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ASTR 1102-002 2008 Fall Semester Joel E. Tohline, Alumni Professor Office: 247 Nicholson Hall [Slides from Lecture21] Slide 2 Chapter 24: Galaxies and Chapter 26: Cosmology Slide 3 Individual Galaxies Location in Space Coordinate (angular) position on the sky Distance from Earth Motion through Space Motion across the sky (proper motion) None observable! Motion toward/away from us (radial velocity) Extremely revealing discovery made by Slipher, Hubble, Humason Slide 4 Distance Ladder Hubble used Pop I Cepheid variables to determine distances to nearby galaxies. Slide 5 Hubble used Doppler-shift measurements to determine radial velocities of nearby galaxies. Slide 6 Determining Radial Velocities from Measured Redshifts Textbook Box 24-2 z = redshift = / 0 If z is sufficiently small, then v/c = z If z not small, use relativistic formula to obtain v from z. Slide 7 Plotting measured velocities versus measured distances, Hubble discovered a strong correlation: Slide 8 Hubbles Revealing Discovery All galaxies are moving away from us! (The exception is our nearest neighbor galaxy, Andromeda.) The farther away a galaxy is, the faster it is receding from us! Correlation described by simple formula: v = H 0 d H 0 is the slope of the line in the Hubble Law diagram: H 0 = 73 (km/s)/Mpc Slide 9 Slide 10 Hubbles Revealing Discovery All galaxies are moving away from us! (The exception is our nearest neighbor galaxy, Andromeda.) The farther away a galaxy is, the faster it is receding from us! Correlation described by simple formula: v = H 0 d H 0 is the slope of the line in the Hubble Law diagram: H 0 = 73 (km/s)/Mpc Slide 11 Determining Distance from z If the Hubble Law continues to be relevant at all distances throughout the universe, then we can turn the formula around and use measurements of z to give us distances to remote galaxies! d = v/H 0 = (zc)/H 0 Slide 12 So in order to determine distances to all of the galaxies in some region of the sky, all we have to do is measure the (Doppler-shift) redshift of each galaxy. Slide 13 Slide 14 Slide 15 Slide 16 The Hubble Constant H 0 Lets examine more closely the meaning of the so-called Hubble Constant, H 0 H 0 = (73 km/s)/Mpc = (73 km/s)/(3.085 x 10 19 km) = 2.37 x 10 -18 /s That is, 1/H 0 = 4.23 x 10 17 s = 13.4 billion yrs Slide 17 The Hubble Constant H 0 Lets examine more closely the meaning of the so-called Hubble Constant, H 0 H 0 = (73 km/s)/Mpc = (73 km/s)/(3.085 x 10 19 km) = 2.37 x 10 -18 /s That is, 1/H 0 = 4.23 x 10 17 s = 13.4 billion yrs Slide 18 The Hubble Constant H 0 Lets examine more closely the meaning of the so-called Hubble Constant, H 0 H 0 = (73 km/s)/Mpc = (73 km/s)/(3.085 x 10 19 km) = 2.37 x 10 -18 /s That is, 1/H 0 = 4.23 x 10 17 s = 13.4 billion yrs Slide 19 Interpretation of Hubbles Law Hubbles Law appears to put us in a special location in the Universe: Everything appears to be expanding away from us! Einsteins general theory of relativity provides a context for interpreting (& understanding) Hubbles Law that does not put us in a special location.