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Classification of Galaxies1 Name _____________________________________ Date ____________ Section _____________ Classification of Galaxies1 Hubble’s Tuning Fork was once thought to represent the evolutionary sequence, with galaxies starting out as ellipticals and evolving to having more structure as they aged into spirals (top of right fork) or barred spirals (bottom of right fork). In this activity we investigate the defining characteristics of the different types of galaxies (ellipticals versus spirals) and work to understand the types better by studying the ages of stars, the amount of dust present, and gas and star formation. Step 1 Classifying galaxies Table 1. The galaxies shown in order of the World Wide Telescope tour in the planetarium. Galaxy ID Constellation Defining Characteristics M87 (NGC 4486) Virgo M61 (NGC 4303) Virgo NGC 4643 Virgo M60 (NGC 4649) Virgo M88 (NGC 4501) Coma Berenices M91 (NGC 4548) Coma Berenices NGC 4881 Coma Berenices NGC 4874 Coma Berenices 1 Inspired by Lecture Tutorials for Introductory Astronomy, 2nd ed., Pearson Education Inc. (2008) 8/4/14 l08_classifying_galaxies.docx 1 Step 2 Applying Hubble’s classification scheme based on a galaxy’s morphology 1. Using the images displayed in the planetarium tour, sort the 8 galaxies into either the elliptical or spiral category. Fill in the table that follows. Be sure to note a few defining characteristics that you used (shape, color, size, or any other distinctive features). Hubble’s Galaxy ID Defining characteristics category numbers M87 (NGC 4486) NGC 4881 Elliptical NGC 4874 M60 (NGC 4649) M61 (NGC 4303) NGC 4643 Spiral M88 (NGC 4501) M91 (NGC 4548) Step 3 Understanding the underlying physical characteristics of these types of galaxies What can the morphology of a galaxy tell us about its physical characteristics? These characteristics include a. the ages of the stars in the galaxy; b. the presence of dust; and c. the presence or absence of gas and star formation. All of these properties are linked together in a physical way. Here we want to find out how these characteristics are related to a galaxy’s classification. We start with a review of stars and the H-R Diagram to answer this question about the morphology of galaxies. If you were to view a cluster of stars that contained thousands and thousands of stars, could you decide what kinds of stars were dominating the light coming from each cluster? Shown at the right is a typical HR diagram showing all of the possible regions where stars are found. This diagram could not be for a single cluster of stars as there would not be both massive stars AND white dwarfs and red giants in the same cluster. But, we can use it to infer the kinds of stars dominating the light from a cluster. 2 2. You observe a cluster of stars that has many stars radiating blue light. What kind of stars must be dominating this cluster? a. white dwarfs b. massive main sequence stars and blue giants c. red supergiants and red giants d. low mass main sequence stars 3. A new cluster of stars has been discovered where all of the stars seem to be reddish-orange (no star ever appears to be pure red). You conclude that this cluster’s light is dominated by a. white dwarfs. b. massive main sequence stars and blue giants. c. red giants. d. low mass main sequence stars. Step 4 The ages of stars in the galaxies Now, refer to the galaxies numbered 1 - 8 in the images being shown either through slides, handouts, or a colored version of this document. Assume that the colors shown represent the true colors of the galaxies. Answer the questions that follow. 4. List the number and classification for each galaxy that appears to be mostly reddish-orange or orangish-yellow. Using the same star groupings given on the HR diagram above, what is dominating the light coming from these galaxies? 5. List the number and classification for each galaxy that appears to be mostly blue. Why do you think these galaxies appear blue? 6. Which type of galaxies (spiral or elliptical) appears to have many young stars as indicated by their overall color: elliptical, spiral, or both? 7. Do the type of galaxies you identified in question 6 also appear to contain old stars? How do you know? 3 8. Would you state that the galaxies identified in question 4 above contained mostly old or mostly young stars? Explain your reasoning. Step 5 Dust in galaxies Besides stars, galaxies sometimes contain dust. This dust produces dark bands across or patches in the image of the galaxy. 9. Which of the galaxies show evidence of dust? Note each galaxy’s number and classification. Step 6 Gas and star formation in galaxies 10. A galaxy that is experiencing active star formation must contain abundant gas. Which type of galaxy appears to have abundant gas available? What criteria did you use when examining the images to answer this question? 11. Which type of galaxy (elliptical or spiral or both or neither) is likely to contain all spectral types of stars, from O to M? Explain your choice. 4 .
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