Trigonometric parallax is the main distance-finding method that you have learned about thus far. Although trig parallax is very accurate for finding distances to nearby stars it is not very useful for determining distances to more distant objects such as the star clusters you are studying in this lab. Different techniques are required to determine distances to clusters. You will now learn about one such technique. You will use it to calculate the distance to two star clusters. We will start with the open cluster called the Pleiades.
Look at the plot below.  There are two groups of points.  From our understanding of the physics of stars, we know how luminous a Main Sequence star of a certain temperature (or Spectral Type) should be.  This is what the black points show (except we plot Absolute Magnitude instead of Luminosity).  The blue points are the apparent magnitudes (that is, how bright the stars appear to be) for Main Sequence stars in the Pleiades cluster, whose distance we want to know.
Now recall that the difference in the apparent and absolute magnitude of a star gives you its distance (this is just the inverse-square law for light expressed in terms of magnitudes).  So you just need to measure the vertical size of the gap to find the distance to the cluster.  Do this for stars of a few different temperatures and use the average.  Use THIS LINK to convert differences in magnitude to distance.

3. What is your estimate of the distance to Pleiades?

4. If we were to move the Pleiades closer to us, describe what the plot on the Distance page would look like.  Would the BLUE or BLACK points move, or both, and in which direction?



When you have determined the distance to the Pleiades apply the same technique to the globular cluster M5, using its H-R Diagrams on the "Cluster H-R Diagram" page. Remember to compare magnitudes for the same temperature. Do you get a very different distance?

5.  Look at the H-R diagram of M5 on the "Cluster H-R Diagram" page. Near which letter are the stars still on the Main Sequence?

6.  Determine the distance to the globular cluster M5 using the same method you used for the Pleiades. Remember, the difference in magnitudes is determined at the same temperature.