Transcript Document
The Milky Way More than just a candy bar. 15 15 Goals • • • • Structure of our Galaxy. Its size and shape. How do stars and things move through it? The Galactic Center. 15 The Milky Way • • • • • • • • Stars Dust Gaseous Nebulae Open Clusters Globular Clusters Pulsars Black Holes How do they all fit together to make our galaxy? 15 Optical emission from stars and nebulae 15 Near-Infrared emission from stars – copyright E. L. Wright and COBE 15 Far-Infrared dust emission – copyright E. L. Wright and COBE 15 Radio emission from neutral hydrogen – copyright J. Dickey 15 X-ray emission from hot gas – copyright S. Digel and ROSAT 15 Gamma-ray emission from pulsars and black holes – copyright NASA 15 Where are We? • Not at the center of the Milky Way. • Where is the center? • Globular Clusters point the way. M10 – copyright Credner and Kohle 15 You Are Here 15 Near-Infrared stellar emission – copyright E. L. Wright and COBE 15 Galactic Distances • How do we know the distance to stars and clusters in our galaxy? • Stellar parallax: – Parallax of nearby stars relative to background stars. – Good out to ~500 pc. • What about the background stars? • How far are they? 15 Standard Candles • • • • • “Standard Candles” If we know how bright something looks, And we know how bright it should be, Result Distance We do this everyday with size. 15 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). • m – M give you distance. 15 Example Distance = 1000 pc • Deneb is A2Ia star m = 1.25 A2 Blue star Ia Supergiant M = -8.8 distance m M 5log10 10pc 15 Standard Candles • Other “Standard Candles” • Variable stars. • Stars that change in luminosity. – RR Lyra stars – Cepheid variables 15 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! 15 30 kpc 8.5kpc 500 pc 15 Variables in Clusters 15 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 15 The Sun’s “age” R V • We define our age by trips around the Sun. • How many trips of Sun around Milky Way? R = 8.5 kpc V = 220km/s P = 2.5x108 yrs • 20 trips. 15 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. 15 200 pc 5 pc Sagittarius A* - Sgr A* 15 Supermassive Black Hole • 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! • Event Horizon ~ 0.02 AU! • Probably in the centers of all spiral galaxies. Copyright Eckart & Genzel 15 Homework #15 • Read: Bennett Ch20.1 - 20.3: • Do Ch20: – Problems 3, 12 15