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The Milky Way Galaxy Seeds chapter 12 Objective Investigate our home galaxy and its stellar community Introduction Our galaxy ~75,000 ly diameter ~100 billion stars One of the largest star systems in the cosmos via lactea “milky way” How did it form and what is its structure?

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the milky way galaxy

The Milky Way Galaxy

Seeds chapter 12

objective
Objective
  • Investigate our home galaxy and its stellar community
introduction
Introduction
  • Our galaxy
    • ~75,000 ly diameter
    • ~100 billion stars
    • One of the largest star systems in the cosmos
  • via lactea “milky way”
  • How did it form and what is its structure?
the discovery of our galaxy
The Discovery of our Galaxy
  • Until this century astronomers assumed stars are scattered uniformly through space
variable stars
Variable Stars
  • Variable stars were not understood until the 20th century
  • Instability strip
  • RR Lyrae variables
  • Cepheids
  • Period-luminosity relation
the size of the galaxy
The Size of the Galaxy
  • Open clusters distributed along Milky Way
  • Globular star clusters centred on Sagittarius - the centre of our galaxy
an analysis of the galaxy
An Analysis of the Galaxy
  • Disc component
    • Sun 8.5kpc from centre
    • spiral arms
  • Spherical component
    • halo
    • nuclear bulge
the mass of the galaxy
The Mass of the Galaxy
  • Differential rotation
  • Rotation curve
  • Dark matter
  • Dark halo ~7  visible radius~10-100 visible mass
the origin of the milky way
The Origin of the Milky Way
  • Spherical component provides fossil evidence
the age of the milky way
The Age of the Milky Way
  • Dating prone to uncertainties
  • Best estimates are:
    • Disc ~ 7 Gyr age
    • Halo ~ 11 Gyr age
stellar populations
Stellar Populations
  • Population I
    • Young metal-rich disc population stars
  • Population II
    • Old metal poor halo population stars
the element building cycle
The Element-Building Cycle
  • Lighter metals commonly produced in evolved stars
  • Heavier metals rarer, produced in SNe
  • Very metal poor stars are old galactic fossils
the history of the milky way galaxy
The History of the Milky Way Galaxy
  • Traditional model
    • single cloud forms sphere & then collapsed to a disc
  • Revised model
    • include mergers?
spiral arms and star formation
Spiral Arms and Star Formation
  • Spiral arms contain hot blue stars, dust, gas, young clusters
  • Why are they there despite differential rotation?
tracing the spiral arms
Tracing the Spiral Arms
  • Young objects are spiral tracers
  • Arms are associated with star formation
radio maps of spiral arms
Radio Maps of Spiral Arms
  • 21cm radiation from cool hydrogen and Doppler effect enable arms to be mapped
  • Arms irregular but dense
the density wave theory
The Density Wave Theory
  • Arms are waves of compression
  • Trigger star formation
  • How does wave start?
  • What causes flocculent (woolly) arms?
star formation in spiral arms
Star Formation in Spiral Arms
  • Self-sustaining star formation affects arms
  • Grows clumps and spurs of new stars
the nucleus
The Nucleus
  • Most mysterious part of galaxy
  • 30 magnitudes visual extinction
  • Radio/IR reveal crowded fast-moving stars
observations
Observations
  • Sagittarius A* is powerful radio source only 4AU diameter marking galaxy’s core
  • Stars only 1000AU apart, gas/dust ring, cavity & stars orbiting compact 3 million solar mass object
the massive black hole hypothesis
The Massive Black Hole Hypothesis
  • Compact - 13 times larger than Sun
  • Accretion via disc can provide radio energy
  • Swirl of matter 3pc across around it
  • Eats few tenths solar mass/year