The Milky Way Galaxy

1. The Milky Way Galaxy • Levels of organization: • Stellar Systems • Stellar Clusters • Galaxies • Galaxy Clusters • Galaxy Superclusters • The Universe • Everyone should know where they live: • The Solar System • (we don’t life in a cluster) • The Milky Way Galaxy • The Local Group • The Virgo Supercluster • The Universe

2. What we see Closeup view:

3. Outside view: Tilted view: Edge on view:

4. Studying Galaxies: • We cannot get outside our galaxy • The distances are too great – we cannot send a spacecraft even to the nearest stars • The “outside” views are of other galaxies, probably similar to our own • We are inside our galaxy • This lets us see details of our galaxy with unparalleled precision • However, there is dust in the plane of the galaxy – makes it hard to study within the plane of our galaxy • It makes it very difficult to see the overall shape and distribution of our galaxy • We infer other galaxies have many details similar to ours • We infer our galaxy has an overall shape and structure similar to others

5. Stars in Our Neighborhood: Thin Disk Population I stars: stars like the Sun • The majority (90%) of the stars in our neighborhood are very similar to the Sun • Typical masses 0.1 – 10 solar masses • Lower masses are effectively invisible • Higher masses are so rare they are no longer in our neighborhood • Metallicity 0.4% or more (Sun = 1.6%) • Velocities relative to the Sun typically 50 km/s or less • A bit higher for the oldest stars (> 10 Gyr) • These stars are not uniformly distributed: • Most are within 250 pc of a plane called the “galactic plane” • Within this plane, there are more stars in the direction of Sagittarius than in the opposite direction • Collectively, these stars make up the “thin disk”

6. Characterizing locations • The density of any type of star is roughly described by two parameters • How high (vertically) they are from the disk • How far out they are from the center

7. Clusters • Stars are often grouped into tight groups called clusters • Presumably, stars born together • Typically have almost all the same age and similar metallicity • Two types, open clusters and globular clusters • In shape, they are roughly spherical • Typically: an inner core with high density • Density gradually drops off rc rt • Shape of a cluster • Typically roughly spherical • Dense inner region • Core radius rc • Sharp dropoff at large radius • Tidal disruption radius rt • Region where other gravitational objects have stripped stars away

8. Lots of Stuff Missing Hope to add it eventually

9. Open Clusters NGC 290 M35 NGC 2158 Pleiades M6 M36

10. Globular Clusters M3 M80 M10 M2 M13

11. The Bulge • Must be studied via infrared, because view blocked by gas and dust • High metallicity stars, comparable to the Sun • Almost all older stars, 1 Gyr or more • Little or no gas and dust – no current star formation • Some rotation with galaxy, but lots of random motion as well

12. The Bulge • Disk appears blue from young stars • Bulge appears red from old stars

13. The Bulge • The bulge is approximately 2 kpc in radius and 1 kpc thick • Flattened sphere? • One side of the bulge looks thicker than the other • Best guess – this side is closer to us • This implies our galaxy is a barred spiral galaxy • Bulge is bar shaped

14. The Nucleus • Near the center of our galaxy lies a complex region • Fast star formation • Recent supernovae remnants • Hot gas • Fast motion • Density of stars is very high here • Intense radio sources can penetrate the gas and dust

15. The Nucleus Radio Image • Closer in we see streamers of gas apparently flowing in towards the center • Near the center is a strong radio source called Sagittarius A* • There are also stars orbiting it very quickly X-ray image

16. The Monster in the Middle • We can use the motion to find the distance • Doppler shift tells us the velocity • Period & velocity tells us the radius • Apparent size tells us the distance • 7.9  0.4 kpc • Radio waves can’t come from black hole itself • Gas from nearby attracted by gravity • Accelerates to near light speed • Friction creates heat/X-rays/etc. • More efficient than any other power source • We can also determine the mass of Sagittarius A* • About 4 million MSun • Black hole • 4.0 million MSun

17. The Mass of the Galaxy • How much mass is there in the whole galaxy? • Method #1: Count stars • Method #2: Measure Orbits ObjectMass (MSun ) Disk Stars 60  109 Disk Gas ~10  109 Bulge 20  109 Halo Stars 1  109 Nucleus diddly squat MACHOS ???? Dark Matter ???? • Counting stars indicates a total mass of about 100 billion MSun • Maybe a little bit more • Almost all of this mass is closer than the Sun • From orbital motion of the Sun (homework #1) • Mass closer than Sun is about 90 billion Msun • Expect as we go outwards, this mass will remain about the same • This results in rotation curves the fall off at large r