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You Be the Astrophysicist: What is the Velocity of M31?

optical image of Andromeda galaxy
Credit & Copyright: Jason Ware (

You have been given a challenge question by your astronomy professor. The first person to solve the question, using astronomical experiments and data (instead of looking up the answer in a book) will be excused from exams for the rest of the year! With this motivating factor, you set out to answer the question, "what is the radial velocity (the velocity in a straight line towards or away from us) of the galaxy M31 with respect to our galaxy?"

Info Click here for more information on M31.

Resources available to you include the University's intro astronomy equipment (for example an optical telescope), as well as astronomical data available in print and online. Use your good understanding of the laws of physics to select an experiment which will help you to find the answer.

With which experiment would you like to begin?

Experiment Use the 1/r2 relationship of light and the galaxy's intensity to determine its distance at two different times and divide the distance traveled by delta t.

Experiment Observe the spectrum of M31 and compare its emission lines to those of calcium and hydrogen in the laboratory.

Experiment Use Hubble's Law (the relationship between a distant galaxy's distance and its speed) to try and solve the problem.

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