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Chapter 15

Chapter 15. Galaxies. What do you think?. Do galaxies all have spiral arms? Are most of the stars in a spiral galaxy in its arms? Are galaxies isolated objects? Are all galaxies moving away from the Milky Way?. Galaxies seem to take one of four different appearances. SPIRALS.

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Chapter 15

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  1. Chapter 15 Galaxies

  2. What do you think? • Do galaxies all have spiral arms? • Are most of the stars in a spiral galaxy in its arms? • Are galaxies isolated objects? • Are all galaxies moving away from the Milky Way?

  3. Galaxies seem to take one of four different appearances SPIRALS

  4. Galaxies seem to take one of four different appearances SPIRALS

  5. Galaxies seem to take one of four different appearances SPIRALS

  6. The tightness of a spiral galaxy’s arms is correlated to the size of its nuclear bulge Type Sa Type Sb Type Sc

  7. Variety of Spiral Arms Flocculent spirals (fleecy) Grand-design spirals (highly organized)

  8. We easily see these spiral arms because they contain numerous bright O and B stars which illuminate dust in the arms. However, stars in total seem to be evenly distributed throughout the disk.

  9. Self-propagating star formation and spiral density waves produce spiral arms

  10. Galaxies seem to take one of four different appearances BARRED SPIRALS

  11. Galaxies seem to take one of four different appearances BARRED SPIRALS

  12. Galaxies seem to take one of four different appearances BARRED SPIRALS

  13. Bars of stars run through the nuclear bulges of barred spiral galaxies Type SBa Type SBb Type SBc

  14. Type E0 Type E3 Type E7 Galaxies seem to take one of four different appearances ELLIPTICALS

  15. Galaxies seem to take one of four different appearances ELLIPTICALS

  16. Elliptical galaxies display a variety of sizes and masses • Giant elliptical galaxies can be 20 times larger than the Milky Way • Dwarf elliptical galaxies are extremely common and can contain as few as a million stars

  17. Galaxies seem to take one of four different appearances IRREGULAR

  18. Galaxies seem to take one of four different appearances • Spirals • Barred Spirals • Ellipticals • Irregulars

  19. This classification scheme is known as the Hubble Tuning Fork Scheme

  20. Galaxies are gravitationally bound into clusters and superclusters • Galaxies are not scattered randomly throughout the galaxy • Galaxies are found in clusters • The Milky Way is part of the Local Group of about 35 galaxies • Clusters of galaxies are clustered as well in groups called superclusters • Our Local Group is part of the Local Supercluster • The majority of space is empty - called voids.

  21. The Local Group

  22. Virgo Cluster

  23. Fornax Cluster

  24. Hercules Cluster

  25. Each dot represents a single galaxy

  26. Galaxies in a cluster can collide and combine NOTE: There is so much space between stars in a galaxy that the probability of two stars crashing into each other is extremely small.

  27. Colliding galaxies …. • Do not involve colliding stars • Might form hot intergalactic gas • Could initiate rapid star formation - called STARBURST GALAXIES • Cause galaxy mergers called “galactic cannibalism”

  28. Three galaxies, M81 (big), M82 (medium), and NGC 3077 (small).Are they related to one another?

  29. Galactic Cannibalism Computer Simulation

  30. Non-luminous material in galactic halos may account for some dark matter in the universe NOTE: No cluster or supercluster of galaxies contains enough visible matter to provide enough gravitational “glue” to keep the spinning clusters bound together.

  31. The redshifts of remote superclusters indicate that the universe is expanding • During the 1920s, Edwin Hubble and Milton Humanson were studying galaxy spectra • They found that galaxy spectral lines were redshifted • They measured the distance to galaxies by finding Cepheid variables and using the period-luminosity relationship • They found that the most distant galaxies had the largest redshifts • This is called the HUBBLE FLOW.

  32. Hubble’s Law slope = 75 km/s/Mpc called Hubble’s Constant

  33. Astronomers now measure distances to galaxies using supernovae and the Tully-Fisher relation • Standard candle method • If you know how bright something really is and compare that to how bright it looks, then the distance can be determined • Supernovae • All Type II a supernovae have the same luminosity • Tully-Fischer Relation • The broader the spectral line, the faster it is rotating and the more luminous the galaxy.

  34. What did you think? • Do galaxies all have spiral arms? No, galaxies may be either spiral, barred spiral, elliptical, or irregular. Only spirals and barred spirals have arms. • Are most of the stars in a spiral galaxy in its arms? No, the spiral arms contain only 5% more stars than the regions between the arms. • Are galaxies isolated objects? No, galaxies are grouped in clusters, and clusters are grouped in superclusters. • Are all galaxies moving away from the Milky Way? No, only the galaxies in other superclusters are necessarily receding from us.

  35. Self-Check 1: Draw and label the tuning fork diagram, showing the principal Hubble classes for galaxies, and describe the criteria for the assignment of subclasses. 2: Discuss the differences between flocculent spirals and grand-design spirals. 3: Explain what spiral arms are and how they are sustained. 4: Compare and contrast the contents of the various Hubble classes of galaxies. 5: Describe the distribution of galaxies in space, and distinguish between regular and irregular clusters of galaxies. 6: Explain the observational basis for the dark matter problem in spiral galaxies and in clusters of galaxies. 7: Discuss the role of collisions and mergers in our understanding of galaxies. 8: State the Hubble law and describe how the Hubble constant is evaluated and how it is used to determine the distances of galaxies.

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