     In this lab, you will be using two photographs of the Pleiades to
construct a color-magnitude diagram of this star cluster.
A color-magnitude diagram is similar to a Hertzsprung- Russell diagram,
but it uses the observed parameters color (instead of temperature) and
magnitude (instead of luminosity).  After constructing the color-magnitude
diagram, you will be asked some questions about star clusters.  This lab
is designed to be completed AFTER you have learned about HR diagrams in
your class.  If you don't know what an HR diagram is, wait.

     This lab entails judging stellar magnitudes and colors from different
sizes of star points on two photographs.  Once the magnitudes are
determined, you will have to plot the magnitudes and colors to create
a color magnitude diagram.  This extra-credit lab is worth a full lab
grade due to its complexity and difficulty.

     One picture was taken without a filter, and one was taken with an
amber filter---be sure you know which picture is which!  The magnitude
template consists 9 circles of different sizes.

The lab procedures are as follows: 

\begin{enumerate}
\item   Number the circles on the magnitude template from 1 to 9, with
1 being the largest circle.  These represent the magnitudes of stars.
The brightest star has a magnitude of 1, the dimmest has a magnitude of 9.

\item   Measure the magnitude of a star on the picture taken without a filter
(B image), by comparing it to the magnitude template you have constructed.
If the star size falls between, say magnitude 2 and 3, call it 2.5.
Number the star as \#1 on the image, to make sure you don't measure
that star again.  Enter the magnitude in the B magnitude column of the
table below.

\item   Measure the magnitude of the same star used in step \#2, this time
using the picture taken with the amber filter (V image).  Again, number
the star on the image, and then enter the magnitude in the V magnitude
column of the table.

\item  Repeat steps 2 \& 3 for 20-25 stars.  Try to stay in the center of
the picture, and don't measure any stars that you can't separate out
into a single star.

\item  For all of the stars measured, compute the B-V magnitude by
subtracting the V magnitude from the B magnitude.  The B-V magnitude
represents a star's color.  High values of B-V mean the star is very blue.
Low or negative values of B-V mean the star is more red.

\item  Now, plot B magnitude vs. B-V magnitude on the plot on the following
page.  For each star, make a point on the graph.  Do not connect the
points with a line.
\end{enumerate}

\section{Questions}

\begin{enumerate}
\item  The plot you have just made is also called a Hertzsprung-Russell
Diagram.  One of these axes represents temperature, and one represents
luminosity.  Label the x-axis as temperature and the y-axis as luminosity,
and show which way each quantity increases with an arrow.

\item  Now that the plot looks like an HR diagram, label the main sequence.
If there are stars that don't seem to fall on the main sequence, label
what type of star they probably are (e.g. red giant, white dwarf, etc).
If you don't have any stars that fall outside of the main sequence,
then show where red giants and white dwarfs should be on this plot.

\item  Based on your HR diagram for this cluster, is the Pleiades an open
cluster or a globular cluster, and how can you tell?  (You may need to
read about the different types of star clusters in your textbook before
answering this question.)

\item  Why don't you find many red giants or white dwarfs in this star
cluster?  (Again, your book may be helpful.)

\end{enumerate}

\begin{tabular}{|c|l|l|l|}\hline
Star Number&B magnitude&V magnitude&B-V magnitude\\\hline
1&&&\\			\hline
2&&&\\			\hline
3&&&\\			\hline
4&&&\\			\hline
5&&&\\			\hline
6&&&\\			\hline
7&&&\\			\hline
8&&&\\			\hline
9&&&\\			\hline
10&&&\\			\hline
11&&&\\			\hline
12&&&\\			\hline
13&&&\\			\hline
14&&&\\			\hline
15&&&\\			\hline
16&&&\\			\hline
17&&&\\			\hline
18&&&\\			\hline
19&&&\\			\hline
20&&&\\			\hline
21&&&\\			\hline
22&&&\\			\hline
23&&&\\			\hline
24&&&\\			\hline
25&&&\\			\hline
\end{tabular}

\cleardoublepage

\begin{figure}[p]
\caption{}
\label{fig:cluster}
\centering
\leavevmode
\epsfysize=1.0\textheight
\epsfbox{EXTRA/cluster.ps}
\end{figure}

\cleardoublepage

