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 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
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”
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
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
Hope to add it eventually
Open Clusters
M35
NGC 290
NGC 2158
M6
M36
Pleiades
Globular Clusters
M3
M80
M10
M2
M13
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
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
We can also determine the mass of
Sagittarius A*
•About 4 million MSun
•Black hole
•4.0 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
The Mass of the Galaxy
How much mass is there in the whole galaxy?
•Method #1: Count stars
•Method #2: Measure Orbits
•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
Object
Disk Stars
Disk Gas
Bulge
Halo Stars
Nucleus
MACHOS
Dark Matter
Mass (MSun )
60  109
~10  109
20  109
1  109
diddly squat
????
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