deletions | additions       diff --git a/Now_its_time_time.tex b/Now_its_time_time.tex  index 39343c3..a60b59e 100644  --- a/Now_its_time_time.tex  +++ b/Now_its_time_time.tex 
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`Colour' in astronomy is not colour like the colours of the rainbow. It is defined as the \emph{ratio of how bright a star is when viewed through two different filters}. Remember that ratio of brightnesses is the same as the difference in magnitudes (just from the usual logarithm laws). If, for example,  the magnitude of a star observed through the green filter is $V$, and its  magnitude when observed through a blue filter is $B$, then its colour  could be measured as $B-V$. {\color{red}   \begin{itemize}  \item  If$B-V$ is large,  the object is relatively brighter in green light, compared to blue light. light, then its $B-V$ will be a positive number.  Astronomers would say it had a ``red'' colour, even though it is actually the blue and green light which is being considered. \item  If the object is brighter in blue light than green, its  $B-V$ was negative, it will be negative. It  would then  have a ``blue'' colour. \end{itemize}  }  For all the objects in your CCD frames, compute colours, and plot  these colours against one of the absolute magnitudes (eg. $M_V$  against $B-V$). This is an HR diagram!