Dr Paul Allen

NGC 5584 (Photo credit: Wikipedia ) " Billions and billions " of stars in a galaxy (after a quote often mistakenly attributed to Carl Sagan) is how many people imagine the number of stars you would find in one. Is there any way to know the answer for sure? "It's a surprisingly difficult question to answer. You can't just sit around and count stars, generally, in a galaxy," said David Kornreich, an assistant professor at Ithaca College in New York State . He was the founder of the "Ask An Astronomer" service at Cornell University. Even in the Andromeda Galaxy — which is bright, large and relatively close by Earth, at 2.3 million light-years away — only the largest stars and a few variable stars (notably Cepheid variables ) are bright enough to shine in telescopes from that distance. A sun-size star would be too difficult for us to see. So astronomers estimate, using some of the techniques below. Massive investigation The primary way astronomer...

English: Albert Einstein Français : Portrait d'Albert Einstein (Photo credit: Wikipedia ) Hubble came into astronomy in the 1920’s at a very propitious time. It was at just the time that astronomers were gaining access to these large dome telescopes that were able to resolve very tiny pinpoints of light in the night sky. Prior to Hubble and the scientists who were looking into the night sky in the 1920’s, there was debate among astronomers as to whether or not the Milky Way galaxy , in which our solar system resides, was the only galaxy, or whether there might be others beyond it. Hubble resolved that issue as he also resolved these points of light, because as he looked through this great dome telescope at the Palomar Observatory , he was able to determine that little points of light that had been viewed through ordinary telescopes before, and just looked like little points, actually revealed galaxies—whole galaxies with hundreds and millions of stars. The picture behind us is...