1. Chapter 20 Galaxies And the Foundation of Modern Cosmology

2. 20.1 Islands of Stars Our goals for learning
How are the lives of galaxies connected with the history of the universe?
What are the three major types of galaxies?
How are galaxies grouped together?

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

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

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
Elliptical Galaxy
Spiral Galaxy

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

13. Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
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
Spheroidal Component:
bulge & halo, old stars,
few gas clouds

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

20. Lenticular Galaxy (S0 class):
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

22. Elliptical Galaxy:
All spheroidal component, virtually no disk component
Red-yellow color indicates older star population-no dust, with few exceptions

23. Irregular Galaxy, Irr Class, Peculiar (Pec) regular with ding.

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

25. Spheroid
Dominates
Disk Dominates
Hubble’s galaxy classes:
HUBBLE TUNING FORK

26. How are galaxies grouped together?

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

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

29. 20.2 Measuring Galactic Distances
Our goals for learning
How do we measure the distances to galaxies?

30. How do we measure the distances to galaxies?
The Distance Ladder

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

32. Step 1
Determine distances of objects in solar system using radar.
Time how long it takes to get there and back.

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

34. Luminosity passing through each sphere is the same
Area of sphere:
4π (radius)2
Divide luminosity by area to get brightness

35. Luminosity Brightness = 4π (distance)2
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:
Distance =
4π x Brightness
A standard candle is an object whose luminosity we can determine without measuring its distance

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

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

38. Cepheid variable stars are very luminous

39. Step 4
Because the period of a Cepheid variable star tells us its luminosity, we can use these stars as standard candles. Discovered by John Goodricke, deaf and mute and 17.

40. Cepheid variable stars with longer periods have greater luminosities

41. White-dwarf supernovae (SN Ia) can also be used as standard candles

42. Step 5
Apparent brightness of white-dwarf supernova Ia tells us the distance to its galaxy
(up to 12 billion light-years)

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

44. We measure galaxy distances using a chain of interdependent techniques

45. How did Hubble prove that galaxies lie far beyond the Milky Way?

46. The Puzzle of “Spiral Nebulae”
Before Hubble, some scientists argued that “spiral nebulae” were entire galaxies like our Milky Way, while others maintained they were smaller collections of stars within the Milky Way
The debate remained unsettled until someone finally measured their distances

47. Hubble settled the debate by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles. He was off by a factor of 2 because of dust, but convinced other astronomers.

48. What is Hubble’s Law?

49. The spectral features of virtually all galaxies are redshifted  They’re all moving away from us

50. Hubble’s Law: velocity = H0 x distance
Redshift yields recession velocity (vertical)

51. Redshift of a galaxy thus tells us its distance through Hubble’s Law:
velocity H0
However, this is not very accurate, particularly at great distances.

52. Distances of farthest galaxies were calculated
from redshifts, but now
SN Ia’s give us much more precision all
the way out.

53. The expansion rate appears to be the same everywhere in space
The universe has no center and no edge (as far as we can tell)

54. One example of something that expands but has no center or edge is the surface of a balloon

55. Cosmological Principle
The universe looks about the same no matter where you are within it
Matter is evenly distributed on very large scales in the universe
No center & no edges
Consistent with all observations to date, except ‘Flow’ in Microwave backround over about 10% of universal field.

56. Hubble’s constant used to be used for the age of universe because it relates velocities and distances of all galaxies (not valid now).
Age = ~ 1 / H0
Very inaccurate now!
Distance
Velocity

57. How does the universe’s expansion affect our distance measurements?

58. Distances between faraway galaxies change while light travels

59. Distances between faraway galaxies change while light travels
Some Astronomers think in terms of lookback time rather than distance
distance?

60. Expansion stretches photon wavelengths causing a cosmological redshift directly related to lookback time