Participation - From Additional Course Pack Materials

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. Messier 87 Virgo Messier 61 Virgo NGC 4643 Virgo NGC 4643 Virgo Messier 88 Virgo NGC 4649 Coma Berenices Messier 91 Coma Berenices NGC 4881 Coma Berenices NGC 4874 Coma Berenices 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 oher distinctive features). Generalize! Shown at the left 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. Start with star clusters

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. Extend our knowledge of stars to galaxies

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?

M87 (NGC 4486) NGC 4881 NGC 4874 M60 (NGC 4649) Extend our knowledge of stars to galaxies

5. List the number and classification for each galaxy that appears to be mostly blue. Why do you think these galaxies appear blue?

M61 (NGC 4303) NGC 4643 M88 (NGC 4501) M91 (NGC 4548) 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? 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. Today’s goals:

Describe the Hubble “tuning fork” and summarize its use today. State the basic differences between elliptical and spiral galaxies in how they look at visible wavelengths Be able to classify galaxies based upon their morphology Outline the general differences in kinds of stars dominating each type, the presence of gas and dust, and the presence or absence of star formation

21 Hubble tuning fork

22 Describe the Hubble “tuning fork” and summarize its use today. Elliptical galaxies Spiral galaxies

M89 E0

State the basic differences between elliptical and spiral galaxies in how they look. 23 Spiral galaxies: List at least 3 characteristics.

24 State the basic differences between elliptical and spiral galaxies in how they look. Ellipticals: List at least 3 characteristics.

25 State the basic differences between elliptical and spiral galaxies in how they look. Irregular galaxies are - irregular. Options:

Spiral Barred spiral Elliptical Irregular Merging

27 Characteristic Ellipticals Spirals Irregulars These have numbered types ranging from E0 to E7. This type have disks, arms, central bulges. Some in this class have bar-like structures across their bulges. Entire galaxy has stars moving in all possible directions. Most stars follow nearly circular orbits, traveling in the same direction. Contain large amounts of dust and cold, dense gas concentrated in disk. Contain little or no dust and cold gas, just extremely hot X-ray gas. Active star formation is an on-going event. Little or no star formation occurring in these galaxies. These are without symmetry in shape or structure. These contain primarily old stars (red giants) and thus look orangish. These have regions of red (hot hydrogen) and blue (massive stars). These galaxies have been distorted by close encounters or mergers.

Outline the general differences in kinds of stars dominating each type, the presence of gas and dust, and the presence or absence of star formation 28