The Milky Way Our Galactic Home 9 B

The Milky Way Our Galactic Home 9 B

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Goals • • • Structure of our Galaxy. Its size and shape. How do stars and things move through it? Mass and Dark Matter. The Galactic Center. 9 B

The Milky Way • • Stars Dust Gaseous Nebulae Open Clusters Globular Clusters Pulsars Black Holes How do they all fit together to make our galaxy? 9 B

Optical emission from stars and nebulae 9 B

Near-Infrared stellar emission – copyright E. L. Wright and COBE 9 B

Far-Infrared dust emission – copyright E. L. Wright and COBE 9 B

Radio emission from neutral hydrogen – copyright J. Dickey 9 B

X-ray emission from hot gas – copyright S. Digel and ROSAT 9 B

Gamma-ray emission from pulsars and black holes – copyright NASA 9 B

Where are We? • We aren’t at the center of the Milky Way. • Where is the center then? • Globular Clusters point the way. M 10 – copyright Credner and Kohle 9 B

You Are Here 9 B

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Near-Infrared stellar emission – copyright E. L. Wright and COBE 9 B

Galactic Distances • How do we know the distance to stars and clusters in our galaxy? • Trigonometric parallax good out to 100 pc. • We believe galaxy is ~30 kpc wide. • How do we know? 9 B

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). • From Lecture 7 B: m – M give you distance. 9 B

Example • Deneb is A 2 Ia star – – m = 1. 25 A 2 Blue star Ia Supergiant M = -8. 8 Distance = 1000 pc 9 B

Standard Candles • “Standard Candles” • If we know how bright something should be, and we know how bright it looks Distance • Variable stars. – RR Lyra stars – Cepheid variables 9 B

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! 9 B

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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 9 B

… and Formation • Picture the formation of the Sun: – – Spherical cloud Condenses to disk Planets in a plane Oort cloud sphere. • Perhaps the same with the galaxy? 9 B

Missing Mass • From variable stars we know distances. • From Doppler shift we know rotation velocity. • Use Kepler’s Third Law (again) to get mass of the Milky Way. • M = 1011 x Msun 9 B

Dark Matter • What causes the mass to keep on increasing? • Don’t see anything there. Thus “dark” matter. – – Brown dwarfs Planets White dwarfs Strange matter? • Use gravitational lensing (last lecture) to look for these “dark” objects. 9 B

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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. 9 B

200 pc 5 pc Sagittarius A* - Sgr A* 9 B

Stellar Motion • 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! • Supermassive Black Hole! • Event Horizon < 0. 05 AU! • Probably in the centers of all spiral galaxies. Copyright Eckart & Genzel 9 B