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Chapter 19 Galaxies And the Foundation of Modern Cosmology

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Chapter 19 Galaxies And the Foundation of Modern Cosmology

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  1. Chapter 19Galaxies And the Foundation of Modern Cosmology

  2. How are the lives of galaxies connected with the history of the universe?

  3. Hubble Deep Field • Our deepest images of the universe show a great variety of galaxies, some of them billions of light-years away

  4. If the entire sky has a similar galaxy distribution there must be over 100 billion galaxies visible to existing telescopes. Spirals are more visible due to the luminosity of hot young stars. Ellipticals are more common, just harder to see. Irregulars are only about 25%

  5. Galaxies and Cosmology • A galaxy’s age, its distance, and the age of the universe are all closely related • The study of galaxies is thus intimately connected with cosmology— the study of the structure and evolution of the universe

  6. What are the three major types of galaxies?

  7. Hubble Ultra Deep Field

  8. Hubble Ultra Deep Field

  9. Hubble Ultra Deep Field Spiral Galaxy

  10. Hubble Ultra Deep Field Spiral Galaxy

  11. Hubble Ultra Deep Field Elliptical Galaxy EllipticalGalaxy Spiral Galaxy

  12. Hubble Ultra Deep Field Elliptical Galaxy EllipticalGalaxy Spiral Galaxy

  13. Hubble Ultra Deep Field Elliptical Galaxy EllipticalGalaxy Irregular Galaxies Spiral Galaxy

  14. halo disk bulge Spiral Galaxy

  15. Disk Component: stars of all ages, many gas clouds Spheroidal Component: bulge & halo, old stars, few gas clouds

  16. Disk Component: stars of all ages, many gas clouds Blue-white color indicates ongoing star formation Spheroidal Component: bulge & halo, old stars, few gas clouds Red-yellow color indicates older star population

  17. Thought Question Why does ongoing star formation lead to a blue-white appearance? A. There aren’t any red or yellow stars B. Short-lived blue stars outshine others C. Gas in the disk scatters blue light

  18. Thought Question Why does ongoing star formation lead to a blue-white appearance? A. There aren’t any red or yellow stars B. Short-lived blue stars outshine others C. Gas in the disk scatters blue light

  19. Barred Spiral Galaxy: Has a bar of stars across the bulge

  20. Lenticular Galaxy: Has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and elliptical)

  21. Elliptical Galaxy: All spheroidal component, virtually no disk component Red-yellow color indicates older star population

  22. Irregular Galaxy Blue-white color indicates ongoing star formation

  23. Hubble’s galaxy classes Spheroid Dominates Disk Dominates

  24. Spiral Galaxies NORMAL SPIRALS Range from Sa to Sc: Sa: little gas and dust, large nuclear bulges, tightly wound spiral arms To Sc: large clouds of gas and dust, small nuclear bulges, loosely wound arms Spiral Galaxies A mixture of stars hot/young and cool/older Short lived hot stars produce most of the light.

  25. Barred-Spiral Galaxy Shapes Barred Spirals have an elongated nucleus with spiral arms extending from the ends. Our galaxy is a barred spiral. Halos are not visible in these images but are thought to be there. See rotational curves.

  26. Elliptical Galaxy Shapes Elliptical galaxies are round or elliptical contain no visible gas and dust and lack hot, bright stars. Range from E0 to E7. Dwarf ellipticals are the most common, but giant ellipticals account for most of the mass.

  27. E index Elliptical Galaxies are classified by a numerical index 0 to 7. Index is calculated from ratio 10(a-b)/a where a and b are the semi-major and semi-minor axes of the elliptical cross section rounded to the nearest whole number. E0 s are round. E7s are highly elliptical p. 257

  28. M87 Giant Elliptical M 87 is classified as an E1. It is surrounded by over 500 globular clusters. p. 257

  29. Irregular Galaxy Shapes Irregulars are a chaotic mix of gas and dust and stars with no obvious bulge or spiral arms.

  30. How are galaxies grouped together?

  31. Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies)

  32. Elliptical galaxies are much more common in huge clusters of galaxies (hundreds to thousands of galaxies)

  33. Clusters of Galaxies • Rich galaxy clusters have more than a 1000 galaxies in a roughly spherical distribution , r~ 3Mpc(10 million lyr.) EG Virgo Cluster, 2500 galaxies home to the giant elliptical M87 Poor galaxy clusters have less than 100 EG the Local Group

  34. Local Group What we have nearby. Some 45 galaxies in 1Mpc of us. Only a few spirals, most are dwarf-ellipticals and irregulars.

  35. The Local Group Figure 13.8: (a) The Local Group. Our galaxy is located at the center of this diagram. The vertical lines giving distances from the plane of the Milky Way are solid above the plane and dashed below. Fig. 13-8a, p. 268

  36. Sagittarius Dwarf Galaxy Figure 13.8: (b) The Sagittarius Dwarf Galaxy (Sgr Dwarf) lies on the other side of our galaxy. If we could see it in the sky, it would be 17 timeslarger than the full moon Fig. 13-8b, p. 268

  37. Spiral galaxy Dwingeloo 1 Figure 13.8: (c) Spiral galaxy Dwingeloo 1, as large as our own galaxy, was not discovered until 1994 because it is hidden behind the Milky Way. Note the faint spiral pattern. All individual stars here are in our galaxy. (Courtesy Shawn Hughes and Steve Maddox, Royal Greenwich Observatory, and Ofer Lahav and Andy Loan, University of Cambridge) Fig. 13-8c, p. 268

  38. Local Supercluster

  39. How do we measure the distances to galaxies?

  40. Step 1 Determine size of solar system using radar

  41. Step 2 Determine distances of stars out to a few hundred light-years using parallax

  42. The relationship between apparent brightness and luminosity depends on distance: Luminosity Brightness = 4π (distance)2 We can determine a star’s distance if we know its luminosity and can measure its apparent brightness: Luminosity Distance = 4π x Brightness A standard candle is an object whose luminosity we can determine without measuring its distance

  43. Step 3 Apparent brightness of star cluster’s main sequence tells us its distance

  44. Knowing a star cluster’s distance, we can determine the luminosity of each type of star within it

  45. Variable Stars a) RR Lyrae periods vary from 0.5 to 1.0 day b) Cepheid periods range from 1 to 100 days. c) Cepheid photos from successive nights offset slightly.

  46. Cepheid variable stars with longer periods have greater luminosities

  47. 0 The Cepheid Method Allows us to measure the distances to star clusters throughout the Milky Way

  48. Tully-Fisher Relation Entire galaxies can also be used as standard candles because galaxy luminosity is related to rotation speed