Hubble’s Law Lab Activity

Hubble’s Law Lab Activity

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 Lab Activity

Hubble’s Law


In the 1920’s Edwin Hubble investigated the relation between distance to galaxies and their velocity toward or away from us. He found that most galaxies are moving away from us. In a paper published in 1929, Hubble reported a linear relation between distance and velocity, now known as the Hubble law. One of the implications of Hubble’s law is that our universe is expanding and thus had a beginning.


To prepare for this lab activity you should carefully read through pages 413-419 of your textbook which covers Hubble’s law. In this lab activity you will make use of a simulation to explore various aspects of Hubble’s law.


  1. Understand the concept of Hubble’s law
  2. Measure the redshift of several galaxies which Hubble observed by using galactic spectra
  3. To measure the angular size of these galaxies
  4. Determine if any relationship exists between the galactic redshift and angular size
  5. Explore what that relationship means and what it implies about our universe


Hubble’s Law Theory:

Hubble used the redshift of galaxies he observed to determine their velocity. All galaxies emit a wide spectrum of light of various wavelengths. But these spectra have something in common: the calcium H and K absorption lines. These are particular wavelengths of light that have rest values of 3933 angstroms (for K) and 3968 angstroms (for H) and are produced by all galaxies.

To the right is an absorption spectrum of galaxy NGC 2775. Note the predominate Calcium K and H lines. These same lines and their relative position always appear in galactic spectra. However they will be shifted either right or left depending on whether the galaxy is moving away from or toward the earth.



The amount of redshift due to the velocity of an observed galaxy can be calculated using the following equation:


Redshift  = [Observed wavelength – Rest wavelength] / Rest wavelength

To determine the redshift, you need to calculate from the spectra the ratio of the observed wavelengths of the H and K lines to their rest wavelengths. For example, suppose the observed wavelength of the H line is 3988 angstroms for a distant galaxy moving away from us. In this case the redshift would be:

redshift = (3988 – 3968)/3968 = 0.005


The faster a galaxy is moving away from us the larger its redshift. This lab makes use of a java applet in which you will measure the observed wavelength of the H line for several galaxies. In fact you will be using the same galactic data that Hubble used! You will also measure the angular size of the galaxies which correlates to their distance away from earth. Your goal is to see if there is a relationship between the redshift of these galaxies and their angular size, just as Hubble did.

Hubble’s Law Review Questions:

To make sure you understand the key concepts essential to this lab, answer the following questions based on your textbook reading assignment:


  1. What debate did Hubble put to rest with his discovery?



  1. What is redshift (p124)?



  1. How did Hubble use redshift to measure the speed of galaxies he observed?



  1. Write the mathematical expression for Hubble’s law:



  1. What does the constant Ho in Hubble’s law represent?



  1. What are two problems astronomers encounter when trying to use Hubble’s law to measure galactic distances?



  1. List the six techniques astronomers typically use to measure cosmic distance. In what way are they part of a chain?


Hubble’s Law Lab Activity Procedure

  1. Open the following web page in your browser. Note that you will be using real data obtained by the University of Arizona.




  1. Select a galaxy image to measure it angular size. Follow the directions and record the galaxy name and angular size (milliradians) in the table below.


  1. Next, go back to the main page and measure the wavelength of the K line by clicking the galaxy spectra. You are only interested in measuring the wavelength of the K line which is labeled on the left spectra. Follow the directions to measure its wavelength and record in the table below.


  1. Calculate and record the redshift using the equation from the theory section.


  1. Repeat steps 2 through 5 so that you have data for at least 20 galaxies.

Data Table:

Galaxy Galaxy Name Angular Size K Wavelength Redshift


  1. Create a graph of redshift versus angular size for your galactic data. Draw a straight line that best fits your data points. You can use any resource to do this including a spreadsheet program such as Microsoft Excel, a graphing calculator, or you may construct the graph by hand on paper. Attach your graph to this document. If necessary scan or photograph your graph and attach the image.


  1. Do you observe any relationship between redshift and angular size from your graph? If so explain.


  1. How is your graph similar to the one Hubble constructed?


  1. What is the meaning of the slope of the line in your graph? Hint: There is a connection to Hubble’s equation.


  1. How does your graph show that the universe is expanding?




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