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?
The Discovery of our Galaxy • Until this century astronomers assumed stars are scattered uniformly through space
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 • Open clusters distributed along Milky Way • Globular star clusters centred on Sagittarius - the centre of our 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 • Differential rotation • Rotation curve • Dark matter • Dark halo ~7 visible radius~10-100 visible mass
The Origin of the Milky Way • Spherical component provides fossil evidence
The Age of the Milky Way • Dating prone to uncertainties • Best estimates are: • Disc ~ 7 Gyr age • Halo ~ 11 Gyr age
Stellar Populations • Population I • Young metal-rich disc population stars • Population II • Old metal poor halo population stars
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 • Traditional model • single cloud forms sphere & then collapsed to a disc • Revised model • include mergers?
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 • Young objects are spiral tracers • Arms are associated with star formation
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 • Arms are waves of compression • Trigger star formation • How does wave start? • What causes flocculent (woolly) arms?
Star Formation in Spiral Arms • Self-sustaining star formation affects arms • Grows clumps and spurs of new stars
The Nucleus • Most mysterious part of galaxy • 30 magnitudes visual extinction • Radio/IR reveal crowded fast-moving stars
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 • 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