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

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

Hubble’s Law y = mx + b Hubble’s Law v = Ho x d Hubble Diagram Equation of a Straight Line y = mx + b m = slope = Hubble’s Constant (Ho) y - recessional velocity(v) x - distance(d) Hubble’s Law v = Ho x 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) Recessional Velocity Observed Wavelength of H Line c (λobs – λo) λo v = λobs = 388.8 nm + (112.7 nm) x L

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

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