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 Near-Infrared stellar emission – copyright E. L. Wright and COBE 15 John P. Gleason M51 - Copyright HST 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 Concept Test • If the Earth were located at the center of the galaxy, we would see: a. b. c. d. e. Globular clusters in only one direction. Globular clusters in all directions. Many more supernovae than we do now. Many more stars being formed than we do now. None of the above. 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 (luminosity), • Result Distance • We do this everyday with size. 15 m versus M • 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 Variables in Clusters M3 Copyright – K. Stanek (Harvard ) 15 30 kpc 8.5kpc 500 pc 15 Concept Test • A standard candle can be any object (or class of object) that: a. Always has the same luminosity. b. Has some means of knowing its luminosity without first needing to know its distance. c. Can vary in brightness (as long as it always has the same average luminosity). d. Has a known absolute magnitude. e. Always gives off the same amount of energy, regardless of distance from us. 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 • For Wednesday: • Read: Bennett Ch20.1 - 20.3: • Do Ch20: – Problems 3, 12, 13 15