 In 1923, after finding the V1 C-feed variable in Andromeda and determining that Andromeda was an entire galaxy over two million light-years from our own, Hubble turned his sights on other spiral and elliptical nebula and found that they were galaxies as well. In his studies of these galaxies, he mapped their radial velocity as determined by the shift in spectral lines against their distance from us. He found what we see here in the Virgo supercluster. NGC 1068 is 35 million light-years away and receding at 784 km per second. NGC 3949 is 50 million light-years away and receding at 1,120 km per second. NGC 44414, a galaxy studied by the key project on extragalactic distance scales, is 62 million light-years away and receding at 1,336 km per second. NGC 4319, a galaxy with both C-feed variables and Type 1A supernova, is 80 million light-years away and receding at 1,792 km per second. NGC 1309, also a galaxy with both C-feed variables and Type 1A supernova, is 100 million light-years away and receding at 2,244 km per second. Hubble found that, except for a few nearby local group galaxies, all the spectra shifts were to the red. All the galaxies were moving away from us. And more than that, he found that the further away from us they are, the faster they are moving away from us. And even more than that, he found that the relationship between velocity and distance is linear. The graph is a straight line. The equation is simple. The receding velocity of a galaxy is equal to the slope of the line, a constant. Times the distance the galaxy is away from us. Today that constant is known as the Hubble constant. And the equation is known as Hubble's law. If we measure the red shift of a galaxy, we can determine its receding velocity. And knowing its receding velocity, this equation tells us how far away it is. This gives us a new rung on our cosmic distance ladder called redshift. The accuracy of this rung depends entirely on the value of the Hubble constant. That's why it's one of the most studied numbers in astronomy and cosmology. This constant has been refined over time and the distance is used to see how far it holds has increased by orders of magnitude with our modern ability to determine distances with space telescopes like Hubble, analyzing Type 1A supernova, out to billions of light years. The box at the lower left shows the region that Hubble probed. The current best value for the Hubble constant using this approach is 22.4 km per second per million light years, plus or minus 3.2. That's around 13 miles per second per million light years. That is a receding velocity of a galaxy goes up by 22.4 km per second for each additional million light years away from us it is. This slow and steady movement of galaxies away from us is called the Hubble flow. This Hubble flow, where galaxies are getting further away with time, also implies that in the past they were closer together. It follows that we can ask how long would it take a galaxy to reach its current distance from us given its current velocity. That's simply the distance divided by the velocity or 1 over the Hubble constant, 13.4 billion years. That's the age of the universe. We'll see later in our chapter on the cosmos that the Hubble constant turned out to not be constant over large enough times and distances. In modern cosmology it is called the Hubble parameter and it gives us a slightly larger age for the universe around 13.8 billion years.