15 The Milky Way More than just a candy bar.

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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 Deneb is A2Ia star m = 1.25 A2  Blue star Ia  Supergiant M = -8.8 Distance = 1000 pc

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 500 pc 8.5kpc

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” 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 = 2.5x10 8 yrs 20 trips. R V

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.

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 Ch : Do Ch20: –Problems 3, 12