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Chapter 15 A Universe of Galaxies

Chapter 15 A Universe of Galaxies

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Chapter 15 A Universe of Galaxies

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  1. Chapter 15A Universe of Galaxies

  2. In this chapter we will see: • What are the three types of Galaxies? • How do we measure the distances to galaxies? • How did galaxies form? • What are quasars/supermasive blackholes?

  3. 15.1 Islands of Stars • Our Goals for Learning • What are the three major types of galaxies? • How are galaxies grouped together?

  4. What are the three major types of galaxies?

  5. Hubble Ultra Deep Field: all of this picture is just a tiny dot on our sky. Hubble telescope collected light for about 10 days from this point on the sky, and was able to resolve all this galaxies. How old is the Universe? When did first galaxies form? Some of the Galaxies we see here are one of the first formed…

  6. Hubble Ultra Deep Field

  7. Hubble Ultra Deep Field Spiral Galaxy Spiral Galaxy

  8. Hubble Ultra Deep Field Spiral Galaxy Spiral Galaxy

  9. Hubble Ultra Deep Field EllipticalGalaxy Elliptical Galaxy Spiral Galaxy Spiral Galaxy

  10. Hubble Ultra Deep Field Elliptical Galaxy Elliptical Galaxy Spiral Galaxy

  11. Hubble Ultra Deep Field Elliptical Galaxy Elliptical Galaxy Irregular Galaxies Spiral Galaxy Spiral Galaxy

  12. Types of galaxies:

  13. halo disk bulge Spiral Galaxy

  14. There are many variations on the theme of spiral galaxies… Barred Spiral Galaxy: galaxies bulge elongated with spiral arms starting at the end of this bar. Milky way might be a barred spiral galaxy!

  15. The sombrero Galaxy: Spheroidal Component (halo & bulge) Disk Component

  16. Disk Component: stars of all ages, many gas and dust clouds Spheroidal Component: bulge & halo, old stars, little gas and dust

  17. 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

  18. 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

  19. 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

  20. 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

  21. Elliptical Galaxy

  22. Elliptical Galaxy All spheroidal component, virtually no disk component: looks much like the bulge and halo of spiral galaxy.

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

  24. Irregular Galaxy

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

  26. Hubble invented a system for classifying galaxies: Spheroid Domina-tes Disk Dominates Astronomers hoped that classification of galaxies might yield deeper insight (as it was the case with the stars), but galaxies turned out to be far more complex than the stars…

  27. How are galaxies grouped together?

  28. Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies) As for example our local group!

  29. Elliptical galaxies are much more common in huge clusters of galaxies (hundreds to thousands of galaxies). This will help us understand the origin of galaxy types…

  30. What have we learned? • What are the three major types of galaxies? • (1) Spiral galaxies have prominent disks and spiral arms. • (2) Elliptical galaxies are rounder and redder than spiralgalaxies and contain less cool gas and dust. • (3) Irregular galaxies are neither disklike nor rounded in appearance.

  31. What have we learned? • How are galaxies grouped together? • Spiral galaxies tend to collect in groups of galaxies, whichcontain up to several dozen galaxies. • Elliptical galaxies aremore common in clusters of galaxies, which contain hundredsto thousands of galaxies, all bound together by gravity.

  32. 15.2 Distances of Galaxies • Our Goals for Learning • How do we measure the distances to galaxies? • What is Hubble’s Law? • How do distance measurements tell us the age of the universe?

  33. How do we measure the distances to galaxies?

  34. Brightness alone does not provide enough information to measure distance

  35. We measure distances ‘in steps’: basic idea is to determine distance to some closer object, and then use that knowledge to measure distances bigger yet. First two steps we have seen already:

  36. Step 1 Determine size of solar system using radar

  37. Step 2 Determine distances of stars out to a few hundred light-years using parallax. What is the size of Milky Way?

  38. Step 3 To measure distances within Milky Way (100,000 ly) we use apparent brightness of stars in star clusters. We know the distances to the Hydes star cluster through parallax. Comparing the apparent brightness of its main sequence stars to those stars in other clusters, gives us the distances to other clusters!

  39. If Pleiades are 7.5 times dimmer than Hyades, how much farther Pleiades are?

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

  41. Thought Question Which kind of stars are best for measuring large distances? A. High-luminosity stars B. Low-luminosity stars

  42. Thought Question Which kind of stars are best for measuring large distances? A. High-luminosity stars B. Low-luminosity stars

  43. Cepheid variable stars are very luminous! We determined distances to star clusters in our Galaxy, and noticed peculiar Cepheid stars and studied them well. When we find now a Cepheid star in a more distant star cluster, belonging to other Galaxy, we can determine their distance (because now we know their luminosity!) In this way we use the “standard candle” approach. Standard candle is an object whose luminosity we know. Then by measuring its apparent brightness we can find the distance to that object. What would be a standard candle in this case?

  44. Edwin Hubble, using Cepheids as standard candles, was the first to measure distances to other galaxies

  45. Measuring distances using Cepheids has been a key mission of the Hubble Space Telescope

  46. Hubble’s extra-sharp vision allows us to observe individual Cepheid variable stars in galaxies up to 100 million light-years away Galaxy M100

  47. Hubble’s extra-sharp vision allows us to observe individual Cepheid variable stars in galaxies up to 100 million light-years away Galaxy M100

  48. Cepheid variable star in M100 with period ~ one month

  49. White-dwarf supernova are even brighter than Cepheides! Step 5 Apparent brightness of white-dwarf supernova tells us the distance to its galaxy (up to 10 billion light-years) Supernova

  50. We measure galaxy distances using a chain of techniques