Who was Charles Messier?
Charles Messier was a French astronomer during the late 1700s. He was particularly interested in finding comets, but the limited technology of the time frustrated his efforts. Space is full of things that can look like comets when viewed with a weaker telescope, so he found himself wasting a lot of time trying track these objects.To deal with this problem, he began compiling a list of "not comets" - things like star clusters, galaxies, and nebulae - so he could easily ignore them while he hunted for comets. Eventually this list was published in the 1781 issue of Connaissance des temps, a yearly publication of astronomical research.
Unfortunately, as much as he wanted to be known for discovering comets, he ended up becoming known for this catelogue of things that aren't comets.
What Are Messier Objects?
A Messier Object is one of the many "not comets" that Charles Messier included on his list. Of course, the list was updated over time, growing from his initial 103 objects to a total of 110.These objects are often known by other names - the Crab Nebula and the Andromeda Galaxy are M1 and M31 respectively for example - but astronomers still like to reference them by the numbers Messier assigned to them.
About M13
The target of the Arecibo Message was M13, a large globular cluster of stars found in the constellation Hercules. To find it, first locate the constellation and then look for the four stars that make up the square representing his body. This square is known as the "Keystone", and M13 is located in the middle of Keystone's top side.M13 is visible to the unaided eye, though you can see it more clearly with binoculars or a hobby telescope.
Although this just looks like a big ball of stars, upon closer inspection you should be able to make out three dark "lanes" fanning out in a triangluar or Y shape. This feature is known as M13's "propeller", and it's something unique to this cluster.
Why was M13 Chosen?
M13 was chosen as the recipient of the Arecibo signal for one very simple reason:It was available.
Nobody really expects or cares if the signal actually reaches alien life, and it's highly unlikely that there will be any stars with inhabited planets in the path of the signal.
