the red line. (Do this by positioning the cursor over the peak corresponding to the red line.) Note that the position of this peak is no longer where it was in the laboratory sample of hydrogen in step 3. Instead, the peak has been shifted towards the longer wavelength partof the spectrum, which is the redder end of the spectrum. This phenomenon is called a "redshift."
Based on your experiments with the Doppler effect, would you conclude that Galaxy 1 is moving away from Earth or towards Earth?
STEP 6. "CLOCKING" A GALAXY
Now select Galaxy 3 from the Source menu. The lines are redshifted even more than for Galaxy 1. Based on your investigation of the Doppler effect, what does this tell you about the speed of Galaxy 3, compared to Galaxy 1?
It turns out that the amount of the observed redshift is proportional to the speed of the source (for speeds that are not close to the speed of light). For example, for a galaxy moving away from us at 10% of the speed of light, its light will be redshifted by 10%. So, for this example, the hydrogen line that was at 656 nanometers will be redshifted by about 65 nanometers.
Can you tell how fast Galaxy 3 is receding from us? Use the spectroscope to measure the redshift of this galaxy. First determine the wavelength of the red hydrogen line, and then compare it to the wavelength of this line in the laboratory sample of hydrogen gas. By how much has the line been shifted? What fraction of the original wavelength is this? What fraction of the speed of light is the galaxy moving?
Congratulations! It's one thing to measure the speed of a car or a baseball pitch... but you've just measured the speed of a galaxy from millions of trillions of miles away!
Images of Galaxies. You can obtain galaxy images that are all at the same scale, at the Digitized Sky Survey from the Space Telescope Science Institute.
Speeds of Galaxies. You can compare your results with published measurements from astronomers, at NASA's Extra-Galactic Data Base (NED).
NASA's Office of Space Science by the
Smithsonian Astrophysical Observatory
© 2001 Smithsonian Institution
Open source: https://www.cfa.harvard.edu/seuforum/galSpeed/
Submit your lab answers to the dropbox.