1. The Nature of Galaxies Chapter 17

2. Other Galaxies External to Milky Way –established by Edwin Hubble –used Cepheid variables to measure distance M31 (Andromeda Galaxy) far outside Milky Way Three basic types: –elliptical –spiral –irregular Elliptical Spiral Irregular

3. Spiral Galaxies Similar to Milky Way: –thin disk + nuclear bulge + halo –Disk contains: dust and gas –H II regions, H I regions, molecular clouds spiral arms active star formation open clusters mixture of young & old stars –Halo contains: old stars Globular Clusters –Bulge contains: old stars

4. Spiral Galaxies

5. Barred Spiral Galaxies Some spirals have bar in center –“barred spirals”

6. Elliptical Galaxies Shape ranges from: –spherical to ellipsoidal Characteristics: –no disk or spiral arms –old reddish stars similar to halo or bulge in spirals –little gas or dust –little star formation

7. Irregular Galaxies No specific shape –often appear chaotic Often have intense star formation –gravitational interaction with other galaxies? Mixture of old and new stars

8. Interacting Galaxies

9. Cartwheel Galaxy Simulation by C. Mihos et al., CWRU NASA/HST

10. Galaxy Masses For spirals: –use Doppler shift; measure galaxy rotation –make rotation curve –calculate mass using Kepler’s Law For ellipticals: –use Doppler shift; measure stellar orbital velocities –calculate mass using Kepler’s Law Results –Giant ellipticals and spirals are most massive; –irregulars & dwarf ellipticals least massive

11. Mass-to-Light Ratios ratio of mass to luminosity –for Sun, M/L = 1 –average star M/L = 2 to 3 –for entire Galaxy M/L ~ 100 90% of galaxy mass is unseen

12. Extragalactic Distances Compare apparent and absolute brightness Variable Stars: –Cepheids, RR Lyrae Standard Candles: –brightest stars, supernovae, planetary nebulae Galaxy techniques: –For spirals: rotation rate gives mass mass depends on number of stars, hence luminosity –For ellipticals: range of stellar velocities depends on mass (hence luminosity)

13. Fifteen years ago, a quasar was observed that was found to be located 8 billion light years away. If our universe is approximately 15 billion years old, when did the quasar emit the light that we observe? A.15 years ago B.7 billion years ago C.8 billion years ago D.15 billion years ago

14. Galaxy Motion Galaxy spectra: –absorption lines redshifted –more distant galaxies have larger redshift ALL galaxies moving away

15. The Hubble Law Hubble Law: –The more distant the galaxy, the faster it is moving away. v = H d (H is the Hubble constant, d is distance)

16. Implications of Hubble Law Every galaxy moving away Farther away = faster Conclusion: –Universe is expanding Predicted by Einstein’s Theory of Relativity Are we at center? NO –universe same in all directions –there is no center!

17. The Expanding Universe A uniformly expanding universe –explains Hubble law example: expanding loaf of raisin bread Galaxies (like raisins) not moving, not expanding Space is expanding