Observing Projects
Use the Starry Night™ program to explore the alignment of the Milky Way with respect to our Earth-based coordinate system and its appearance at different electromagnetic wave-lengths. Select Favourites > Explorations > Milky Way to display a wide-field view of our spiral galaxy from the center of a transparent Earth at 6:30 a.m. on September 1, 2014. The view is similar to that seen by observers on Earth and shows an edge-on view of this galaxy in a direction toward the galactic center, represented by the marked position of a foreground star, HIP86948. (a) You can use this view to estimate the alignment of the galactic plane with our reference plane in the sky, the Celestial Equator, which is the projection of the Earth’s equator on to the sky. Display the plane of the galaxy by clicking on View > Galactic Guides > Equator and then show the Celestial Equator by clicking on View > Celestial Guides > Equator. Estimate the angle between these two planes. (b) This visible-light image of the Milky Way shows significant structure, with many dark regions. These dark regions do not show the absence of material in these directions but the presence of dust and gas clouds obscuring the distant stars. Visible light is scattered and absorbed by these dense clouds. You can examine this galactic plane at other electromagnetic wavelengths. Open the Options menu and choose Stars > Milky Way… to display Milky Way Options. Use the brightness slider to show maximum brightness. Click the Wavelength box to expand the list of possible wavelengths and display the image of the galactic plane at each wavelength. At which wavelengths does the Galactic Center show up most prominently? Why do you think this region shows up brighter at these wavelengths?
There are two distinct populations of stars in our galaxy, the Population I young, hot stars, associated with dust and gas clouds in which active star formation continues, and the Population II older stars with no associated dust and gas. You can explore the distribution of these populations of stars by examining the distribution of clusters of stars of each population. For example, Open Clusters are composed of Population I stars while Globular Clusters consist of older Population II stars. You can use the Starry Night™ program to explore the distribution of clusters in and around our galaxy. Select Favourites > Explorations > Milky Way to view the galaxy edge-on from the center of a transparent Earth. Before displaying the clusters, ensure that each type of cluster is assigned a different color by opening Options > Stars > Globular Clusters… and, in the Globular Clusters Options panel, change the Outline Colour to red and close the panel. Repeat this process to change the color of all clusters by opening Options > Stars > Star Clusters… and changing the Outline Color in the Star Clusters Options panel to blue. Open the View menu and click on Stars > Globular Clusters to display the distribution of these old Population II star clusters around the galactic center. (Early measurements of distances to many of these clusters, using variable stars with known intrinsic brightness as beacons, were used to determine the position of the center of the galaxy.) You can now display clusters of all types, both the Open Clusters of young stars (represented by dotted circles) and the older Globular Clusters (represented by solid circles), by selecting View > Stars > Star Clusters from the menu. With the above color selections for clusters, the globular clusters will change to purple and will be distinguishable from the remaining open clusters depicted in blue. (a) What do you notice about the distribution of the globular clusters compared to the open clusters? Use the hand tool to move along the Milky Way plane to examine the distribution of each type of cluster, and hence each population of stars, across the sky. (b) Note in particular the two extra concentrations of clusters off the galactic plane. Why do you think that these extra concentrations are in these positions in our sky? (Hint: Move the cursor over these regions to identify them.) (c) You can examine several of these star clusters in detail. Use the Find facility to look at and zoom in on the globular clusters M13, M80, M5, M2, NGC4590, and NGC7089 and the open clusters, the nearby Pleiades, NGC2264 and M7. Comment on the noted differences in the nature and structure of these different types of clusters.
Use the Starry Night™ program to measure the dimensions of the Milky Way Galaxy. Select Favourites > Explorations > Milky Way Galaxy to display a face-on view of a simulation of the Milky Way Galaxy from a distance of 0.128 Mly from the Earth. The position of the Sun is labeled, directly below the center of the Galaxy in the view on the screen. Note the spiral arm structure of the galaxy. (Recent research has revealed that our Galaxy appears to have a bar structure surrounding the galactic center.) (a) Click on the cursor selection tool to the left of the toolbar and activate the angular separation tool to measure the angular separation between the Sun and the center of the Galaxy as seen from this vantage point, and find the corresponding distance in light years (ly). (Both values are shown in the display beside the line drawn by the angular separation tool.) Convert the angular separation to a decimal number in degrees (1° = 60′ = 3600″); then find the scale of the image of the Galaxy in ly/degree. (b) Change the cursor to the location scroller and use it to view the Galaxy edge-on and oriented vertically on the screen, with the Sun still below the center. To do this, place the location scroller tool at the center of the right-hand edge of the screen, hold down the mouse button (on a two-button mouse, hold down the left mouse button), and move the location scroller directly to the left, toward the center of the Galaxy. Then change to the angular separation tool and use it to measure the angular separation of the Sun from the center of the Galaxy. This value should be approximately the same as you found in part a. (c) Use the angular separation tool to find the total angular diameter of the Milky Way Galaxy as seen from this viewpoint, measured from one end to the other of this edge-on view (zoom out if necessary). Round off the measurement to the nearest degree and then use the scale that you calculated in part (a) to find the approximate diameter of the Galaxy in ly. (d) In a similar fashion, use this viewpoint to measure the distance in degrees and ly for the following quantities: the diameter (in the plane of the Galaxy) of the central bulge, the thickness (perpendicular to the plane of the Galaxy) of the central bulge, and the thickness (perpendicular to the plane of the Galaxy) of the disk of the Galaxy at the location of the Sun. (e) In the edge-on view of this simulated galaxy, note the effect of the dust and gas clouds that obscure the light from the more distant spiral arms. Compare this simulated view with that of the edge-on view of a distant galaxy by clicking on Home to return to your present sky and using the Find facility to display NGC4565. (Note: The image of this galaxy is slightly offset from the position found by Starry Night™.) How similar is this galaxy to the Milky Way Galaxy, as represented by the Starry Night™ simulation?
Collaborative Exercises
Student book bags often contain a wide collection of odd-shaped objects. Each person in your group should rummage through their own book bags and find one object that is most similar to the Milky Way Galaxy in shape. List the items from each group member’s belongings and describe what about the items is similar to the shape of our Galaxy and what about the items is not similar; then indicate which of the items is the closest match.
One strategy for identifying a central location is called triangulation. In triangulation, a central position can be pinpointed by knowing the distance from each of three different places. First, on a piece of paper, create a rough map showing where each person in your group lives. Second, create a circle around each person’s home that has a radius equal to the distance that each home is from your classroom. Label the place where the circles intersect as your classroom. Why can you not identify the position of the classroom with only two people’s circles?
Figure 22-13 shows how emission spectra from hydrogen clouds would be shifted due to their motion around the Galaxy. Create a similar sketch showing an oval automobile racetrack with four cars moving on the track and a stationary observer outside the track at one end. Position and label the four moving cars all sounding their horns: (1) one that would have its horn sound shifted to longer wavelengths; (2) one that would have its horn sound shifted to shorter wavelengths; and (3) two cars moving in opposite directions so that their horn sounds would have no Doppler shift at all.
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