Levels of organization: Stellar Systems Stellar Clusters
Galaxies Galaxy Clusters Galaxy Superclusters The Universe Everyone
should know where they live: The Solar System (we dont life in a
cluster) The Milky Way Galaxy The Local Group The Virgo
Supercluster The Universe
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What we see Closeup view:
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Outside view: Tilted view: Edge on view:
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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
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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
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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
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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
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Hope to add it eventually
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M35 NGC 2158 NGC 290 Open Clusters M36 M6 Pleiades
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M80 M10 M3 Globular Clusters M13 M2
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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
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The Bulge Disk appears blue from young stars Bulge appears red
from old stars
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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
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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
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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
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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 cant 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
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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