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The Milky Way Physical Astronomy Professor Lee Carkner Lecture 20 The Milky Way We can see the band of the Milky Way on a dark night Nature of galaxy not known until early 20 th century Basic structure Central dense bulge Young disk with spiral arms Old halo with dark matter Disk

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the milky way

The Milky Way

Physical Astronomy

Professor Lee Carkner

Lecture 20

the milky way2
The Milky Way
  • We can see the band of the Milky Way on a dark night
  • Nature of galaxy not known until early 20th century
  • Basic structure
    • Central dense bulge
    • Young disk with spiral arms
    • Old halo with dark matter
slide3
Disk
  • Most visible area of the MW
  • Sun is ~8 kpc from center
  • Two components
    • Height ~ 350 pc
    • Site of current star formation
  • Thick disk of older stars
    • Fainter and has fewer stars (few % of thin disk)
metallicity
Metallicity
  • We use metal abundance as a proxy for age
  • Normally use the iron to hydrogen ratio compared to the sun

[Fe/H] = log [(NFe/NH)star / (NFe/NH)sun]

  • Range:
    • 0 (exactly like the sun)
  • Not perfectly reliable
    • Not completely mixed
age of disk
Age of Disk
  • Thin disk has broad range of metallicities
    • -0.5 to 0.3
    • -0.6 to -0.4
    • Formed from episode of star formation between 10 and 11 Gya
spiral arms
Spiral Arms
    • Gas, dust, young stars, bright stars, blue stars all concentrated in arms
    • Hard to map in our galaxy
  • From via density waves
    • As clouds orbit the Milky Way, they get stuck in areas of greater density
the bulge
The Bulge
  • The central part of the MW is a thickened bar-shaped bulge
    • Hard to see due to extinction
    • Due to several waves of star formation
    • Region within which ½ of the light is emitted
slide8
Halo
  • Above and below the disk are the globular clusters About 150 total
    • Metallicity around -0.8
    • May be associated with thick disk
    • Or else would have broken up over the last ~12 Gyr
rotation curve
Rotation Curve
    • period of sun ~ 230 million years
    • Rotation speed should fall off with distance
  • Instead galaxy has flat rotation curve
    • Rotational velocity constant with increasing distance from center
dark matter
Dark Matter
  • However, orbits of stars exterior to the sun indicate that there must be a total of about 1012 Msun
  • Dark matter is about 95% of total galactic mass
    • Cannot be dust, gas or stars
mass to light
Mass to Light
  • Ratio of mass in solar masses to light in solar luminosities
  • For Milky Way ~ 60
dark matter candidates
Dark Matter Candidates
  • MACHOs
    • MAssive Compact Halo Objects
    • White dwarfs, neutron stars, black holes, red dwarfs, brown dwarfs
    • Should pass in front of other stars, momentarily brightening them
  • WIMPs
    • Very low probability of interaction
    • Should be able to detect in very large isolated detector arrays
galactic center
Galactic Center
  • Galactic center is 8 kpc from the sun in the constellation of Sagittarius
    • Can find from distribution of halo globular clusters
    • Best data from radio, IR and X-ray (not visible)
    • stars are “isothermal”
radio observations
Radio Observations
  • A complex series of thermal and non-thermal sources
  • At the center is a very bright, unresolved source, Sgr A*
    • Less than ~2 AU in size
x ray observations
X-ray Observations
  • Sgr A* corresponds to a bright X-ray source
  • Explosions of material must have occurred in the past
ir observations
IR Observations
  • The K band at 2.2 mm is used to observe stars close to Sgr A*
  • Can use Kepler’s third law to find mass of Sgr A*
the core
The Core
  • Sgr A* has a mass of 3.7X106 Msun in a space less than 2 AU in size
  • Destroys near-by stars to provide mass for accretion disk and outflows
  • Black hole is fairly quiescent
next time
Next Time
  • Read 25.1-25.4
  • Homework: 24.2, 24.30, 24.33, 25.2b, 25.8a, 25.8b