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Announcements •The last two Dark Sky Nights are two weeks away on Monday April 17 and Wednesday April 19. Set-up will start at 7:30pm. •Don’t forget about your project. Presentations will be Monday May 1 at 3:20pm. A written paper is also due at the same time. Exam 4 is after the presentations The biggest black holes are located at the centers of galaxies Watch Journey to a Galactic Black Hole video Black holes often form jets of relativistic particles Watch Formation of a Black Hole Jet video The clearest evidence of a black hole is from spectra Extreme redshift to extreme blueshift in a small space is the fingerprint of stuff orbiting a black hole Quasars are supermassive black holes at the centers of galaxies The closest quasar is about 2.4 Gly. Locally we find their distant cousins: AGN’s Chapter 10: The Expanding Universe Measuring the expanding Universe The Classical Doppler Effect v obs rest z c rest Spectroscopic binaries are examples of the classical Doppler effect The Relativistic Doppler Effect 1 z 1 1 v c v c For objects moving away or towards you at speeds close to the speed of light a different Doppler formula must be used. For the classical Doppler effect z must always be less than 1. For the relativistic Doppler effect z can be anything greater than zero. So how big is the universe? The Milky Way is the biggest thing we can see with our naked eyes. Is this the extent of the universe? If not, how big is the Milky Way compared to the universe? The Spiral Nebulae of Lord Rosse Is this something inside the Milky Way or outside it? William Hershel’s Milky Way Count stars in each direction and then plot it out. The Milky Way is a wide, flat disk shape with the Earth located close to the center. Harlow Shapley’s Milky Way Herschel’s Milky Way Measure the distance to the globular clusters using RR Lyrae stars and then plot it out. Can’t tell the shape of the Milky Way but it is much larger than Herschel’s Milky Way. The Shapley – Curtis Debate April 1920 •Harlow Shapley…The Milky Way is huge so the spiral nebulae must be located within the Milky Way •Heber Curtis…The spiral nebulae are island universes like our Milky Way and thus are not part of it •No definitive conclusion was reached The Big Problem pointed out by the Shapley – Curtis Debate No one had good measurements of the distance to the spiral nebulae. Without knowing how far away they were we can’t know how big they are. So how do you measure the distance to a celestial object? Stellar Parallax…too small to measure with the naked eye Defining the parsec Problem: the largest measured parallax angle is less than one arcsecond. Parallax can only be measured accurately for stars less than 1600 lightyears away. Farther than that and the parallax angle is too small to measure. Luminosity Distance L dL 4b If we know the intrinsic luminosity of something we can measure its apparent brightness and calculate its distance. Luminosity can be found for pulsating variables by measuring their period of pulsation The Tulley-Fisher Relation works for spiral galaxies If we observe a Type Ia Supernova we can use it to determine the distance to the galaxy The “Distance Ladder” Complicating factor to the luminosity distance: extinction Edwin Hubble looked for Variables in the Andromeda “Nebula” Since the period-luminosity relationship for Cepheid's had been recently determined, their luminosity could be calculated.
