Andromeda Galaxy, also known as "Andromeda Nebula": the most distant object that human eyes can see without the help of technical instruments.
If you gaze at the sky during a clear autumn night from a location far from city lights, you will notice a faint, diffuse, luminous spot right behind the constellation Andromeda (you should first give your eyes about half an hour to adjust to the darkness). This spot is the Milky Way's neighboring galaxy,* located about 2.7 million light-years away from us. To behold its rather unspectacular appearance is to come as close as the unaided human eye can come to seeing across an infinite distance.** Each particle of the light that you see from this galaxy began its journey of 25 quintillion kilometers from some distant sun about 2.7 million years ago, only to land eventually in your right or left eye.
Andromeda Galaxy (NASA, Hubble Space Telescope)
Andromeda is the closest galaxy*** to our own; its diameter of 150,000 light-years makes it the largest member of the Local Group, a group of galaxies that also includes our Milky Way. It was first described in the tenth century A.D. by the Persian astronomer Al-Sufi, who named it "Small Cloud". In 1917, the Mount Wilson Observatory telescope discovered the first evidence of individual stars existing inside the nebula. Only then did we realize it to be a galaxy.
The Andromeda Galaxy moves toward us at a speed of approximately 500,000 km/h and will probably collide with the Milky Way in about 4 billion years. However, this should not worry us: by that time, the Earth will have long been uninhabitable due to the ever-increasing radiant power of the sun (see ►end).
* A galaxy is a collection of billions of stars held together by gravity and usually rotating around a joint center. Stars and galaxies are not distributed randomly in the universe; rather, gravity has arranged them hierarchically into star clusters, galaxies, galaxy clusters, and galaxy superclusters.
** Telescopes, by contrast, are able to see up to 40 billion light-years into outer space. You can find images of some of the things that can be seen there under ►telescope.
*** Some dwarf galaxies in the constellations Sagittarius and Canis Major are even closer to us and will be devoured by the Milky Way in a few million years. These, however, are so small that they can be observed only in the infrared region with strong telescopes.