The Milky Way

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The Milky Way

• Our home galaxy, full of stars, gas and
  mysterious dark matter
• We decompose it into a disk and a halo and a few
  other parts

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Second Exam Results

• Mean 53.7 20 pt curve 73.9
• Standard deviation 10.4
• Maximum 76 raw, 92 curved
• Minimum 36 raw, 56 curved.
• Distribution gt90 4 gt80
  5 gt70 9 gt60 5 gt50 2

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Almost a View of our Milky Way
NGC 4526, a spiral galaxy like the MW but about
30 Mpc away it has a similar size, luminosity
and structure
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Edge-on View and View of MW
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Key Parts of the Milky Way

• HALO Contains most globular clusters, and most
  Pop II stars roughly 30 kpc (105 lt-yr) in
  diameter.
• THICK DISK roughly 5 kpc thick, and 30 kpc in
  diameter contains Pop I stars (but low density).
• THIN DISK 500 pc thick contains MOST stars
  includes spiral arms and great majority of
  luminosity.
• DUST DISK only 50 pc thick new stars are born in
  the molecular clouds found within this very thin
  disk.
• SPIRAL ARMS are wrapped within the dust/thin
  disk contain almost all hot, luminous (O and B)
  stars.

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Overall Structure of the Milky Way
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Inner Parts

• GALACTIC BULGE roughly 2 kpc in radius
  around center highest concentration of stars,
  including many globular clusters.
• GALACTIC CENTER in the direction of the
  constellation Sagittarius, some 8 kpc from the
  Solar System (SS).
• Multiwavelength Milky Way

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MAPPING the MILKY WAY

• Dust, mainly in molecular clouds, shrouds the
  Disk we see few stars beyond 2 kpc from SS in
  the thin disk, where the number of stars is much
  greater
• Originally astronomers thought the Milky Way WAS
  the Whole Universe SS central to it (because
  of visible light extinction by dust)
• Location of Globular Clusters in halo implied
  center towards Sagittarius and SS actually
  towards one side in early 20th century.
• Atomic Hydrogen gas sends 21 cm radio waves that
  allow us to map the far side of the galaxy, and
  the outer reaches where there are few stars

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Our Nearest Big Neighbor, M31, the Andromeda
Galaxy
Andromeda, about 30 kpc across down to nucleus
only 15pc
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A Limited Conception of the MW
Herschels map of the Galaxy from star
counts More in the MW plane, but thought the Sun
near the center and got the size too small
didnt understand dust
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Distribution of Globular Clusters
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How do stars orbit in our galaxy?
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Stars in the disk all orbit in the same direction
with a little up-and-down motion
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Orbits of stars in the bulge and halo have random
orientations
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Thought Question

• Why do orbits of bulge stars bob up and down?
• A. Theyre stuck to interstellar medium
• B. Gravity of disk stars pulls toward disk
• C. Halo stars knock them back into disk

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Thought Question

• Why do orbits of bulge stars bob up and down?
• A. Theyre stuck to interstellar medium
• B. Gravity of disk stars pulls toward disk
• C. Halo stars knock them back into disk

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Distances from Variable Stars

• Certain stars act as standard candles with
  fixed LUMINOSITY (M)
• So, MEASURED BRIGHTNESS (m) lets us compute their
  distances.
• RR Lyrae stars all have similar absolute
  magnitudes (around -0.5 to -1.5). Their periods
  are all less than one day. They can be seen in
  nearby galaxies outside the MW.
• Can be seen out to 10s of Mpc. Cepheid variables
  are even more luminous, and have longer periods
  (1-50 days).

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Light Curves of RR Lyrae and WW Cgyni (a
Cepheid Variable)
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The Instability Strip

• Both RR Lyrae and Cepheid variable stars are
  post-main sequence stars (subgiants and giants)
  whose atmospheres pulsate strongly due to opacity
  variations

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Standard Candles via Period-Luminosity Relations
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Variable Stars and the Distance LadderThey take
us out to moderately distant galaxies
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Motions Near the Sun

• Measure Doppler shifts of many stars to get
  velocities near the Sun
• Motions are faster closer to the galactic center
  so, on the average, stars ahead of Sun and inside
  get ahead (redshifted) while those behind and
  outside fall behind (also redshifted)

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Gas Velocities from 21 cm Lines
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Rotation Gives Mass Distribution

• ROTATION CURVES plot the stellar or gas velocity
  (v) against distance from center of galaxy (r).
  Mostly measured by 21 cm emission from H I gas
• Rigid body curve v ? r (like CD in a player or a
  rigid arm swinging)
• Keplerian curve v ? 1/r1/2 ? most mass centrally
  concentrated. This would be like Mercury
  orbiting fastest and Neptune slowest around the
  Sun.
• Flat curve v ? constant ? M rises significantly
  specifically M ? r

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Galactic Rotation Curve
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DARK MATTER SEEMS TO REALLY MATTER

• For the MW a FLAT rotation curve implies there is
  MISSING MASS or
• DARK MATTER that isn't Stars or Gas seen out to
  20 kpc from galactic center.
• Essentially ALL other Spiral galaxies for which
  Rotation Curves can be measured ARE ALSO FLAT, so
  DM is EVERYWHERE!
• More evidence for DM comes from CLUSTERS OF
  GALAXIES we'll discuss this later.
• Yet more evidence comes from COSMOLOGICAL
  measurements of the structure of the universe as
  a whole (last couple of lectures!)

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Dark Matter Candidates

• Missing Red Dwarfs (not enough next slide)
• Planets or Brown Dwarfs on the loose (unlikely to
  be enough gravitational lensing)
• Isolated black holes (very unlikely to be enough)
  
• Massive neutrinos (evidence for then having a
  tiny mass makes them a good candidate, but very
  unlikely that they dominate the DM)
• Snowballs (very difficult to form them, unpopular
  choice)
• As yet undiscovered particles (Axions
  Supersymmetric particles WIMPs Weakly
  Interacting Massive Particles) MOST popular now
  BUT no convincing detections yet.

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Few Red Dwarfs Seen in Globular Cluster 47 Tucanae
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Gravitational Lensing by Brown Dwarfs

• Temporary increase in stars brightness due to a
  dark mass moving in front
• A rare detection is shown in the right

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Key Properties of MW

• We are about r 8 kpc from the center.
• We orbit the center at v 220 km/s
• That makes for a galactic year (circumference
  divided by velocity) of
• (2 ?) x 8,000 x (3.0857 x 1013 km) / 220 km/s
  7.1 x 1015 s 2.24 x 108 yr.
• So, roughly 225 million years is ONE GALACTIC
  YEAR.
• How old is the solar system in galactic years?
• At nearly 4.6 billion years of age, the SS is
  only about 20 galactic years old!

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Weighing the Galaxy
Orbital speed depends on mass inside at a
particular radius. This can be used with any
galaxy for which motions can be measured. Mass
vs. Distance Applet
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Orbital Velocity Law

• The orbital speed (v) and radius (r) of an object
  on a circular orbit around the galaxy tells us
  the mass (Mr) within that orbit

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Mass of the Milky Way

• Mgal ?r3/P2 from Newtons laws.
• This is dominated by DARK MATTER, but total mass
  can be estimated by the velocity of stars at
  different distances.
• Out to solar distance (about 8 kpc) the mass is
  about 1 x 1011 M? (mostly stars)
• Out to 15 kpc, (the visible radius) a good
  estimate for the mass is nearly 4 x 1011 M? (now
  mostly DM).
• Out to about 70 kpc (gt 90 dark matter) 2
  x 1012 M?

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Spiral Galaxies
M101 is seen face on (similar to MW) NGC 4565 is
edge on
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Stellar Populations

• Pop I Stars Have compositions like the sun 70
  H, 28 He, 2 "metals" these metals are mostly
  Carbon, Oxygen and Nitrogen
• Use the CNO cycle to generate Main Sequence
  energy if M gt 1.5 M?
• Are almost all younger than 8 billion years.
• Most are in the thin disk the rest are in the
  thick disk.

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Stellar Populations, 2

• Pop II Stars Have compositions with much less
  heavy elements than the Sun 72H, 28 He, 0.2
  metals is typical
• Use the pp-II on the MS if M gt 1.5 M?
• Are almost all older than 8 billion years.
• Most are in the halo and galactic bulge however
  plenty pass through the thick disk too.
• Pop III Stars The very earliest born
• they have essentially NO METALS,
• formed from only H and He made in the BIG BANG
• Only a few possible detections.

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Spiral Arms

• Fundamentally produced by Gravitational
  Perturbations to the galactic disk
• Produced either by a CENTRAL BAR or by a
  COMPANION GALAXY
• TWO ROUTES to Spiral Arms
• First, DENSITY WAVES
• Think traffic jam in space
• Second, STAR FORMATION CHAIN REACTION

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Density Wave Analogy to Traffic Jam

• Small extra density holds stars/gas up, like a
  broken down truck on the side of the road
  --Molecular clouds compressed, stars born
  --This best explains beautiful smooth
  ("grand design") spirals

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Density Waves Can Make Spiral Arms
NGC 1566 shows density wave features with dust
lanes and nearby young star clusters
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• Spiral arms are waves of star formation
• Gas clouds get squeezed as they move into spiral
  arms
• Squeezing of clouds triggers star formation
• Young stars flow out of spiral arms

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So Can Stochastic Star Formation

• Random birth of Massive Stars
• Their SN explosions compress nearby clouds make
  new stars
• Differential rotation of galaxy yields spiral
  appearance by streching the stars out
• This best explains "rattier", broken-up spirals
  (like the Milky Way, though some Density Wave
  contribution is OK.)

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Stochastic or Self-Propagating Star Formation
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Spiral Arm Facts

• Typically, spiral arms have dark, DUSTY CLOUDS on
  their edges.
• Some of these are compressed enough to form
  bright O-B STAR CLUSTERS, which can in turn
  ionize and light up parts of the clouds into H II
  regions.
• Stars older than about 20-30 Myr are usually
  outside the arms.
• NOTE the arms are barely denser in stars than
  the rest of the disk but they stand out because
  they have nearly all the hot, bright, young
  stars.
• Spiral Arms Applet

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Stellar Clusters

• ALL clusters contain many more stars than average
  within diameters of 3-20 pc. We usually define
  three types
• O-B ASSOCIATIONS
• OPEN (or Galactic) CLUSTERS
• GLOBULAR CLUSTERS

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O-B ASSOCIATIONS

• usually lt 100 stars,
• found in the THIN DISK
• definitely Pop I -- higher metallicity (similar
  to the Sun)
• stand out because these massive MS stars are so
  powerful
• ages usually lt 30 Myr
• definitely BLUE in color because they have many
  hot (O and B) MS stars

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OPEN (or GALACTIC) CLUSTERS

• 100's to 1000's of stars,
• found in the DISK and BULGE
• definitely Pop I -- higher metallicity (similar
  to Sun)
• stand out because of some pretty massive MS stars
  and LOTS of stars
• ages range from 5 Myr up to 3 Gyr (M Mega,
  million, G Giga, billion)
• colors are BLUE through YELLOW from dominant MS
  stars

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Open Cluster Pleiades
Only 120 pc from the Sun, the Seven Sisters have
many fainter companions only the most massive
have left the MS
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GLOBULAR CLUSTERS

• 104 to gt 106 stars
• MOSTLY found in the HALO (plenty in the BULGE
  too, and a few found passing through the DISK)
• All Pop II -- much lower heavy element abundance
  than the Sun
• stand out because of HUGE number of stars in them
  
• ages all gt 5 Gyr
• RED in color low mass (red) MS stars and higher
  mass Red Giants provide most of their light.
  Blue stars are gone from the MS.

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Globular Cluster Omega Centauri
Higher mass stars have become RGs, MS are low
mass So the globular clusters look RED since they
are OLD.
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Why do astronomers love to study star clusters?

• First, because all the stars in a given cluster
  are nearly the SAME DISTANCE from us.
• So differences in apparent magnitude translate
  directly to absolute magnitude differences
• Plot color-magnitude diagram for the cluster
• Compare it with a H-R diagram made from stars of
  known distances
• Slide MS part up or down until cluster MS
  overlaps known MS
• Then can get the distance to the cluster (and ALL
  its stars) m - M 5 log (d/10 pc)
• This is a version of what is called SPECTROSCOPIC
  PARALLAX.

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Equal Distances and Equal Ages

• Second, because all the stars in a given cluster
  are nearly the SAME AGE.
• Theoretical H-R diagrams have the higher mass
  stars reaching ZAMS first It takes 107 years
  before 2-3 M? stars reach ZAMS
• meanwhile highest mass stars have left MS to
  become SN
• by 108 years many high mass stars have become RGs
  and SGs, but lowest mass stars still not on ZAMS.
  
• By 109 yr all low mass stars on ZAMS but TURN-OFF
  down in A stars I.e., all O, B and some A will
  have evolved off MS by then.
• By 1010 yr, all stars down to about Sun's mass
  will have left the MS, and the cluster will have
  big RG, Horizontal Branch and WD contributions.
• THE FURTHER DOWN THE TURN-OFF IS, THE OLDER THE
  CLUSTER.
• Plots of individual clusters H-R diagram confirm
  this evolution!

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H-R Diagrams of ClustersTurn-offs are lower for
older clusters as highest mass stars leave MS
first
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Halo Stars 0.02-0.2 heavy elements (O, Fe,
), only old stars
Halo stars formed first, then stopped
Disk Stars 2 heavy elements, stars of all
ages
Disk stars formed later, kept forming
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THE GALACTIC CENTER

• Until the past 20 years, it was very mysterious,
  mainly because VISIBLE light CANNOT PENETRATE
  all the DUST in the DISK
• UV light is absorbed even more strongly!
• Confused by stars between us and the Center

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New Tools Radio

• RADIO maps show H I gas, supernova remnants,
  synchrotron emission from filaments of strong
  magnetic fields

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Infrared Shows Fast Moving Stars

• penetrates dust much better,
• IR from tall mountains, planes, satellites
• some emission from very center and also quite a
  few INDIVIDUAL STARS (RGs, mostly)
• over the past decade the ORBITS of some such RGs
  have been determined

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Stars appear to be orbiting something massive but
invisible a black hole? Orbits of stars
indicate a mass of about 4 million MSun
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IR Movie of Stellar Orbits
This is from the Max-Planck Institute for
Extraterrestrial Physics in Germany, based on
their measurements over 10 years. Similar
results have come from a Caltech group.

• http//www.mpe.de/ir/GC/index.php
• Milky Way Center Zoom

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X-rays from the Galactic Center

• Since the earth's atmosphere blocks them, we need
  SATELLITES!
• The lower energy (soft) X-rays are absorbed by
  gas,
• BUT Higher energy (hard) X-rays can penetrate out
  to us
• Some are from SNRs near the Galactic Center, but
  a modest amount from the very center, also seen
  as the strong radio source Sagittarius A
• Q. WHAT DO ALL THESE MEASUREMENTS TELL US?

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A. THERE'S A SUPERMASSIVE BLACK HOLE in the
Galactic Center, Sgr A

• gas moving very fast (from radio measurements)
• orbits of some nearby RGs very fast those
  further away are slower
• X-rays consistent with weak emission from
  accretion disk
• MSMBH 3.6 x 106 M?