1. ASTR 1020 – April 11 . Final Problem Set Due Today
Next Observatory Night April 17
Next Exam April 20
Website

2. M31 The Great Nebula in Andromeda

3. Our Region
15,000,000pc
Local Group
Virgo Cluster
M81,82

4. Hubble Sequence
(not an evolutionary sequence!)

5. Elliptical Galaxy

20. M87 in Virgo Cluster

24. Mass of Galaxies From Orbital Considerations Orbital velocity of stars
binary galaxy orbital velocities
clusters of galaxies
They All Show Dark Matter

25. Rotation Curves Without Dark Matter
Hydrogen emits a radio emission line at l = 21cm
Doppler shift allows us to measure orbital velocity of gas clouds vs radius

26. Dark Matter Mass to Light Ratio:
Mass in Solar Masses divided by Luminosity in Solar Luminosities
Sun has M/L of 1 (by definition)
Rigel M=20 L=  M/L = .01
Pros Cen M=.1 L =  M/L = 100
Galaxies typically have M/L of 7 to 10
Something like 85% of the mass is dark matter

27. Dark Matter
Dark matter is in Ball  We can measure its distribution even though we don’t know what it is.
Dark Matter
Regular Matter

28. What is Dark Matter? WHAT IS DARK MATTER????? Some possibilities:
Ionized Gas
Small Stars
Planets
Baseballs
Black Holes
Neutrinos
Neutralinos
A flaw in Newton’s Laws
MAssive Compact Halo Objects (Machos)
Weakly Interacting Massive Particles (Wimps)
Astronomers don’t know what most of the matter in the universe is!

29. Galaxies are Close Together
Unlike stars, they run into each other.
30,000pc wide, but only 500,000pc apart
Stars million miles wide but trillions apart

30. Bullseye!

33. Cluster of Galaxies

34. X-ray Clusters Over last 10 Billion years there have been many
galaxy collisions in these crowded clusters.
When two galaxies pass through each other stars
will continue on their original path – more or less.
Interstellar gas clouds collide and cannot pass through
each other. They get stripped and pass into the
gravitational well of the cluster. This fills with very
hot shocked gas over time.
So hot it emits x-rays.
Shows matter distribution. (Mostly dark matter again.)

35. Coalescence
After galaxies collide, on average, the friction causes them to drop
down toward the center. Eventually they settle to the bottom.

36. CD Galaxy
A Galactic Cannibal
M87 has eaten 100 of its own.

37. Gravitational Lenses Einstein predicted light bends
as it goes around massive objects.
Just like an orbiting body.

40. The Distance Scale
All the things astronomers have done to gain a handle on distance

41. Solar System Distances to 3AU
Radar
Measure time of flight of signal bouncing off planets
Transit of Venus
Captain Cook in Hawaii
Time that Venus hits limb of Sun

42. Parallax to 100pc
I year cycle

43. Main Sequence Fitting to 55,000pc
Plot enough stars and the
Main Sequence becomes
clear.
This works out to Magellanic
Clouds.
Beyond that, MS stars too
faint

44. Cepheid Variables to 100Mpc

45. Period-Luminosity Relationship
Cepheids are bright and can be seen at very large distances.

46. Type I Supernovae
All Type Ia supernovae are the same brightness. That white dwarf that implodes
and then explodes is always about the same. And they’re bright. You can see them
at billions of parsecs!
Apparent magnitude can be converted to distance.

47. Tully-Fisher Relation
The brighter the galaxy is,
the faster it rotates.
Use radio spectrum to measure
Doppler width.
Not super accurate, but it
works.

48. The Distance Scale

49. Redshift of Galaxies Hubble found that galaxies are redshifted.
The absorption lines are
those of the stars that
make up the galaxy.

50. Hubble’s Law The more distant the galaxy, the greater the redshift.
The more distant is the galaxy,
the faster it is flying away from
us.
H0=70 km/s/Mps

51. Example Galaxy in Virgo Cluster is at 15Mpc
will recede at v = 70 km/s/Mpc x 15Mpc = 1050km/s
dl = l x v/c = 4000Å x 1050/300,000 = 14Å
a 4000Å line will appear at 4014Å

52. Example 2
l0 = 1216Å but is observed at l = 1228Å
dl = 12

53. A Modern Hubble Diagram

54. Galaxies are Flying Apart
Galaxies remain same size. They just get farther apart.
Effect is the same no matter which position you occupy

55. Expanding Universe Current Universe The volume of the universe is
increasing.
Early Universe

56. The Age of the Universe Notice: At time zero, d = 0
All galaxies are at zero distance!
The universe has zero volume!
when t=1/H

57. The Beginning Before 13 billion years ago, there was no before.
Time and space started with the Big Bang
We will return to this in a few more lectures

58. Galaxies are Moving
Gravity attracts them to mass centers.

59. Redshift Survey Galaxies across the sky. Distance from redshift.
“The Great Wall”

60. More Galaxies

61. The Texture of the Universe

63. Theory of Structure Structure grew out of minor fluctuations.
Gravity causes density regions
to enhance.
The very origin of structure.

64. The Cosmic Web