1. Chapter 15
Our Galaxy

2. All sky view of the Milky Way

3. Questions How many stars are in the Milky Way? About 200 billion
How many galaxies are there?
About 100 billion – 2 trillion (based on Hubble Ultra Deep Field)

4. Questions How old is the Milky Way Galaxy?
It is 15 billion years old and will remain active for a another 10 billion years.
Recall: The Sun is ~ 4.5 billion years old.

5. Are all stars members of the Milky Way?
No, most stars in the universe are in other galaxies.
What is the structure of the Milky Way?
We know in general, that it has the shape of a disk with spiral arms and a central bulge.

7. Where in the Milky Way is our solar system located?
The solar system is located in a spiral arm about 28,000 light years (8kpc) from the center of the Milky Way.
The orbital period of the disc:
~ 225 million years

8. Sketch of the Milky Way Galaxy

9. Artist’s conception of the Milky Way from its outskirts

10. Edge on schematic view of the Milky Way

13. Galaxy Formation

14. The Star-Gas-Star Cycle
Generations of stars continually recycle the same galactic matter.
Supernova stir and heat the interstellar medium and contribute new heavy elements.
Stars return mass to interstellar medium in two ways
Stellar winds (solar wind)
Death events (planetary nebula and supernova)

15. The Star-Gas-Star Cycle

16. Evidence of Star-Gas-Star Cycle
A dying low mass star returns gas to the interstellar medium in a planetary nebula. (HST)

17. X-ray emission from a supernova remnant Cassiopeia A
X-ray emission from a supernova remnant Cassiopeia A. (Chandra X-ray Observatory)

18. Atomic Hydrogen Gas
Most of the intersellar medium is composed of atomic hydrogen gas (70%).
Atomic hydrogen emits a spectral line with a wavelength of 21 cm – in the radio!
Using radio telescopes, Astronomers can detect this 21 cm radiation coming from all different directions.

19. Molecular Clouds
Cool clouds of molecular gas form out of hydrogen and other elements.
Molecular clouds are formed from:
H2 - Hydrogen gas
CO – Carbon Monoxide
H2O – Water
NH3 – Ammonia
Gravitational forces in molecular clouds collect molecules into dense cores, eventually becoming protostars.

20. Molecular Cloud in the constellation Orion.

21. Completing the Cycle
After a few stars begin to form in a cluster, their UV radiation begins to ionize the surrounding gas of the cloud from which they have formed.
The surrounding cloud material begins to ionize and give off its own light.
We now have an ionization or emission nebula.

22. The Eagle Nebula: a complex of clouds where new stars are forming
New stars forming
Molecular clouds

24. Distribution of gas in the Milky Way

25. The Milky Way in Motion

27. Spiral Arms
Observational evidence suggests that they result as a consequence of waves generated by star formation.
Theoretical models indicate that disturbances in the disk form spiral density waves that are the source of the galaxy’s spiral arms.

28. Galaxy M51 - A spiral galaxy with two prominent spiral arms. (HST)
Red blobs are ionization (emission) nebulae

29. Relationship of dust, gas, and new star clusters in a spiral arm

30. The Galactic Center
In visible light, dusty gas obscures the galactic center.

31. Radio image of Sgr A* shows bright filaments that trace out magnetic field lines around a central black hole
Radio image of galactic center shows ionized gas swirling around Sgr A*

32. Stars appear to be orbiting something massive but invisible …
a black hole?
Orbits of stars indicate a mass of about 4 million Msun.
Insert ECP7e, figure here
Download a great movie of star motions from
© 2015 Pearson Education, Inc.

33. Star Velocities Near The Galactic Center

34. Galactic Center -Evidence of Black Hole (artist’s rendering)

35. How do we know that we are located in the spiral arm?
In 1917 Harlow Shapley discovered that the globular clusters form a huge spherical system that is not centered on the Earth.

36. Finding Our Place Early view Sun at center Sun Harlow Shapley
Globular Clusters evenly distributed
Sun
Globular Clusters unevenly distributed
Harlow Shapley

38. The Great Debate - April 26, 1920
A galaxy is an
island universe!
And many nebulae
are galaxies.
These are just nebulae
within the Milky Way!
Heber Curtis
Harlow Shapley

39. RR Lyrae and Cepheid Variable Stars
These are both pulsating variable stars.
Their pulsation periods are on the order of a few days.
Using the period-luminosity relationship, distances to other galaxies can be estimated

40. RR Lyrae & Cepheids on H-R diagram

41. Period-luminosity relationship AKA
“The Leavitt Law”

42. Light curves for RR-Lyrae and Cepheid Variables

43. New Distance Ladder

44. Messier Catalogue

45. Many Messier objects are actually other galaxies

46. … and more galaxies still

48. In 1923, Edwin Hubble was examining photographic plates of the Andromeda Nebula M31
Hubble located three novae, each marked with an "N.“
(he was looking for Novae)
One of these novae, however, turned out to be a Cepheid variable, a star that changes predictably in brightness
The "N" was crossed out and the star was relabeled "VAR!“
This Cepheid, and others subsequently discovered in the Andromeda Nebula, enabled Hubble to prove that the Nebula was not a star cluster within our own Milky Way, but a galaxy more than a million light years away.

49. Edwin Hubble

50. Edwin Hubble

52. Right Ascension
00 : 42.7 (h:m)
Declination
+41 : 16 (deg:m)
Distance
2900 (kly)
Visual Brightness
3.4 (mag)
Apparent Dimension
178x63 (arc min)

53. Andromeda Nebula M31, is actually another spiral galaxy

54. Open clusters are found in
The Galactic Halo
The Galactic Disc
The Center of the galaxy.
Everywhere in the Milkyway.

55. End of Section

56. Prelude to the Dark Matter Problem

57. Rotation Curve for a merry go round
v = R v R

58. Rotation Curve for the planets in our solar system
The curve declines with radius because the mass is concentrated at the center (the Sun)

59. The Milky Way’s Rotation Curve is flat, indicating that the Milky Way’s mass is not concentrated in the center bulge
This implies the existence of “Dark Matter”

60. Dark Matter/Missing Mass ?? Stay tuned - more about this mystery later…