Some 1.7 billion years after the Big Bang, galaxies began to form, appearing in clusters across the expanding bubble of the Universe. Hydrogen and helium comprised 98% of this primordial matter and the fusion of these atoms within stars would produce the other elements that now comprise the building blocks of planets and, where existent, life itself.
Our home galaxy, the Milky Way, formed about 12 billion years ago, one member of a cluster that contains thirty major galaxies and many smaller ones. As galaxies go, the Milky Way is very large, harboring 600 billion stars (ten times the average); on the other hand, our galaxy cluster is quite small (the Virgo Cluster, 50 million light years away, contains 2500 galaxies). In all, the Universe is thought to contain 100 billion galaxies, many of which have collided and changed form through time; indeed, scientific evidence suggests that the Milky Way has impacted at least two other galaxies and is on a collision course with the massive Andromeda Galaxy.
The Milky Way, a disc-shaped, spiral galaxy, is centered on a massive black hole; our sun, which developed 5 billion years ago, lies within the periphery of the galaxy, in the Orion Arm. The entire galaxy revolves around the black hole and, since we are 26,000 light years from the center, it takes our sun 200 million years to complete one revolution; the sun is now on its 25th lap (its 24th circuit began at the dawn of the Mesozoic Era). Since our solar system is near the outer edge of the Milky Way, we have a limited view of the Galaxy and far fewer stars in the night sky than we would if we were closer to its center (in which case we would have no night).
It's truly difficult to ponder these facts without feeling insignificant! To believe that we are the only intelligent beings in the Universe is either blind arrogance or pure folly.
