Lecture 40 Observing the Milky Way

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Lecture 40Observing the Milky Way
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Galaxies

• Large collections of stars, dust and gas
• Held together by their gravity.
• Contain millions to billions of stars.
• Stars rotate around the center of the galaxy.
• Our galaxy is the Milky Way

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Structure of the Milky Way

• Hard to observe our Galaxy -- we are inside.
• William Herschel -- tried to find where Sun was
  in the galaxy by counting stars
• Found same density of stars on all sides
• Concluded we are in the center.

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Dust Blocks Our View of the Center of the Milky
Way
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Shapley Uses Globular Clusters to Find the Center

• Globular Clusters often orbit outside disk of MW.
• Shapley observed them mainly on one side of the
  sky.
• We are not at the center of the galaxy.

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Milky Way
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Components of the Milky Way

• Disk
• Young and older stars.
• Much gas and dust.
• Extends 30 kpc in diameter
• Sun is 2/3 out from center of the disk.

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• Bulge
• Young and older stars
• Gas and dust.
• Near center of disk.

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• Halo
• Mostly old stars.
• Many stars in globular clusters orbiting center.
• Little gas so no new star formation.

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Edge-on Spiral Galaxy
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Motions of gas and stars

• Disk
• Takes 225 myrs for Sun to go about the center.
• Stars rotate differentially
• Takes different amounts of time for stars to get
  around the center

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• Halo and bulge
• Stars move in all directions around the center

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Mapping the Structure of the Milky Way

• Dust obscures view of galaxy
• Need to look at Radio, IR wavelengths of light
• 21 cm Emission Line
• Can observe neutral hydrogen (not ionized)
• Light emitted in radio.
• H very common

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• Hydrogen emits light because of a spin-flip
  transition.

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Milky Way
Optical
Near - IR
21 - cm
Radio
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Distribution of Gas

• Much of the gas is distributed in arms in the
  disk spiraling around the center

Milky Way in 21 cm
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Spiral Structure in the Milky Way
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Formation of Spiral Structure
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Density Waves Form Spriral Arms

• Spiral structure may be caused by waves of higher
  density.
• Same material is not always in the arm, just
  temporarily compacted.
• Slinky
• Traffic on the highway

Lower density Area
Lower density Area
Slow moving car
Density wave
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Spiral Arm Formation

• Stars form in arms since they have higher
  density.
• Recent star formation some O and B stars.
• Bright, blue regions

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Spiral Arm Formation

• Between arms, star formation less recent, less
  bright.
• Arms have only 5 more stars than the other area,
  but they are all very luminous (O and B MS types)

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Self-Propagating Star Formation

• Stars form in higher density area.
• SN later on compress nearby areas, start new star
  formation
• May explain star formation in flocculent galaxies

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Structure of the Milky Way
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Center of the Galaxy

• Center suspected to contain billions of stars
• Cannot observe center in visible light due to
  dust.
• Use IR and radio observations
• Bright source near center in radio - Sagittarius
  A
• Possible black hole in the center?

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Galactic Center
Radio image of Galactic center. 60 parsecs
across Filaments may be associated with magnetic
field.
Radio image of Galactic center 7 parsecs
across Sagittarius A at center.
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Black Hole at the Center

• Stars are orbiting center very quickly.
• Estimate mass of 2.6 million solar masses in area
  1 AU across
• Likely black hole.

IR image of Galactic Center