Imagine the Universe!
Imagine Home | You Be the Astrophysicist! |

Observing the Light Intensity of M31

You know that the apparent brightness of an object changes as its distance changes, so you should be able to see how much the galaxy M31 moves by observing its change in brightness over a given time period. Finding its velocity is then just a matter of dividing this distance by the time interval. A snap. You can use a light curve of M31 to discern how the galaxy's brightness changes over time.

The function that describes how apparent brightness changes with distance is given by the relationship:

formula for intensity as a function of distance

The intensity of the light varies as the inverse square of the separation from the light source (r).

Info Tell me more about the relationship between intensity and distance
Info Tell me more about lightcurves.
Data Click here to see a light curve for M31 and to use this technique to solve for its velocity.
Experiment Click here to return to the beginning and try another experiment.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

DVD Table of Contents
Educator's Index