1. Galaxies

2. Galaxies A galaxy is a large system of stars and interstellar matter.
Stars can be classified by heavy metal concentration. Two types of star populations:
Population I – hot, luminous and young stars, seeded with heavy elements
Population II – older and cooler with fewer heavy elements (formed earlier than newer stars)

3. Types of Galaxies
Galaxies can be classified in several ways. The most common is a system developed by Edwin Hubble, who discovered that the universe is filled with many galaxies beyond our own Milky Way.

5. Types of Galaxies
Galaxies can be classified into four broad categories:
1) spiral
2) lenticular
3) elliptical
4) irregular

6. Types of Galaxies - Spirals
Spirals are flat disks of stars with bright bulges in their centers. Spiral arms wrap around these bulges.
Spiral arms probably form as the result of waves that sweep through the galactic disk.
In some spirals, a wave organizes the stars in the center into a bar. The arms of barred galaxies spiral outward from the ends of the bar.

7. Types of Galaxies - Spirals
The nucleus or bulge consists of population II stars, the disk consists of population I stars.

8. The Milky Way
Our galaxy is a barred spiral. It has at least four spiral arms.
The name was derived from Greek, then Latin.

9. The Milky Way – Spiral Arms
Four main arms: Perseus, Norma/Cygnus, Crux/Scutum, Carina/Sagittarius
The Sol system is in a spur, the Orion spur.

13. Andromeda

15. NGC 4414

17. NGC 1300
Barred Spiral

19. Very much like our Milky Way galaxy
NGC 3949
Very much like our Milky Way galaxy

21. NGC 7331
“The Milky Way’s Twin”

23. A band of absorbing dust make an “evil eye”

25. One of my favourites; I have a summer home there
M104 – Sombrero Galaxy
One of my favourites; I have a summer home there

27. Infrared view from Spitzer telescope
Sombrero Part 2
Infrared view from Spitzer telescope

29. Types of Galaxies – Lenticular
Smooth disk galaxies.
Stellar formation in these galaxies has stopped, so they are populated by population II stars.

30. NGC 2787

32. Types of Galaxies – Elliptical
Like fat, fuzzy eggs or footballs.
Instead of spreading out into a thin disk, as they do in spirals, the stars in ellipticals wrap completely around the galaxy's heart in all directions.
Mostly population II stars

33. Types of Galaxies – Elliptical
The largest galaxies in the universe are giant ellipticals.
They can contain a trillion stars or more, and span as much as two million light-years (20 times the Milky Way).
Some of them appear to contain "supermassive" black holes at their hearts —three billion times as heavy as the Sun.

34. M110

36. M32

38. Likely cause by a collision of two gas-rich galaxies
NGC 1316
Likely cause by a collision of two gas-rich galaxies

40. M87
Elliptical galaxy with super massive black hole jet – one that has absorbed over 2 billion solar masses of material

42. Types of Galaxies – Irregular
Irregulars exhibit peculiar shapes.
Often distorted by gravitation by other objects .

43. “Baby” galaxy – the youngest galaxy found so far
I Zwicky 18
“Baby” galaxy – the youngest galaxy found so far

45. Being pulled apart by a nearby galactic cluster
NGC 1427A
Being pulled apart by a nearby galactic cluster

47. Naming Galaxies Many conventions exist for naming galaxies.
Conventions include:
1) IAU traditional names
2) Messier object names
3) New General Catalogue names

48. Hubble Classification Scheme
Elliptical:
E  elliptical
#  ellipticity (0 is spherical, 7 is cigar)
Spiral:
S  Spiral
B  Barred
letter  Bulge and Arms

50. Hubble Classification Scheme
When naming galaxies (especially ellipticals), geometry or viewing angles can play a large role.
The ‘tuning fork’ is not related to galactic evolution.

51. Sometimes galaxies collide!
Colliding Galaxies
Sometimes galaxies collide!

52. NGC 2207 and IC 2163

54. NGC 4038/4039: The Antennae

58. NGC 4676: The Mice

60. Spiral galaxy sliding through a much larger galaxy
NCG 1275
Spiral galaxy sliding through a much larger galaxy

62. Andromeda and the Milky Way
In about 5 billion years, Andromeda and the Milky Way may collide resulting in an elliptical (this will take about a billion years to form)

64. Clusters and Superclusters
Galaxies are grouped into clusters.
Our cluster is known as the Local Cluster.

66. Clusters and Superclusters
Clusters of galaxies are further grouped into superclusters.
Our supercluster is known as the Virgo Supercluster, or Local Supercluster.