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 locationsThe 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

ClustersStars 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 rc

• Sharp dropoff at large radius• Tidal disruption radius rt

• Region where other gravitational objects have stripped stars away

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Hope to add it eventually

M35 NGC 2158NGC 290

Open Clusters

M36

M6Pleiades

M80 M10M3

Globular Clusters

M13M2

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 MSun

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 MSun

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 GalaxyObject Mass (MSun )Disk Stars 60 109

Disk Gas ~10 109

Bulge 20 109

Halo Stars 1 109

Nucleus diddly squatMACHOS ????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

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