15 the milky way more than just a candy bar.. 15
Post on 19-Dec-2015
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TRANSCRIPT
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GoalsGoals
• Structure of our Galaxy.• Its size and shape.• How do stars and things move through
it?• The Galactic Center.
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The Milky Way• Stars• Dust• Gaseous Nebulae• Open Clusters• Globular Clusters• Pulsars• Black Holes• How do they all fit together to make our
galaxy?
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Where are We?• Not at the center of
the Milky Way.• Where is the
center?• Globular Clusters
point the way.
M10 – copyright Credner and Kohle
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Galactic Distances
• How do we know the distance to stars and clusters in our galaxy?
• Stellar parallax:– Parallax of nearby stars relative to
background stars.– Good out to ~500 pc.
• What about the background stars?• How far are they?
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Standard Candles• “Standard Candles”• If we know how bright something looks,• And we know how bright it should be,• Result Distance• We do this everyday with size.
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Spectroscopic Parallax• If you know how luminous a star REALLY
is and how bright it looks from Earth, you can determine how far away it must be to look that faint.
• For any star in the sky, we KNOW:– Apparent Magnitude (m)– Spectral Type (O, B, A, F, G, K, M)– Luminosity Class (Main Sequence, Giant,
etc…). These are denoted by a roman numeral (V, III, I,…).
• Combine spectral type and luminosity class to get absolute magnitude (M).
• m – M give you distance.
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Example • Deneb is A2Ia starm = 1.25 A2 Blue starIa SupergiantM = -8.8
10pc
distancelog5 10Mm
Distance = 1000 pc
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Standard Candles• Other “Standard Candles”• Variable stars.• Stars that change in luminosity.
– RR Lyra stars– Cepheid variables
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Variable Stars• For RR Lyrae stars:
– Average luminosity is a standard candle
– Always ~ 100 x Sun
• For Cepheid variables:– Pulsation period is
proportional to average luminosity
– Observe the period find the luminosity
• Good to 15 Mpc!
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Rotation …
• Objects in the disk, rotate in the disk.– Nebulae– Open clusters– Young stars
• Objects in the halo, swarm in a halo.– Old stars– Globular
clusters
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The Sun’s “age”
• We define our age by trips around the Sun.
• How many trips of Sun around Milky Way?
R = 8.5 kpcV = 220km/sP = 2.5x108 yrs
• 20 trips.
RV
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The Heart of the Galaxy
• Because of all the dust in the Galaxy, we can’t see its center in visible light.
• Can use IR and radio to pierce the dust.
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Supermassive Black Hole
• Infrared images of stars in the Galactic Center over 8 years.
• The “+” is the radio source Sgr A*
• Conclusion: Must be over one million solar masses within less than 1/5 of a light year!
• Event Horizon ~ 0.02 AU!
• Probably in the centers of all spiral galaxies.
Copyright Eckart & Genzel