1. Calculating the Hubble Constant
Lab 37
Calculating the Hubble Constant
From our point of view nearly all the galaxies are rushing away at high speeds. Hubble and Humason (1931) showed that the recessional velocities of galaxies are proportional to their distances. 14

2. The Hubble Law The Hubble law is v = H r
where v is radial velocity, r is distance, and H is the present day value of the Hubble constant.
The Hubble constant sets the scale of recessional velocities of galaxies to their distances.
Knowing distances to an uncertainty of 10% or less is very good.
To calculate the value of H, you need to know the recessional speeds and the distances for a set of galaxies. 17

3. Constructing a Hubble plot
How do you measure distances? The best way is to look for Type 1A supernovae.
Type 1A supernovae are standard candles—you know how bright they must be, so you compare that to how bright they look in the sky.
MANY other methods exist, but the Type 1A supernovae are certainly the best method known. 17

4. Lab Procedure
Plot the data from Table I (Galaxy distance [Mpc] vs. velocity [km/sec])
On the graph, make each major tick mark 50 Mpc and 2500 km/sec;
Determine a best fit value for H (km/sec per Mpc), Table II;
Calculate likely high and low values for H, Table II
Best fit line
v = Hr V x X
H = v/r
Velocity (km/sec)
Example:
v = 36,000 km/s
r = 600 Mpc
H = 36,000/600 = 60 km/s/Mpc
Distance (Mpc) r

5. Lab Procedure
Calculate
From Graph
From Graph
Calculate
Calculate
Once H is determined, it can be used to calculate the distance r to other galaxies! (v = Hr → r = v/H)