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 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