Galaxies

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. NGC 3949 Very much like our Milky Way galaxy

21. NGC 7331 “The Milky Way’s Twin”

23. M64 A band of absorbing dust make an “evil eye”

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

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

45. 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.