The Milky Way Galaxy

1. The Milky Way Galaxy

3. The Milky Way Galaxy Panoramic View

4. We now know that our Milky Way is a highly flattened system of stars, about 25,000pc across, with the sun about 2/3 of the way from the center.

5. Studies of Galactic Structure The first serious study of the Milky Way was carried out by William and Caroline Herschel in the late 1700’s. They attempted to find the structure of the Milky Way Galaxy by counting the numbers of stars they could see through their telescope in different directions in the sky.

6. The model of the Galaxy they derived is known as the Grindstone model; they believed the galaxy was slightly flattened with the sun near the center. Sun We now know that this model is wrong because the Herschels did not know about interstellar dust.

7. In the early 1900’s, a Dutch astronomer, J.C. Kapteyn, repeated the Herschel’s study photographically. His model, now known as the Kapteyn Universe, was equally wrong. It situated the sun near the center of the Milky Way Galaxy. Again, Kapteyn was unaware of the presence of interstellar dust.

8. Modern Studies of the Milky Way Galaxy The first real progress in determining the structure of the Milky Way came about in 1908 when Henrietta Leavitt discovered that Cepheid Variable stars (and their lower-luminosity counterparts, the RR-Lyrae stars) obey a Period-Luminosity Relationship.

10. Harlow Shapley discovered RR Lyrae variable stars in Globular Clusters associated with the Milky Way. He used the Period-Luminosity relationship to calculate the distances to these globular clusters.

11. Shapley reasoned that the center of the Globular Cluster system should coincide with the actual center of our Milky Way Galaxy. This gave him a way to find the position of the center of our galaxy, even though it is obscured by dust as seen from the Earth. Sun

12. Shapley found that the true size of our Milky Way Galaxy is much larger than found by both the Herschels and Kapteyn. Those earlier studies had been flawed because they did not take into account absorption of light by interstellar dust. The distance to the Galactic center is now known to be about 8400pc (8.4 kpc), from the sun, and the diameter of the disk of the galaxy is about 25 kpc.

15. Where is most of the interstellar dust in the Milky Way Galaxy found? In the halo In the nuclear bulge In the nuclear bulge and halo In the disk In the disk and nuclear bulge

16. Components of the Milky Way The Disk Component: The disk is flattened like a pancake and contains gas, dust, young & middle-aged stars and open star clusters. In our Galaxy, the disk is organized into Spiral Arms

17. Components of the Milky Way The Spherical Component: Includes the Halo and the Nuclear Bulge. The Halo: Contains old stars, globular clusters and little gas and dust. The Nuclear Bulge: Contains a mixture of old and young stars, along with gas and dust.

18. In the Milky Way Galaxy, stars of all ages are uniformly distributed through the galaxy stars of different ages are found in different parts of the galaxy, with young stars in the halo and old stars in the disk stars of different ages are found in different parts of the galaxy, with young stars in the disk and old stars in the halo.

19. Astronomers identify two populations of stars in our Galaxy, called Population I and Population II • Population I: Are young and middle-aged, metal-rich • stars found in the disk of the Galaxy. • Population II: Old and metal-poor stars found in the • halo and nuclear bulge of the galaxy

21. Population I Population II

22. The Formation of the Galaxy • The galaxy probably began as a roughly spherical • rotating gas cloud composed entirely of hydrogen • and helium, shortly after the Big Bang. • Gravity caused this cloud to collapse, primarily • along the rotational axis, leading to flattening of • the galaxy. Some early star formation occurred, • and some of these stars became supernovae and • enriched the interstellar medium with metals. • The flattening continued, forming the disk of the • galaxy. The older (first generation) of stars retain • their spherical distribution. Young, metal-rich • stars are found only in the disk.

23. Our Galaxy is a Spiral Galaxy. How do we know this? The spiral structure of our galaxy can be observed using: • Optical Tracers: These are young stars (O and B-type stars), OB Associations and young open clusters. These • objects should not have moved far from their places of birth, and so should still trace out the spiral arms.

24. Local Spiral Arms from optical tracers

25. Spiral Arms in our Galaxy can also be observed using • Radio Tracers: Clouds of neutral hydrogen concentrate • along spiral arms. They emit 21 cm radiation. Visible 21 cm

26. 21 cmRadiation is emitted by Neutral Hydrogen in interstellar clouds When a Hydrogen atom flips from Parallel to Anti-Parallel, it emits a radio photon with a wavelength of 21 cm.

27. Spiral Arms in the Galaxy (21 cm radiation)

29. Rotation of the Galaxy Our Sun is in orbit around the center of the galaxy. The orbital velocity is about 230 km/s. It takes about 200 million years for our sun to orbit the galaxy (“Galactic year”). We can use this information and Newton’s form of Kepler’s Third law to estimate the mass of the galaxy. Interior to the orbit of the sun, this mass is about 9  1010 M.

30. Rotation of the Galaxy We can observe the velocities of objects at different positions in our galaxy and form the Rotation Curve of our galaxy

31. Rotation of the Galaxy The astonishing thing about this rotation curve is that it continues to be flat well beyond the position of the sun, implying that there is a large amount of matter in the outer parts of the Galaxy. The rotation curve => total mass of the Galaxy  1  1012M. However, this matter cannot be accounted for in terms of stars, gas or dust. It is not, apparently, luminous. Thus it is known as Dark Matter or Missing Matter. It may account for 90% or more of the mass in our Galaxy!!

32. The Origin of Spiral Structure When we look at external spiral galaxies we find that these galaxies come in two types. • Grand Design spiral galaxies in which spiral arms • are well developed and can be traced over nearly • 360o. • Flocculent spiral galaxies in which the spiral arms • appear to be made of small spiral segments.

33. Grand Design Galaxies

34. Flocculent Spirals

35. What causes the spiral arms? Density Wave Theory: Spiral arms are compression or density waves which rotate around the disk of the galaxy. These density waves cause the compression of molecular clouds, leading to star formation. Density waves are thought to be responsible for the Grand Design spiral galaxies.

36. What causes spiral arms? Self-Sustaining Star Formation: In this scenario, star formation triggers nearby interstellar clouds to contract, leading to more star formation. This self-sustained star formation leads to clumps of new stars which are drawn out into spiral arms by the differential rotation of the galaxy. This mechanism is thought to be responsible for the flocculent spiral galaxies.

37. The Galactic Nucleus • Is very complex and not well understood – the nucleus • is hidden behind thick clouds of dust. Observations can • only be made in the Radio and the Infrared. The nuclear • region is characterized by: • High Star Densities • Clouds of neutral hydrogen organized into two • expanding “arms” • A powerful radio source, Sagittarius A at the center • emitting synchrotron radiation and thermal radiation • The core of Sagittarius A (A*) is associated with high • velocity gas clouds.  a 106 M Black Hole??

38. Radio image of Galactic center. Stellar motions near the galactic center