[BLANK_AUDIO] Hi gang and welcome back to Analyzing the Universe. One of the most remarkable and useful characteristics of our universe is that as we look out into space, we are also looking backwards in time. Since light travels about 186,000 miles in one second. When we look at the sun, whose distance is about 93 million miles, we are seeing it as it was about eight minutes ago. Thus, our astronomical observations can yield a history of our universe by examining objects very far away. However, we had no idea of the vastness and hence antiquity of the universe ,until the 1920's when Edwin Hubble discovered some innocuous appearing stars in a nebula or cloud called M31. These objects were Cephei variable stars which we have encountered before, and which have the unique feature that they all say, actually growing and shrinking over a period of several days. Moreover the amount of time it takes for these starts to pulsate is directly related to their intrinsic luminosity. How bright they really are. Once we know an objects true brightness it is a simple matter to use it's apparent brightness to calculate it's distance from us. It was undoubtedly with a trembling hand that Hubble calculated the distance to M31, our nearest spiral galaxy neighbor. It was over one million light years away. Just imagine light that can travel over seven times around the world in one second takes over a million years to reach us from M31. Thus, when you go out on a clear Autumn night and find M 31 in the Constellation of Andromeda you are seeing that object as it actually was when the first human life creatures began to walk the earth. With the knowledge of the distance to far flung astronomical bodies, a wonderful side benefit emerged. We were able to find the dimensions of an object from it's angular size in the sky. We did this recently when we discussed Cass-A. But here we'll calculate the size of M31. Modern, more accurate measurements place it about two million light years away and its apparent size in the sky is three degrees. About the same apparent diameter as six full moons, placed side by side. What is the size of the Andromeda Galaxy? So we have a big skinny triangle. The angle of opening is three degrees. Which is equal to about 1 20th of a radian. We have to get everything in units or non-units of radian so that we can make the comparison of the size in the sky and distances together. And we know we have six full moons spanning M31. And if the distance to M31 is 2 million light years. Then this distance here, the size of andromeda actually is going to be 2 million light years divided by 20. Or approximately 100,000 light years across. With this awesome revelation about the size of the universe. And some of its constituents and avalanche of observations opened our eyes to yet more astounding facts. With this study of other galaxies and its exceedingly interesting relationship emerged. The further a galaxy was away from us measured by the seffiad variables and other techniques, the faster it appeared to be moving, as measure from the doppler shift of it's spectral features. It was as if the entire universe was somehow exploding into space. The faster each piece was moving, the further it would be away from anywhere else after a given amount of time. Because of this unique relationship between velocity and distance, known as the Hubble Law in honor of its discoverer, we now had a new method for measuring distances to really remote objects. All we need to do is measure the doppler red shift from the energy spectrum and deduce the distance from a very simply equation. Velocity equals a constant H, the Hubble constant, times the distance it is away. Here, H is the Hubble constant and has been experimentally determined to be about 70 kilometers per second per mega parsec. Thus for each million parsecs in distance away from us, which is about three million light years, the object's velocity increases by 70 kilometers per second. So for example, an object whose spectrum exhibits a velocity of 500 kilometers per second, would be at a distance of about 7 mega-parsecs from us. The stage was now set for the discovery of the quasars. [BLANK_AUDIO]