by Chet Raymo
"Here's another APOD photo that deserves a few more words: the Perseus Cluster of Galaxies. What are we looking at?
Click image for larger size.
The photo is of a part of the northern sky in the constellation Perseus that you could cover with the tip of your little finger held at arm's length- a part of the sky in which you would see nothing with the naked eye. So let's start with that. Until the invention of the telescope, nothing.
All of the well-defined dots in the photo, mostly bluish, are stars in our own Milky Way Galaxy, some nearer than others, most of them the very hot blue-white stars that show up at enormous distances. Many more Milky Way stars in this part of the sky- cooler, more reddish stars- are simply not bright enough to show up in the exposure. Beyond these foreground stars we see the mostly yellowish galaxies of the cluster.
Get this. The foreground Milky Way stars are typically several thousand light-years away. The Perseus galaxies are 250 million light-years away. That is, if one of the Milky Way stars was at the tip of your outstretched arm, the Perseus Cluster galaxies would be 60 miles away. And each of the galaxies is another Milky Way, another swarm of hundreds of billions of stars. And- and- a longer exposure of the same part of the sky would show more galaxies beyond the Perseus Cluster, hundreds of thousands of galaxies, reaching back to the beginning of time.
When Galileo turned his telescope on the "Beehive" nebulosity in Cancer, he counted thirty-six stars. To the three stars of Orion's belt he added fifty. When he examined the Milky Way with his instrument, the stars he saw defied enumeration. Those starry nights in the winter of 1609-10 were- or should have been- a turning point in history, an awakening to the vast territories of our ignorance. And Galileo didn't know the half of it.”