Why did many nineteenth-century astronomers think that the “spiral nebulae” are part of the Milky Way?
What was the Shapley-Curtis “debate” all about? Was a winner declared at the end of the “debate”? Whose ideas turned out to be correct?
How did Edwin Hubble prove that the Andromeda “Nebula” is not a nebula within our Milky Way Galaxy?
What is the Hubble classification scheme? Which category includes the largest galaxies? Which includes the smallest? Which category of galaxy is the most common?
Which is more likely to have a blue color, a spiral galaxy or an elliptical galaxy? Explain why.
Why are Type Ia supernovae useful for finding the distances to very remote galaxies? Can they be used to find the distance to any galaxy you might choose? Explain your answers.
What is the Tully-Fisher relation? How is it used for measuring distances? Can it be used for galaxies of all kinds? Why or why not?
Some galaxies in the Local Group exhibit blueshifted spectral lines. Why aren’t these blueshifts violations of the Hubble law?
What is the difference between a cluster and a supercluster? Are both clusters and superclusters held together by their gravity?
What are starburst galaxies? How can they be produced by collisions between galaxies?
What evidence is there for the existence of dark matter in clusters of galaxies?
What is gravitational lensing? Why don’t we notice the gravitational lensing of light by ordinary objects on Earth?
When quasi-stellar radio sources were first discovered and named, why were they called “quasi-stellar”?
Earth is composed principally of heavy elements, such as silicon, nickel, and iron. Would you be likely to find such planets orbiting stars in the disk of a spiral galaxy? In the nucleus of a spiral galaxy? In an elliptical galaxy? In an irregular galaxy? Explain your answers.
Discuss the advantages and disadvantages of using the various standard candle distance indicators to obtain extragalactic distances.
Describe what sorts of observations you might make to search for as-yet-undiscovered galaxies in our Local Group. How is it possible that such galaxies might still remain to be discovered? In what part of the sky would these galaxies be located? What sorts of observations might reveal these galaxies?
In the early twentieth century, there was considerable debate about the nature of spiral nebulae and their distance from us, but the debate was resolved by improvements in technology. As a group, list three issues that we, as a culture, did not understand in the past but understand today, and explain why we now have that understanding.
Even though there are billions of galaxies, there are not billions of different kinds. In fact, galaxies are classified according to their appearance. As a group, dig into your book bags and put all of the writing implements you have (pens, pencils, highlighters, and so on) in a central pile. Remember which ones are yours! Determine a classification scheme that sorts the writing implements into at least three to six piles.
Write down the scheme from Exercise 2 and the number of items in each pile. Ask the group next to you to use your scheme and sort your materials. Correct any ambiguities before submitting your classification scheme.
Imagine your company, Astronomical Artistry, has been contracted by the local marching band to create a football halftime show about spiral galaxies. How exactly would you design the positions of the band members on the field to represent the different spiral galaxies of classes Sa, Sb, and Sc? Create two columns on your paper by drawing a line from top to bottom, drawing sketches in the left-hand column, and writing a description of each sketch in the right-hand column. Also include what the band’s opening formation and final formation should be.
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Use Starry Night™ to visit a variety of galaxies and determine whether they are spiral, barred spiral, elliptical, or irregular. Click on Home to see the sky from your home location. Click on the Options tab, expand the Deep Space layer and click Off all images except Messier Objects and Bright NGC Objects. Type Ctrl-H (Cmd-H on a Mac) or select View > Hide Horizon from the menu to remove the horizon. Also select View > Hide Daylight to remove daylight from the view. Use the Find pane to visit each of the galaxies listed below. First click the icon in the search box of the Find pane and choose Search All from the menu. Then, for each object, type its name in the search box of the Find pane and press the Enter key. (Hint: To go to the galaxy without slewing, press the spacebar.) Use the Zoom buttons to examine each galaxy in detail and then classify it as a spiral (S), barred spiral (SB), elliptical (E), or irregular (Irr), and the subclassification of each galaxy (e.g., Sa, E5).
Clusters of galaxies contain different numbers and distributions of galaxies and harbor significant amounts of the mysterious dark matter. In this exercise, you can use Starry Night™ to compare a few of these groupings and see the gravitational effect of dark matter. You can start by looking at one of the largest galaxy clusters, the Virgo cluster. Select Favourites > Explorations > Virgo Cluster-Milky Way from the menu. You are looking at this group of galaxies from a very large distance out in space, at about 66 Mly from the Sun. Our Milky Way Galaxy is labeled at the bottom left of the view, across a void in space from this cluster. Use the location scroller to move around the Virgo cluster and consider its overall shape and its relationship to neighboring galaxies.
Use the Starry Night™ program to investigate the large-scale structure of the universe. Open Favourites > Explorations > Large Scale Structure, and select the location scroller tool. The main window shows a view looking toward Earth from a location 311 million ly away. (For comparison, the Andromeda Galaxy is only about 2 million ly away from Earth.) Each dot on the screen is a galaxy. If you position the cursor anywhere on the screen and click and hold the mouse button (left mouse button on a two-button mouse), a small, circular arrow appears near the bottom center of the screen. The Milky Way Galaxy, with Earth and the Sun in it, is near the center of this circular arrow, although Earth and the Sun are too small to be visible in this view.
Use the Starry Night™ program to observe a peculiar galaxy, a galaxy that shows features not included in the Hubble classification scheme. Click the Home button in the toolbar to place yourself at your home location at the present time and click the Stop button. Use the View menu or button bar to hide the horizon and hide daylight.