Transcript Document
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 • Deneb is A2Ia star Example – – – – 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 Kepler’s Third Law (again) to get mass of the Milky Way. • M = 1011 x Msun P a 2 3 4 3 P a GM 2 2 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 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 9B