Transcript Ch24
Chapter 24 Galaxies Nick Devereux 2006 Birr Telescope - Ireland Built in 1845 by the Third Earl of Rosse In terms of size, it is as large as the Hubble Space Telescope Mirror size is 72 inches ~ 6 feet ~ 2 m. Most important discovery was spiral structure in the Messier nebula, M51; Determining Distances to Galaxies (In approximately the order that they were introduced); 1. Cepheids 2. Velocities 3. Supernovae Nick Devereux 2006 The Distance Ladder Nick Devereux 2006 Cepheids • Henrietta Leavitt (1868 – 1921), working on the stellar spectra project at Harvard, discovered variable stars by inspecting photographic plates. • She found 2400 Cepheids. Nick Devereux 2006 Cepheids undergo periodic changes in brightness Nick Devereux 2006 Which in turn is correlated with the stars luminosity – extremely useful as a distance indicator Nick Devereux 2006 The luminosity is a measure of absolute brightness. The fact that the apparent brightness is less than the absolute brightness is a consequence of distance. Hence the distance, D. Nick Devereux 2006 With a distance D = 0.7 Mpc, Andromeda is the nearest spiral galaxy to us The Cepheids yielded distances to the Andromeda Galaxy, and other galaxies, which enabled Hubble to calibrate his new distance determination technique based on the recession velocities of galaxies. Nick Devereux 2006 Velocities Nick Devereux 2006 Redshifts are converted into recession velocities using the Doppler equation Doppler Equation v/c = / Nick Devereux 2006 The Hubble Law v = Ho D Nick Devereux 2006 The Hubble Constant • Ho = 50 kms-1/Mpc according to Sandage • Ho = 100 kms-1/Mpc according to DeVaucouleurs • Ho = 75 kms-1/Mpc according to modern measurements. The way to use the equation is to measure the recession velocity, and divide by Ho to get D in Mpc. Nick Devereux 2006 Supernova Nick Devereux 2006 Type 1a Supernova attain approximately the same peak brightness Nick Devereux 2006 Different types of Supernovae, but the Type Ia’s are the best as they reach the same maximum brightness Nick Devereux 2006 Type Ib Nick Devereux 2006 Type Ic Nick Devereux 2006 Type II Nick Devereux 2006 Supernova light curves Nick Devereux 2006 Maximum brightness for a Type 1a supernova is MB = -19.6 +/- 0.2 mag One Major Complication is Dust • The major problem with all distance determinations is the extinction of starlight due to dust in our Galaxy and other galaxies. Nick Devereux 2006 The amount of dust extinction depends on which direction you look Nick Devereux 2006 The consequence of dust extinction is to make the “star” appear further away than it actually is, so we have to correct for this effect. Nick Devereux 2006 Because the extinction changes the color of the object as well as making it fainter. Nick Devereux 2006 Nick Devereux 2006 So, if you know what the intrinsic color of the object is you can figure out what the extinction coefficient is from the observed color. Nick Devereux 2006 Not all Galaxies are Spirals however…. Galaxies collide and merge Distant galaxies look chaotic How did they become the beautiful symmetric objects we see today? - an active area of current research