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The Milky Way Galaxy Astronomy 315 Professor Lee Carkner Lecture 17.

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Presentation on theme: "The Milky Way Galaxy Astronomy 315 Professor Lee Carkner Lecture 17."— Presentation transcript:

1 The Milky Way Galaxy Astronomy 315 Professor Lee Carkner Lecture 17

2 Upcoming Extra Credit  Sigma Xi Science Café  6:30 pm, Thursday, April 21  New Hall 013, St. Ambrose University  More info at:  Planetarium Open House  8:30-10:00, Saturday, May 7  More info at:  Sign in at event for extra credit

3 Via Lactia   The band forms a complete ring  We seem to be in the center of a disk of stars    Clouds of gas and dust block our view so it is hard to see beyond our local region

4 Our View of the Milky Way

5 The Milky Way from Outside

6 Discovering The Galaxy  In the early part of the century Harlow Shapley found the distance to globular clusters using Cepheid variables  Globular clusters  Unlike stars in the disk, we can see distant globulars  Cepheid variables  If we can find luminosity and flux we can get distance (F = L/4  d 2 )

7 The Center  Shapley found the distance to the globulars and plotted their positions  He found:    We are not at the center of the galaxy and the disk extends out much further than we can easily see

8 Changing Views of the Galaxy

9 How Do We Learn About The Milky Way?  Optical observations   Radio observations   Infrared observations   Observing other galaxies  Since we are in the middle of the Milky Way we can’t get an overview of it

10 Structure of the Milky Way  Disk   Nucleus   Halo  Spherical distribution of old stars and globular clusters around disk and bulge

11 Edge-on and Face-on

12 The Disk  The disk is very thin   Younger stars and star forming regions near the center, older stars above and below   Disk exhibits differential rotation (inner parts rotating faster than outer)

13 Differential Galactic Rotation

14 Mass and Orbits  Finding the properties of a star’s orbit allows us to find the mass internal to the orbit  M = a 3 /P 2  M = mass (in solar masses)  P =  a =  You must use the correct units!

15 Spiral Structure  We know that other galaxies have spiral structure, but it is harder to see the Milky Way’s  We find spiral arms by tracing:    They are not uniformly distributed but are found in a loose spiral structure  How do spiral arms form?

16 Local Spiral Arms

17 Density Waves  Spiral arms are like traffic jams   This can trigger star formation in the arms   The clouds eventually move out the other side   The spiral arm material changes, only the pattern stays the same

18 Density Wave

19 At the Core   The nucleus is the hardest part of the galaxy to observe due to all the gas and dust   One, Sagittarius A* (Sgr A*), may be the center of the galactic core  What is it?

20 Properties of the Core  Stars near the core are very close together and moving very fast   Sgr A* is emitting enormous amounts of energy   A black hole

21 Massive Black Holes  Sgr A* does not move and may have jets and an accretion disk   Can get mass from orbit of star S2:  Period =  Semimajor axis =  Formed from stars and clouds of material falling into the center   Our own is very hard to observe

22 The Halo  The halo are stars orbiting in a large sphere around the galaxy  The halo is composed of old stars   Halo stars -- Population II -- metal poor  Disk stars -- Population I -- metal rich   Halo stars formed formed early from relatively unprocessed material

23 Globular Clusters   Size:  Shape:  Contents:  Globulars are in elliptical orbits around the galactic center  Unlike open clusters in the disk, globular clusters are very tightly gravitationally bound

24 History of the Milky Way  How did the galaxy form?    Basic theory has Milky way forming from smaller protogalaxies  Gas stripped out to form disk   Remains of protogalaxies form halo 

25 Next Time  Quiz #2  Covers lectures 10-16

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