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.
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Presentation on theme: "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."— Presentation transcript:
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!
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 = 10 11 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.
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