1. Galaxies

2. What is a Galaxy?
A galaxy is a collection of billions of stars, dust, and gas all held together by gravity.
Galaxies are scattered throughout the universe.
They vary greatly in size and shape. Not all galaxies look alike.

3. A bit of History
In 1995, the Hubble Space telescope observed a patch of sky in the Big Dipper for 10 days
Many galaxies, each one an island of stars like our own Milky Way, were detected
By counting them and correcting for the entire sky, we estimate that there are over 80 billion galaxies in the observable Universe

4. A bit of History
We study the life cycle of the galaxies in the same way we do stars
We piece together a life story by observing many different galaxies at various stages of their lives
Studying life cycles of galaxies is more difficult than that of stars
Study using the Electromagnetic spectrum

5. Absorption line spectrum
                  
dark lines in a continuous spectrum => light absorbed at certain wavelengths
source: cooler gas with a continuous source behind it
Emission line spectrum
                                         
                   
bright lines at certain wavelengths (no continuous spectrum)

6. Three major components:
A thin disk consisting of young and intermediate age stars
this disk also contains gas and is actively forming new stars.
Dust in the disk makes it appear orange in the picture.

7. Three major components:
(cont.)
Dust absorbs blue light more than red light and thus makes stars appear reddish.
Our Galaxy has spiral arms in its disk - these spiral arms are regions of active star formation.

8. Three major components:
2. a bar of older stars (white in the COBE picture).

9. Three major components:
An extended dark halo whose composition is unknown.
Since the matter in the halo does not consist of luminous stars, it does not show up in the COBE image.
The existence of the dark halo is inferred from its gravitational pull on the visible matter.

10. Galaxy Size The sizes of all three types span a wide range, from
Dwarf galaxies (contain 100 million stars)
Giant galaxies (contain 1 trillion stars)

11. Classification of Galaxies
Edwin Hubble looked at many galaxy pictures and decided to group, or classify them.
He could have use size, color, shape or any other feature that he noticed but decided on shape or form as the deciding factor
He used letters to represent the different types

13. Three Shapes:
1. Elliptical: oval shaped

14. Elliptical galaxy- “E”
The word elliptical refers to its degree of “roundness”.
Most galaxies are elliptical
The shape can go from nearly circular to flattened or elongated.
These variations are denoted by number {E0 – E8}

15. Lenticular Galaxies
Lenticular galaxies are sometimes called "armless spiral galaxies." Lenticular galaxies have a central bulge, but no spiral arms.
If the central bulge is not very bright, it can be very difficult to tell the difference between a lenticular galaxy and an E0 galaxy - where the tines meet the handle.

16. 2. Spiral: looks like it has arms. The Milky Way is a spiral galaxy.

17. Spiral Galaxy
Draw this diagram

18. Spiral Galaxy “S”
Named so because it reminded Hubble of a pin wheel or whirlpool
Those with spiral arms where divided into 3 sub-groups depending on how tightly the arms were wound around the nucleus
Tightly wound around the nucleus are type “Sa” and loosest type “Sc”

19. Andromeda Galaxy (M31)
The Andromeda Galaxy (M31) is the nearest spiral galaxy to our own, the Milky Way. It lies approximately 2,500,000 light-years away in the constellation Andromeda.

20. Barred- Spiral Galaxy “SB”
Named so because it reminded Hubble of a spiral with a solid bar across the center
In about 1/3 of spiral galaxies, the arms spiral out not at the center but from a straight bar of stars, gas, and dust that extends from both sides of the bright nucleus

21. Barred- Spiral Galaxy “SB”
The sub-groups “a” – “c” are in order of increasing openness of the arms but with a “B” for barred inserted.

22. Barred - Spiral Galaxy
Draw this diagram

23. 3. Irregular: A big blob of stars

24. Irregular Galaxy
These are galaxies that feature neither spiral nor elliptical morphology.
Hubble defined two classes of irregular galaxy

25. Irregular Galaxy
Irr I galaxies- have asymmetric profiles and lack a central bulge or obvious spiral structure; instead they contain many individual clusters of young stars
Irr II galaxies- have smoother, asymmetric appearances and are not clearly resolved into individual stars or stellar clusters

26. The Large and Small Magellanic Clouds are irregular galaxies close to the Milky Way Galaxy

27. For the “E” or Elliptical galaxies, he used the amount of flattening or elongation, from E0 (E zero) to E7, to put the galaxies into smaller groups.
For the “S” or spiral galaxies, he used how tightly the arms were wound around the bright core, to further classify these galaxies Sa, Sb, or Sc.
For the “SB” or barred spiral galaxies, he used the increased openness of the arms top group these galaxies as SBa, SBb, SBc.

28. Galaxy Groups
Galaxies are not spread uniformly throughout the universe. They tend to group together and form clusters. The Milky Way Galaxy belongs to a small cluster called the Local Group

29. Galaxy Groups
The diameter of the Local Group is about 2 million light years
The Milky Way and Andromeda Galaxies are the largest
Some clusters are much larger ranging 5 to 30 million light years

30. Galaxy Groups
Galaxies in clusters often collide and merge to form strangely shaped ones. It is believed that the Milky Way and Andromeda Galaxies are destined to collide and merge some time in the future

31. Galaxy Groups
When galaxies collide stars are not destroyed; more stars are actually created by the new gravity motions