M13 is the brightest globular star cluster in the northern sky and was discovered as early as 1714 by the English astronomer Sir Edmond Halley. It is about 25,100 light years away from Earth (the data vary between 23,000 and 26,000 Lj) and has 300,000 times the luminosity of the sun with a diameter of 150 light years.

M13 has an apparent brightness of 5.8 mag and contains about 300,000 stars. It can already be seen as a foggy spot in prism binoculars. In a telescope with an opening of about 10 cm it can be resolved into individual stars at the edge and is therefore a worthwhile object for amateur astronomers.

Like all globular star clusters, they are gravitatively bound to our Milky Way and so M13 orbits the center of our galaxy. But not on their plane, but strongly inclined, so that the Hercules cluster crosses and even passes through the galactic plane twice during each revolution. You could call globular clusters cosmic suburbs to our own galaxy.

A message from SETI

M13 was the destination of the radio signal of the Arecibo message. A strong signal was sent out by the Arecibo radio telescope to contact a possible alien civilization. The globular cluster was chosen because many stars are gathered in a relatively small space, which should increase the chance of encountering an inhabited world. Taking into account the long signal propagation time, however, the response of a technically developed civilization would not arrive on Earth until at least 45,600 years later.