The Milky Way Our Galactic Home 9B 9B Goals Structure of our Galaxy. Its size and shape.
How do stars and things move through it? Mass and Dark Matter. The Galactic Center. 9B The Milky Way
Stars Dust Gaseous Nebulae Open Clusters Globular Clusters
Pulsars Black Holes How do they all fit together to make our galaxy? 9B Optical emission from stars and nebulae 9B
ar-Infrared stellar emission copyright E. L. Wright and COBE 9B ar-Infrared dust emission copyright E. L. Wright and COBE
9B adio emission from neutral hydrogen copyright J. Dickey 9B -ray emission from hot gas copyright S. Digel and ROSAT
9B mma-ray emission from pulsars and black holes copyright NASA 9B Where are We? We arent at the
center of the Milky Way. Where is the center then? Globular Clusters point the way. M10 copyright Credner and Kohle
9B You Are Here 9B 9B ar-Infrared stellar emission copyright E. L. Wright and COBE
9B 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?
9B 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 7B: m M give you distance. 9B Example
Deneb is A2Ia star m = 1.25 A2 Blue star Ia Supergiant M = -8.8
distance m M 5log10 10pc Distance = 1000 pc 9B 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 9B 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! 9B 9B 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
9B 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? 9B Missing Mass
From variable stars we know distances. From Doppler shift we know rotation velocity. Use Keplers Third Law (again) to get mass of the Milky Way. M = 1011 x Msun 2 P a 3
2 4 3 P a GM 2 9B
Dark Matter What causes the mass to keep on increasing? Dont see anything there. Thus dark matter. Brown dwarfs
Planets White dwarfs Strange matter? Use gravitational lensing (last lecture) to look for these dark objects. 9B 9B
The Heart of the Galaxy Because of all the dust in the Galaxy, we cant see its center in visible light. Can use IR and radio to pierce the dust.
9B 200 pc 5 pc Sagittarius A* - Sgr A* 9B 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 9B
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