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Chapter 14 The Milky Way Galaxy

Chapter 14 The Milky Way Galaxy. Chapter 14 The Milky Way Galaxy. Units of Chapter 14. Our Parent Galaxy Measuring the Milky Way Galactic Structure The Formation of the Milky Way Galactic Spiral Arms The Mass of the Milky Way Galaxy The Galactic Center. The Milky Way.

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Chapter 14 The Milky Way Galaxy

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  1. Chapter 14The Milky Way Galaxy

  2. Chapter 14The Milky Way Galaxy

  3. Units of Chapter 14 Our Parent Galaxy Measuring the Milky Way Galactic Structure The Formation of the Milky Way Galactic Spiral Arms The Mass of the Milky Way Galaxy The Galactic Center

  4. The Milky Way • 200+ billion stars • 100k light years across • 10k light years thick at the galactic bulge • 1k light years thick in the disc. • 1 of 100s of billions

  5. Women In Astronomy • Williamina Fleming • Catalog of brightness and spectra • Antonia Maury • Stellar spectra leading to H-R Diagram • Annie Cannon • Spectra classification system • Henrietta Leavitt • Cepheid variable stars

  6. Williamina Fleming Maid to Curator of Astronomical Photographs

  7. Antonia Maury Classification of stellar spectra leading to H-R Diagram

  8. Annie Cannon spectral classes O, B, A, F, G, K, M

  9. Henrietta Leavitt 'period-luminosity relationship‘ of Cepheids

  10. Human Computers Harvard Observatory 1910

  11. Cepheid and RR Lyrae Stars • Two stars played a key role in creating a model of the universe. • Variable stars allowed measurement of luminosity and thus distance. luminosity Apparent brightness distance2

  12. Measuring the Milky Way The variability of these stars comes from a dynamic balance between gravity and pressure – they have large oscillations around stability.

  13. Measuring the Milky Way • This allows us to measure the distances to these stars. • RR Lyrae stars all have about the same luminosity; knowing their apparent magnitude allows us to calculate the distance. • Cepheids have a luminosity that is strongly correlated with the period of their oscillations; once the period is measured, the luminosity is known and we can proceed as above.

  14. Measuring the Milky Way The usefulness of these stars comes from their period–luminosity relationship.

  15. Question 4 a) measuring distances with Cepheid variable stars. b) identifying the mass of the Galaxy’s central black hole. c) determining the masses of stars in an eclipsing binary system. d) using spectroscopic parallax to measure distances to stars. The period – luminosity relationship is a crucial component of

  16. Question 4 a) measuring distances with Cepheid variable stars. b) identifying the mass of the Galaxy’s central black hole. c) determining the masses of stars in an eclipsing binary system. d) using spectroscopic parallax to measure distances to stars. The period – luminosity relationship is a crucial component of Cepheid variable stars with longer periods have higher actual luminosities; short-period Cepheids are dimmer.

  17. Measuring the Milky Way Many RR Lyrae stars are found in globular clusters. These clusters are not all in the plane of the galaxy, so they are not obscured by dust and can be measured. This yields a much more accurate picture of the extent of our Galaxy and our place within it.

  18. Measuring the Milky Way We have now expanded our cosmic distance ladder one more step.

  19. Question 1 a) supernova remnants. b) white dwarf stars in the spiral arms. c) red giant variable stars in globular clusters. d) bright O and B stars in open clusters. e) X-ray sources. The location of the galactic center was identified using

  20. Question 1 a) supernova remnants. b) white dwarf stars in the spiral arms. c) red giant variable stars in globular clusters. d) bright O and B stars in open clusters. e) X-ray sources. The location of the galactic center was identified using Harlow Shapley used pulsating RR-Lyrae variables as distance indicators to the globular clusters. He then deduced the distance and direction of the Milky Way’s center.

  21. Question 2 a) about 30 Kpc from the center in the halo. b) 30,000 light-years from the center in a globular cluster. c) at the outer edge of the galactic disk, in the plane. d) about halfway from the center, in the spiral arms. e) in the bulge, near the Orion arm. Our Sun is located in the Milky Way Galaxy

  22. Question 2 a) about 30 Kpc from the center in the halo. b) 30,000 light-years from the center in a globular cluster. c) at the outer edge of the galactic disk, in the plane. d) about halfway from the center, in the spiral arms. e) in the bulge, near the Orion arm. Our Sun is located in the Milky Way Galaxy The Sun orbits the center of the Galaxy within the disk, taking about 225 million years to complete one orbit.

  23. Our Parent Galaxy From Earth, we see few stars when looking out of galaxy (red arrows), many when looking in (blue and white arrows). Milky Way is how our Galaxy appears in the night sky (b).

  24. Question 3 a) a spiral galaxy. b) a barred spiral galaxy. c) an elliptical galaxy. d) a quasar. e) an irregular galaxy. Detailed measurements of the disk suggest that our Milky Way is

  25. Question 3 a) a spiral galaxy. b) a barred spiral galaxy. c) an elliptical galaxy. d) a quasar. e) an irregular galaxy. Detailed measurements of the disk suggest that our Milky Way is Measurements of stellar motion in and near the bulge imply that it is football shaped, about half as wide as it is long, characteristic of a barred spiral galaxy.

  26. Spiral Galaxies M31 M101 Andromeda which can be seen with the naked eye. 2.5Mly away NGC 4565

  27. Finding Andromedanear Pegasus

  28. Measuring the Milky Way One of the first attempts to measure the Milky Way was done by Herschel using visible stars. Unfortunately, he was not aware that most of the galaxy, particularly the center, is blocked from view by vast clouds of gas and dust.

  29. Measuring the Milky Way We have already encountered variable stars – novae, supernovae, and related phenomena – which are called cataclysmic variables. There are other stars whose luminosity varies in a regular way, but much more subtly. These are called intrinsic variables. Two types of intrinsic variables have been found: RR Lyrae stars and Cepheids.

  30. Measuring the Milky Way The upper plot is an RR Lyrae star. All such stars have essentially the same luminosity curve, with periods from 0.5 to 1 day. The lower plot is a Cepheid variable; Cepheid periods range from about 1 to 100 days.

  31. Overlay of Images

  32. Galactic Structure This artist’s conception shows the various parts of our Galaxy, and the position of our Sun.

  33. Galactic Structure The galactic halo and globular clusters formed very early; the halo is essentially spherical. All the stars in the halo are very old, and there is no gas and dust. The galactic disk is where the youngest stars are, as well as star formation regions – emission nebulae, large clouds of gas and dust. Surrounding the galactic center is the galactic bulge, which contains a mix of older and younger stars.

  34. Galactic Structure This infrared view of our Galaxy shows much more detail of the galactic center than the visible-light view does, as infrared is not as much absorbed by gas and dust.

  35. Galactic Structure Stellar orbits in the disk are in a plane and in the same direction; orbits in the halo and bulge are much more random.

  36. The Formation of the Milky Way Any theory of galaxy formation should be able to account for all the properties below.

  37. The Formation of the Milky Way The formation of the galaxy is believed to be similar to the formation of the solar system, but on a much larger scale.

  38. Question 5 a) the spiral arms formed first. b) the globular clusters formed first. c) the disk component started out thin and grew. d) spiral density waves formed first. e) the bar in the bulge formed first. In the formation of our Galaxy

  39. Question 5 a) the spiral arms formed first. b) the globular clusters formed first. c) the disk component started out thin and grew. d) spiral density waves formed first. e) the bar in the bulge formed first. In the formation of our Galaxy Globular clusters contain very old stars, no gas or dust, and orbit around the center randomly.

  40. Galactic Spiral Arms Measurement of the position and motion of gas clouds shows that the Milky Way has a spiral form.

  41. Galactic Spiral Arms The spiral arms cannot rotate along with the galaxy; they would “curl up.”

  42. Galactic Spiral Arms Rather, they appear to be density waves, with stars moving in and out of them much as cars move in and out of a traffic jam.

  43. Galactic Spiral Arms As clouds of gas and dust move through the spiral arms, the increased density triggers star formation. This may contribute to propagation of the arms. The origin of the spiral arms is not yet understood.

  44. The Mass of the Milky Way Galaxy The orbital speed of an object depends only on the amount of mass between it and the galactic center.

  45. Question 6 a) the Sun’s mass and velocity in orbit around the galactic center b) the rotation of the bulge and disk components c) the Sun’s age and age of globular cluster stars d) the motion of spiral arms and the mass of the central black hole e) the Sun’s orbital period and distance from the center What two observations allow us to estimate the Galaxy’s mass?

  46. Question 6 a) the Sun’s mass and velocity in orbit around the galactic center b) the rotation of the bulge and disk components c) the Sun’s age and age of the globular cluster stars d) the motion of spiral arms and mass of the central black hole e) the Sun’s orbital period and distance from the center What two observations allow us to estimate the Galaxy’s mass? Use the modified form of Kepler’s law to find the mass: Total mass = (orbital size)3 / (orbital period)2

  47. The Mass of the Milky Way Galaxy Once all the galaxy is within an orbit, the velocity should diminish with distance, as the dashed curve shows. It doesn’t; more than twice the mass of the galaxy would have to be outside the visible part to reproduce the observed curve.

  48. Question 7 a) 21-cm maps of the spiral arms b) the rotation curve of the outer edges of the Galaxy c) orbits of open clusters in the disk d) infrared observations of new star- forming regions e) X-ray images of other galaxies What suggests that the mass of our Galaxy extends farther than its visible disk?

  49. Question 7 a) 21-cm maps of the spiral arms b) the rotation curve of the outer edges of the Galaxy c) orbits of open clusters in the disk d) infrared observations of new star- forming regions e) X-ray images of other galaxies What suggests that the mass of our Galaxy extends farther than its visible disk? The outer edges of the Galaxy’s disk rotate much faster than they should. Most of the mass of the Galaxy must be dark matter.

  50. The Mass of the Milky Way Galaxy • What could this “dark matter” be? It is dark at all wavelengths, not just the visible. • Stellar-mass black holes? • Probably no way enough could have been created • Brown dwarfs, faint white dwarfs, and red dwarfs? • Currently the best star-like option • Weird subatomic particles? • Could be, although no evidence so far

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