Notes to the Pleiades Lab

This lab begins with some graphing.  I have copied those graphs to the back of
the lab form.

1.  Graph 1 is a Color-Magnitude graph from the data of Eggen.  In this graph
only the stars in a particular cluster are used - in this case the Pleiades
(M45).  The Color-Magnitude diagram is a replacement for the H-R Diagram.
Notice that color index (B-V) replaces spectral type and mV replaces Mv.  I
spent some time in the lecture discussing why this works.

2.  The second graph is the standard Main Sequence.

One of the uses of the Color-Magnitude Diagram is in the determination of
distance.  We do this by noting by how much the vertical scales of these two
graphs differ.  This is called distance modulus and is computed in the sense
(mV - MV).  We will compute the distance modulus in two ways and compare the
results.

In Method 1 we simply pick the data points that are circled on the Color-Magnitude
diagram for the Pleiades (Graph 1).  For each point read off the color index
(horizontal scale) and the apparent magnitude (vertical scale).  Using that
color index, go to the Standard Main Sequence (Graph 2) and read off the 
Absolute Magnitude of a Main Sequence star with that color index.  As an example
look at the data point directly below the circled point 22.  The data table for it
would look like...


(mB - mV)               mV                  MV                     mV - MV
----------------------------------------------------------------------------
   0.0                 7.75                1.5                      6.25


In Method 2 we use the entire color-magnitude diagram and match it to the
standard Main Sequence.  Here we rely on the fact that we have the same
scales on the two graphs.  Overlap the horizontal scales and slide
vertically until the standard Main Sequence lies along the bottom envelope
of points in the color-magnitude graph.  A light table is handy, but maybe
your computer screen will work.  A direct readout of the distance modulus is
to note where M=0.0 of graph 2 intersects the vertical scale on graph 1
after the alignment is made between the graphs.

Question 1 - Use the distance modulus determined by each of the methods
and follow the procedure.  We indicate in italics the calculation involved in each
step.  If I recall an answer around 160 pcs is about right.

Question 2 - We are generally safe in making several assumptions about stars
that fall in clusters
a.  they have the same composition
b.  they have the same age
c.  they have the same distance

Which of these are important in the work of this lab?

Question 3 - dust reddens the light and notice that we are observing color
(B-V)  Think about how that affects the results of the distance
determination.  Dust also dims the stars.  How does this affect the distance
determination?