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Chapter 15

Chapter 15. A Universe of Galaxies. The Hubble Deep Field 10 day exposure –field located in the Big Dipper. Edwin Hubble. Edwin Hubble. Hubble’s Galaxy Classification . Spiral Galaxies.

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Chapter 15

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  1. Chapter 15 A Universe of Galaxies

  2. The Hubble Deep Field10 day exposure –field located in the Big Dipper

  3. Edwin Hubble

  4. Edwin Hubble

  5. Hubble’s Galaxy Classification

  6. Spiral Galaxies • galaxies like the Milky Way with arcing structures lying in a plane and emanating from the nuclear bulge

  7. M51 M81

  8. Lenticular Galaxies • Galaxies that have disks but no spiral arms.

  9. Barred Spiral Galaxies • galaxies with a bar of stars running through the nuclear bulge

  10. NGC 1097 M91 NGC 4123 NGC 4477

  11. Elliptical Galaxies • galaxies with an elliptical shape, no spiral arms, and little interstellar matter

  12. M32 - E2 NGC 4125 M87 - E1

  13. Irregular Galaxies • galaxies that are asymmetrical and are sometimes just two or more galaxies colliding

  14. Large Magellanic Cloud – a small irregular galaxy that orbits the MMilky Way

  15. Centaurus A M87

  16. Measuring Cosmic Distances • Distances to other galaxies are measured using Main Sequence Fitting. • This entails the use of a light source of known, standard luminosity called a standard candle. • The distance can be found using the luminosity-distance formula • Apparent brightness = luminosity/4d2

  17. Comparison of the apparent brightness of stars in the Hyades Cluster with those of the Pleiades Cluster The Pleiades Cluster is 2.75 times farther away because (2.75)2 = 7.5 times dimmer The same luminosities are assumed for all main sequence stars of the same color.

  18. RR Lyrae and Cepheid Variable Stars • These are both pulsating variable stars. • Their pulsation periods are on the order of a few days. • Using the period-luminosity relationship, distances to other galaxies can be estimated

  19. Cepheid Period-luminosity Relation. Cepheids of a particular period have very nearly the same luminosity.

  20. Edwin Hubble

  21. Edwin Hubble

  22. http://www.seds.org

  23. Right Ascension 00 : 42.7 (h:m) Declination +41 : 16 (deg:m) Distance 2900 (kly) Visual Brightness 3.4 (mag) Apparent Dimension 178x63 (arc min)

  24. Andromeda Nebula M31, is actually another spiral galaxy Andromeda Nebula M3, is actually another spiral galaxy

  25. Tully-Fisher Relation – using galaxies as standard candles • Astronomers discovered that the faster a spiral galaxy rotates, the more luminous it is. • This relationship is called the Tully-Fisher relation, after its discoverers

  26. The Tully-Fisher relation.

  27. Galaxy Observations • During the 1920's Edwin Hubble and Milton Humason photographed the spectra of many galaxies with the 100 inch telescope at Mount Wilson. • They found that most of the spectra contained absorption lines with a large redshift.

  28. Red Shift and Distance 24 Mpc 1200 km/s 300 Mpc 15,000 km/s 780 Mpc 39,000 km/s 1220 Mpc 61,000 km/s

  29. Galaxy Observations • Using the Doppler effect, Hubble calculated the velocity at which each galaxy is receding from us. • Using the period and brightness of Cepheid variables in distant galaxies, Hubble estimated the distances to each of the galaxies.

  30. The Tully-Fisher Relation

  31. Hubble’s Law • Hubble noticed that there was a linear relationship between the recessional velocity and the distance to the galaxies. • This relationship is know as Hubble’s Law: V = H D recessional velocity = Hubble’s Constant  Distance

  32. Hubble’s Law • H is known as the Hubble constant. • It’s true value appears to be somewhere between 55 to 75 km/s/Mpc. • This means that a galaxy that is 1 megaparsec from Earth will be moving away from us at a speed somewhere between 55 to 75 km/s.

  33. The Distance Chain or Ladder

  34. Measuring Cosmic Age

  35. Measuring Cosmic Age Raisin Cake Model Like raisins in rising raisin cake, galaxies move away from each other in our expanding universe.

  36. Typical Cube of Galaxies

  37. Homogeneous, isotropic universe? NO!

  38. The Birth of The Universe-“The Big Bang” A very rough estimate for the age of the universe

  39. All of space and time were created in the Big Bang, which then expands. Analogous to the surface of a balloon.

  40. Cosmological Red Shift As the universe expands, photons of radiation are stretched in wavelength, giving rise to the cosmological redshift.

  41. Elliptical, Spiral and irregular galaxies at different ages.

  42. Modeling Galaxy Birth • The most successful models are based on the following assumptions: • Hydrogen and helium gas filled all of space fairly uniformly early in the universe. • The near uniformity had small perturbations which allowed for dense regions to exist.

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