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\subsection{Hubble's Constant}  Reference: Zeilik, Gregory \& Smith, Introductory Astronomy \& Astrophysics,  Chapter 22.  The universe is expanding; Observations of distant galaxies made by Edwin Hubble in the 1920s turned up an interesting fact:  that  every galaxy is moving away from every other galaxy, and the further apart  two galaxies are apart, are,  the faster they are receding from each other. This observation  is known as Hubble's Law, after explained by realising that  the famous American astronomer who discovered velocity is due to  the expansion of the space   between galaxies, rather than relative motion between the galaxies, i.e. the  Universe back in is expanding.   A common analogy is how  the 1920's. It distance between two points drawn on an uninflated balloon increases  as the balloon is blown up.  This is known as Hubble's Law, and it  can be written as: expressed as  \begin{equation}  v = H_0 d  \label{eq:hubble}  \end{equation}  \noindent  where $v$ is the apparent recessional  velocity of a galaxy  at which something is moving away from us, $d$ its a  distance $d$  from us, and the constant $H_0$ is known as Hubble's constant. $H_0$ has the interesting units of km/s/Mpc (provided $v$ and $d$ are given in  km/s and Mpc respectively), which implies that the further away an object  is from us the faster its recessional velocity.  This means that the Hubble  constant is basically a measure of tells us  how fast quickly  the Universe space in between galaxies  is expanding. If it is large, then  the universe Universe  is growing like crazy. expanding rapidly.  We are going to use the distance we measured to the Hydra I cluster to estimate $H_0$. We have already measured the distance $d$ to Hydra I. Now we need the  velocity at which it is rushing away from us $v$. This is measured using