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How does this answer compare with the earlier one? If they are different,  what is this telling us about the relative sizes of galaxies in Hydra I  and our galaxy?\subsection{Hubble's Constant}  Reference: Zeilik, Gregory \& Smith, Introductory Astronomy \& Astrophysics,  Chapter 22.  The universe is expanding; every galaxy is moving away from every other  galaxy, and the further two galaxies are apart, the faster they are  receding from each other. This is known as Hubble's Law, after the  famous American astronomer who discovered the expansion of the Universe  back in the 1920's. It can be written as:  \begin{equation}  v = H_0 d  \label{eq:hubble}  \end{equation}  \noindent  where $v$ is the velocity at which something is moving away from us, $d$ its  distance from us, and the constant $H_0$ is known as Hubble's constant.  This constant is basically a measure of how fast the Universe is expanding.  If it is large, the universe is growing like crazy. 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  the Doppler effect. The gas in most Hydra I galaxies emits emission-lines  of Oxygen, Hydrogen and other elements. With a spectrometer on a telescope,  we can measure the Doppler shift in these lines, and hence the velocity $v$.  These velocities are listed in Table \ref{tab:hydragalvecolity}, for several of the brightest  galaxies in Hydra I.