deletions | additions       diff --git a/Find_the_average_vel.tex b/Find_the_average_vel.tex  index e5a4856..aa18cfc 100644  --- a/Find_the_average_vel.tex  +++ b/Find_the_average_vel.tex 
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and the rest of the solar system is probably similar. The age of our  galaxy is estimated as about $10^{10}$ years.   \subsection{Advanced problems}  Make a map of Hydra 1. Get some overhead projector sheets and washable  marker pens. Put the transparent sheet on top of the Schmidt plate, and  mark the positions of the galaxies. You might like to use a colour code,  with different colours for spirals, ellipticals, and things just too darn  faint to decide. Don't bother mapping the edges of the plate \ -- \ concentrate  on the centre where the cluster is.  You can't possibly map every galaxy on the plate \ -- \ most are so  small you would need a microscope to tell them apart from the stars. So only  do the bigger ones, but try and be consistent; if you include smaller   galaxies in one part of the plate than another, it will introduce spurious  features into your map.  How big is the cluster (in Mpc, using your distance to the cluster)? How  many galaxies does it have? What fraction are spirals and ellipticals?  Do both types of galaxies live equally in all parts of the cluster? Is  the cluster nice and round, or is it distorted? If a cluster has distortions  and lumps, it tells you that it is a newly formed cluster, that has not  had time to settle down in equilibrium.  \subsection{Appendix 1: Galaxy types}  Galaxies come in two main types, spirals and ellipticals. Nobody knows