Milky Way Galaxy

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Milky Way Galaxy
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Learning GoalsWhat We Should Know

• Size, Shape and Properties of the Milky Way
• Determination of Distance to Stars in the Milky
  Way
• Measuring Gas in the Milky Way
• Determination of the Mass of the Milky Way
• Spiral Arms Structure and Star Formation
• Population I and II Stars
• Density Wave Theory
• The Galactic Center
• Formation of the Milky Way

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17 Milky Way Galaxies

• Reading Assignments
• Pathways to Astronomy, Schneider Arny
• Units 70 73
• Online Resources Course Website

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Milky Way Properties

• Number of Stars
• Mass
• Spatial Distribution Shape
• Star Properties

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Number of Stars in Milky Way

• 100 to 200 billion stars
• Mass
• Stellar mass 2x1011 Solar Masses
• Gas 8x1010 Solar Masses
• Total Mass 6x1011 to 1012 Solar Masses
• Distribution of Spectral Classes

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Milky Way GalaxyProperties

• Determination of Spatial Distribution of Stars
• Observations of numbers of stars and locations
• Problem of the effect of dust

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Observations of Milky Way

• Distances to Clusters (Globular) using Variable
  stars

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Observations

• Distribution of Globular clusters defines halo
  and is centered far from sun
• Corrections for Interstellar dust increased size
  estimates
• 21 cm emission line of Hydrogen

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21 cm Hydrogen Line
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21 cm Hydrogen Line

• Maps out the hydrogen gas in galaxy (HI regions)
• Not affected by dust
• Show regions of interstellar clouds and star
  formation
• Structure of spiral arms
• Doppler shift reveals motion of arms

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Milky Way GalaxyProperties

• Shape
• Thin Disk with central bulge
• Halo
• Spiral Arms

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Size

• 30 kpc diameter
• 200- 400 pc thickness in disk
• Central bulge 10 kpc diameter

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Location of the Sun

• 8.5 kpc from center
• period to orbit the center of galaxy is 250
  million years
• Determination of the mass of galaxy
• Keplers Law
• P250 million years
• A8.5 kpc
• Mass inside Suns orbit 2 x 1011 Solar masses

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Galactic Rotation

• Orbital speed vary faster closed to galactic
  center
• Does not rotate as a solid object

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Types of Stars in Milky Way

• Population I and Population II

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Types of Stars in Milky Way

• Population I
• Younger stars in disk
• Enriched in heavy elements (up to 5)
• O B stars, Type I Cepheids, Sun
• Type II supernova
• Interstellar matter

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Type of Stars

• Population II
• Older stars in bulge and halo
• Low in heavy elements (less 1)
• Type I supernova
• Type II Cepheids, RR Lyrae stars
• Planetary nebula

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Problem of Pop I and Pop II Stars

• Why are there no pure Hydrong/Helium Stars
• Stellar Enrichment
• Sun is probably a 5th generation star
• Explanations
• Original stars (K and M) have incorporated heavy
  elements from nebula from early O and B stars

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Stellar Motions

• Population II Stars
• Population II stars not exactly circular orbits
  about center
• Eccentric orbits randomly oriented
• Orbit above and below through plane of disk
• Population I Stars
• Circular orbits about the center at speeds of
  about 250 km/s
• Motion of population II relative to population I
• Population II high velocity stars

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Role of Spiral Arms

• Star Formation regions
• Interstellar gas
• Stellar recycling
• More heavy elements in middle of disk less in
  outer regions

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Spiral Arms

• Are they static long lived feature?
• Rotational wrap up
• Sun 40 t0 50 trips about center of galaxy
• Density Wave theory
• Gravitational disruption of orbits

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Disruption of Orbits
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Density Wave theory

• Density increases where orbits become close
• Density gradient
• Spiral shape
• Enhanced star formation (visible by O and B
  stars)
• Stars move faster than arms and end up the region
  between spirals
• O and B burn out leaving older stars between
  spiral arms

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Galactic Center

• Hidden by clouds and general dust in galaxy
• Center is brightest object in the heavens
• Evidence that the center of the galaxy is a bar
• draws gas to the inner region of the galaxy
• Inner 300 pc has 100 million solar masses of gas
• About 100x average density of galaxy
• High temperature gas with x-ray emissions

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Center of Galaxy

• Intense radio emissions ( Sagittarius A)
• Dense star cluster
• Central pc of galaxy contains million stars
• Compact radio source
• Possible black hole
• gravity necessary to explain a dense central
  cluster

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Age of Galaxy

• 12 14 billion years based on clusters

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Formation of Galaxy

• Gravitation contraction of large cloud of gas
• Formation of halo and population II stars and
  clusters
• Contraction and rotation to form disk
• Population I stars and spiral arms
• Importance of Galactic Collisions

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Questions