GLOBULAR STAR CLUSTERS

Messier 13, the great globular cluster in the constellation of Hercules is generally considered the most spectacular globular cluster in the northern skies. It was discovered in 1714 by Edmond Halley. At magnitude 5.8, M 13 is barely visible to the naked eye on very dark nights with little light pollution. Even small telescopes will resolve it into an extensive, beautiful mass of stars. One of the reasons it appears quite large and bright is that it is relatively nearby, at about 25,000 light years away. At that distance, its angular diameter of about 23 minutes corresponds to a distance of about 145 light years. It has a total luminosity of around a quarter of a million suns and contains several hundred thousand stars. The night sky seen from a planet near the center of this cluster would be filled from horizon to horizon with thousands of stars brighter than our planet Venus or the brightest star in our skies, Sirius. In 1974, the Arecibo radio telescope transmitted a message toward M 13, intended to communicate the existence of human intelligence to hypothetical extra-terrestrials in the cluster. The message will not reach M 13 for another 25,000 years and when it does arrive, M 13 will no longer be where the message was aimed. Thus, the Arecibo message was really more of a technological demonstration than a serious attempt to contact alien life. Technical/capture data is here.