ζ Herculis

Zeta Herculis is a binary (3.0, 5.4; 145, 1.1") whose companion makes an orbit every 34.5 years.

The star forms a corner of the 'keystone' of the constellation, the central quadrangle. To find zeta, move southwest eight degrees from pi (the easiest star to find in this part of the skies): binoculars.

Zeta and epsilon (seen in the same field) form the southern portion of the 'keystone', the lopsided square that is the most recognisable feature of the constellation. The keystone is too large to get more than two stars in a binocular field at one time.


Due north of zeta a little over a binocular field is eta, which forms the northwest corner of the keystone: binoculars.

In the same field as eta Herculis is the bright globular cluster M13, the Great Hercules Cluster, considered the finest globular cluster in the Northern Hemisphere.

M13 is thought to be at least 10 billion years old. Well over 300,000 stars make up the cluster; it has an apparent magnitude of 5.8, nearing naked-eye visibility in perfect conditions. A fine sight in binoculars, but it takes a small telescope to resolve its stars.

M13 was the destination of the famous Arecibo message, written by Carl Sagan and others, sent 16 November 1974 with appropriate fanfare. The message, which described mankind and a bit of his culture, was proportedly designed to be read by extra-terrestrials, should they come across the transmission. However the M13 cluster, at 25,000 light years away, means that the message would not arrive until 25,000 years later, by which time the cluster itself would have dispersed and moved on; there is no way the message could ever be received, a fact which the message-senders didn't disclose to the public. It was more a nod to the scientists and engineers who devised the exercise than it ever was about contacting extra-terrestrial life forms.


Moving far to the east now, we have several outstanding binaries to examine.

The region is most easily found with the naked eye. Start at zeta, move down southeast to delta, then follow a line of bright stars northeast, the same apparent distance between each other: lamda and mu.
      mu Her is a striking binary (Struve 2220) with an orange-yellow primary and a red companion, A-BC: 3.5, 9.8; 247, 35.2".

Keeping xi Her in your field of view, move east half a field: binoculars .

Two binaries are in this field, 99 Herculis and 100 Herculis. 99 Her is a multiple star system of wide but faint companions (except for AB):
      AB: 5.3, 8.9; 317, 1.2" (orbit)
      AC: 5.3, 10.7; 59, 96.7"
      AD: 5.3, 11.1; 103, 140.1"
      AE: 5.3, 10.8; 78, 169.1"
      AF: 5.3, 10.7; 165, 162.4"
      AG: 5.3, 11.4; 0, 177.5".

100 Herculis has a curious colour contrast (yellow-white and blueish-white): 5.8, 5.8; 183, 14.2".

Now click on 95 Herculis on the map.

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1999-2014 by Richard Dibon-Smith.