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

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

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

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

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

9B Where are We? We aren’t 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 Near-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 = 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!

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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 Kepler’s Third Law (again) to get mass of the Milky Way. M = x M sun

9B 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.

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9B 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.

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