1. Hubble’s Law

2. Edwin Hubble ( )
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!

3. We measure galaxy distances using a chain of techniques!

4. Hubble’s Law
v = Ho x d

5. 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!!

7. 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

8. 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

9. Sample Galaxies
Distance
images
Recessional Velocity
spectra

10. 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

11. Measuring Recessional Velocity
H line
388.8 nm
501.5 nm
112.7 nm
L – measure distance (mm) between the nm and the nm emission lines
x – measure distance (mm) between the nm line and the H line
λo = nm (laboratory wavelength of the H line)
Recessional Velocity
Observed Wavelength of H Line
c (λobs – λo) λo
v =
λobs = nm (112.7 nm) x L

12. 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

13. End