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\subsection{Distance to Hydra I}  In the first half of this exercise, we will estimate the distance to  the Hydra I cluster. Getting distances is always the most a  difficult exercise in astronomy; in many ways it resembles a black art more than  a science. We we  can't take a ruler to anything outside our Solar System (our rockets are much too slow), and triangulating only works to  distances of about 20 parsecs, so we have to use guesswork. The other techniques.   A common  method for measuring distances  is this; we assume that we know some property of so-called `standard candles'.   As an analogy, imagine you are looking through  a distant object as  it really is. For example, we might assume that we know how bright telescope at  a particular distant star is. Then we very far-away  lightbulb.   If you can  measure the amount of light coming into your telescope from the  lightbulb, and you know that it is a 60W tungsten filament bulb, you can  calculate exactly  how bright far away  it appears is by comparing the measured brightness in your telescope  to what you get  from the Earth. same kind of lightbulb in your desk lamp one metre away.  This technique works great for lightbulbs, but its a little trickier for stars and galaxies.  The ratio biggest problem  of the course is that no  two gives us stars or galaxies are exactly alike, so you  don't always have a nearby reference point that is totally reliable.  Another issue, especially when using brightness scales, is that  the distance to Universe between source and  telescope isn't empty, and the stuff in between distorts  the star. image you see, biasing  your results.  The method we will use in this exercise is galaxy diameters. If we  knew how big a galaxy really is, and could measure how big it appears,