Hubbles Law Edwin Hubble 1889 1953 Measured distances

Hubble’s Law

Edwin Hubble (1889 -1953) • Measured distances to nearby galaxies using Cepheid variables • Galaxies are islands of stars • Developed a classification scheme for galaxies. • Discovered the Expansion of the Universe • Space telescope named after him!

We measure galaxy distances using a chain of techniques!

Hubble’s Law v = Ho x d

Recessional Velocity is Proportional to Distance Milky Way 1400 700 40 Mpc 30 Mpc 2800 2100 10 Mpc A V=0 km/sec 20 Mpc Milky Way 1400 20 Mpc B 700 10 Mpc A 700 30 Mpc C 1400 2100 Alien’s Galaxy B V=0 km/sec 40 Mpc 10 Mpc 2800 20 Mpc C 700 1400 The Universe is Expanding!!

Hubble’s Law Hubble Diagram y = mx + b Equation of a Straight Line m = slope = Hubble’s Constant (Ho) y - recessional velocity(v) Hubble’s Law v = Ho x d x - distance(d)

Finding a Galaxy’s Distance Hubble’s Law Hubble Diagram v = Ho x d d = v / Ho To Find Distance: Measure recessional velocity (red shift) 132 Mpc

Sample Galaxies • Distance – images • Recessional Velocity – spectra

Measuring Distance • Elliptical galaxies - same physical size (0. 032 Mpc) • Distances inversely proportional to angular size • Virgo – known distance of 15. 6 Mpc d = 15. 6 Mpc 49. 4 mm S

Measuring Recessional Velocity H line 388. 8 nm 501. 5 nm 112. 7 nm L – measure distance (mm) between the 388. 8 nm and the 501. 5 nm emission lines x – measure distance (mm) between the 388. 8 nm line and the H line λo = 396. 8 nm (laboratory wavelength of the H line) Observed Wavelength of H Line x λobs = 388. 8 nm + (112. 7 nm) L Recessional Velocity v= c (λobs – λo) λo

Hubble Diagram v = Ho x d Hubble Diagram x x rise x - draw best fit line - find slope (Ho) x x Procedure - plot data run slope = rise/run

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