LAB 10 - HUBBLE'S LAW

Cosmology is that branch of astronomy that is concerned with the composition and evolution of the entire universe.  This branch of astronomy can trace its roots back to the remarkable discoveries made by Edwin Hubble.  In the 1920s, Edwin Hubble was an astronomer working with the 100-inch telescope on Mount Wilson in California.  Hubble photographed images of hundreds of galaxies.  He also collected their spectra.  In the photographs, he was able to identify Cepheid variable stars in each galaxy.   Knowing the period-luminosity relationship for Cepheid variable stars, Hubble was able to determine the distance to each galaxy.  From the spectra, Hubble noticed that each galaxy’s spectral lines were shifted towards the red end of the spectrum.  This indicated that each galaxy was speeding away.  The amount of the red shift is directly related to speed at which each galaxy was moving away.  Hubble plotted the data he collected – galactic distance vs. galactic recessional velocity – and noticed that there was a linear relationship between these variables.  This relationship became known as Hubble’s Law.

V = H x D

Recessional Velocity = Hubble’s Constant x Distance

Hubble’s constant quickly became one of the mostly hotly contested values in science.   An accurate measurement of Hubble’s constant not only allows to you to understand the relationship between galactic speed and distance, but can also be used to determine the age of the universe.   This is based on the assumption that the expansion rate of the cosmos (which is what Hubble's constant represents) has been constant (recently discoveries are challenging this assumption!!!).  

The following video provides instruction related to this lab:

Part One:  Construct the Graph

The table below contains the distance and recessional velocity for six galaxies. 

In this lab, you are going to graph this information.  Your graph should be constructed such that distance (Mpc = megaparsecs) is on the horizontal (x) axis and recessional (going away) velocity (km/s) is on the vertical (y) axis. Here are the instructions for creating your graph:

(1) Go to the following online graphing program and input the numbers from the table above to create your graph: Quick Graphing Program 5.0

(2) Input the Distance (Mpc) into the x column.  You can change the name of the x column to "Distance (Mpc)" as well.

(3) Input the Velocity (km/s) into the y column.  You can change the name of the y column to "Velocity (km/s)" as well.

(4) After you plot all the points, you are then going to ask the program to draw a best fit line through the collection of dots on the graph.  One end of the line has to start at (0,0) because that represents our position in the Milky Way Galaxy.   To do this, click on Curve Fit, then select Proportional graph option. 

When you are done with the graph, it should look similar to the one below:

(5) Now that you've drawn the line, the program automatically calculates the equation of the line.  You'll find this equation directly under the data columns.  The number that you see in parentheses is the slope of the line.  Calculating the slope of the line in this way, with this kind of data, gives you the value of the Hubble Constant. 

(6) Take a screen capture of your graph and submit it below. 

This video provides a nice overview of the important concepts of this particular lab:

Part Two: Answer The Questions

Now that you've completed the graph, you are ready to answer some questions.