Name Lab Partners: Section Date Stars Above, Earth Below By Tyler Nordgren Laboratory Exercise for Chapter 10 Equipment: Balloon Ruler THE EXPANDING UNIVERSE Purpose: To create a simple universe and observe its expansion. We will measure the speed of expansion of this universe and determine its Hubble Law and age. We will then apply these methods to actual observations of galaxies in our own universe and thus calculate Hubble s Law and from it the age of our universe. INTRODUCTION Between 1912 and 1917, the astronomer Vesto Slipher at Lowell Observatory first noted that the majority of galaxies he observed had spectra that were red shifted. If these shifts to the red were the result of Doppler shifts, the conclusion is that these galaxies were all moving away from us. In the 1920s, Edwin Hubble of the Mt. Wilson Observatory measured distances to many galaxies using Cepheid variables and compared these distances to the velocity with which the galaxies were moving away. He found that the more distant the galaxy the faster it was moving. This is now called Hubble s Law. This law implies that our Universe is expanding. We shall see how this is the case by first creating model Universes out of balloons. We shall place galaxies on them, let them expand, measure their velocities and construct our own Hubble Laws. Once done, we will use these laws to measure the age of our model Universes. Following this experiment we will then use actual spectra from several real galaxies to repeat this procedure for our own Universe. PART 1: OUR MODEL UNIVERSE Instructions Your balloon is your Universe. Inflate and deflate it a couple times to get it limber. Now, inflate the balloon partially. Use the permanent marker to draw ten galaxies (or simple dots, if you wish) on the surface of the balloon. Choose one galaxy to be your home galaxy. Label it and then each of the other nine galaxies. Write the names of the nine galaxies in Table 1. Use the ruler provided to carefully measure the distance between your home galaxy and each of the other nine galaxies. We will pretend that each centimeter on your balloon is really a megaparsec. In Table 1 write the distances in megaparsecs (Mpc) between each galaxy and your home galaxy. Now, inflate your universe further, while being careful not to pop it. Measure the distances between
Page 2 galaxies again and enter the new distances in Table 1. TABLE 1 Galaxy Distance 1 Distance 2 Velocity Name Mpc Mpc km/s 1. 2. 3. 4. 5. 6. 7. 8. 9. We will pretend that 5 billion years passed between when you made your first set of measurements and when you made the second set. Calculate how fast each galaxy is moving away from your home galaxy in kilometers per second. Hint: 1 Mpc = 3.1 x 10 19 km. 1 year = 30 million seconds. Enter the velocity of each galaxy in the last column of Table 1.
Page 3 1.1 Are all galaxies moving away from the home galaxy? 1.2 Would this still be true if you had chosen a different home galaxy? 1.3 Do you see any relation between how fast a galaxy moves and its distance from your home galaxy? 1.4 Where is the center of your Universe s expansion? Is it on the surface of the balloon? In Figure 1 plot the second set of distances you measured for each galaxy (these are the distances the galaxies are currently at from your home galaxy) on the x-axis. On the y-axis plot the velocity. Be sure that both axes have zero at the origin. Draw the single best straight line you can that goes through the origin and the set of data points. This line is simply the equation: V = H o x D where H o is called Hubble s constant. Draw a circle around the equation above so you won t forget it. You will lose ten points for this lab if you don t. Do not discuss this with others, or you will lose ten points. 1.5 What is the value for your Hubble s constant? Find it by measuring the slope of your line in km/s per megaparsec.
Page 4 FIGURE 1
Page 5 Run your balloon experiment backwards in your mind. If you could take all of the air out of your balloon and shrink it to a point, all of the galaxies would have zero separation. This would be the moment of birth of your balloon universe. We can calculate how long your Universe has been around using your value for Hubble s constant. The time it has taken for each galaxy to get from a single point to where it is on your balloon now is simply: time = distance / velocity Your Hubble s Law says: So: Or: velocity = H o x distance time = distance / (H o x distance) time = 1/H o We measured the Hubble constant in km/sec/mpc. These are strange measurement units: distance per time per distance! If we can get both of the units of distance to be the same, then they will cancel, and we will be left with just the units of 1/time. In fact, the real units of the Hubble constant are 1/sec. 1.6 Convert the units of the Hubble constant to1/sec by dividing it by 1 Mpc = 3 x 10 19 km. Hubble constant 1/sec. 1.7 Calculate the age of your Universe in seconds by finding 1 over the Hubble constant. Age sec 1.8 Finally, how many years is that? (Hint: There are about 30 million seconds in a year.) The age of your Universe is years How many billions of years is that? WAIT for the rest of the class to finish part 1.
Page 6 PART 2: THE REAL UNIVERSE Instructions In Figure 2 are spectra of five relatively nearby galaxies. Distances to these galaxies have been derived by other methods and are given in the middle column of Figure 2. The spectra show two separate sets of spectral features. Notice that the copy here is a positive so that absorption lines appear dark, emission lines and the background continuum appear white. The emissions lines at the top and bottom are those of a helium and hydrogen comparison standard (like the ones in the spectroscopy lab). The diagram below identifies the hydrogen and helium lines and their appropriate wavelengths. In Figure 2, the central spectrum is that of the galaxy itself. The most prominent features are two absorption lines of singly ionized calcium. The one on the right is called the calcium H-line, while the one on the left is the K-line. Notice that the lines do not appear in the same place (relative to the comparison lines) on the different spectra, because each of the galaxies has a different velocity and hence a different Doppler shift. The H- and K-lines have the following wavelengths (in the laboratory, at rest). H - 3968 Å K - 3933 Å The average rest wavelength is 3951 Å. For each galaxy spectrum, a horizontal arrow indicates the shift of the center of the H and K pair. The base of the arrow is the center of the H and K pair (3951 Å) in the comparison spectrum; the tip is at the center of the pair in the galaxy spectrum. 2.1 Determine the plate scale; that is, the number of Angstroms per millimeter in this reproduction. First, measure the distance in millimeters from the left-most comparison line to the right-most comparison line in one of the galaxy spectra in Figure 2. Distance mm 2.2 Take the difference of the wavelength values between those same two lines as given in the diagram at the bottom of Figure 2. Wavelength difference Å 2.3 The plate scale is just the wavelength difference divided by the distance. Plate scale Å/mm
Page 7 FIGURE 2
Page 8 For each galaxy, write the distance in Mpc in Table 2 (these distance are given in Figure 2). Measure with a ruler (in millimeters) how far the midpoint of the H- and K-lines has shifted from its stationary position. (The amount of red shift is shown by the length of the arrow.) Multiply this number by the plate scale to get the redshift in angstroms. Finally, use the Doppler formula to get the recessional velocity. Write all of these in Table 2. TABLE 2 Galaxy Distance in Mpc Shift in mm Shift in Å Velocity in km/sec #1 #2 #3 #4 #5 Doppler formula (where the speed of light = 3 x 10 5 km/s): Velocity Speed of light Shift in A Rest wavelength
Page 9 FIGURE 3 Just as you did for Figure 1 in Part 1, use Figure 3 to plot the distance and velocity of each of
Page 10 the galaxies in Table 2. We are now going to use the exact same technique as in Part 1 to find Hubble s Law and the age of the real Universe. 2.4 Draw a straight line passing through (or as close as possible to) the data points. Be sure the line goes through the origin. Determine the Hubble constant as you did before by finding the slope of the line. Hubble constant km/sec/mpc 2.5 Convert the units of the Hubble constant to 1/sec. (1 Mpc = 3 x 10 19 km.) Hubble constant 1/sec. 2.6 Calculate the age of the Universe in seconds by finding 1 over the Hubble constant. Age sec 2.7 Finally, how many years is that? (Again, there are about 30 million seconds in a year.) The age of the Universe is years Geologists estimate the age of the Earth to be 4.5 x 10 9 years. Is the Universe younger or older than the Earth? Sections of this lab are taken from the Astronomy 101 course at Cornell University.