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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 l.jpg

The Milky Way

Physical Astronomy

Professor Lee Carkner

Lecture 20


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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


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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)


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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


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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


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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


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    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


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    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


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    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


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    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


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    Mass to Light

    • Ratio of mass in solar masses to light in solar luminosities

    • For Milky Way ~ 60


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    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


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    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”


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    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


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    X-ray Observations

    • Sgr A* corresponds to a bright X-ray source

    • Explosions of material must have occurred in the past


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    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*


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    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


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    Next Time

    • Read 25.1-25.4

    • Homework: 24.2, 24.30, 24.33, 25.2b, 25.8a, 25.8b


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