Observing Projects
In an attempt to explore the far reaches of the universe, the Hubble Space Telescope (HST) took long-exposure images of very dark regions of space that appear to contain no bright stars or galaxies. These images, known as the Hubble Deep Field and Hubble Ultra Deep Field images, reveal very rich fields of faint and very distant galaxies. The light now arriving at Earth from some of these galaxies has traveled for more than 13 billion years and was collected by the HST at a rate of a few photons per minute! This light was emitted very early in the life of the universe, only a few hundred million years after the Big Bang. You can examine and measure these two images. (a) In Starry Night™, open the Options pane and ensure that the Hubble Images option is checked in the Deep Space layer. Open the Find pane, ensure that the search edit box is empty, and click on the icon in this box to display a list of image sources. Click on Hubble Images and double-click on Hubble Deep Field to center the view on this dark region of space. Note its position with respect to the Big Dipper. (Note: If you cannot identify this region of the northern sky, click on View > Constellations > Asterisms and View > Constellations > Labels. Remove these indicators after you have identified the region.) Zoom in to a field of view about 3° wide and note that the region still appears to be devoid of objects. Zoom in again until the Hubble Deep Field (HDF) fills the view. One-quarter of the full HDF, with dimensions of 1.15′ × 1.15′, is displayed in Starry Night™.
The bright object with spikes radiating from it is a star in our own galaxy (the spikes are caused by diffraction by the supports for Hubble’s secondary mirror), but all of the other objects that appear on this long-exposure image are galaxies containing millions of stars. Examine this image carefully and attempt to identify some examples of each kind of galaxy—spiral, barred spiral, elliptical and irregular—in this field. Choose 5 or 6 of the largest galaxies in this field, record their shapes and galaxy types, and use the angular separation tool to measure carefully and record their angular dimensions. (b) Click the Zoom panel in the toolbar and select 90° from the dropdown menu. Return to the Find pane and the list of Hubble Images and double-click on Hubble Ultra Deep Field (HUDF) to center the view of this “dark” region of the sky. Zoom in on this region and note that, even at a field of view as small as 2º, no objects can be seen in the position of this long-exposure image. Zoom in further until the full HUDF, with dimensions of 3.3′ × 3.3′, fills the field of view to see this rich field of faint and very distant galaxies. Examine this image carefully and attempt to identify each kind of galaxy—spiral, barred spiral, elliptical and irregular—in this field. Again, select 5 or 6 of the largest galaxies in this field, record their shapes and galaxy types and use the angular separation tool to measure their dimensions. (c) Consider the mix of different kinds of galaxies and assess whether the proportions of different kinds are the same in these two images. Compare the angular sizes of the largest galaxies in these two images.
Use Starry Night™ to compare the distances of objects in the Tully Database with the radius of the limit of our observable universe, the Cosmic Light Horizon. As you will find, the most distant galaxies in this database are a long way away from Earth and yet these distances are only a small fraction of the distances from which we can see light in our universe. Select Favourites > Explorations > Tully Database to display this collection of galaxies in their correct 3-dimensional positions in space around the Milky Way Galaxy, as seen from a distance of about 0.5 Mly from the Sun. Note the Zone of Avoidance surrounding the Milky Way, where material within the arms of the galaxy prevents us from seeing objects in these regions of our sky. Now use the Increase current elevation button (to the left of Home button) in the toolbar to increase the distance from the Milky Way to about 900 Mly, when the outer extent of the Tully Database becomes visible. Open File > Preferences, select Cursor Tracking (HUD) in the dropdown menu and ensure that Distance from observer, Name, and Object type are selected. Select the angular separation tool from the cursor tool selection at the upper-left of the screen. (a) Use this tool to measure the distances from the Milky Way to a selection of outer galaxies. (b) How do these distances compare with the Cosmic Light Horizon? (c) What fraction of the radius of the observable universe is covered by the Tully Database?
Collaborative Exercises
As a group, create a four- to six-panel cartoon strip showing a discussion between two individuals describing why the sky is dark at night.
Imagine your firm, Creative Cosmologists Coalition, has been hired to create a three-panel, folded brochure describing the principal observations that astronomers use to infer the existence of a Big Bang. Create this brochure on an 8½ × 11 piece of paper. Be sure each member of your group supervises the development of a different portion of the brochure and that the small print acknowledges who in your group was primarily responsible for which portion.
The three potential geometries of the universe are shown in Figure 25-14. To demonstrate this, ask one member of your group to hold a piece of paper in one of the positions while another member draws two parallel lines that never change in one geometry, eventually cross in another geometry, and eventually diverge in another.
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