Galaxies and the Universe

1. Galaxies and the Universe The Milky Way 31.1 Other Galaxies in the Universe 31.2 Cosmology 31.3 Chap. 31

2. Objectives The Milky Way • determine the size and shape of the Milky Way, as well as Earth’s location within it. • describe how the Milky Way formed.

3. Discovering the Milky Way • Distances to clusters determined using variable stars.

4. Discovering the Milky Way • Distances to clusters determined using variable stars.

5. Variable Stars Stars in the ‘giant’ branch of HR diagram that pulsate in brightness

6. Discovering the Milky Way • Distances to clusters determined using variable stars. • RR Lyrae variables Brightness pulsates between 1.5 hours and 1 day

7. Discovering the Milky Way • Distances to clusters determined using variable stars. • RR Lyrae variables • Cepheid variables (graph this) Brightness pulsates between 1 and 100 days

8. Discovering the Milky Way • Distances to clusters determined using variable stars. • RR Lyrae variables • Cepheid variables • These stars make good standard candles The larger the period (time) of pulsation the greater the luminosity. (graph this)

9. Calculating distance If a star is really bright (__________ magnitude) but it appears to be dim (_________ magnitude), you know it’s far. The dimmer it looks the farther it is.

10. Locating the Center of the Galaxy

11. Locating the Center of the Galaxy • Globular clusters are centered around a point about 28,000 ly away

12. Locating the Center of the Galaxy • Globular clusters are centered around a point about 28,000 ly away • Center has high density of stars

13. Locating the Center of the Galaxy • Globular clusters are centered around a point about 28,000 ly away • Center has high density of stars • Center is toward Sagittarius constellation http://www.esa.int

14. Shape of Milky Way • The MW is a flattened disk shape

15. Shape of Milky Way • The MW is a flattened disk shape • Galactic center (nucleus) surrounded by nuclear bulge

16. Shape of Milky Way • The MW is a flattened disk shape • Galactic center (nucleus) surrounded by nuclear bulge • A spherical-shaped halo containing older stars surrounds the disk.

17. Shape of Milky Way • Four major spiral arms (and several minor spiral arms) have been identified

18. Mass of the Milky Way

19. Mass of the Milky Way • Might be found by measuring luminosity Remember that luminosity is related to mass. Stars that are bigger are also _________.

20. Mass of the Milky Way • Might be found by measuring luminosity • Mass is usually found by using our orbital speed

21. Calculating Mass (M1 + M2)P2 = a3 Kepler’s 3rd law M1 is sun’s mass (measured in “sun masses”) M2 is universe’s mass (measured in “sun masses”) P is orbital period (years) = 240 million y a is distance (in AU) 1 ly = 63,200 AU

22. Mass of the Milky Way • Might be found by measuring luminosity • Mass is usually found by using our orbital speed • Since the MW is about 100 billion times the mass of the Sun, an average sized star, the MW must contain about stars.

23. Mass of the Milky Way • Might be found by measuring luminosity • Mass is usually found by using our orbital speed • Since the MW is about 100 billion times the mass of the Sun, an average sized star, the MW must contain about 100 billion stars.

24. Mass of the Center of the Milky Way • Stars near the center orbit center very fast – this indicates a very center

25. Mass of the Center of the Milky Way • Stars near the center orbit center very fast – this indicates a very massive center

26. Mass of the Center of the Milky Way • Stars near the center orbit center very fast – this indicates a very massive center • It is thought that there is a super black hole at the center of our galaxy This center is about 2.6 million times the Sun’s mass

27. Age of Stars in Milky Way

28. Age of Stars in Milky Way • Young stars form in the arms of the MW

29. Age of Stars in Milky Way • Young stars form in the arms of the MW • Old stars are found in the halo/nuclear bulge.

30. Formation of Milky Way

31. Formation of Milky Way • MW was originally round. Notice the arrangement of the oldest stars.

32. Formation of Milky Way • MW was originally round. • The MW cloud collapsed and flattened into a disk shape.

33. The End

34. Objectives Other Galaxies – 30.2 • Describe how astronomers classify galaxies • Identify how galaxies are organized into clusters and superclusters • Describe the expansion of the universe

35. Identifying

36. Identifying • Astronomers saw other galaxies before they knew what they were.

37. Identifying • Astronomers saw other galaxies before they knew what they were. • Edwin Hubble measured their distances to confirm they were not in MW. He used variable stars to do it.

38. Classifying

39. Classifying • Spiral M74 in pisces “Cosmic Frisbee”

40. Classifying • Spiral • Normal spirals (S)

41. Classifying • Spiral • Normal spirals (S) • Barred spirals (SB) NGC 1300 – in Eridanus

42. Classifying • Spiral • Normal spirals (S) • Barred spirals (SB) • These are further divided by how tightly wound arms are (a, b, c) Type a represents tightly wound arm with bright nucleus.

43. Classifying • Spiral • Ellipticals “Cosmic Football”

44. Classifying • Spiral • Ellipticals • Divided from E0 to E7.

45. Classifying • Spiral • Ellipticals • Divided from E0 to E7. • E7 has a large ratio of major axis/minor axis, E0 is circular.

46. Classifying • Spiral • Ellipticals • Irregular Galaxies (Irr) http://www.nasa.gov

47. Classifying • Masses

48. Classifying • Masses • Dwarf ellipticals have few stars (about 1 million).

49. Classifying • Masses • Dwarf ellipticals have few stars (about 1 million). • Large spirals, like MW, have about 100 million stars.

50. Classifying • Masses • Dwarf ellipticals have few stars (about 1 million). • Large spirals, like MW, have about 100 million stars. • Giant ellipticals have mass of 100 trillion x the sun.