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

What we see Closeup view:

Outside view: Tilted view: Edge on view:

Studying Galaxies: 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 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

Stars in Our Neighborhood: Thin Disk 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) Population I stars: stars like the Sun 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”

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

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 Shape of a cluster Typically roughly spherical Dense inner region Core radius r c Sharp dropoff at large radius Tidal disruption radius r t Region where other gravitational objects have stripped stars away rcrc rtrt

Hope to add it eventually

M35 NGC 2158 NGC 290 Open Clusters M36 M6 Pleiades

M80 M10 M3 Globular Clusters M13 M2

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

The Bulge Disk appears blue from young stars Bulge appears red from old stars

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

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

The Nucleus 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 Radio Image

Black hole 4.0 million M Sun 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 The Monster in the Middle We can also determine the mass of Sagittarius A* About 4 million M Sun 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

How much mass is there in the whole galaxy? Method #1: Count stars Method #2: Measure Orbits The Mass of the Galaxy ObjectMass (M Sun ) Disk Stars60  10 9 Disk Gas~10  10 9 Bulge20  10 9 Halo Stars1  10 9 Nucleusdiddly squat MACHOS???? Dark Matter???? Counting stars indicates a total mass of about 100 billion M Sun 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 M sun Expect as we go outwards, this mass will remain about the same This results in rotation curves the fall off at large r