Transcript Milky Way
The Milky Way Our Galactic Home 14 14 Goals • • • • • Structure of our Galaxy. Its size and shape. How do stars and things move through it? Mass and Dark Matter. The Galactic Center. 14 The Milky Way • • • • • • • • Stars Dust Gaseous Nebulae Open Clusters Globular Clusters Pulsars Black Holes How do they all fit together to make our galaxy? 14 Optical emission from stars and nebulae 14 Near-Infrared stellar emission – copyright E. L. Wright and COBE 14 Far-Infrared dust emission – copyright E. L. Wright and COBE 14 Radio emission from neutral hydrogen – copyright J. Dickey 14 X-ray emission from hot gas – copyright S. Digel and ROSAT 14 Gamma-ray emission from pulsars and black holes – copyright NASA 14 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 14 You Are Here 14 14 Near-Infrared stellar emission – copyright E. L. Wright and COBE 14 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? 14 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. 14 • Deneb is A2Ia star Example – – – – m = 1.25 A2 Blue star Ia Supergiant M = -8.8 distance m M 5log 10 10pc Distance = 1000 pc 14 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 14 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! 14 14 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 14 … 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? 14 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. More than what we see. • M = 1011 x Msun P a 2 3 4 3 P a GM 2 2 14 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. 14 14 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. 14 200 pc 5 pc Sagittarius A* - Sgr A* 14 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 14